characterization of the adsorbed species of molecular iodine ...

CHARACTERIZATION OF THE ADSORBED SPECIES OF

MOLECULAR IODINE ON METAL SUBSTRATES

By

CHELSIE LEE BECK

A dissertation submitted in partial fulfillment of the requirements for the degree of

DOCTOR OF PHILOSOPHY

WASHINGTON STATE UNIVERSITY Department of Chemistry

MAY 2021

© Copyright by CHELSIE LEE BECK, 2021 All Rights Reserved

© Copyright by CHELSIE LEE BECK, 2021 All Rights Reserved

ii

To the Faculty of Washington State University: The members of the Committee appointed to examine the dissertation of CHELSIE LEE

BECK find it satisfactory and recommend that it be accepted.

Sue B. Clark, Ph.D., Chair

Brian H. Clowers, Ph.D.

Samuel A. Bryan, Ph.D.

Brian J. Riley, Ph.D.

Mark D. Engelmann, Ph.D.

iii

ACKNOWLEDGMENT

This work would not have been possible without the support and encouragement of many

people. Foremost, I would like to thank Dr. Brian Riley who was instrumental in the day to day

experimental planning and project execution. His selflessness and willingness to help others is

truly admirable. I would like to thank my committee chair Dr. Sue Clark; she is the smartest

person I have ever met and an inspiration to female scientists. I would also like to thank my other

committee members; Dr. Sam Bryan, Dr. Mark Engelmann, and Dr. Brian Clowers. Over the

years, I have been fortunate to work with many amazing and talented scientists at Pacific

Northwest National Laboratory (PNNL). There are many colleagues at PNNL that inspired and

encouraged me to pursue a PhD, in particular Dr. Samuel Morrison, who is one of the hardest

workers I have met. There are numerous colleagues at PNNL that directly contributed to my

understanding of iodine, specifically Dr. Sarah Saslow and Dr. Hilary Emerson. PhD research

can be a frustrating experience and I would like to thank Dr. Derrick Seiner and Dr. Brienne

Seiner who both listened to my crazy ideas and attempts at understanding the data. Dr. Nathaniel

Smith was instrumental in the development of the flow through system. There are also many

experts at PNNL that provided characterization for my samples including Dr. Mark Engelhard,

Dr. Saehwa Chong, Dr. Luke Sweet and Dr. Nabajit Lahiri. For my funding I would like to

acknowledge the Chemical Dynamics Initiative at PNNL.

I would also like to thank my family. My husband, Kyle, has always been supportive in

everything I decide to do. My three children, Callie, Oliver and Everly, they bring me so much

joy and remind me what is truly important. My mother who taught me the value of education and

that I was capable of anything. My father who always demonstrated a strong work ethic. My

iv

aunt, who taught me to be open minded and question everything. To my extended family of

grandparents, aunts, and uncles, thank you for always supporting me.

v

CHARACTERIZATION OF THE ADSORBED SPECIES OF

MOLECULAR IODINE ON METAL SUBSTRATES

Abstract

Chelsie Lee Beck, Ph.D Washington State University

May 2021

Chair: Sue B. Clark

Molecular iodine is one volatile species of iodine which is released in the event of a

nuclear accident. The release of radioiodine poses a concern to public health because the human

body relies on iodine for normal metabolic processes and will bioaccumulate iodine from food

and other sources. If radioiodine are present in these sources, they are accumulated with stable

iodine in the thyroid and can lead to thyroid cancer or other thyroid disorders. Iodine has one

stable isotope with 127 amu. There are two major isotopes of radioiodine: 131I and 129I. 131I has a

half-life of 8.04 days and poses the greatest threat to human health due to its high specific

activity. 129I has a half-life of 15.7 mil years and consequently becomes a long-term problem for

the environment. Other isotopes of radioiodine exist but they have much shorter half-lives.

Molecular iodine, I2, is one gaseous species of iodine and the closer to the source of the accident

the larger the fraction of gaseous iodine which is expected to be in this form. Further away the

fraction goes down and that is due to the reactivity of I2. Molecular iodine will react with most

surfaces to physisorb and with metal surfaces it is known to chemisorb. The chemical

vi

transformation that occurs upon chemisorption changes the species of iodine and therefore its

properties, including the volatility and the chance for resuspension.

This work seeks to understand the transformations that iodine undergoes upon adsorption

to understand how the transport of radioiodine is impacted and in turn the concern for public

health and safety. Stainless steel is used throughout reactors and is therefore a surface of interest

for the interaction with iodine. This manuscript focuses on characterizing and understanding the

species of adsorbed iodine on stainless steel and its metal constituents. As well as determining

the parameters impacting the speciation and adsorption such as the humidity and oxygen.

vii

TABLE OF CONTENTS

Page

ACKNOWLEDGEMENTS ........................................................................................................... iii

ABSTRACT .................................................................................................................................... v

TABLE OF TABLES ..................................................................................................................... x

TABLE OF FIGURES .................................................................................................................. xii

CHAPTER ONE ............................................................................................................................. 1

1.1 Concern of Radioiodine ................................................................................................... 1

1.2 Iodine Chemistry .............................................................................................................. 4

1.3 Photolytic Reactions ......................................................................................................... 5

1.4 Adsorption ........................................................................................................................ 6

1.5 Reactivity ......................................................................................................................... 7

1.6 Adsorption on Metal Surfaces ........................................................................................ 12

1.7 Exposure Conditions ...................................................................................................... 14

1.8 Goals of Work ................................................................................................................ 16

1.9 References ...................................................................................................................... 17

1.10 Attributions..................................................................................................................... 21

CHAPTER TWO .......................................................................................................................... 23

2.1 Abstract .......................................................................................................................... 24

2.2 Introduction .................................................................................................................... 25

2.3 Experimental .................................................................................................................. 26

2.4 Results and Conclusions................................................................................................. 29

2.5 Conclusions .................................................................................................................... 44

viii

2.6 Acknowledgements ........................................................................................................ 44

2.7 References ...................................................................................................................... 45

CHAPTER THREE ...................................................................................................................... 49

3.1 Abstract .......................................................................................................................... 51

3.2 Introduction .................................................................................................................... 52

3.3 Experimental .................................................................................................................. 53

3.4 Results and Discussion ................................................................................................... 58

3.5 Summary and Conclusions ............................................................................................. 70

3.6 Funding Sources ............................................................................................................. 72

3.7 Acknowledgements ........................................................................................................ 72

3.8 References ...................................................................................................................... 72

CHAPTER FOUR ......................................................................................................................... 76

4.1 Abstract .......................................................................................................................... 77

4.2 Introduction .................................................................................................................... 77

4.3 Experimental .................................................................................................................. 80

4.4 Results and Discussion ................................................................................................... 84

4.5 Summary and Conclusions ........................................................................................... 101

4.6 Acknowledgements ...................................................................................................... 102

4.7 References .................................................................................................................... 102

CHAPTER FIVE ........................................................................................................................ 105

5.1 Conclusions .................................................................................................................. 105

5.2 Ongoing (Unpublished) Work ...................................................................................... 108

5.3 Future Work ................................................................................................................. 110

ix

5.4 References .................................................................................................................... 111

APPENDIX ................................................................................................................................. 112

6.1 Chapter 2 Supporting Information ............................................................................... 112

6.2 Chapter 2 Supporting Information References............................................................. 119





x

LIST OF TABLES

Page

Table 1-1. Isotopes of iodine and half-lives given in minutes (m), hours (h), days (d) and years

(y) [2]. *EC = electron capture, ß- is beta decay, ß+ is positron emission. ..................................... 2

Table 1-2. Physical properties of some metal iodides [18]. “Sol.” denotes water solubility where

“S” is soluble, “VS” is very soluble, “SS” is slightly soluble, “I” is insoluble, “R” reacts with

water, and “-” means no data was found. “Hygro” is the hygroscopic nature of the compound. ... 5

Table 2-1. Starting sample mass (mi) for SS316L+I and SS304L+I samples and gravimetric

iodine uptake (m%I,g; mass adsorbed per mass of starting material). At the bottom of the table

averages and standard deviations (in parenthesis) are provided for each set. .............................. 32

Table 3-1. Summary of sample names and descriptions including atmosphere and temperature

(T). RT denotes room temperature (22±3°C). ............................................................................... 55

Table 3-2. Summary of iodine uptake experiment on Fe granules including the atmosphere

(atm.), the sample mass (ms), the iodine mass (mI), the iodine uptake in mass% (m%I), the post-

desorption uptake (mI,pd), and the post-desorption iodine uptake in mass% (m%I,pd). Both

experiments were conducted at 60°C. ........................................................................................... 65

Table 3-3. EDS data of Fe granules. Compositional data is in atomic% and 1σ standard

deviations are provided in parenthesis below the values .............................................................. 68

Table 4-1 Results of control studies.............................................................................................. 91

Table 4-2. Summary of comparison of materials data for experiments run at RH = 45.0 ± 2.5%.

....................................................................................................................................................... 92

Table 5-1. Summary of mass uptake for Ni-200 coupons exposed to similar amounts of iodine

with light and without light. Relative humidity was 25-29% for both samples. ........................ 109

xi

Table 5-2. SEM-EDS data of Ni-200 coupons exposed with and without light. ........................ 110

xii

LIST OF FIGURES

Page

Figure 1-1. Picture depicting some of the possible gaseous iodine species that can be released in

a nuclear accident. I2 is circled to indicate that it was the focus of this work. ............................... 1

Figure 1-2. Diagram of gas phase iodine species released from a reactor accident and the

subsequent uptake pathways for humans [12]. ............................................................................... 3

Figure 1-3. Reactions of iodine in the atmosphere [21].................................................................. 6

Figure 1-4. Iodine interactions with stainless steel in air. This figure was taken from Wren et al.

[7] and reprinted with permission from Elsevier © 1999. ............................................................ 11

Figure 1-5. An example of halogens diffusing on surfaces with a low Miller index. This figure

was taken from Andryushechkin et al. [37] and reprinted with permission from Elsevier © 2018.

....................................................................................................................................................... 13

Figure 1-6 Dowben [38] reprinted with permission from Taylor and Francis © 1987. ............... 14

Figure 1-7. From Abrefah et al. [9] reprinted with permission from Taylor and Francis © 1994.

....................................................................................................................................................... 15

Figure 2-1. SEM images of as-received stainless steels including (a,c) SS304L and (b,d) SS316L

shown at different magnifications. The top row is 100× magnification and the bottom row is

1000×. ........................................................................................................................................... 30

Figure 2-2. Pictures of triplicate iodine exposures of (a) SS304L+I and (b) SS316L+I in separate

containers. The numbers next to each image denote the replicate number and the scalebar is valid

for all pictures. SS304L was exposed for 34 days and SS316L for 30 days. ............................... 31

xiii

Figure 2-3. Picture of SS316L+I (a) one minute after the iodine exposure chamber was opened

and (b) two hours after being exposed to atmosphere. The arrows in (b) are callouts for the

regions most affected by air exposure. ......................................................................................... 34

Figure 2-4. SEM micrographs of (a) unexposed SS304L and (b-d) iodine-loaded SS304L+I at (b)

500×, (c) 2500× showing the corrosion layer, and (d) 10,000× showing the corrosion layer ...... 34

Figure 2-5. SEM micrographs of SS316L+I replicate 1 and replicate 2 including (a) 500× of

SS316L+I-1, (b) 1,000× of SS316L+I-1, (c) 500× of SS316L+I-2, and (d) 1,000× of SS316L+I-

2..................................................................................................................................................... 35

Figure 2-6. (a) BSE-SEM micrograph of SS304L+I-1 at 7500× and (b-g) EDS phase map and

elemental maps. EDS spot analysis regions are shown in (a) with the corresponding data

provided in Figure 2-7. The scalebar shown in (a) is valid for all images. A different version of

this figure is provided in Figure S2-2 (Supporting Information) that shows a higher-

magnification view of the EDS regions and phase map in (a) and (b), respectively. ................... 36

Figure 2-7. Summary of EDS data from spot analysis in Figure 2-6a showing the high-Fe and

high-Ni regions. Regions 1-3 were averaged for the high-Fe data and regions 4-8 were averaged

for the high-Ni data. The minimum and maximum values can be found in Table S2-4. ............ 37

Figure 2-8. XRD patterns for SS304L, SS316L, SS304L+I, and SS316L+I. The term “AR”

denotes “as-received”, “*” denotes the diffraction peaks associated with the base materials and

“” denotes those associated with fougerite (see text for more information). ............................. 40

Figure 2-9. (a) Comparison of ICP-OES and EDS of SS304L+I-2 (taken at 350× magnification).

Error bars are 1 standard deviation. (b) The comparison of ICP-OES leachate and EDS analysis

of SS304L+I-2 to the EDS data of the base SS304L material. Error bars are 1 standard deviation.

The minimum and maximum values can be found in Table S2-5. ............................................... 43

xiv

Figure 3-1. SEM regions for Fe+I at different magnifications (EDS locations are shown in Figure

S3-4 for this sample). .................................................................................................................... 60

Figure 3-2. XPS spectra of nickel particles before and after exposure including the (a) C 1s

regions, (b) the O 1s regions, and (c) the I 3d regions. For (a), the intensities were normalized to

the carbon 285.0 eV carbon peak. For (b), the intensities were normalized to the 531.4 eV

oxygen peak. For (c), the intensities were normalized to the I 3d5/2 peak at 619.3 eV. ................ 62

Figure 3-3. Mass uptake (m%I,g) by Fe granules over an ~18-day exposure. Error bars are 1σ of

three replicate samples. ................................................................................................................. 63

Figure 3-4. SEM micrographs of as-received materials used in room temperature exposures. The

top row is 100× magnification and the bottom row is 1000×, with the (a) Ni, (b) Fe, (c) Cr, 9d)

SS304L, and (e) SS316L. Note that sample packing on the stub is not consistent. The four

images on the far right for SS304L and SS316L were taken from Beck et al. [8] and reprinted

with permission. ©2020 American Chemical Society. (f) SEM micrographs of three Fe granules

showing the differences in appearance and size. .......................................................................... 64

Figure 3-5. SEM-EDS data for Fe granules FeGN60 (8c-2) after exposure showing (a) BSE-SEM

micrograph, (b) Fe map, (c) O map, (d) I map, and (e) the phase map. ....................................... 68

Figure 3-6. (a) Fe granule (FeGA60) exposed in air at 60ºC. (b) Fe granule (FeGN60) exposed in

N2(g) at 60ºC. (c) Fe granule (FeGN60) exposed in N2(g) at 60ºC with different morphology than

(b) and at higher magnification, inset shows full particle. (d) Localized area from image (c)

showing concentric rings of oxidation with iodine adsorption in between oxide layers. (e) higher

magnification of image (d) where a dot map was collected, which is shown in (g)-(j) with boxed

region corresponding to line scan region shown in (f). ................................................................ 70

Figure 4-1 Schematic of flow through system where “FM” denotes flow meter. ........................ 82

xv

Figure 4-2. Mass uptake relative to the surface area (SA) of the five materials over a 14-day

period of exposure to I2(g) in ambient conditions. Vertical lines corresponding with data markers

indicate when I2 was removed from the system in each experiment. ........................................... 85

Figure 4-3 SEM micrographs showing the edge views of the I2(g)-exposed Ni-200 coupon taken

at different orientations. ................................................................................................................ 86

Figure 4-4. SEM micrographs (500×) of different materials (a) before iodine exposure, (b) post

iodine exposure, and (c) after leaching. ........................................................................................ 88

Figure 4-5. Exposure of metal coupons to I2(g). For the first 30 min (t ≤ 30 min), the atmosphere

contained dry air, humidity was introduced at t = 30 min (45.0 ± 2.5% RH), and t ≥ 60 min was

dry air until weight loss plateaued. ............................................................................................... 93

Figure 4-6. SEM at 75× magnification for Ni-200 exposed to various levels of humidity. ......... 97

Figure 4-7. Summary of mass uptake and iodine sublimation rate for experiments run under

different relative humidity (RH) levels. ........................................................................................ 99

Figure 4-8. SEM of SS316 exposed in (a,c) air and (b,d) argon................................................. 101

Figure 5-1. Ni-200 coupons exposed to iodine at 27% RH run in (left) no light or (right) with

light. ............................................................................................................................................ 109

xvi

Dedication

I dedicate this thesis to my mother, Niki Swanson. I would not be the person I am today without

your love and encouragement.

1

CHAPTER ONE

INTRODUCTION

1 1

1.1 Concern of Radioiodine

A nuclear accident such as Fukushima in 2011 is a potential concern to public health.

Volatile fission products can be released through multiple pathways. Iodine is a fission product

with various volatile species, shown in Figure 1-1, that have the potential to be released. The

inventory of iodine in used nuclear fuel is relatively low, with only 12 kg of iodine in 1000 kg of

fission products [1]. However, the multitude of short-lived isotopes shown in Table 1-1 represent

a large portion of the activity [1, 2]. One of the isotopes of greatest radiological significance in a

nuclear accident is 131I because it has high specific activity and a t1/2 long enough to be a concern

for public health [3, 4].

2

Figure 1-1. Picture depicting some of the possible gaseous iodine species that can be released in a nuclear accident. I2 is circled to indicate that it was the focus of this work.

The radioisotope of 131I is a fission product with a cumulative fission yield of 2.89×10-2

to 3.16×10-4 atoms from thermal-induced fission of 235U [5] with t1/2 = 8.025 d (see Table 1-1).

Iodine has one stable isotope with 127 amu, i.e., 127I. There are also a variety of radioiodine

isotopes that are produced from fission of uranium. Iodine can be volatile and, in the case of a

nuclear accident, it is one of the fission products that are a major concern due to relatively long

half-life and high fission yield [6-10].

Table 1-1. Isotopes of iodine and half-lives given in minutes (m), hours (h), days (d) and years (y) [2]. *EC = electron capture, ß- is beta decay, ß+ is positron emission.

Isotope Half-Life (t1/2) Decay mechanism* 123I 13.27 h EC 124I 4.18 d EC, ß+ 125I 59.41 d EC 126I 13.11 d EC, ß-, ß+ 127I Stable – 128I 24.99 m ß-, EC, ß+ 129I 1.57×107 y ß-

3

130I 12.36 h ß- 131I 8.02 d ß-, ß- 132I 2.30 h ß- 133I 20.8 h ß-, ß- 134I 52.5 m ß- 135I 6.57 h ß-, ß-

The 129I radioisotope is the longest-lived isotope of iodine (t1/2 = 1.57×107 y) and is the

greatest concern for long-term environmental implications. Stable iodine is an essential nutrient

for both animals and humans and is accumulated in the thyroid from food sources [11]. In the

case of radioiodine, this leads to multiple pathways for ingestion and bodily incorporation, such

as direct ingestion from particles in the air, drinking water, or plants, as illustrated in Figure 1-2.

Figure 1-2. Diagram of gas phase iodine species released from a reactor accident and the subsequent uptake pathways for humans [12].

There is also the possibility of ingestion from milk from animals that have ingested

radioiodine [13]. The amount of 131I released to the environment from nuclear accidents can vary

and has ranged from 1.5×1017–1.6×1017 Bq in Fukushima [6, 14] to 2.7×1017 Bq in Chernobyl

[15]. The massive amounts of radioiodine can be a serious threat to human health, especially for

4

children. Children are more suspectable than adults because their thyroids are actively

accumulating iodine. The region of Gomel, which is north of Chernobyl, received the largest

plume of radioactivity and saw an increase in thyroid cancer in children from 1-2 cases per year

before Chernobyl to 38 cases in 1991 [16]. The increase was not as drastic in regions further

from Chernobyl such as Brest and Grodno, however the cases in all the regions combined went

from 4 cases per year in 1987 to 55 cases in 1991 [16]. Thyroid cancer from other factors is often

treatable but the thyroid cancer caused by radioiodine was extremely aggressive and sometimes

deadly [16]. Acute injury causing loss of thyroid function is also possible [17].

1.2 Iodine Chemistry

Iodine is the heaviest stable halogen with many similarities to bromine and chlorine in

terms of possible oxidation states (i.e., -1, 0, +1, +3, +5, +7), binding environments, and chemical

reactivities. The high oxidation states are generally found in iodine compounds with very

electronegative elements such as oxygen and fluorine, i.e I2O5 [18]. Unlike chlorine, which is a

gas [i.e., Cl2(g)] at standard temperatures and pressures (STP), and bromine, which is a liquid [i.e.,

Br2(l)] at STP, iodine is a blue/black solid [i.e., I2(s)] with a metallic luster. However, iodine solid

has a high vapor pressure of 0.031 kPa at 25°C and sublimes to form I2(g) [18]. Molecular iodine

is a large linear molecule that due to its size has a long bond length of 133 pm (covalent radius)

and low bond strength, with a dissociation energy of 151 kJ/mol [18].

All halogens are one electron from a noble gas electron configuration, so they easily accept

electrons and are consequently good oxidizers. However, the electronegativity decreases from

fluorine to iodine making iodine the weakest oxidizer and reactions between iodine and the other

5

halogens will lead to iodine being oxidized, as shown in Equation 1-1. On the opposite side of that

trend, iodine is the strongest reducing agent.

2I-(aq) + Cl2(g) → I2(s) + 2Cl- (1-1)

Although it is less electronegative than the lighter halogens, iodine still forms iodides

with most elements except for noble gases. The iodide anion is one of the largest with a radius of

220 pm and it is also less hydrophilic than the smaller halogen anions. The metal iodide bonds

range from ionic to covalent. Ionic bonds are formed with the alkali metals such as sodium, NaI

and covalent bonds with transition metals such as titanium, TiI4. The ionic bonds are largely

soluble in water and other polar solvents but the non-polar covalently bonded metal iodides, such

as PbI3 and AgI are not. There are many transition metals which are covalently bonded but with a

large enough (>0.5) difference in electronegativity to be categorized as polar covalent and those

compounds i.e NiI2 and FeI2 can be water soluble. For metals with multiple oxidation states, the

reducing power of iodine, mentioned previously, generally leads to metal iodides of the lower

oxidation state for the cation, e.g.. FeI2 is more favorable than FeI3. The solubility of some metal

iodides and other properties are noted in Table 1-2.

Table 1-2. Physical properties of some metal iodides [18]. “Sol.” denotes water solubility where “S” is soluble, “VS” is very soluble, “SS” is slightly soluble, “I” is insoluble, “R”

reacts with water, and “-” means no data was found. “Hygro” is the hygroscopic nature of the compound.

Compound Physical form (crystal) Sol. Hygro. Tm ∆𝑯𝑯𝒇𝒇

° ∆𝑮𝑮𝒇𝒇°

(°C) (kJ/ mol)

(kJ/ mol)

CrI2 Red/brown S Yes 867 -156.9 – CrI3 Dark green hexagonal SS No 500(d) -205.0 -202.5 CuI I 591 -67.78 -69.45 FeI2 Reddish violet S Yes 594 -104.60 -111.74 FeI2*4H2O Black leaflets S Yes 90(d) – – MoI2 Back crystal I Yes 700 – – MoI3 Black solid I No 927 – –

6

MoI4 Black crystal I No 100(d) – – NbI3 Black solid – 510 (d) – – NbI4 Gray orthogonal crystal R 503 – –

NbI5 Yellow black monoclinic R 327

NiI2 Black hexagonal S Yes 800(s) -78.241 -76.061 NiI2*6H2O Green monoclinic S Maybe 43(lw) – – MnI2 White hexagonal S Yes 80(d) – – MnI2*4H2O Red VS – – – –

1.3 Photolytic Reactions

Molecular iodine due to its high bond length and low bond dissociation energy can easily

disassociate from photolytic effects as shown in Equation (1-2) [19-21]. This leads to a variety of

reactions in the atmosphere (Figure 1-3).

I2 + hν→ 2I (1-2)

Figure 1-3. Reactions of iodine in the atmosphere [21].

The adsorption region for I2 is in the visible to near ultraviolet region of the electromagnetic

spectrum, which means that there are abundant sources in the photon region necessary to dissociate

molecular iodine [20]. While, photoinduced reactions are a key parameter in atmospheric

7

chemistry, in this work, the focus was on reactions inside of a facility so photoinduced reactions

were not studied.

1.4 Adsorption

The property of iodine of most interest to this work is the “stickiness” or the behavior of

iodine to adsorb to most surfaces, whether by chemisorption, physisorption, or a combination

thereof. Adsorption refers to the enrichment of a material in the vicinity of an interface.

Adsorption refers to a solid surface in contact with a fluid phase which can be either gas or

liquid. Adsorption can sometime be accompanied with absorption which is penetration of the

fluid into the solid. In the case of both adsorption and absorption occurring, the term sorption is

used to cover both [22]. For iodine, in most cases, the adsorption mechanism is physisorption,

which refers to adsorption that occurs via van der Waals interactions or in other words

adsorption without chemical bonding [22]. Iodine has the strongest van der Waals forces out of

halogens due to the large size. The order of strength is F2 < Cl2 < Br2 < I2. These interactions are

relatively weak, and the iodine can desorb intact. Depending of the surface, iodine can also

chemisorb, which is where iodine chemically bonds with the surface of a substrate. In this case

the bond is much stronger, and the species and oxidation state of the iodine is changed. Several

different metals have been shown to chemisorb iodine including Fe, Ni, Cu, Zn, Al and Ag [7,

23-26]. As mentioned previously, iodine can form iodides with many elements, and it is expected

that with metal substrates the metal iodide is formed.

1.5 Reactivity

The chemical form of iodine impacts its reactivity, deposition velocity, and ability to

spread, so one area of research has focused on the partitioning of iodine species in the gas phase

8

[3, 4, 11, 27]. Even before radioiodine is released to the environment during a nuclear accident,

the conditions of the damaged reactor impact the species of iodine [14]. A series of experiments

were done to model the forms of iodine in fission product releases and found that the three main

forms were molecular iodine, organic iodine, and aerosol-bound iodine [27]. Other studies have

noted that molecular iodine can desorb from aerosol-bound iodine, which means that the aerosol

fraction can be a source of additional molecular iodine [15]. These three forms were always

measured but the relative amounts changed depending on the accident conditions. Previous

sources have reported that CsI would be the main form of iodine that would come from fuel and

be released into the containment vessel [8]. CsI is water soluble as are many metal iodides and it

would, therefore, be dissolved in the coolant or the safety spray. The iodide could be oxidized

via hydrogen peroxide, a radiolysis product of water, or oxygen as shown in Equations (1-3) and

(1-4), respectively. However, in accident scenarios, it has been shown that kinetics, not

thermodynamics, govern the speciation of iodine [8]. Thus, Equation (1-3), which has slow

kinetics, would not be the main reaction pathway, instead oxidation via radicals such as ⸰OH

would be faster. The volatile I2 then partitions between the aqueous and gas phase.

2 H+ + 2 I- + H2O2 → I2 + 2 H2O (1-3)

2 I- + 2 H+ + O2 → I2 + 2 H2O (1-4)

Molecular iodine has higher reactivity compared to organic iodides and is the form with

the highest deposition velocity as it rapidly becomes adsorbed onto surfaces [23]. Organic

iodides, e.g. CH3I and C2H5I, are much more volatile than I2 as noted by their lower boiling

points of 42.8 and 72.2 °C respectively[28] and they are less reactive than iodine which makes

their deposition velocity much lower.

9

The deposition velocity relates to the net mass flux toward the ground, which can be

shown by Equation (1-5) below where Fd is the flux toward the ground, vd is the deposition

velocity and Ca is the air concentration [14].

Fd = vd • Ca (1-5)

The deposition velocity of molecular iodine is strongly dependent on the type of surface on

which it is depositing and the adsorption/desorption process(es) with that surface [14]. In nature,

it can range from 1×10-3 m s-1to 20×10-3 m s-1 [29] to 3×10-3 m s-1 to 30×10-3 m s-1 on grass [30].

Furthermore, the deposition velocity relates to the residence time of gaseous iodine in the

atmosphere before it deposits on the ground. This varies for the different chemical species but

has been estimated at 10 d for inorganic iodine (i.e., I2), 14 d for particulate iodine, and 18 days

for organic iodine [11]. The iodine can be removed from the atmosphere via wet or dry

deposition. Dry deposition is directly connected to the deposition velocity, but wet deposition

relies on precipitation in the form of rain or snow to remove the iodine. Different species are

more easily removed via wet deposition, with organic iodine being the least removed [11]. The

behavior of iodine once it is released from the reactor in an nuclear accident is complex and the

focus of this study was on the behavior of iodine before it leaves the reactor, specifically factors

that could decrease the amount that is eventually released.

Lebel et. al. [14] states four parameters affecting the total airborne concentration of

radioiodine released during a nuclear accident including:

1. Deposition of molecular iodine [i.e., I2(g)]

2. Deposition of organic iodine (e.g., CH3I)

3. Deposition of aerosol bearing iodine

10

4. Agglomeration of radioiodine aerosols with natural atmospheric aerosols

This dissertation focuses on the first parameter, i.e., the deposition of molecular iodine on

various metal substrates. As mentioned previously, the deposition is dependent on net

adsorption, chemisorption, and desorption. The possibilities of desorption events that could

reintroduce radioiodine to the atmosphere are dependent on the type of adsorption (i.e.,

physisorption vs. chemisorption) as well as the stability and vapor pressure of the chemisorbed

species. Therefore, the species of adsorbed iodine is key to understanding the possibility of

resuspension from desorption. This work has focused on chemisorption of molecular iodine with

different metals and metal alloys.

Multiple variables affect the adsorption behavior of iodine on metal substrates including

specific surface area (m2 g-1) of the substrate, temperature, atmosphere, humidity, and radiolysis.

For this study, we focused on ambient temperature and did not include radiolytic effects.

However, preliminary data on the importance of humidity led to an emphasis on understanding

the role of humidity dependence on iodine speciation and extent of substrate corrosion. There is

not a large body of literature available specifically investigating and addressing the adsorption of

molecular iodine on metal surfaces at ambient conditions. There is a wide range of papers

focused on high-temperature conditions that would be representative of normal operating

conditions for a nuclear reactor [31, 32]; however, adsorption is not consistent over temperature

ranges and consequently high-temperature studies are not necessarily relevant to room

temperature conditions.

In a paper by Wren et al., 131I was used as the source of iodine and the adsorbed

concentration was measured using a NaI γ detector [7]. In this paper, they found that adsorption

11

of iodine was a rapid process even under ambient temperature and pressure conditions and that

further corrosion by oxygen was extensive. They proposed a two- step adsorption process in the

presence of air as shown in Equations (1-6) and (1-7). The mechanism is shown in Figure 1 and

illustrates the diffusion of I2 through the corrosion layer to react with Fe and the O2 reaction with

the metal iodide to form an oxidated iron iodide species during loading. During purging O2

continues to react with the surface. This mechanism does not account for the desorption of iodine

during purging but the paper states that O2 is responsible for the loss and significant desorption

does not occur when purging with N2. Another assessment in the article is that moisture mediated

reactions are not a major mechanism based on the difference in adsorption between loading in N2

vs air where the humidity was relatively the same. It is also noted that the adsorption with N2 is

unexpected based on the lack of a liquid electrolyte since migration of Fe through the alloy at

room temperature should be minimal. It seems that this neglects the possibility that water

mediated reactions are a mechanism but the addition of O2 in air leads to different species being

formed than in N2.

(Step-1) Fe + I2 → FeI2 (1-6)

(Step-2) FeI2 + y/2 O2 → FeIxOy (1-7)

12

Figure 1-4. Iodine interactions with stainless steel in air. This figure was taken from Wren et al. [7] and reprinted with permission from Elsevier © 1999.

The benefit of using radioiodine for the Wren et al. [7] experiment was that a very low

iodine concentration could be achieved, ranging from 9×10-13 to 1.2×10-11 mol•cm-3 and still

quantified. This is the range of maximum iodine concentrations expected in a fuel handling

accident according to Wren et al. [8], however later studies state a complete melt down would

result in much higher concentrations of 10×10-8 mol•cm-3. Glanneskog et al. [17] looked at

individual metals of Cu, Al and Zn instead of an alloy like Wren et al. [7] and saw two reaction

rates; this included a faster initial rate than the later rate. The observation of two separate rates

was explained by Glanneskog et al. [17] by an initial iodine interaction with the metal followed

by a slower reaction when iodine had to diffuse through the metal iodide layer to react with the

pristine underlying metal substrate. In our experiments, we attempted to be within the ranges of

those used by Glanneskog et al. [17] and Wren et al. [7] whenever possible.

13

The work by Wren et al. [7] led to a further paper on the kinetics of corrosion, which

used pseudo-first-order approximations [33]. One specific item of importance noted in this paper

was that the concentrations of Fe and I, which were the input parameters based on the reactions

in Equations (1-6) and (1-7), did not lead to an accurate assessment of the expected I2 removal

from the gas phase, they added another variable to account for this, labeled as impurities. This

estimation led to one of the first hypotheses for this work, which is whether or not metal iodides

can form with all of the components of stainless steel (e.g., Ni and Cr) and not only Fe.

1.6 Adsorption on Metal Surfaces

Adsorption of halogens onto metal surfaces falls into three categories: chemisorbed

monolayer, surface halide, and bulk-like halide with surface halides as an intermediate between

the other two. Initial adsorption results in a chemisorbed monolayer and after that a thin-thick

halide film develops depending on the concentration of the halogen. Several studies have been

done [34-36] to study halogen adsorption on pure surfaces and it is generally agreed upon that

adsorption is dissociative in nature such as the simplified schematic shown in Figure 1-6 [36].

The halogen molecule is dissociated as it approaches the surface of the metal. More recent

studies have shown more complicated system where even the monolayer chemisorbed layer

shows non uniform atomic structures for the halogen and reordering of the substrate [37]. The

activation barrier for dissociative adsorption of halogens on metals is so low that even at room

temperature the metal surface is able to dissociate the iodine [37]. Another factor in adsorption is

diffusion of the halogen over the substate occurs via the hopping method shown in Figure 1-5,

however experimental data to support the diffusion process is difficult to obtain. The adsorption

of iodine on Ni is a special case because iodine is a large molecule and Ni(100) has a small

lattice. This results in the distance between the adsorbed NiI2 in the monolayer coverage case

14

being too small so stable structures can’t be formed; instead complex systems are formed on the

surface [37].

Figure 1-5. An example of halogens diffusing on surfaces with a low Miller index. This figure was taken from Andryushechkin et al. [37] and reprinted with permission from

Elsevier © 2018.

Therefore, it is possible that a different mechanism is responsible. Humidity has often

been part of the gas stream for experiments but authors do not find a direct correlation between

relative humidity and iodine adsorption [7]. Wren, et al. [7] did experiments using N2 and air

with the same amount of humidity and since the results were different, they determined that

moisture mediated reactions were not the primary mechanism. Other room temperature studies of

iodine adsorption also had humidity present but did not vary or control it [17]. It seems to be a

common parameter that humidity is present, at various levels, depending on the study but is not

considered a parameter worth testing. Thus, the role of humidity was a primary focus of the work

contained in this dissertation.

15

Figure 1-6 Dowben [38] reprinted with permission from Taylor and Francis © 1987.

1.7 Exposure Conditions

There are two ways to expose iodine. The simplest approach is a static exposure which

uses a solid source of iodine in a closed container to produce iodine gas over time, either with

heating to increase the sublimation of I2 or at ambient temperatures. The other exposure is a

dynamic exposure where a gas stream containing iodine is flowed over the sample. Iodine

exposures using flow through systems are the most useful design for iodine experiments, since

they allow for control of gases, however they are also more expensive and difficult to engineer.

Either method requires that care be taken to choose container materials which will not react with

the iodine. This can be difficult since iodine reacts with most plastics. A good option is a glass

that physisorbs iodine, like most things, but the iodine has a relatively short residence time. An

important thing to avoid are surfaces which could create organic iodides thereby changing your

reactive species such as painted surfaces [1, 3, 4]. An example of a flow through system is

16

shown in Figure 1-7. A microbalance is critical for in-situ kinetics measurements if stable iodine

is being used instead of radioiodine, which precludes the ability to use in situ gamma detectors to

monitor iodine concentration. The main concerns with such a system is protecting the

microbalance. The system pictured in Figure 1-7uses helium gas to do that.

Figure 1-7. From Abrefah et al. [9] reprinted with permission from Taylor and Francis © 1994.

1.8 Goals of Work

The purpose of this work was to increase the understanding of the adsorption behavior of

iodine as it relates to potential iodine release in the event of a nuclear accident. From review of

the literature it was found that characterization of the adsorbed iodine species was often lacking

in detail, likely due to the difficulty in measuring iodine using surface techniques that rely on

17

ultra-high vacuum, UHV, such as XPS and SEM-EDS. However, understanding the species is

key to understanding the stability of the adsorbed species and the possibility of resuspension due

to high volatility or other characteristics. Chapter 2 and Chapter 3 focus on characterization of

the adsorbed species on two common stainless-steel alloys, 304L and 316L as well as individual

metals that are dominant in the alloys, i.e. Fe, Cr and Ni. A destructive technique was used to

assist in elucidating the species when surface techniques did not allow for complete

understanding.

After characterizing the adsorbed species, it was found that humidity plays a key role in

the adsorption process. Chapter 4 focuses on the deployment of a flow through system that

allowed for control of the gas phase humidity and oxygen content, as well as quantifiable iodine

concentrations. To understand the role of O2, both air and argon were used as carrier gases. The

iodine concentration range was kept constant between 5-7x10-9 mol*cm-3. Using a water bubbler

and controlling the flow rate, relative humidity levels between 3-87% could be achieved.

18

1.9 References

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on iodine chemistry. Paris, France: Organisation for Economic Co-Operation and Development;

2007. Report No.: NEA/CSNI/R(2007)1 Contract No.: NEA/CSNI/R(2007)1.

2. Hou X, Hansen V, Aldahan A, Possnert G, Lind OC, Lujaniene G. A review on

speciation of iodine-129 in the environmental and biological samples. Analytica Chimica Acta.

2009;632(2):181-96.

3. Simondi-Teisseire B, Girault N, Payot F, Clément B. Iodine behaviour in the containment

in Phébus FP tests. Annals of Nuclear Energy. 2013;61:157-69.

4. Haste T, Payot F, Manenc C, Clément B, March P, Simondi-Teisseire B, et al. Phébus

FPT3: Overview of main results concerning the behaviour of fission products and structural

materials in the containment. Nuclear Engineering and Design. 2013;261:333-45.

5. JEFF-3.3 2021 [Available from: https://www.oecd-nea.org/dbdata/jeff/jeff33/index.html.

6. Hou X, Povinec PP, Zhang L, Shi K, Biddulph D, Chang C-C, et al. Iodine-129 in

Seawater Offshore Fukushima: Distribution, Inorganic Speciation, Sources, and Budget.

Environmental Science & Technology. 2013;47(7):3091-8.

7. Wren JC, Glowa GA, Merritt J. Corrosion of stainless steel by gaseous I2. Journal of

Nuclear Materials. 1999;265(1):161-77.

8. Wren JC, Ball JM, Glowa GA. The Chemistry of Iodine in Containment. Nuclear

Technology. 2000;129(3):297-325.

9. Abrefah J, de Abreu HFG, Tehranian F, Kim YS, Olander DR. Interaction of iodine with

preoxidized stainless steel. Nucl Technol. 1994;105(2):137-44.

19

10. Guentay S, Cripps RC, Jäckel B, Bruchertseifer H. Iodine Behaviour During a Severe

Accident in a Nuclear Power Plant. CHIMIA International Journal for Chemistry.

2005;59(12):957-65.

11. Whitehead DC. The distribution and transformations of iodine in the environment.

Environment International. 1984;10(4):321-39.

12. Transport - diffusion - reaction 2021 [Available from: http://www.labex-

cappa.fr/sites/default/files/images/CAPPA/workpackages/WP6_illus6.png.

13. Thorne M. Estimation of animal transfer factors for radioactive isotopes of iodine,

technetium, selenium and uranium. Journal of environmental radioactivity. 2003;70(1-2):3-20.

14. Lebel LS, Dickson RS, Glowa GA. Radioiodine in the atmosphere after the Fukushima

Dai-ichi nuclear accident. Journal of Environmental Radioactivity. 2016;151:82-93.

15. Noguchi H, Murata M. Physicochemical speciation of airborne 131I in Japan from

Chernobyl. Journal of environmental radioactivity. 1988;7(1):65-74.

16. Baverstock K, Egloff B, Pinchera A, Ruchti C, Williams D. Thyroid cancer after

Chernobyl. Nature. 1992;359(6390):21-2.

17. Glänneskog H. Interactions of I2 and CH3I with reactive metals under BWR severe-

accident conditions. Nuclear engineering and design. 2004;227(3):323-9.

18. Kaiho T, editor. Iodine Chemistry and Applications. Hoboken, New Jersey: John Wiley

& Sons, Inc.; 2014.

19. Saiz-Lopez A, Plane jMC, Baker A, Carpenter LJ, Von Glasow R, Gomez Martin JC, et

al. Atmsopheric Chemistry of Iodine. Chemical Reviews. 2012;112:1773-804.

20. Simpson WR, Brown SS, Saiz-Lopez A, Thornton JA, von Glasow R. Tropospheric

Halogen Chemistry: Sources, Cycling, and Impacts. Chemical Reviews. 2015;115(10):4035-62.

20

21. Vogt R. Iodine Compounds in the Atmosphere. In: Fabian P, Singh ON, editors. Reactive

Halogen Compounds in the Atmosphere. Berlin, Heidelberg: Springer Berlin Heidelberg; 1999.

p. 113-28.

22. Rouquerol J, Rouquerol F, Llewellyn P, Maurin G, Sing KSW. Adsorption by Powders

and Porous Solids: Principles, Methodology and Applications: Elsevier Science; 2013.

23. Chamberlain A, Eggleton A, Megaw W, Morris J. Behaviour of iodine vapour in air.

Discussions of the Faraday Society. 1960;30:162-9.

24. Riley BJ, Kroll JO, Peterson JA, Matyáš J, Olszta MJ, Li X, et al. Silver-Loaded

Aluminosilicate Aerogels As Iodine Sorbents. ACS Applied Materials & Interfaces.

2017;9(38):32907-19.

25. Beck CL, Riley BJ, Chong S, Karkamkar A, Seiner DR, Clark SB. Molecular iodine

interactions with metal substrates: Towards the understanding of iodine interactions in the

environment following a nuclear accident. Journal of Nuclear Materials. 2021;546:152771.

26. Beck CL, Riley BJ, Chong S, Smith NP, Seiner DR, Seiner BN, et al. Molecular iodine

interactions with Fe, Ni, Cr and stainless-steel alloys. Ind Eng Chem Res. 2021.

27. Ramsdell JVJ, Simonen CA, Burk KW. Regional Atmospheric Transport Code for

Hanford Emission Tracking. In: Energy Do, editor. Richland WA: Battelle; 1994.

28. Cumming WM, Hooper IV, Wheeler TS. Systematic Organic Chemistry: Modern

Methods of Preparation and Estimation: D. Van Nostrand Company; 1924.

29. Sehmel GA. Particle and gas dry deposition: a review. Atmospheric Environment (1967).

1980;14(9):983-1011.

30. Heinemann K, Vogt K. Measurements of the deposition of iodine onto vegetation and of

the biological half-life of iodine on vegetation. Health Physics. 1980;39(3):463-74.

21

31. Tigeras A, Bachet M, Catalette H, Simoni E. PWR iodine speciation and behaviour under

normal primary coolant conditions: An analysis of thermodynamic calculations, sensibility

evaluations and NPP feedback. Progress in Nuclear Energy. 2011;53(5):504-15.

32. Hoinkis E. Review of the Adsorption of Iodine on Metal and Its Behavior in Loops. Oak

Ridge, TN: Oak Ridge National Laboratory; 1970. Report No.: ORNL-TM-2916 Contract No.:

ORNL-TM-2916.

33. Wren JC, Glowa GA. Kinetics of Gaseous Iodine Uptake onto Stainless Steel during

Iodine-Assisted Corrosion. Nucl Technol. 2001;133(1):33-49.

34. Simpson WC, Yarmoff JA. Fundamental studies of halogen reactions with III-V

semiconductor surfaces. Annual review of physical chemistry. 1996;47(1):527-54.

35. Jones RG. Halogen adsorption on solid surfaces. Progress in Surface Science. 1988;27(1-

2):25-160.

36. Grunze M, Dowben P. A review of halocarbon and halogen adsorption with particular

reference to iron surfaces. Applications of Surface Science. 1982;10(2):209-39.

37. Andryushechkin B, Pavlova T, Eltsov KN. Adsorption of halogens on metal surfaces.

Surface Science Reports. 2018;73:83-115.

38. Dowben P. A review of the halogen adsorption process on metal surfaces. Critical

Reviews in Solid State and Material Sciences. 1987;13(3):191-210.

22

1.10 Attributions

Chapter 2 Molecular Iodine Interactions with Metal Substrates: Towards the

Understanding of Iodine Interactions in the Environment Following a Nuclear Accident. The

experiments, data interpretation and writing were completed by Chelsie Beck. Revisions of the

manuscript were carried out by Brian Riley and Derrick Seiner. A variety of instrumentation was

utilized to characterize the samples and experts in those instruments carried out the analysis.

SEM-EDS analysis of samples was conducted by Brian Riley. XRD analysis was done by

Saehwa Chong. TGA by Abhijeet Karkamkar and ICP-OES analysis by Chelsie Beck. Derrick

Seiner and Brian Riley assisted with experimental planning and data interpretation. Sue Clark is

the committee chair. This manuscript was published in the Journal of Nuclear Materials

2021;546:152771 (DOI: 10.1016/j.jnucmat.2020.152771).

Chapter 3 Molecular iodine interactions with Fe, Ni, Cr and stainless-steel alloys. The


manuscript were carried out by Brian Riley, Derrick Seiner, Brienne Seiner and Nathaniel Smith.

A variety of instrumentation was utilized to characterize the samples and experts in those

instruments carried out the analysis. SEM-EDS analysis of samples was conducted by Brian

Riley. XRD analysis was done by Saehwa Chong. XPS by Mark Engelhard and ICP-OES

analysis by Chelsie Beck. Derrick Seiner, Nathaniel Smith, Brienne Seiner and Brian Riley

assisted with experimental planning and data interpretation. Sue Clark is the committee chair.

This manuscript was published in Industrial Engineering and Chemistry Research 2021;60:2447-

2454 (DOI: 10.1021/acs.iecr.0c04590).

Chapter 4 Adsorption of Iodine on Metal Coupons in Humid and Dry Environments. The


23

manuscript were carried out by Brian Riley and Nathaniel Smith. A variety of instrumentation

was utilized to characterize the samples and experts in those instruments carried out the analysis.

SEM-EDS analysis of samples was conducted by Brian Riley. XRD analysis was done by

Saehwa Chong. Nathaniel Smith and Brian Riley assisted with experimental planning and data

interpretation. Sue Clark is the committee chair. This manuscript was submitted to Journal of

Nuclear Materials.

24

CHAPTER TWO

Molecular Iodine Interactions with Metal Substrates: Towards the Understanding of Iodine

Interactions in the Environment Following a Nuclear Accident

2

Chelsie L. Beck, Brian J. Riley, Saehwa Chong, Abhijeet Karkamkar, Derrick R. Seiner, Sue B.

Clark

Pacific Northwest National Laboratory, Richland, WA 99354

Source: Beck CL, Riley BJ, Chong S, Karkamkar A, Seiner DR, Clark SB. Molecular iodine


environment following a nuclear accident. J Nucl Mater. 2021;546:152771.

25

2.1 Abstract

In order to evaluate the potential impacts to the public from radioiodine in a nuclear

event, it is vital to expand our understanding of the interaction of molecular iodine with various

surfaces. There are many potential surfaces that iodine could interact with in and around a

nuclear facility, including stainless steel. This study, carried out at ambient temperature, pressure

and humidity, demonstrates the highly adsorptive nature of molecular iodine on two types of

austenitic stainless steel, SS304L and SS316L. By using a novel approach which combines

inductively coupled plasma-optical emission spectroscopy (ICP-OES) with surface energy

dispersive X-ray spectroscopy (EDS) there is evidence of the formation of metal iodides that

have not previously been verified or quantified. Samples exposed to gaseous molecular iodine

formed an iodine containing corrosion product visible by scanning electron microcopy (SEM).

Evaluation of the metals in the corrosion region using EDS was compared to a water leach of the

same samples analyzed using ICP-OES. The combination of techniques provides indication of

metal iodide formation with minor stainless-steel constituents including: NiI2, MnI2 and CrI2.

26

2.2 Introduction

Stable iodine exists in the environment and is a key element used by the thyroid to

regulate human metabolic processes [1]. Radioactive iodine is produced during fission of nuclear

fuel including radioisotopes with short and long half-lives (t½), e.g., 131I with t½ = 8.04 days and

129I with t½ = 1.57×107 years, respectively [2]. The volatility of iodine makes it a major source of

radiotoxicity in material released during an accident scenario such as ruptured fuel rods [3]. Due

to half-life and specific activity, the releases of 131I from an extreme accident, such as the

Fukushima Daiichi Nuclear Power Plant event in 2011, can pose a threat to human health for 1-2

months post release [4]. Iodine is volatile and extremely labile, so it can travel long distances [5].

For example, 131I from Chernobyl was detected in Japan, some 8000 km away [6].

The interaction between molecular iodine and potential adsorption surfaces is an

important component in understanding potential sites of accumulation of radioactive iodine in

the event of a nuclear accident. Nuclear facilities and industrial areas are comprised of a variety

of stainless steel surfaces which have the potential to interact and accumulate iodine [7]. Iodine

physisorbs to most surfaces but it is generally understood to chemisorb to metal surfaces [7].

Many studies detailing the adsorption capacities of various materials have been documented, but

few provide identification and/or characterization of the chemisorbed species [8-13].

Furthermore, many studies are at elevated temperatures that would be indicative of operations for

a nuclear facility and not the ambient conditions that may be present after the initial release [14-

16]. Ambient conditions would be most relevant to areas outside of the main operating areas as

well as structural materials outside of the nuclear power plant. The species of iodine that may be

released in the event of a nuclear accident is an area of active study [17], however it is agreed

that at least a portion of the gas phase iodine will be I2(g) [6, 15, 18]. The current study includes

27

an evaluation of I2(g)-metal interactions at room temperature (RT); the metals included in the

study were metal alloys; SS316L and SS304L. Generally, metal coupons are used to evaluate

substrate-analyte interactions, but particles should have similar properties and were chosen to

increase substrate specific surface area with the intent on increasing the amount of iodine species

formed. Coupons would have a lower specific surface area for similar sample sizes and may have

required higher iodine loadings to reach measurable amounts of reaction products. The main

goals of this work were to (1) evaluate metal-iodine interactions based on gravimetric uptake

during RT-exposures, (2) quantify uptake through chemical analysis techniques, and (3) identify

any metal-iodine phases present in the iodine-reacted metals. Extensive characterization was

performed during the analysis including scanning electron microscopy (SEM; for

microstructure), energy-dispersive X-ray spectroscopy (EDS; for chemical analysis), X-ray

diffraction (XRD; crystal structure), inductively coupled plasma optical emission spectroscopy

(ICP-OES; quantification of elements in water soluble species), and thermogravimetric analysis

(TGA; quantification of physiosorbed iodine on exposed samples) to provide novel insights into

the interaction of iodine with surfaces.

2.3 Experimental

2.3.1 Sample Preparation

The samples SS304L (40 µm, 99.9%) and SS316L (40 µm, 99.9%) were purchased from

US Research Nanomaterials, Inc. (Houston, TX) and used as received. The specific surface areas

of as-received materials were calculated using N2 adsorption isotherms at 77 K with a

Quadrasorb EVO/SI automatic gas sorption system (Quantachrome Instruments). The samples

were degassed under vacuum at 100 °C for 4 hrs before the adsorption measurements. The

specific surface area was determined using 5 points BET (Brunauer-Emmett-Teller) method.

28

The materials were also analyzed by TGA, XRD, SEM/EDS and ICP-OES and compared

with exposed materials. For the iodine exposures, 100–200 mg of each sample was placed in

tared glass petri dishes (BRAND®; BR455701) in a thin layer and weighed on an analytical

balance (±0.1 mg; ME204E Mettler Toledo; Columbus, OH). The petri dishes were arranged in a

circle around an open small glass vial containing iodine crystals (99.999%, Alfa Aesar,

Haverhill, MA) inside of a 350 mL perfluoroalkyl (PFA) jar (100-0350-01, Savillex, Eden

Prairie, MN). Two PFA jars were used with each containing three 200-mg samples of SS304L or

SS316L and iodine. No heat was applied and, in both cases, the amount of iodine in the gas

phase was due solely to room temperature sublimation. After 30 days for SS316L, 34 days for

SS304L of exposure time, the samples were removed from the jars and weighed. Iodine uptake

was quantified using gravimetric uptake (m%I,g) based on Equation (2-1) where ms is the mass of

the initial sample, mI is the mass uptake following iodine-loading experiments based on mass

change during the experiment defined in Equation (2-2), where mf is the final mass after iodine

loading (note that this does not account for any oxygen uptake during the loading resulting in

surface oxides).

m%I,g = mI / ms (2-1)

mI = mf – ms (2-2)

2.3.2 Sample Characterization

Thermogravimetric analysis (TGA) was performed by heating the samples under flowing

nitrogen on an STA 449 Jupiter Netzsch instrument. The samples were heated from 25°C to

400°C at a heating rate of 1 K min-1. The samples were analyzed with SEM/EDS. The SEM

analysis was performed using a JSM-7001F field emission gun microscope (JEOL USA, Inc.;

Peabody, MA). Here, samples were mounted to aluminum stubs with carbon tape and coated

29

with 2.5 nm of Pt using a Quorum 150T ES Ar-plasma sputter coater (Electron Microscopy

Sciences, Hatfield, PA). The EDS analysis was performed using a Bruker xFlash 6|60 (Bruker

AXS Inc., Madison, WI). Both imaging and EDS mapping were conducted at an acceleration

voltage of 15 kV.

The XRD analysis was conducted on samples loaded into zero-background quartz holders

(MTI Corporation). Samples were loaded into the 10-mm diameter and 1-mm deep cavities or

placed on the flat side and run in a Bruker® D8 Advance (Bruker AXS Inc., Madison, WI) XRD

with Cu-Kα emission. The detector used was a LynxEyeTM position-sensitive detector with a

collection window of 3° 2θ. Typical scan parameters were 5–70° 2θ with a step of 0.015° 2θ and

a 0.3-s or a 2-s dwell at each step. Phase identification was performed with EVA (v4) software

(Bruker AXS Inc.).

To quantify the amount of metal iodide(s) formed, the exposed particles were leached

using ultrapure 18-MΩ deionized water (DIW). Metal iodides are unique in their water

solubilities (see Table S2-1 in supporting information), which makes a selective water leach an

ideal method for assessing and quantifying the formation of metal iodides as well as metal

distribution in the iodide phase. Unexposed materials were leached with the same conditions to

verify that there were no pre-existing water-soluble compounds in the base metals. First, ~ 10

mass% of each sample was weighed and then leached by adding 10 mL of DIW, vortexing for 10

s, and allowing to sit for 20 minutes. After 20 minutes, the samples were filtered through a 0.45-

µm syringe filter. The leachate was diluted using water and analyzed for I, Cr, Ni, Mn, Fe and

Mo.

Another experiment was performed to evaluate the leach time used. In this experiment,

another 10% portion was weighed, and 10 mL of DIW was added. The samples were vortexed

30

and allowed to sit for 20 minutes, at which point they were centrifuged at 2500 rpm for 3 min.

An aliquot of the leachate was taken, and the samples were allowed to sit for another 3 hrs. After

3 hrs, the samples were filtered using a 0.45-µm syringe filters and the leachate was collected

and diluted for analysis. They were analyzed with the initial samples to determine if the longer

time allowed more material to be dissolved.

All of the leachate samples were analyzed using a Thermo iCAP7600 ICP-OES in axial

mode. A standard quartz sample introduction system was used. A calibration curve was

generated using at least three calibration standards prepared by diluting 1000 µg/mL standards

from Inorganic Venture (Christiansburg, VA); the Fe, Cr, and Ni were in a custom mix called

PNNL-15, Mn was in a custom mix called PNNL-16, Mo was in custom mix called PNNL-17,

and iodine was a single element solution (CGICI1-125ML). All reported sample concentrations

were within the calibration range for the given analyte. The calibration curve was verified using

calibration check standards after calibration and then every 10 samples.

2.4 Results and Conclusions

2.4.1 Characterization of Received Materials

The metal particles were analyzed by SEM and micrographs from two magnifications are

provided for the SS304L, and SS316L particles in Figure 2-1. The stainless steels are mostly

spherical with some nodes and other imperfections. The measured surface area was 0.030 ±

0.003 m2/g for 304L and 0.036 ± 0.004 m2/g for 316L. The specified metal content in SS304L

and SS316L are very similar, however the SS316L has the addition of 2–3% Mo to increase

corrosion resistance (see Table S2-3 in the Supporting Information).

31

Figure 2-1. SEM images of as-received stainless steels including (a,c) SS304L and (b,d) SS316L shown at different magnifications. The top row is 100× magnification and the

bottom row is 1000×.

2.4.2 Iodine Exposure of SS304L and SS316L in Triplicate

An experiment was performed where SS304L and SS316L were exposed in triplicate in

separate containers. The samples post exposure are shown in Figure 2-2.

32

Figure 2-2. Pictures of triplicate iodine exposures of (a) SS304L+I and (b) SS316L+I in separate containers. The numbers next to each image denote the replicate number and the scalebar is valid for all pictures. SS304L was exposed for 34 days and SS316L for 30 days.

The SS316L+I sample shows noticeable sample loss to the walls of the jar as well as

heterogenous adsorption on the material (see Figure S2-1 in the Supporting Information). The

SS304L+I samples showed uniform color change from a silver to a dark black, indicative of

chemical change and, potentially, the formation of metal iodides and/or metal oxides (see Table

S2-1, in supporting information). In the second exposure for SS316L+I, 231.9 mg of iodine was

sublimed in 30 days. In the second exposure for SS304L, 170.2 mg of iodine was sublimed in 34

33

days. The two exposures were started at the same time, so they had the same amount of relative

humidity, which is estimated at < 40%. The jars were tightly capped so it was not known if

changes in the humidity in the room over the course of the experiment changed the conditions in

the jar but it is assumed that they did not. The water vapor in the air was likely critical to the

adsorption of iodine, as noted by Abrefah et al. [19] and it is expected that some of the mass

uptake could be due to hydrated metal iodides, which would not be distinguishable from

dehydrated metal iodides using SEM/EDS or ICP-OES of the leachate. The difference in the

sublimed mass of iodine may be due to more rapid removal of the iodine from the gas phase for

the SS316L+I samples, which adsorbed more iodine overall than the SS304L+I samples. The

gravimetric mass uptake was highly variable for SS316L+I (i.e., m%I,g = 39.0±38.3 mass%; see

Table 2-1), which may be due to differences in particle sizes and morphologies as seen in the

SEM micrographs of the base material (see Figure 2-1).

Table 2-1. Starting sample mass (mi) for SS316L+I and SS304L+I samples and gravimetric iodine uptake (m%I,g; mass adsorbed per mass of starting material). At the bottom of the

table averages and standard deviations (in parenthesis) are provided for each set.

Sample mi (mg) m%I,g

SS304L+I-1 207.3 34.2%

SS304L+I -2 200.3 26.8%

SS304L+I-3 200.7 27.4% SS304L+I-ave

202.8 (3.9)

30.5% (5.2%)

SS316L+I-1 205.3 82.5%

SS316L+I-2 192.7 10.7%

34

The mass uptake of SS304L+I was much more consistent with m%I,g = 30.5±5.2 mass%

for the three replicates (see Table 2-1). In the exposure, the sublimed mass of iodine could be

fully accounted for by the mass gain of the metal samples, and no evidence was found to suggest

that the system was leaking or that a measurable amount of iodine adsorbed to the walls of the

container. It should be noted that, in some cases, the Savillex containers did have noticeable

color changes associated with the iodine uptake (see Figure S2-1, Supporting Information), but

mass tracking before and after of the containers did not show a measurable uptake.

2.4.3 Surface Analysis of the Iodine-Loaded Materials

The SS304L+I and SS316L+I samples from Section 2.4.2 were analyzed using a

SEM/EDS to determine the distribution of Ni, Fe, Cr, Mn, O, I, and other minor additives in the

metals (e.g., Mo,) – these micrographs are shown in Figure 2-4 and Figure 2-5, respectively.

Low-magnification SEM micrographs of both materials (Figure 2-4b and Figure 2-5a,c) show

the spherical stainless-steel particles surrounded by a corrosion layer containing iodine. The

micrographs of SS316L+I (Figure 2-5a) show a more diffuse iodine layer, which may be a result

of the iodine containing compounds deliquescing post exposure. This possibility of

deliquescence was further evidenced by the visual appearance of the SS316L+I after exposure to

air as shown in Figure 2-3. The gray color is consistent with the as-received particles and the

black regions show high iodine adsorption. The regions indicated by the arrows in Figure 5 are

regions where high iodine regions (dark spots) appeared to be deliquescing.

35

Figure 2-3. Picture of SS316L+I (a) one minute after the iodine exposure chamber was opened and (b) two hours after being exposed to atmosphere. The arrows in (b) are callouts

for the regions most affected by air exposure.

The 500× and 1,000× micrographs for SS316L+I replicates 1 and 2 show a corrosion layer,

which is very flat with very few discernably faceted regions, compared to SS304L+I (Figure

2-4c,d) where highly faceted crystals were observed.

Figure 2-4. SEM micrographs of (a) unexposed SS304L and (b-d) iodine-loaded SS304L+I at (b) 500×, (c) 2500× showing the corrosion layer, and (d) 10,000× showing the corrosion

layer

36

Figure 2-5. SEM micrographs of SS316L+I replicate 1 and replicate 2 including (a) 500× of SS316L+I-1, (b) 1,000× of SS316L+I-1, (c) 500× of SS316L+I-2, and (d) 1,000× of

SS316L+I-2.

Amongst the faceted structures in SS304L+I2 (see Figure 2-4c,d), higher magnification

micrographs revealed two distinct phases in the corrosion layer. The EDS dot map of this region

at 7,500× followed by spot EDS analysis showed some compositional heterogeneity, i.e., distinct

high-Ni and high-Fe regions (see Figure 2-6).

37

Figure 2-6. (a) BSE-SEM micrograph of SS304L+I-1 at 7500× and (b-g) EDS phase map and elemental maps. EDS spot analysis regions are shown in (a) with the corresponding data provided in Figure 2-7. The scalebar shown in (a) is valid for all images. A different version of this figure is provided in Figure S2-2 (Supporting Information) that shows a

higher-magnification view of the EDS regions and phase map in (a) and (b), respectively.

The EDS data of these different regions are shown in Figure 2-7. The high-Ni region

coincided with the rigid (dense) rectangular prismatic crystals and the more porous and

amorphous regions coincided with the high-Fe EDS data. The Cr and O are relatively low in both

regions. Phase maps and EDS data indicate that the high-Ni and high-Fe phases are possibly NiI2

and FeI2, respectively. The phase map and individual elemental maps for SS316L+I (see Figure

S2-4 in supporting information) show highly concentrated Ni and Mn correlated with the high-

iodine region. The densest regions of Fe and Cr correspond with the base material, however that

is to be expected given the high starting fractions of Fe and Cr in the SS316L alloy.

38

Figure 2-7. Summary of EDS data from spot analysis in Figure 2-6a showing the high-Fe and high-Ni regions. Regions 1-3 were averaged for the high-Fe data and regions 4-8 were

averaged for the high-Ni data. The minimum and maximum values can be found in Table S2-4.

2.4.4 Leachate Analysis with ICP-OES Compared with EDS Data

Portions of SS304L+I (replicates 1, 2 and 3) were leached with water to dissolve any

metal iodide of Fe, Cr, Ni and Mn that may be present. As a baseline, the material before

exposure to iodine was leached using the same conditions to verify that no water-soluble metals

were already present in the material. The ICP-OES analysis of the leachate of the base material

showed no measurable Fe, Cr, Ni, Mn, or I. Analysis of the leachates from iodine-exposed

samples after 20 minutes and 3 hours show the same concentration of metals with 95%

confidence, which indicates that a 20-minute leach was sufficient time to dissolve all water-

soluble phases that are present. A 100× dilution of the leachate with water was used to analyze

the iodine in the calibration range. The metal content was lower and a 20× dilution in water was

used to quantify the metals. Water was used as the diluent to avoid oxidation of I- and

consequent loss due to volatility [20].

39

The elemental analysis of the leachate revealed chemistry that was in good agreement

with the EDS spot analysis results of the corrosion layer in SS304L+I-2 taken at 350×

magnification (see Figure 2-9a and Figure S2-3 in the Supporting Information). The Fe:I, Cr:I,

Ni:I molar ratios (see Figure 2-9a) were consistent between the two techniques and the total

metal-iodine (M:I) ratio is 0.56 for both ICP-OES and EDS (see Figure 2-9a). Both techniques

summed Fe, Cr, Mn and Ni for the total metals. Manganese is a very minor component at <2%

by mass however it was measured in the water leach by ICP-OES as well as by EDS. A

comparison of the EDS and ICP-OES data for all the elements is shown in Figure S7 with the

corresponding EDS micrographs in Figure S8 in Supporting Information.

Figure 2-9b shows the Fe:Ni and Fe:Cr molar ratios for SS304L+I-2 with Fe being the

major constituent of stainless steel. The EDS data for both the unexposed (base) material and the

exposed replicate 2 is shown as well as the ICP-OES leachate. The Fe:Ni ratio decreased after

iodine exposure indicating a larger fraction of Ni in the corrosion layer than in the base material.

This likely indicates a highly favorable interaction between Ni and I2. The Fe:Cr ratio increased

following exposure, which indicates a decrease in the Cr content in the corrosion layer. Wren et

al. [8, 21, 22] have multiple papers on the topic of iodine interaction with SS304. The authors

hypothesize that an increase in Cr on the surface is effective at reducing the adsorption of iodine,

stating that Cr forms a stable oxide layer, which acts as a barrier to iodine vapor. In this study,

the presence of Cr in the water leachate does indicate the presence of CrIx complexes in the

iodine-exposed sample, presumably CrI2, since as shown in Table S2-1 in the supporting

information, only CrI2 is water soluble [23-25]. On the other hand, the favorable interaction with

Ni, a minor constituent of stainless steel was suggested by Wren et al. [21], but was not

confirmed, and possible reactions were excluded from the kinetic model developed by the

40

authors to describe iodine adsorption on stainless steel [8, 22]. Manganese was not considered in

the work by Wren et al. [8, 21, 22]; this is likely because Mn is a very minor component, but as

mentioned previously it was measured both by ICP-OES and EDS of the corrosion layer which

shows it is readily reacting with the iodine in these experiments. In the authors review of

available literature, only Fe from the metals available in these stainless steels is assumed to react

with gas-phase molecular iodine [8, 26].

2.4.5 Diffraction Analysis

Bulk XRD was performed on all exposed samples, following iodine exposure and

resulted in multiple diffraction peaks in addition to those from the base (untreated) metals – see

Figure 2-8.

41

Figure 2-8. XRD patterns for SS304L, SS316L, SS304L+I, and SS316L+I. The term “AR” denotes “as-received”, “*” denotes the diffraction peaks associated with the base materials

and “” denotes those associated with fougerite (see text for more information).

Note that SS304L, SS316L, SS304L+I, and SS316L+I all showed the same diffraction

peaks for the base material, which correspond to ~43.6° and ~50.8° 2θ (i.e., 𝐹𝐹𝐹𝐹3�𝐹𝐹, cubic space

group 225; ICSD# 632501) [27]. The additional diffraction peak locations for SS304L+I and

SS316L+I were consistent with differences in intensities; some of these peaks match that of

fougerite [i.e., 𝑅𝑅3�𝐹𝐹𝑚𝑚, trigonal space group 166; Fe(OH)2(OH)0.25(H2O)0.5; ICSD# 159700] [28]

but additional diffraction peaks observed were not found in the databases provided by the

International Centre for Diffraction Data (ICDD) nor the Inorganic Crystal Structure Database

(ICSD); see Figure S2-5, Figure S2-6, and Table S2-2 in the Supporting Information. The

diffraction patterns were compared to fits for FeI2, MnI2, CrI2, CrI3, and NiI2, but no overlaps

42

were identified. The formation of fougerite may be due to the hygroscopic nature of iron iodine

which could be deliquescing. Furthermore, the FeI2·4H2O, if formed, is relatively unstable and

decomposes at 90ºC [29]. Either circumstance could allow for hydroxylation of the iron.

2.4.6 Thermogravimetric Analysis

The TGA analysis of the SS304L+I-1 and SS316L+I-1 samples from the triplicate studies

were measured and the results are shown in Figure S2-9 in supporting information. The mass

loss indicates that the entire adsorbed iodine is volatilized by 250°C. There are several steps in

the curves, which could be due to loss of water adsorbed after the sample had been weighed or

from the base material that had reacted with iodine. The initial weight loss, which appears to

briefly plateau around 86°C, is likely loss of physisorbed iodine. The subsequent losses could be

many different processes including the dehydration or decomposition of hydrated metal iodides

and/or sublimation of metal iodides.

2.4.7 Iodine Interactions with Metals

Based on the limited thermodynamic data available for Cr-I, Fe-I, Ni-I, and Mn-I

complexation, it is difficult to identify which metal-iodine (MIx) complexes would be preferred.

Looking at the data from Table S1 in supporting information, CrI3, FeI2, and NiI2 all show

spontaneous reactions at room temperature (i.e., 298.15 K) [24, 25] with negative Gibb’s free

energies of formation (∆𝐺𝐺𝑓𝑓°). Since these iodine exposures were conducted in air, metal-oxide

(MOy) is also possible; however; the metal-oxide ratio decreased rather than increased before and

after the iodine exposures. The reactions for MIx and MOy formation are shown in Equations (3)

and (4), respectively. In all cases, for a given metal oxidization state (i.e., Cr3+, Fe2+, Ni2+), MOy

formation is preferred over MIx, i.e., ∆𝐺𝐺𝑓𝑓,MO𝑦𝑦° < ∆𝐺𝐺𝑓𝑓,MI𝑥𝑥

° . However, the interaction with stainless

steel is more complex and stainless steel is designed to be “corrosion resistant” with respect to

43

oxygen but is known to be corroded by iodine and other halogens that attack and penetrate the

MOy passivation layer.

2 Cr(s)0 + 3 I2(g) → 2 CrI3(s); ∆𝐺𝐺𝑓𝑓° = -202.5 kJ/mol (3a)

Fe(s)0 + I2(g) → FeI2(s); ∆𝐺𝐺𝑓𝑓° = -111.74 kJ/mol (3b)

Ni(s)0 + I2(g) → NiI2(s); ∆𝐺𝐺𝑓𝑓° = -76.061 kJ/mol (3c)

2 Cr(s)0 + 3 O2(g) → 2 Cr2O3(s); ∆𝐺𝐺𝑓𝑓° = -1058.966 kJ/mol (3a)

Fe(s)0 + O2(g) → FeO(s); ∆𝐺𝐺𝑓𝑓° = -251.4 kJ/mol (3b)

Ni(s)0 + O2(g) → NiO(s); ∆𝐺𝐺𝑓𝑓° = -211.60 kJ/mol (3c)

Based on the ICP-OES and EDS data discussed in Section 2.4.4 and shown in Figure

2-9a and Figure 2-9b, it is likely that the actual corrosion phase is a compound containing Fe, Ni,

Cr, Mn, I, and O; it is possible that at least two different phases exist based on the high-

magnification SEM-EDS data provided in Figure 2-6 and Figure 2-7.

44

Figure 2-9. (a) Comparison of ICP-OES and EDS of SS304L+I-2 (taken at 350× magnification). Error bars are 1 standard deviation. (b) The comparison of ICP-OES

leachate and EDS analysis of SS304L+I-2 to the EDS data of the base SS304L material. Error bars are 1 standard deviation. The minimum and maximum values can be found in

Table S2-5.

Wren et al. [8] proposed a FeIxOy compound as the stable species that does not desorb. In

our study, EDS of the corrosion layer did not indicate an increase in the at% of oxygen that

would indicate MOy formation; however, it is possible that the existing oxygen from the

passivation layer was converted to a MIxOy compound. As mentioned in Section 3.5, Wren, et al.

did not account for the interaction between iodine and the other metals that we have observed in

this study. It is likely that we produced a compound containing multiple metals as shown in the

leachate, which could be responsible for the unidentified diffraction peaks seen in the SS304L+I

45

and SS316L+I and as evidenced by the consistent data obtained from ICP-OES and EDS on

these iodine-loaded materials.

2.5 Conclusions

This study done at ambient temperatures and humidity shows the high adsorption nature

of molecular iodine on two types of austenitic stainless steel, SS304L and SS316L. Combining

destructive analysis with surface EDS is a novel approach to provide evidence of the formation

of metal iodides that have not previously been verified or quantified. The use of selective

leaching with destructive analysis provides a highly selective and quantitative method for

measuring metal iodides. The comparison of high magnification EDS of the corrosion layer and

the ICP-OES analysis of the water leachate provide solid evidence that the water leachate is

representative of the corrosion layer and not the base material. Furthermore, it provides

confirmation of metal iodide formation with minor stainless-steel constituents including: Ni, Mn

and Cr in addition to the predominate Fe. Prior studies focused entirely on the interaction

between Fe in stainless steel and I2. This study has shown that there is likely interaction between

iodine and all the metal components of stainless steel. This new information could impact the

models that have been developed to determine adsorption kinetics of gas phase molecular iodine

on stainless steel which in turn effect our understanding of the environmental impacts from

radioiodine in stainless steel.

2.6 Acknowledgements

This research was supported by the Chemical Dynamics Initiative (CDI) at Pacific

Northwest National Laboratory (PNNL). PNNL is operated by Battelle for the U.S. Department

of Energy (DOE) under Contract No. DE-AC05-76RL0-1830. The authors acknowledge support

and helpful discussions from Neil Henson, Nathaniel Smith, Brienne Seiner and Hilary Emerson.

46

Authors thank Luke Sweet for his assistance with XRD data interpretation. Authors thank Mike

Perkins for help with the graphical abstract. Authors thank Xiaohong (Shari) Li for BET analysis

of the particles. PNNL draws on signature capabilities in chemistry, earth sciences, and data

analytics to advance scientific discovery and create solutions to the nation's toughest challenges

in energy resiliency and national security.

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2. Lin CC, Chao JH. Radiochemistry of iodine: Relevance to health and disease. In: Preedy

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Chernobyl. Journal of environmental radioactivity. 1988;7(1):65-74.

7. Chamberlain A, Eggleton A, Megaw W, Morris J. Behaviour of iodine vapour in air.

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9. Evans GJ, Nugraha T. A Study of the Kinetics of I2 Deposition on Stainless Steel

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10. Deitz VR. Interaction of Radioactive Iodine Gaseous Species with Nuclear-Grade

Activated Carbons. Carbon. 1987;25(1):31-8.

11. Scott SM, Hu T, Yao T, Xin G, Lian J. Graphene-based sorbents for iodine-129 capture

and sequestration. Carbon. 2015;90:1-8.

12. González-García CM, Román S, González JF, Sabio E, Ledesma B. Surface free energy

analysis of adsorbents used for radioiodine adsorption. Applied Surface Science. 2013;282:714-

7.

13. Nandanwar SU, Coldsnow K, Utgikar V, Sabharwall P, Eric Aston D. Capture of

Harmful Radioactive Contaminants from Off-Gas Stream Using Porous Solid Sorbents for Clean

Environment – A Review. Chem Eng J. 2016;306:369-81.

14. Hudswell F, Furby E, Simons JG, Wilkinson KL. The Sorption and Desorption of Iodine.

United Kingdom Atomic Energy Authority Research Group, Division C; 1960. Report No.:

AERE-R-3183 Contract No.: AERE-R-3183.

15. Tyler JW. Surface analysis using X-ray photoelectron spectroscopy of iodine deposits on

17% Cr/12% Ni and mild steel surfaces oxidised in CO2CH3I gas mixtures. J Nucl Mater.

1989;161(1):72-88.

16. Lobb R. Observations on the Microstructure of 20Cr-25Ni-Nb Stainless Steel after

Exposure to Iodine Vapor During Creep at 750° C. Oxid Met. 1981;15(1-2):147-67.

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17. Soelberg NR, Garn TG, Greenhalgh MR, Law JD, Jubin R, Strachan DM, et al.

Radioactive Iodine and Krypton Control for Nuclear Fuel Reprocessing Facilities. Sci Technol

Fuel Inst. 2013;2013:702496.

18. Nair SK, Apostoaei AI, Hoffman FO. A radioiodine speciation, deposition, and

dispersion model with uncertainty propagation for the Oak Ridge dose reconstruction. Health

physics. 2000;78(4):394-413.



20. Niedobová E, Machát J, Kanický V, Otruba V. Determination of iodine in enriched

chlorella by ICP-OES in the VUV region. Microchim Acta. 2005;150(2):103-7.


Technology. 2000;129(3):297-325.



23. Pankratz LB. Thermodynamic properties of elements and oxides. Washington, D.C.: U.S.

Department of the Interior, Bureau of Mines; 1982.

24. Pankratz LB. Thermodynamic properties of halides. Washington, D.C.: U.S. Department

of the Interior, Bureau of Mines; 1984.

25. Pechtl S, Schmitz G, von Glasow R. Modelling iodide - iodate speciation in atmospheric

aerosol: Contributions of inorganic and organic iodine chemistry. Atmos Chem Phys.

2007;7(5):1381-93.

49

26. Hoinkis E. Review of the Adsorption of Iodine on Metal and Its Behavior in Loops. Oak

Ridge, TN: Oak Ridge National Laboratory; 1970. Report No.: ORNL-TM-2916 Contract No.:

ORNL-TM-2916.

27. Oomi G, Mōri N. Magnetovolume effect of invar alloys under high pressure. Phys B+C.

1983;119(1):149-53.

28. Trolard F, Bourrié G, Abdelmoula M, Refait P, Feder F. Fougerite, a new mineral of the

pyroaurite-iowaite group: Description and crystal structure. Clays Clay Min. 2007;55(3):323-34.


& Sons, Inc.; 2014.

50

CHAPTER THREE

Molecular iodine interactions with Fe, Ni, Cr and stainless-steel alloys

3

Chelsie L. Beck, Brian J. Riley, Saehwa Chong, Nathaniel Smith, Derrick R. Seiner, Brienne N.

Seiner, Mark H. Engelhard, Sue B. Clark


Source: Beck CL, Riley BJ, Chong S, Smith NP, Seiner DR, Seiner BN, et al. Molecular iodine

interactions with Fe, Ni, Cr and stainless-steel alloys. Accepted to Ind Eng Chem Res. 2021; doi:

10.1021/acs.iecr.0c04590.

51

3.1 Abstract

The adsorption behavior of molecular iodine is important for understanding the spread of

radioiodine in a nuclear accident. Prior experiments indicate that, in addition to the interaction

with Fe, molecular iodine [i.e., I2(g)] also interacts with the next most abundant components of

austenitic stainless steel (i.e. Ni, and Cr) at room temperature. In this study, we investigate iodine

adsorption on three Fe, Ni, and Cr, while focusing on understanding the variables affecting

adsorption as well as the iodine compounds that are formed during adsorption. Scanning electron

microscopy and energy dispersive X-ray spectroscopy were used to characterize the surfaces of

exposed metal particles and aid in the understanding of the morphology and chemistry of iodine

interactions with the substrates. Inductively coupled plasma-optical emission spectroscopy was

used to detect low levels of metal iodides and X-ray photoelectron spectroscopy was used to

confirm the formation of the metal iodides. The role of environmental factors (e.g., humidity and

oxygen content) for iodine adsorption on metal substrates is addressed. The individual metals

demonstrated formation of metal iodides for Fe and Ni particles from interaction with I2(g). The

formation of metal iodides may be indicating the affinity of iodine for the respective metal. In

this study, the iodine affinities ranked Fe > Ni > Cr as determined by the quantity of

chemisorbed iodine. This trend is also supported by the distributions and proportions of metals in

the corrosion product of the stainless steels. The exposures without oxygen and humidity indicate

the potential of a multistep iodine adsorption process where iodine first attacks the oxide layer

and then chemisorbs to the exposed metal.

52

3.2 Introduction

Iodine is a natural material found in the environment and is used within the human

metabolic process [1]. In addition to natural iodine (i.e., 127I), radioactive isotopes of iodine (e.g.,

131I and 129I) are present in irradiated nuclear fuel and can be released in the event of a nuclear

accident; the half-lives (t½) are 8.04 days for 131I and 1.57×107 years for 129I [2]. Also, iodine

compounds tend to be volatile, increasing their release to the environment during an accident

(e.g., from ruptured fuel rods), and iodine is highly mobile [3], which further increases transport

through the atmosphere.. Any 131I released from a nuclear accident, such as Chernobyl or the

Fukushima Daiichi Nuclear Power Plant event, can affect human health for several months [4].

The chemistry of iodine is very complex, and it can readily convert between gas and condensed

phases through adsorption/desorption processes. Molecular iodine [i.e., I2(g)] is expected to be the

most prevalent species of iodine released and has demonstrated potential for chemisorption on

stainless steel, and its elemental constituents, found in and around nuclear facilities [5].

Investigations into adsorption on stainless steels are often focused on temperatures >200°C to

mimic operating conditions of a nuclear power plant [6]. However, limited studies have been

done on iodine adsorption onto metal substrates at lower temperatures which are more relevant to

accident scenarios, primarily due to the assumed lower specific surface area available [7]. The

limited studies conducted have shown significant and complex interaction of I2(g) with stainless

steels, although gaps remain regarding the chemical nature of the adsorbed iodine [6, 8, 9]. The

current study focuses on evaluating the chemical nature of adsorbed iodine on metal surfaces at

ambient and near-ambient temperatures within various atmospheric conditions.

Prior investigations revealed the formation of metal iodides from exposure of stainless

steels (i.e., SS304L and SS316L) to I2(g) under ambient temperatures and pressures [8]. The three

53

major constituents of these stainless steels are Fe, Cr, and Ni. This study, therefore, is focused on

the interaction of I2(g) with Fe, Ni, and Cr to better elucidate the specific metal-iodine (i.e., M–I)

interactions. A suite of analytical techniques were used to investigate these M–I interactions with

the main goals of this work being to (1) compare gravimetric uptake on individual metals and

alloys, (2) identify surface elements on substrates following iodine interactions through chemical

analysis techniques, and (3) compare iodine uptake and surface structures on Fe following

exposure within different environments, e.g., temperature, humidity, and O2 content. Extensive

characterizations were performed during the analysis to provide insights into the interaction of

iodine with surfaces including scanning electron microscopy (SEM) for microstructure, energy-

dispersive X-ray spectroscopy (EDS), inductively coupled plasma optical emission spectroscopy

(ICP-OES) for quantification of water-soluble elements, X-ray photoelectron spectroscopy

(XPS) to confirm formation of metal iodides measured in the water leachate, and simultaneous

differential thermal analysis with thermogravimetric analysis (DTA/TGA) for quantification of

physisorbed iodine on exposed samples.

3.3 Experimental

3.3.1 Materials

Substrates evaluated within this study included Fe granules and particles of Fe, Cr, Ni,

SS304, and SS316L. The Fe granules were used as-received (Alfa Aesar, 1–2 mm, electrolytic,

99.98%). For the particles, the sizes and purities were the following: Fe (45 µm, 99%), Ni (45

µm, 99.5%), Cr (45 µm, 99.5%), SS304L (40 µm, 99.9%), and SS316L (40 µm, 99.9%), which

were purchased from US Research Nanomaterials, Inc. (Houston, TX) and used as-received.

Details of the material and exposure conditions are documented in Table 3-1. Henceforth, the

54

40–45 µm metals will be referred to as particles and the larger diameter Fe particles will be

referred to as granules.

3.3.2 Exposure to Molecular Iodine

For all experiments, iodine crystals (99.99%, Sigma Aldrich, St. Louis, MO) were used to

generate the iodine vapor for exposure. The Fe granules were exposed to I2(g) at 60°C in an oven

(3511FSQ, Isotemp, Fisher Scientific; Hampton, NH). A 1-L PFA jar (Savillex) was used as the

exposure vessel, containing the samples and iodine in 4 mL glass vials (Qorpak GLC-00980).

The vial containing iodine was placed in the center of the vessel with the sample vials arranged

around it in a concentric pattern. Two experiments were done at 60°C, one in which samples

were iodine-loaded in air and one where samples were purged with N2(g) in a glovebox (M-

Braun, Inc., Stratham, NH) prior to loading into the oven. For the 60°C experiment, samples

were in the oven for four days then removed and weighed. The samples were returned to the jar

without the iodine and placed in the oven for 1 hour to desorb any physisorbed iodine before

beginning characterization of the exposed materials. The mass uptake was determined

gravimetrically after desorption by comparing with pre-reaction samples masses.

The Fe granules were further analyzed in triplicate in a long-term room temperature

exposure. For this exposure, a 250 mL wide-mouth, low-density polyethylene bottle (Thermo

Fisher Scientific) was used as the exposure vessel. The Fe granules were arranged in a circle

around the iodine container, the vessel was sealed, and left at room temperature for a total of 109

days. The Fe samples and iodine were periodically removed and weighed, and solid iodine was

added as needed to replace the sublimed iodine.

55

The particles of elemental metals and alloys were exposed to I2(g) in a sealed container at

room temperature. Here, 100–200 mg of each sample was placed in tared glass petri dishes

(BRAND®; BR455701) in a thin layer and weighed on an analytical balance (±0.1 mg; ME204E

Mettler Toledo; Columbus, OH). The petri dishes were arranged in a circle around an open small

glass vial containing iodine crystals inside of a 350 mL perfluoroalkyl (PFA) jar (100-0350-01,

Savillex, Eden Prairie, MN). A second 350-mL Savillex jar was employed as a control and was

loaded with a similar mass of each powder, but without iodine. The lids were tightly closed, and

the samples were left undisturbed for 32 days, after which all sample dishes and the iodine dish

were weighed.

Table 3-1. Summary of sample names and descriptions including atmosphere and temperature (T). RT denotes room temperature (22±3°C).

Sample Description Sample Name Exposure

Temp Exposure Time Atmosphere

Iron Granules FeGA25 RT Long term (109

days) Air

FeGA60 60 4 days Air FeGN60 60 4 days N2(g)

Fe Powder Fe+I RT 32 days Air Cr Powder Cr+I RT 32 days Air Ni Powder Ni+I RT 32 days Air SS304L SS304L+I RT 32 days Air SS316L SS316L+I RT 32 days Air

Iodine uptake for all samples was quantified using gravimetric uptake (m%I,g) based on

Equation (1) where ms is the mass of the initial sample, mI is the mass uptake following iodine-

loading experiments based on mass change during the experiment defined in Equation (2), where

mf is the final mass after iodine loading (note that this does not account for any oxygen uptake

during the loading resulting in surface oxides).

56

m%I,g = mI / ms × 100 (1)

mI = mf – ms (2)


The as-received particles were analyzed using a particle size analyzer (Partica LA-960,

Horiba, Ltd.) to obtain the geometric mean particle size. The liquid medium used for conducting

measurements was isopropyl alcohol.

The samples were analyzed with SEM/EDS. The SEM analysis was performed using a

JSM-7001F field emission gun microscope (JEOL USA, Inc.; Peabody, MA). Here, samples

were mounted to aluminum stubs with carbon tape and coated with 2.5 nm of Pt using a Quorum

150T ES Ar-plasma sputter coater (Electron Microscopy Sciences, Hatfield, PA). The EDS

analysis was performed using a Bruker xFlash 6|60 detector (Bruker AXS Inc., Madison, WI).

Both imaging and EDS mapping were conducted at an acceleration voltage of 15 kV.

To quantify the amount of metal iodide(s) formed, exposed particles were leached using

ultrapure 18-MΩ·cm deionized water (DIW). Metal iodides relevant to this study are unique in

their water solubilities (see Table S3-1, Supporting Information), which makes a selective water

leach an ideal method for assessing and quantifying the formation of metal iodides as well as

metal distribution in the iodide phase. Unexposed materials were leached with the same

conditions to verify that there were no pre-existing water-soluble compounds in the base metals.

First, approximately 20 mg of each sample was weighed and leached by adding 10 mL of DIW,

vortexing for 10 s, and allowing to sit for 20 minutes. After 20 minutes, the samples were filtered

through a 0.45-µm syringe filter. The leachate was diluted using water and analyzed for I, Cr, Ni,

and Fe.

57

All leachates were analyzed using a Thermo iCAP7600 ICP-OES in axial mode. A

standard quartz sample introduction system was used. A calibration curve was generated using at

least three NIST traceable standards prepared by diluting 1000 µg mL-1 standards from Inorganic

Ventures (Christiansburg, VA). All reported sample concentrations were within the calibration

range for the given analyte. The calibration curve was verified using calibration check standards

after calibration and then after every 10 sample measurements.

The DTA/TGA measurements were performed on a TA Instruments SDT Q600 (TA

Instruments, New Castle, DE, USA). Approximately 25 mg of sample was placed in an alumina

crucible and heated at 10 K min-1 from room temperature to 500°C in air flowing at 10 mL min-1.

An empty crucible was run alongside the sample as a reference and data were collected every

0.1°C. Prior to measurements, the instrument temperature was calibrated with a series of five

high-purity metals (i.e., 99.999% In, 99.99%Sn, 99.999% Zn, 99.999% Al, and 99.999% Au).

The DTA baseline and beam growth expansion calibrations were performed according to

manufacturer specifications.

The XPS measurements were performed with a Physical Electronics Quantera Hybrid

Scanning X-ray Microprobe. This system uses a focused monochromatic Al Kα X-ray (1486.7

eV) source for excitation and a spherical section analyzer. The instrument has a 32-element

multichannel detection system. The X-ray beam is incident normal to the sample and the

photoelectron detector is at 45° off-normal. High energy resolution spectra were collected using

a pass-energy of 69.0 eV with a step size of 0.125 eV. For the Ag 3d5/2 line, these conditions

produced a full width at half max of 0.92 eV ± 0.05 eV. The binding energy (BE) scale is

calibrated using the Cu 2p3/2 feature at 932.62 ± 0.05 eV and Au 4f7/2 at 83.96 ± 0.05 eV.

58

3.4 Results and Discussion

3.4.1 Materials Selection

Particles of ~40–45 µm were used to provide higher specific surface area than would be

achieved with metal coupons to increase the iodine mass uptake and improve the possibility of

detection with the different characterization techniques. The individual metals were exposed at

ambient conditions comparable to the ones used for the stainless-steel exposures from our

previous study [8]. Since Fe is the major constituent of stainless steels and is a primary focus

within most of the available literature, additional experiments were conducted at 60°C in inert

[i.e., N2(g)] and air atmospheres to understand the effects of temperature and atmosphere,

specifically humidity and O2, on MxIy formation with larger (1–2 mm) Fe granules that allowed

for the evaluation of a substrate with a smaller specific surface area.

Previous research showed that halogens increase the normal corrosion of one stainless

steel alloy in the presence of air at 750°C [10]. The metal and alloy particles were used as-

received with no attempt to remove any naturally formed oxide (passivation) layer(s); this was

done because interactions with unoxidized metals and stainless steels has been well researched,

whereas the impact of an oxide layer has not and oxide layers are expected in existing structural

materials [11].

3.4.2 Room Temperature Exposure of Particles

Iodine exposure was completed with Ni, Fe, Cr, SS304L and SS316L particles. The

control set that was not exposed to iodine showed no measurable mass increase indicating that no

measurable oxidation from oxygen in air occurred over the course of the experiment.

Gravimetric uptake results for this experiment are shown in Table S3-4. In the exposure, 233.9

59

mg of iodine sublimed during the 32-d exposure. The combined mass uptake of 304 and 316 was

247.4 mg indicating a large majority of the sublimed iodine was adsorbed and other constituents

of air, such as water vapor, likely account for the additional mass. Due to the high uptake on 304

and 316, it is expected that the iodine concentration in the container never reached vapor

pressure of iodine at room temp (22 °C) of ~27 Pa . The 304 and 316 were likely constantly

removing iodine from the gas phase, however the nonalloy metals would have had an equal

opportunity to interact and did not appear to be as reactive as the alloys. The non-alloy metals

did not have any measurable gravimetric iodine uptake, and no visual change in appearances

were noted (see Figure S1, Supporting Information).

Although gravimetric measurements did not reveal any mass uptake, the SEM/EDS

analysis of the samples allowed for better detection limits for detecting iodine. The Fe+I sample

showed a very small quantity of iodine (i.e., 2.0±0.3 at%) in the spot map at 100× and 6±4 at%

in the 500× EDS analyses (see Figure 3-1 and Figure S3-4, Supporting Information). The

SEM/EDS analysis of the Cr and Ni particles showed that no iodine uptake occurred. However,

as noted previously, it is possible that iodine was on the surface, but the interaction volume was

so large that the iodine signal was overwhelmed by the substrate. It is expected, given the

favorable interaction observed between iodine and Ni in stainless steel, that longer exposures

would lead to the formation of measurable NiIx on the surface. The lack of iodine on the Cr is

expected since sources i.e. Wren et al. [9] stated no interaction between Cr and I and the EDS

data and leaching of the metal iodides formed in the alloys show small quantities of Cr.

60

Figure 3-1. SEM regions for Fe+I at different magnifications (EDS locations are shown in Figure S3-4 for this sample).

It was expected that the Fe and Ni would readily adsorb iodine and the low uptake

observed in the current study was surprising. Multiple variables had the potential to affect the

uptake of iodine. The available specific surface area is one of these, but the uptake affinity was

opposite the particle size trend. The cumulative specific surface area of a fixed volume of

particles increases with decreased particle size; thus, the trend observed here would not be due

61

exclusively to the specific surface area. Thermodynamically, the formation of nickel and iron

oxides is more favorable than the formation of the metal iodides and it is possible that the Ni and

Fe had already reacted with oxygen to form an oxide layer that was thicker than the passivation

layer on the alloys. The particles were not cleaned, as mentioned in section 2.1 and therefore the

oxide layer was not removed, and the thickness was not measured with any of our analytical

techniques. Although the oxide layers are expected to be similar, there are many variables that

could have affected the growth of an oxide layer, such as the age of the particles, the storage, and

the preparation technique which may have exposed them to more oxidation.

Analysis of the Fe+I particles via ICP-OES of the water leachate corroborated the EDS

data indicating the presence of iodine on the Fe+I particles. The molar ratio of the iodine:metal

(MI:Mm) in the water leachate for the Fe+I sample shows a 2.00 ± 0.2 atomic ratio, which

matches the stoichiometry for the formation of FeI2. The FeI2 compound is the predicted

molecule since FeI3 is thermodynamically unfavorable and not expected to exist under standard

temperatures and pressures [9]. The lower detection limits for iodine using a water leach showed

measurable quantities of iodine in the leachate of the Ni+I sample but not in the Cr+I sample.

The Ni+I sample gave a 0.8 ± 0.2 atomic ratio for Ni:I, which is above the expected 1:2 ratio for

NiI2.

The XPS analysis was used to evaluate the Ni powder before and after exposure and

showed the presence of carbon, (20 at%) and oxygen (40 at%) on the surface, indicative of

organic contaminants commonly found on the surfaces of materials (see Table S3, Supporting

Information). The carbon and oxygen spectra are shown in Figure 3-2a and Figure 3-2b,

respectively. There was a small decrease in the oxygen content of ~2 at% after iodine exposure,

62

which is likely due to the incorporation of I on the surface since XPS is very specific for the first

few monolayers. The exposed sample had < 2 at% iodine on the surface. Literature has the I 3d5/2

in NiI2 at 619.0 eV [12, 13], which is in good agreement with the binding energy seen in Figure

3-2c. The iodine peaks confirm the formation of a NiI2 complex, which was inferred from the

water leachate.

Figure 3-2. XPS spectra of nickel particles before and after exposure including the (a) C 1s regions, (b) the O 1s regions, and (c) the I 3d regions. For (a), the intensities were

normalized to the carbon 285.0 eV carbon peak. For (b), the intensities were normalized to the 531.4 eV oxygen peak. For (c), the intensities were normalized to the I 3d5/2 peak at

619.3 eV.

3.4.3 Iron Granules at Room Temperature

The Fe granules were also exposed to I2(g) at room temperature. The granules were much

larger than the previously discussed particles and, subsequently, had a smaller specific surface

area. The long-term exposure at room temperature shows an initial increase in mass until day 12

when the increase began to plateau as shown in Figure 3-3.

63

Figure 3-3. Mass uptake (m%I,g) by Fe granules over an ~18-day exposure. Error bars are 1σ of three replicate samples.

Follow up measurements after two and three months showed that all mass increases

recorded at previous readings was lost and the sample mass was less than the starting mass. This

indicates the possibility of initial formation of FeI2 followed by volatilization of that product,

resulting in the loss of the original sample mass. However, the vapor pressure of FeI2 is very low

(~2.9×10-8 Pa, or ~2.2×10-10 Torr, at 20°C) [14]. It is possible that a hydrated Fe–I compound

formed with a different (higher) vapor pressure. Neither the integration of O2 to form a stable

FeIxOy complex or the formation of a stable iron iodide hydrate under humid conditions (e.g.,

FeI2·4H2O), both suggested by [9] would explain the loss of original sample mass over the

course of this experiment. Desorption of physisorbed I2 would also not explain the decrease in

the starting mass, however it is likely that some portion of I2 was physisorbed and that it is

constantly desorbing and re-adsorbing.

3.4.4 Characterization of Received Materials

The metal particles and granules were analyzed by SEM and micrographs are provided in

Figure 3-4a-f. The Ni, Fe, and Cr particles were largely non-spherical (Figure 3-4a-c). The

64

stainless-steel alloys were mainly spherical but variable in size. The results of particle size (sp)

analysis of the starting materials are shown in Table S3-2 in Supporting Information. Although

the advertised size was 45 µm for the Ni, Fe, and Cr and 40 µm for the stainless steel alloys the

actual particle size distributions varied somewhat and the corresponding differences in specific

surface areas likely contributed to some of the observed variabilities in iodine uptake values.

Figure 3-4. SEM micrographs of as-received materials used in room temperature exposures. The top row is 100× magnification and the bottom row is 1000×, with the (a) Ni,

(b) Fe, (c) Cr, 9d) SS304L, and (e) SS316L. Note that sample packing on the stub is not consistent. The four images on the far right for SS304L and SS316L were taken from Beck

et al. [8] and reprinted with permission. ©2020 American Chemical Society. (f) SEM micrographs of three Fe granules showing the differences in appearance and size.

The thicknesses of the passivation oxide layers on different metals documented in the

literature vary, but they are reasonably similar across the materials tested in the current study.

Literature points to oxide layer thicknesses of ~2.5 nm on Ni [15, 16], ~2–5 nm on Fe [15, 17],

65

~1.5–2.5 nm on Cr [18, 19] and < 4 nm on SS304 [20]. The EDS analysis of the as-received

materials showed similar oxygen content across the materials, from 1–4 at%; however,

accurately quantifying oxygen by EDS is difficult. Furthermore, the interaction volume of the

electrons within the bulk of a sample during EDS analysis is larger than the expected thickness

of an oxide layer (see Figure S3-8, Supporting Information), which means the ratio of

oxygen:metal (i.e., O:M) will not match the expected stoichiometry of the oxide species since the

majority of the sampled area is below the oxide layer.

The XPS data for the Ni material shows the expected 1:1 Ni:O because the measurement is

on the first few monolayers, which confirms that the samples did have an oxide layer before

exposure (see Table S3-3, Supporting Information). The Fe granules showed very different

surface features (see Figure 3-4f). Surface imperfections such as cracks and fissures can be

locations for higher iodine uptake; the variability in surface morphology in the Fe granules was

expected to lead to a non-uniform distribution of iodine adsorbed on the surface.

3.4.5 Iron Granules at Increased Temperatures

The exposure of iron granules at 60ºC in air versus N2(g) showed that initial iodine uptake

was 7.2% for air and not detectable in N2(g). The data from these experiments is summarized in

Table 3-2.

Table 3-2. Summary of iodine uptake experiment on Fe granules including the atmosphere (atm.), the sample mass (ms), the iodine mass (mI), the iodine uptake in mass% (m%I), the post-desorption uptake (mI,pd), and the post-desorption iodine uptake in mass% (m%I,pd).

Both experiments were conducted at 60°C.

Sample Atm. ms (g) mI (g) m%I,g mI,pd (g) m%I,pd

66

FeGA60 Air 0.5539 0.0398 7.2% 0.0245 4.4%

FeGN60 N2(g) 0.3389 ND N/A N/A N/A

In this experiment, enough iodine sublimed that the concentration of iodine in the gas is

expected to have reached the vapor pressure of iodine at 60 ºC (623 Pa or). The gravimetric data

matches with the visual inspection of the samples (Figure S3-2, Supporting Information) that

shows unchanged metallic colored granules after I2(g) exposure in N2(g) and highly oxidized,

reddish, granules after I2(g) exposure in air. The fraction that desorbs should be representative of

the fraction that was physisorbed. In this case, the exposure in air lost 2.8% or about 38% of the

initial gain after the 24 hours of desorption. The DTA-TGA analysis of the sample showed 2.5%

mass loss over the scan (see Figure S3-5, Supporting Information). An initial weight loss event

around 100°C is likely the loss of water since FeI2 is very hygroscopic (Table S3-1, Supporting

Information), and it is not unreasonable to assume that the sample could absorb water from the

air. The FeI2·4H2O complex can potentially decompose, similarly to FeCl2, as shown below in

Reaction (3) [21]. At 100°C, HI would be gaseous and could explain the measured weight loss.

4 FeI2 + 4H2O → 4 FeO + 8 HI (3)

Although gravimetric measurements did not show any iodine uptake in N2(g), the

SEM/EDS data revealed measurable iodine on the surface, as well as visible corrosion. The

SEM/EDS analysis of the Fe granules exposed in air versus N2(g) show drastically different

surface features, likely indicating different reaction mechanisms took place during these

experiments. The iodine exposure conducted in air shows evidence of uniform corrosion and

localized areas of adsorbed iodine (see Figure 3-6j). The backscatter electron SEM micrograph

67

and corresponding EDS dot map shown in Figure 3-5 reveal the adsorption of iodine to the Fe

granules as well as regions of oxidation. The EDS spot analysis data shown in Table 3-3 supports

the visual assessment with a large increase in oxygen and variable ranges of iodine, with high

iodine corresponding to the bright regions shown in Figure 3-6a–e. The dark regions correspond

to the newly formed oxide layers and show 1–9 at% iodine. The SEM/EDS analysis of the

sample exposed to iodine in N2(g) shows a more diverse surface, see Figure 3-6b,c. Three specific

regions were targeted by spot EDS including those high in O, those high in I, and those high in

Fe. Every attempt was made to keep oxygen out of the exposure vessel and the regions of high

Fe and low O likely indicate an initial attack of the oxide layer by I2(g).

68

Figure 3-5. SEM-EDS data for Fe granules FeGN60 (8c-2) after exposure showing (a) BSE-SEM micrograph, (b) Fe map, (c) O map, (d) I map, and (e) the phase map.

Table 3-3. EDS data of Fe granules. Compositional data is in atomic% and 1σ standard deviations are provided in parenthesis below the values

Sample Atmosphere Fe (at%) O (at%) I (at%) O/Fe

As received N/A 84.7 (0.7)

14.6 (0.3) 0 0.172

(0.005)

Dark Regions Air at 60ºC 53.3 (0.9)

41 (5)

5 (4)

0.8 (0.1)

Light Regions Air at 60 ºC 45 (4)

26 (9)

30 (11)

0.6 (0.2)

Oxide Region N2(g) at 60 ºC 40 (4)

58 (4)

1 (1)

1.5 (0.2)

69

Iodide Regions N2(g) at 60 ºC 51

(6) 37 (5)

11 (2)

0.7 (0.2)

Iron Regions N2(g) at 60 ºC 94 (2)

5 (2) <0.5 0.05

(0.02)

Based on the data in Table 3-3, the O/Fe molar ratio in the high-O region is actually higher

in the inert exposure than in air, even though air has significantly more oxygen (~21%) versus

the glovebox (<0.1 ppm), or ~2×106 times more. One possible explanation is that the oxide layer

is being removed and possibly releasing O2(g), where the O2(g) could re-adsorb to different

regions of the surface of the metals. Furthermore, the adsorption of iodine is much lower in the

N2(g) exposure even though both experiments had an excess of I2(g). This supports the theory that

the adsorption of iodine to form MxIy complexes relies, at least in part, on the presence of H2O,

but not completely as suggested by Abrefah, et al. [11] Interestingly, the SEM/EDS of a less-

faceted Fe granule showed the surface of the sample exposed in N2(g) with regions of high iodine

sandwiched between oxide layers, which are produced in a concentric pattern (see Figure

3-6d,e). The concentric pattern is similar to the SEM image of iodine corrosion on

electropolished stainless steel shown by Wren et al. [9] The EDS linescan shown in Figure 3-6f,

illustrates this phenomenon with the spike in iodine content centered between high-oxygen

regions.

70

Figure 3-6. (a) Fe granule (FeGA60) exposed in air at 60ºC. (b) Fe granule (FeGN60) exposed in N2(g) at 60ºC. (c) Fe granule (FeGN60) exposed in N2(g) at 60ºC with different morphology than (b) and at higher magnification, inset shows full particle. (d) Localized

area from image (c) showing concentric rings of oxidation with iodine adsorption in between oxide layers. (e) higher magnification of image (d) where a dot map was collected, which is shown in (g)-(j) with boxed region corresponding to line scan region shown in (f).

3.5 Summary and Conclusions

In these experiments, molecular iodine has been shown to react with particles of metals

and alloys under different conditions which would be relevant in an accident scenario at a

nuclear facility. In the case of stainless steels, the reduction of iodine to form metal-iodide

complexes on the surface occurs quite rapidly at room temperature and metal building

constituents would likely be a source of adsorbed radioiodine. Exposures of the Fe, Cr, and Ni

71

particles to I2(g) showed that iodine interactions did not as readily form metal iodides. In the case

of Cr and Ni, no iodine was measured by EDS analysis, but iodine was observed with Fe. The

higher sensitivity of ICP-OES was able to detect metal-iodide complexation in the Ni+I sample

at a 0.83 molar ratio of Ni:I. The XPS analysis showed iodine on the surface at 1.73 ± 0.16 at%

with a binding energy consistent with that of NiI2.

Multiple variables that can affect adsorption were discussed. Based on these experiments,

the uptake of iodine onto metals appears dependent on the affinity of iodine with the metal or

alloy. The Fe particles exhibited the highest adsorption of molecular iodine, which was

detectable by EDS and ICP-OES. For Ni, the formation of NiIx was detectable by ICP-OES and

XPS. For Cr, which has a low affinity for I2, the I2 did not adsorb at a measurable amount. In this

study, the affinity for metals was Fe > Ni > Cr as determined by the quantity of iodine that was

chemisorbed. This trend is supported by the concentrations of metals in the corrosion product of

the stainless steel. It does not correspond to the distribution of metals in stainless steel which is

Fe > Cr > Ni and, therefore, it is not believed that the affinity for iodine is the same between the

metals.

The iodine exposures conducted in air versus N2(g) on the Fe granules at 60°C showed

differences in adsorption behavior. In air, the Fe granule surface was uniformly corroded by

oxygen with localized areas of high iodine adsorption. In N2(g), evidence was found of iodine

causing reduction in the oxygen content in some areas, as well as visible corrosion from an

increase in oxygen in others. Overall, less iodine was adsorbed to the surface compared to the

sample exposed in air and the areas that did show iodine adsorption were sandwiched between

layers of FexOy. This indicates the possibility of a component of air such as H2O or O2 as a co-

72

reactant or catalyst in the adsorption of iodine on Fe surfaces. More experiments are needed to

explore the dependence of humidity and oxygen on the reaction.

3.6 Funding Sources


Northwest National Laboratory (PNNL). PNNL is operated by Battelle for the U.S. Department

of Energy (DOE) under Contract No. DE-AC05-76RL0-1830.


Authors thank Jaime George for her help with DTA-TGA analysis, Chris Barrett for

particle size analysis and Nathan Canfield for help with EDS data analysis. Authors thank Eugene

Ilton, Hilary Emerson, Sarah Saslow, Neil Henson and Matthew Olszta for valuable scientific

discussion. PNNL draws on signature capabilities in chemistry, earth sciences, and data analytics

to advance scientific discovery and create solutions to the nation's toughest challenges in energy

resiliency and national security. PNNL is operated by Battelle for the U.S. Department of Energy

(DOE) under Contract No. DE-AC05-76RL0-1830. The XPS was performed in EMSL

(grid.436923.9), a DOE Office of Science User Facility sponsored by the Office of Biological and

Environmental Research.

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76

CHAPTER FOUR

Adsorption of Iodine on Metal Coupons in Humid and Dry Environments

4

Chelsie L. Beck, Brian J. Riley, Nathaniel P. Smith, Sue B. Clark


Submitted to Journal of Nuclear Materials

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4.1 Abstract

In this study, five different metal coupons were evaluated for gaseous iodine [I2(g)]

adsorption including two stainless steels (i.e., SS304 and SS316), two Inconel® alloys (i.e., 625

and 718) and pure Ni (i.e., Ni-200) within a dynamic flow-through system where temperature,

iodine concentration, flow rate, atmosphere, and relative humidity were controlled. Humidity

was shown to be critical to iodine adsorption on SS304 and SS316 and Ni-200 at ambient

temperatures. The results presented herein suggest that a moisture mediated reaction is occurring.

However, higher humidity levels decrease the adsorption, suggesting an ideal range of humidity

for highest corrosion. A comparison of the five metal substrates showed the highest I2(g)

adsorption in the following descending order Ni-200 > SS304 > SS316 >718>625. The Inconel

625 and 718 alloys were fairly inert to iodine adsorption under the conditions tested.

Characterization by scanning electron microscopy, energy dispersive X-ray spectroscopy, and X-

ray diffraction of the Ni-200 coupon indicates that NiI2 is formed and flakes off the surface as a

black powder. The SS304 and SS316 coupons showed evidence of extensive reactions with I2(g)

and formed a much more deliquescent corrosion product, which reacted with air when removed

from the flow-through system for weighing on the analytical balance. These findings assist in

predicting iodine adsorption behavior on a variety of metal surfaces under various conditions.

4.2 Introduction

The potential for release of volatile fission products in the event of a nuclear accident is a

major health concern [1]. Radioiodine, specifically 131I, is of the greatest radiological

significance due to biological effects and complex chemistry that can lead to volatile species [2].

Although iodine is only a small portion of the fission products produced during normal fission of

uranium, it has multiple short-lived isotopes that make it a large portion of the activity [3].

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Multiple variables can contribute to the speciation and volatility of iodine and one of those is

adsorption on surfaces in the nearby vicinity, which can act as a sink for volatile iodine or can be

a source of different volatile species such as the reaction of I2 with painted surfaces to produce

organic iodides [3-5]. Adsorption onto metal surfaces such as stainless steel has been shown to

be chemisorption and partially irreversible [6], which essentially removes some volatile iodine

from the potential inventory and lowers the amount that could be released into the atmosphere.

Both dry and wet conditions are possible depending on the type of accident, however deposition

as a function of relative humidity (RH) has not been well established [3].

Recent work on iodine adsorption onto metal substrates focused on characterization of

the adsorbed species using stainless steel particles, as well as unalloyed metal particles [7, 8]. In

the current work, we continue where this previous work left off to investigate the adsorbed

species but also seek to control and understand the parameters impacting adsorption, including

dry vs. humid atmospheres and oxic vs anoxic atmospheres. In this work, five metal/alloy

coupons were exposed to molecular iodine [I2(g)] including two austenitic Fe/Cr stainless steels

(i.e., SS304 and SS316), two Inconel alloys (i.e., 625 and 718), and pure Ni (i.e., Ni-200). The

stainless steels were chosen because they are common steels used in structural materials and, as

such, have been used in previous studies [6, 9]. The Ni and Inconel alloys were selected for two

reasons. The first reason is that prior studies [7, 8] on SS304 and SS316 indicated a large amount

of Ni-Ix formation and we wanted to explore the interaction with nickel without the presence of

additional metals (i.e., Fe and Cr) alloying metals as well as nickel as the dominant metal in an

alloy (i.e., Inconel alloys vs Fe-dominant SS alloys). The second reason is that Inconel alloys

(e.g., 625 and 718) are relevant to the nuclear industry and therefore it is important to understand

their interactions with iodine.

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To investigate the adsorption and desorption processes, the metal coupons were exposed

to I2(g) and then characterized to assess the rate of iodine uptake, the variables affecting uptake

mass/rate (i.e. oxygen, humidity), and the compositions of any corrosion products formed during

the process. Humidity was a specific parameter that was evaluated since some [9] have stated

that it plays an essential role in the adsorption process while others [6] have stated that moisture-

mediated reaction are not primary mechanisms. A specific goal in the current study was to

determine if a specific relative humidity value was necessary to initiate iodine reactions with the

metal substrates since both dry and wet surfaces may exist in the environment depending on the

type of accident [3]. High-temperature studies [10, 11] have shown increased oxidation and

corrosion from water vapor, but they note the lack of understanding of the mechanism to explain

this.

In the current study, a variety of experiments were carried out to understand the role of

atmosphere (e.g., air vs. argon, dry vs. humid), and to compare the results between the different

metal substrates following iodine interactions. Argon was used as an oxygen-free environment to

test the assumption made by Clément et al. [3] that the iodine sorption rate is sensitive to O2

content in the gas phase. This was further shown by Wren et al. [6] who saw differences between

iodine exposures in atmospheres of N2(g) and air . Static exposures were completed as

preliminary experiments, but the results presented in our previous work [8] emphasized the

importance of a dynamic (flow-through) system that allowed for control of the atmosphere and

iodine concentration in real-time.

The metal substrates used in the current study were either used as-cut and washed

(untreated) or electropolished. Electropolishing was used to remove surface imperfections that

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could result in localized sites for adsorption. The flat surface was also used to look at the layers

of adsorption as well as the pitting and deformations caused from leaching the corroded surface.

For the untreated surface preparation, coupons were cleaned of grease or other impurities that

may have been picked up during cutting and handling the materials but did not remove any

surface defects. The untreated surfaces were used as a surrogate to material that could be found

in real-world conditions and these were used in the flow-through studies.

4.3 Experimental

4.3.1 Materials

The metal and alloys were purchased from McMaster-Carr. The certified constituents of

each metal are given in Table S1 (Supporting Information). The compositions of the metals were

measured using energy dispersive X-ray spectroscopy (EDS; described below); these data are

shown in Figure S1 in the Supporting Information. The metals were cleaned and prepared with

two methods; electropolished and untreated. The untreated coupons were cleaned by rinsing with

ultra-pure deionized water (DIW) followed by acetone, then methanol and allowed to air dry. If

residue remained after this protocol, then an oxalic acid-based cleaner (Barkeepers Friend) was

used to wipe away the residue very gently before repeating the cleaning series. The cleaner is

abrasive, so care was taken to not abrade the surface. Electropolishing was used to remove

surface imperfections that could result in localized sites for adsorption. The flat surface was also

used to look at the layers of adsorption as well as the pitting and deformations caused from

leaching the corroded surface. The SEM micrographs of the two surface techniques for Ni-200

are shown in Figure S2 (Supporting Information).

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4.3.2 Static Exposures on Electropolished Coupons

The iodine exposures were carried out in a 350-mL Teflon jar with a screwcap lid (100-

0350-01, Savillex). To produce the samples, each material was cut into 1×1 cm squares and then

the squares were quartered to a rough size of 5×5 mm. Cutting was done using electrical

discharge machining (EDM) (Mitsubishi FA10S) with a 0.010 in. brass wire. The quarters were

electropolished to a mirror finish. Electropolishing was chosen as the surface finish with the goal

of looking at the corrosion layer by scanning electron microscopy (SEM) and EDS. Three of the

5×5 mm of the coupons for each material were placed in a separate glass petri dish. The

remaining 5×5 mm quarter was kept as the control. The control quarter was measured with a

digital calipers (±0.01 mm) and the mass measured with an analytical balance (±0.1 mg;

ME204E, Mettler-Toledo); these data were used to calculate the surface area relative to the mass

for each sample set. The glass dishes were arranged in a concentric pattern equidistant from a

small container of solid iodine inside the Teflon jar (99.99%, Sigma Aldrich). The sample dishes

were removed and weighed 11 times during a 14-day period. With the mass changes reported are

cumulative masses gained on all three coupons of a given sample set.

4.3.3 Dynamic Iodine Exposure System

A dynamic flow-through system was developed that allowed for dynamic iodine

exposures on hanging substrates using different carrier gases; a schematic of this system is

shown in Figure 4-1. The system had a load cell from Sartorius model # WZA224 (Goettingen,

Germany), which allowed for in-situ weighing of the sample (± 0.1 mg). There were two gas

ports into the sample chamber and one exit port. The upper port provided cover gas and its

purpose was to keep the iodine-containing gas from reaching the load cell and potentially

corroding the metal components. The other gas port was slightly below the location of the

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hanging sample to allow the iodine gas to flow over the sample before exiting from the port

above the sample.

Figure 4-1 Schematic of flow through system where “FM” denotes flow meter.

The iodine-containing gas stream was produced by heating solid iodine in a flask within a

water bath, which was heated to control the iodine sublimation rate. The iodine was inside a small

glass vial, which could be easily removed from the flask for weighing so the iodine was not in

direct contact with the bottom of the flask. The mass of iodine sublimed was determined by

weighing the iodine vial before and after the experiment. The sublimed mass and the flow rate

were used to calculate the concentration of iodine in each experiment. The mass adsorbed on the

coupon was monitored by the load cell but also verified by weighing the coupon before and after

the exposure. The carrier gas flowed into the vial and over the subliming iodine carrying I2(g) into

the reaction chamber and out the exit port. The flow-through coupons were rectangular in shape

and were 2×5 cm with a 2.5-mm hole at the top to connect the wire for the load cell in the

comparison study and then 1.8×5 cm for the humidity variation studies. Each coupon was hung

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from the load cell using a nickel-chromel wire (Chromel C compliant to ASTM B267, ASTM

B344) wire. All coupon cutting was done using the EDM process described previously.

4.3.4 Control Studies

Control studies were done on select coupons to study the possible mass gain from

individual variables used in the studies. In this case, the variables were argon, dry air, and humid

air. The SS304 coupons were exposed to 2.8-2.9×109 mol/cm3 iodine for 5 h in dry air, dry air was

achieved by passing the air through a Drierite desiccant column packed with CaSO4,. SS316 was

exposed to 2.8-2.9 ×109 mol/cm3 iodine for 7 h in Ar. The exposure in humidity with no iodine

was done for 0.5 h at 45% humidity, followed by 10 min of dry air, per the normal exposure

protocol.

4.3.5 Humidity Variation Studies

The humidity levels were controlled by splitting the dry air tubing so that half was dry air

and half was humid air as shown in Figure 4-1. The flow rates of the dry air and humid air were

used to control the RH in the sample chamber. The flowmeters for both ranged from 0-1 standard

cubic feet per hour (SCFH; Brooks model #MR3A00SVVt). The combined flow of the two

flowmeters were kept at 1 for all experiments. The RH levels were tested at five different flow

rates ranging from 0 to 1 scfh as shown in Table S4 (Supporting Information). The comparison

of the five metal substrates was conducted with 0.5 SCFH on each flowmeter, which

corresponded to 45-±2.5% RH. The exposures of the five materials started with purging the

system with the coupon hanging for at least 15 min with dry air. The load cell was then tared

before adding iodine to the flask. Dry air was flowed for 30 min before introducing humidity.

The sample was exposed to humidity for an additional 30 min the mass changes were recorded.

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Then, the humidity was turned off and the iodine vial was removed and weighed. Dry air was

flowed without iodine until the mass loss on the coupon had plateaued. The final mass on the

load cell was then recorded and the coupon was removed from the system and weighed on the

analytical balance.


Following exposures, pictures of samples were collected using a digital camera. The

SEM-EDS data were collected using a JSM-7001F field emission gun microscope (JEOL USA,

Inc.; Peabody, MA) and a Bruker xFlash 6|60 (Bruker AXS Inc., Madison, WI) EDS system,

respectively. X-ray diffraction was performed using a Bruker® D8 Advance (Bruker AXS Inc.,

Madison, WI) XRD with Cu-Kα emission. The detector used was a LynxEyeTM position-sensitive

detector with a collection window of 3° 2θ. Samples were mounted on zero-background silicon

quartz holders for analysis. ICP-OES analysis of the leachate was conducted using a Thermo

iCAP7600 in axial mode. Calibration curve was generated for the analytes of interest using NIST

certified standards from Inorganic Venture (Christiansburg, VA). The leachate was produced by

adding 10 mL of 18ΩM deionized water to a vial with 1 exposed coupon. After 1 h an aliquot of

the leachate was removed and diluted for analysis.

4.4 Results and Discussion

4.4.1 Static Exposure of Electropolished Coupons

The exposures carried out in the Savillex jar for two weeks showed drastically different

iodine adsorption behaviors for the five sets of electropolished materials. The mass gain relative

to the surface area of the coupons is shown in Figure 4-2.

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Figure 4-2. Mass uptake relative to the surface area (SA) of the five materials over a 14-day period of exposure to I2(g) in ambient conditions. Vertical lines corresponding with data

markers indicate when I2 was removed from the system in each experiment.

The SS304 and SS316 coupons had linear uptake of 0.5 mg/day and 0.4 mg/day,

respectively, and the continued upward mass gains at the time of termination show that the

coupons were not at saturation after 14 d. The Ni-200, Inconel-625, and Inconel-718 had much

less mass uptake overall and the uptake for those materials is shown in the inset for Figure 4-2.

The Ni-200 had an initial slope of 0.2 mg/day that appeared to plateau after 72 h but then gained

mass again starting at 144 h. The Inconel alloys 625 and 718 had even less uptake than the Ni-

200 and they also appeared to plateau around 72 h. The 625 had mass gain at the end after iodine

was removed (at 144 hours) that indicates oxidation of the metal or hydration of the metal iodide

may be occurring. A closer look at Figure 4-2 shows that the plateau areas from 72-144 h and

247-312 h for Ni-200 correspond to times when the vessel was not opened for multiple days.

This indicated that changing the atmosphere in the exposure jar by opening it to weigh samples

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affected the corrosion rate and in response the subsequent experiments were done using the flow

through system.

4.4.2 Characterization of Electropolished coupons

The SEM-EDS analyses were done on each of the materials from the top views and side

views. Most of the materials had corrosion on the edges that obscured the ability to get a clear

image of the corrosion layer. The Ni-200 coupon was the only coupon that provided a flat edge

and a visible corrosion line that allowed for EDS analysis at the interface. The SEM-EDS data

from Ni-200 are shown in Figure 4-3. Spot EDS was done on the corrosion layer as well as the

base material. The lighter regions on the base material showed some iodine on the surface but

overall, the edge of the coupon had <3 at% iodine. The corrosion region on top of the coupon

had an average of 63 ± 2 at% iodine and 33 ± 3 at% Ni which is a ratio of 0.53 for Ni:I; this

suggests the presence of NiI2-based compounds.

Figure 4-3 SEM micrographs showing the edge views of the I2(g)-exposed Ni-200 coupon taken at different orientations.

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The SEM micrographs showing the top views of the five metal coupons at 500×

magnification are shown in Figure 4-4. The micrographs of the electropolished surfaces are shown

in Figure 4-4a and reveal that the electropolishing was effective at removing almost all surface

imperfections, with only minor pitting and scratches. The exposed coupons shown in Figure 4-4b

reveal complete coverage of the coupon from a corrosion product in all cases, with different

corrosion product morphologies for the different base metals.

Additional SEM-EDS analyses were completed post leaching to determine the surface

metal distribution after water-soluble corrosion products were removed (see Figure 4-4c). Previous

work [6] has shown an increase of Cr on the surface of SS304 due to the formation of iron iodides.

In the case of post leaching, it is expected that the metals forming water soluble iodides FeI2, NiI2,

MnI2, NbI4, NbI5 and CrI2 would be removed during the leach process revealing the surface

underneath of metal compositions that do not readily form metal iodides. Properties of relevant

metal iodides are tabulated in Table S3 in supporting information.

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Figure 4-4. SEM micrographs (500×) of different materials (a) before iodine exposure, (b) post iodine exposure, and (c) after leaching.

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After the water leaching process, the SEM analyses show Figure 4-4c) show surfaces with

large craters from the removal of the corrosion product. The leached coupons of SS304 and SS316

have undissolved corrosion products visible on the surface. During a similar study using particles,

no insoluble corrosion products had formed, so this result with the coupons was unexpected for

SS304 and SS316 [7]. Spot EDS analysis of SS304 showed bright particles on the surface and

those were indicated to be CuI which is poorly soluble in water so it is expected that the precipitates

in the leachate were CuI [1]. The Cu was likely transferred to the coupons during the EDM

machining process used to cut the coupons where a brass wire was utilized. Spot EDS was used to

identify the chemical makeup of the insoluble regions and the micrograph for SS316 is shown in

Figure S4-4 (Supporting Information). The dark solids on the surface had high fractions of I and

Cr. The Fe/Cr molar ratio of the dark solids shown in spots 1-5 in Figure S4-4 (Supporting

Information) was 0.6 ± 0.3 compared to 3.6 ± 0.1 for the unexposed untreated surface. This could

mean that CrI3, which is water soluble but has slow kinetics for dissolution, is forming. However,

based on the reducing nature of iodine which is the strongest of the halogens, it is likely that the

reduced cation CrI2 would be formed [12]. Therefore, it is more likely that regions of high Cr are

arising after removing the metal iodides. Areas of high Cr post after multiple cycles of

adsorption/desorption have been noted by others [6]. The precipitate of 625 was analyzed by SEM-

EDS and the micrograph and regions are shown in Figure S4-5 (Supporting Information). The EDS

showed that the precipitate was 29.6 at% Mo, 27.2 at% Nb, and 25.9 at% Cr and less than 3 at%

Fe, 2 at% I, and 6 at% Ti. The base material untreated was 2.3 at% Nb, 5.8 at% Mo and 25.9 at%

Cr, which means the precipitate was likely not a piece of the alloy that may have been removed

during corrosion. It is also unlikely that the precipitate was comprised of insoluble metal iodides

of Nb, Mo and Cr since the iodine content was so low and Nb and Cr are water soluble. The Cr

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content was unchanged from the base material but the Nb and Mo increased drastically, which

might have been caused by the migration of other elements to form metal iodides leaving enriched

regions of Nb and Mo. If metal iodides formed around the enriched region these could have been

removed during the leaching process. The leached coupon of Inconel 625 also shows high Nb and

Mo in both the dark and light areas shown in Figure S8 of the leached coupon. The Mo was 40 ±

2 at% and the Nb was 16 ± 5 at%. The Nb and Mo content was a large increase from the unexposed

coupon as shown by the ratios in Figure S4-7.

The five electropolished coupons were also leached using deionized water and the

leachates were analyzed by ICP-OES. The Inconel 625 had insoluble precipitates in the leachate,

which were largely Mo, Nb, and Cr as determined by SEM-EDS. The leachate for Ni-200 was the

most straightforward since it contained only Ni and I. The leachate data gave a ratio of Ni:I of 0.47

±0.05, which was consistent with the expected NiI2 species and also consistent with the EDS data

of the corrosion layer. For the SS304 coupon, the leachate showed various transition metals but

the most dominant were Fe and Ni. The ratio of total moles of metal (Fe + Ni) to iodine was 0.44

± 0.04, which is close to the 0.5 expected ratio for FeI2 and NiI2. For the SS316 coupon, the most

prevalent metals in the leachate were Fe > Ni > Mn > Cu. As mentioned previously, Cu is not a

constituent of 316 and it was determined that Cu was transferred to the sample during the cutting

process via the brass EDM wire. The molar ratio of (total metals):iodine for the SS316 coupon

was the same as for SS304, i.e., 0.44 ± 0.04. The leachate data was consistent with what was seen

in prior studies with stainless steel alloy particles [7], where the iron iodide and nickel iodide were

the main iodides formed. The Inconel 625 and Inconel 718 leachate showed only Ni and Zn in the

leachate with iodine. Zn, like Cu, was likely transferred from the brass wire used for cutting the

coupons. By combining Zn and Ni the metal:iodine molar ratios were 0.44 ± 0.04 and 0.43 ± 0.04

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for Inconel 625 and Inconel 718 respectively. It was surprising based on the prevalence of NiI2

formed on the SS304 and SS316 coupons that the extents of corrosion of pure Ni (i.e., Ni-200) and

the high-Ni alloys (i.e., Inconel 625 and 718) were not more severe. This may be due to the higher

amount of Mo in these alloys which has been shown to increase the thickness of the passivation

layer when combined with high Cr in an alloy [13].

4.4.3 Control Studies

Three control studies were conducted, as shown in Table 4-1. Two of them with iodine

but no humidity, to verify that longer time periods didn’t lead to mass gain in those environments

and the third with humidity but no iodine. Humidity with no iodine was conducted to verify that

the mass gain shown after adding humidity was not a result of wetting the coupons but was

instead, actual adsorption of iodine. None of the control studies showed any mass gain on the

coupon, as verified by the analytical balance. Table 4-1 shows that no mass was measured and

using the sensitivity of the balance as the upper bound of possible undetected mass uptake.

Table 4-1 Results of control studies

Material Atmosphere Iodine Present Mass Uptake (g)

SS316 Argon Yes <0.0001

SS304 Dry air Yes <0.0001

Ni-200 Humidity No <0.0001

4.4.4 Dynamic Experiments-Comparison of Materials

The five different metals were all exposed to a dry air then humid air and finally desorbed

with dry air. A comparison of the iodine uptake for the five materials is tabulated in Table 4-1.

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Table 4-2. Summary of comparison of materials data for experiments run at RH = 45.0 ± 2.5%.

Material Iodine Concentration (×10-9 mol/cm3)

Mass uptake (mg/min/cm2)

Ni-200 6.29 0.075 Inconel 625 6.25 0.005 Inconel 718 5.45 0.012 SS304 6.05 0.035 SS316 5.57 0.019

The iodine concentration was ~ 5–7×10-9 mol/cm3 in all experiments. Figure 4-5 showed

no measurable mass gain for SS304, SS316 and Ni-200 until humidity was added to the system.

For Inconel 625 and Inconel 718, minimal measurable mass gains were observed even after

introducing humidity. It should be noted that no mass gain does not necessarily prove that no

adsorption took place, only that it was less than the sensitivity of the load cell, i.e., ~0.1 mg.

Furthermore, the 2×5 cm coupons were very close to the edges of the main glass chamber and

static interactions between the glass and the samples, especially within dry air, were a constant

concern. To verify that no adsorption occurred in dry air, the experiment was repeated with new

coupons of Ni-200 and SS304 that were slightly narrower 1.8×5 cm for 5 h each. The coupons

were weighed with the analytical balance before and after exposure to determine mass changes;

no measurable mass changes were observed, which supports the data gathered within dry air and

humid air exposures.

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Figure 4-5. Exposure of metal coupons to I2(g). For the first 30 min (t ≤ 30 min), the atmosphere contained dry air, humidity was introduced at t = 30 min (45.0 ± 2.5% RH),

and t ≥ 60 min was dry air until weight loss plateaued.

Plots of the mass uptake over the course of the experiment for the materials is shown in

Figure 4-5. Because the control study with humid air and no iodine showed no mass uptake, it is

assumed that mass gain in the plot is due to the adsorption of iodine but may include contribution

from water in a hydrated metal iodide species. All materials show a slight plateau around 20 min

after humidity was introduced followed by a slightly different slope. It is unclear whether the

plateau may have been real or an artifact from the load cell. However, other authors have also

noted two rates in the adsorption of iodine. It is likely that the initial rate is higher due to the

iodine interaction directly with the metal whereas the second rate is slower because the fresh

iodine is diffusing through the metal iodide layer to react with the underlying metal substrate [1].

The initial slopes for the materials were, 0.7 mg/min for SS304, 0.5 mg/min for SS316 and 1

mg/min for Ni-200. It was interesting to note that SS316 and S304 had smaller slopes than Ni-

200 and lower overall mass gain even though the room temperature static exposure showed that

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both had much higher adsorption capacities than Ni-200. This may have been due to the

difference in surface treatments between the two tests or it could be due to the different time

scales of the two tests, one hour vs. 14 days, which would indicate that although Ni-200 has the

most rapid initial adsorption, it has a much lower overall capacity for iodine adsorption.

4.4.5 Characterization

The SEM-EDS analyses were conducted on the materials after exposure in the dynamic

flow-through system. The Ni-200 coupon had black powder, which had fallen off the coupon and

the powder was analyzed. The 500× micrograph of the powder and EDS are shown in Figure S4-

3 (Supporting Information). The EDS data gives a molar ratio of I:Ni = 1.8:1, which is very close

to the intended molar ratio of I:Ni = 2:1 expected from the formation of NiI2. Based on previous

work with SS304 and SS316 particles it is assumed that metal iodides are formed with most of

the metal constituents [7], however the SEM-EDS results of the SS304 and SS316 coupons does

not provide clear evidence of this. One trend that is consistent for both SS304 samples analyzed

is that the highest iodine regions correspond to low Cr content. This is consistent with prior

studies [6] that noted no interaction between I and Cr and eventual enrichment of Cr on the

surface as corrosion compounds with the other metals were removed (e.g., Fe, Ni). Analysis of

Inconel-625 show that the measured mass uptake was likely due to iodine adsorption on Cu

which was transferred to the edges during cutting. The surface of the coupon shows no iodine

and very little Cu, whereas EDS of the edge shows iodine corresponding with detectable levels

of Cu. The SEM micrograph and EDS data is in Figure S4-12 and Table S4-7 (Supporting

Information). Inconel 718 may have had a similar contamination from the brass wire used for

cutting based on zinc on the surface. However, iodine adsorption was on the surface of the

coupon not the edges and not all regions of high iodine correspond to high zinc, see Figure S4-11

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and Table S4-6 in supporting information. Based on EDS it is likely that a small amount of

iodine adsorption may occur on Inconel 718 however it is much lower than for Ni-200, SS3104

or SS316.

4.4.6 Dynamic Exposures and the Role of Humidity

Humidity was found to be a key component in the corrosion of the reactive materials. To

further understand the dependence on humidity, various relative humidity levels were evaluated.

The two goals with these studies were (1) to understand whether higher humidity levels

increased the rate of corrosion and (2) whether there was a threshold humidity level needed for

the corrosion to occur. Recent research at high temperatures, 350ºC have shown that interstitial

protons from dissociation of water on the surface is responsible for increasing oxidation by

decreasing the energy barrier for diffusion through the lattice [10]. Other studies at ambient

temperatures and pressures have also shown the possibility of water dissociation however the

degree of dissociation was strongly dependent on surface hydroxyl groups on the metal which

were able to stabilize the dissociated water and lowered the activation barrier [14]. For SS304

exposed at ambient temperatures and ~30% RH in both air and N2, differences in iodine loading

led the authors to conclude that moisture mediated reactions were not a main driver [6]. For the

purpose of this manuscript, Ni-200, was selected for further tests involving humidity levels

because it was the least complex in terms of composition (i.e., pure Ni). Two lower humidity

tests were conducted at 14% RH and 29% RH as measured with an Omega OM-THA2-U

humidity sensor and two at higher humidity 67% and 87%. The stability of the humidity

measurement was ~±2%) For these tests, the purge was done with humidity at the set flow to

equilibrate the system prior to adding iodine. The results showed no signs of iodine adsorption

for the 14% RH test. This included, no mass gain observed and no visual changes to the coupon

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were seen, and the EDS data showed that the iodine concentration on the surface of the coupon

was <0.5 at%. This indicates that there is a threshold RH necessary for iodine adsorption. The

29% RH showed mass gain and visual changes similar to, but less severe than, what was seen at

47% RH. This could mean that there is a correlation between RH and uptake kinetics, however

further tests at higher humidity levels showed that 47% was the maximum for iodine adsorption.

Lower mass gain and less corrosion as shown in Figure 6 and 8. Based on the necessity for water

to be present to initiate iodine adsorption, it is possible that it is not I2 interacting with the metal

surface but instead a hydrolysis product of I2. The basic hydrolysis of I2 is shown in Equation (4-

1) but several intermediates have been noted by Wren et al. [2]; one such intermediate that has

been identified is I2OH-, which was measured spectroscopically in alkaline solution [15].

I2 + H2O → HOI + I− + H+ (4-1)

It is possible that an intermediate or HOI interacts with the Ni surface to form NiI2 and

H2O. It is also possible that the water lowers the activation barrier for adsorption in a manner

similar to the one noted by Luo et al. [10] for the oxidation of Ni substrates. Both of those

possibilities would lead to an increase in corrosion with higher humidity levels which is not what

experiments showed.

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Figure 4-6. SEM at 75× magnification for Ni-200 exposed to various levels of humidity.

The Ni-200 samples exposed to different RH levels were analyzed with SEM-EDS. As

noted previously, EDS analysis did not show any iodine on the low humidity sample (RH =

14%). The 29% RH Ni-200 sample showed a range of iodine concentrations from a 1:1 to 2:1

98

molar ratio of I:Ni that corresponds to the visual color change on the coupon. Bulk XRD of

scrapings from the coupon match with NiI2•6H2O (see Figure S4-9 in Supporting Information).

The higher humidity tests at 47%, 67% and 87% show decreasing amounts of iodine on the

surface that corroborate the lower mass uptake determined gravimetrically. As mentioned

previously the possible roles of water considered should have led to higher corrosion for higher

humidity. It is therefore possible that higher humidity especially when equilibrating with humid

air prior to adding iodine to the system results in a water layer on the coupon that acts as a barrier

to iodine adsorption (see Figure 4-7). It is also possible that the iodine gas interacts with the

higher humidity and forms more aerosols which may be more likely to adsorb to surfaces in the

exposure vessel. Iodine concentration likely plays a role in the lower adsorption. The plot of

humidity vs. sublimation rate of iodine shown in Figure 4-7shows that iodine sublimation

decreases with higher humidity resulting in a lower concentration of iodine in the gas stream

however the mass uptake does not follow this trend until after 45% humidity which indicates that

lower iodine concentration is not the main factor in the lower mass uptake.

99

Figure 4-7. Summary of mass uptake and iodine sublimation rate for experiments run under different relative humidity (RH) levels.

The Inconel 625 and Inconel 718 coupons were both tested at higher humidity levels in

the flow-through apparatus to see if that would lead to measurable mass gain. For the Inconel

625 coupon, measurable mass gain was observed of ~11 mg at 87% humidity after 15 min.

Unfortunately, the coupon was too close to the walls and some of the liquid was transferred to

the exposure vessel and the experiment was stopped. A final mass post dry air was not obtained

because of the loss to the vessel. However, the test did show that higher humidity levels can lead

to measurable uptake for Inconel 625. For the Inconel 718 coupon, the max humidity of 87% did

not result in any measurable gain; however, SEM-EDS analyses did show iodine on the surface.

The SEM-EDS results for the 718 sample in Figure S4-6 in supporting information shows the

micrograph at 1500× magnification and the locations of spot EDS. Even though no mass uptake

was observed with the load cell, there were areas showing very high iodine content up to 32 at%.

The high-iodine regions correspond to low Cr content and unperturbed Fe content as shown in

the Ni/Cr and Ni/Fe molar ratios.

100

4.4.7 Dynamic Exposures with Argon and the Role of O2

To determine the role of O2 in the corrosion process, an experiment was conducted using

Ar as the carrier gas instead of air. Here, SS316 was chosen as the sample material due to the

high reactivity observed in previous experiments. The experiment was set up the same as for

comparison of materials in section 3.3 but with dry Ar, then humid Ar (RH = 45±2.5), followed

by a desorption step. The mass uptake was very similar to that of the air/humidity experiment for

SS316 with 11.2 mg adsorbed in air and 11.6 mg in Ar; however the similarity should not be

overstated since SS316 deliquesces in air so at least part of the mass gain is likely water. The

SEM micrographs, shown in Figure 4-8 show somewhat similar morphology but more iodine on

the air sample than the argon sample and both have varied metal distribution across the surface,

see Figure S4-10 (supporting information). The extensive corrosion on the SS316 in Ar indicates

that O2 is not required in the adsorption reaction. However, it is possible that the presence of O2

could lead to different adsorbed species over time such as the FeIxOy compound predicted by

Wren et al. [6].

101

Figure 4-8. SEM of SS316 exposed in (a,c) air and (b,d) argon.

4.5 Summary and Conclusions

Humidity was shown to be critical to initiate iodine adsorption at the iodine

concentrations tested. Preliminary data indicates that a threshold humidity level is needed to

facilitate iodine adsorption on metal substrates, and this may be due to a hydrolysis product of I2

being the reactant with the surface. Furthermore, there may be a direct dependence on the uptake

kinetics from the RH level. This would support the possibility of a hydrolysis product being the

reactant, as water would be in the initial reaction step.

A comparison of the five metal substrates showed the highest adsorption (in descending

order) of Ni-200 > SS304 > SS316 > 718 > 625. All of the samples that gained mass showed at

least partial desorption under the dry air purge and this may have been due to dehydration of

hydrated metal iodide complexes or the desorption of physisorbed I2(g). The Inconel alloys 625

102

and 718 were fairly inert to iodine adsorption under the conditions tested. Characterization by

SEM-EDS of the Ni-200 coupon indicates that NiI2 is formed and flakes off the surface as a

black powder. The SS304 and SS316 had extensive iodine adsorption and formed a much more

deliquescent corrosion product, which reacted with air when removed from the system for

weighing on the analytical balance. In contrast, the Ni-200 corrosion product remained dry.

Depending on the type of metal used in a facility the adsorption of iodine and reactions will vary.

Lastly, using alloys and metals with various metal constituents we were able to determine that

metal iodides do not readily form with Cr or Mo which are known to work together to form very

stable passivation layers, with increasing Mo content increasing the thickness of the film [13].



Northwest National Laboratory (PNNL). We would like to thank Saehwa Chong for providing

X-ray diffraction data. PNNL is operated by Battelle for the U.S. Department of Energy (DOE)

under Contract No. DE-AC05-76RL0-1830. PNNL draws on signature capabilities in chemistry,

earth sciences, and data analytics to advance scientific discovery and create solutions to the

nation's toughest challenges in energy resiliency and national security.

4.7 References

1. Glänneskog H. Interactions of I2 and CH3I with reactive metals under BWR severe-

accident conditions. Nuclear engineering and design. 2004;227(3):323-9.


Technology. 2000;129(3):297-325.

103




4. Wren JC, Ball JM, Glowa GA, editors. Studies on the effects of organic-painted surfaces

on pH and organic iodide formation. OECD Workshop on Iodide Aspects of Severe Accident

Management; 1999; Vantaa, Finland: Committee on the Safety of Nuclear

Installations/Organization for Economic Cooperation and Development, NEA/CSNI/R(99)7.

5. Wren JC, Ball JM, Glowa GA. The interaction of iodine with organic material in

containment. Nucl Technol. 1999;125(3):337-62.






8. Beck CL, Riley BJ, Chong S, Smith NP, Seiner DR, Seiner BN, et al. Molecular iodine

interactions with Fe, Ni, Cr and stainless-steel alloys. Ind Eng Chem Res. 2021.



10. Luo L, Su M, Yan P, Zou L, Schreiber DK, Baer DR, et al. Atomic origins of water-

vapour-promoted alloy oxidation. Nature Mater. 2018;17:514–8.

11. Regina JR, DuPont JN, Marder AR. The effect of water vapor on passive-layer stability

and corrosion behavior of Fe-Al-Cr base alloys. Oxid Met. 2004;61:69-90.

104


& Sons, Inc.; 2014.

13. Sugimoto K, Sawada Y. The role of molybdenum additions to austenitic stainless steels

in the inhibition of pitting in acid chloride solutions. Corrosion Science. 1977;17(5):425-45.

14. Yamamoto S, Andersson K, Bluhm H, Ketteler G, Starr DE, Schiros T, et al. Hydroxyl-

induced wetting of metals by water at near-ambient conditions. The Journal of Physical

Chemistry C. 2007;111(22):7848-50.

15. Wren JC, Paquette J, Sunder S, Ford BL. Iodine chemistry in the +1 oxidation state. II. A

Raman and uv–visible spectroscopic study of the disproportionation of hypoiodite in basic

solutions. Can J Chem. 1986;64(12):2284-96.

105

CHAPTER FIVE

CONCLUSIONS AND FUTURE WORK

5

5.1 Conclusions

Radioiodine is a major concern to public health and safety in the event of a nuclear

accident, which makes understanding parameters that could change the amount released critically

important. One such parameter is adsorption/desorption on surfaces inside the facility. Painted

surfaces are prevalent and have been the focus of previous studies [1-3], however bare metal

surfaces also have the potential to chemisorb iodine [1]. Chemisorption of iodine species on

surfaces could potentially reduce the amount of iodine in the gas phase as long as the species are

not easily desorbed. Based on that, characterization of the adsorbed species to understand the

stabilities of the species is very important. However, characterization of the iodine species from

previous studies was often not extensive or thorough [4]. The main focus of Chapters 2 and 3

was to characterize and understand the species of iodine adsorbed onto metal surfaces.

Determining the species of adsorbed iodine, even using nonradioactive iodine, in

experiments can be difficult and problematic. One major limitation to measuring iodine-

containing samples is high volatility of iodine containing species. The volatility and corrosive

nature of iodine poses a risk to all instruments that use ultra-high vacuum (UHV), which is many

surface techniques [5]. If volatile iodine is present, it can corrode the equipment causing long-

term problems. A wide array of analytical techniques were used in this study and short

evaluation of each will be presented.

106

Scanning electron microscopy and energy dispersive x-ray spectroscopy (SEM-EDS)

analyses were heavily utilized to characterize the adsorbed species in the current study. Since

SEM requires UHV, samples that were expected to have physisorbed I2 could not be analyzed

without first desorbing either in a vacuum oven or a vacuum desiccator. Although heating the

sample seems like a minor inconvenience, it resulted in the possibility of altering the species

since many iodine containing compounds are not very stable and melt or decompose at relatively

low temperatures, such as FeI2*4H2O which decomposes at 90ºC at atmospheric pressure (Table

1-2). Therefore, heating was avoided whenever possible and very low temperatures were used

when needed. SEM allowed for very high-resolution micrographs of the samples, which showed

changes to the surface morphology and the coverage of the sample with a corrosion product. The

EDS allowed for identification of the elements on the surface. The main drawback with EDS is

that it is not accurate at quantifying oxygen. Also, EDS does not give any oxidation state

information on the identified species. Furthermore, the interaction (analytical) volume of the

electron beam to generate characteristic X-rays varies as a function of sample density, average

atomic number, and the beam conditions (e.g., acceleration voltage) is ~5–15 µm, which may

have been larger than the expected adsorbed layer and, consequently, the base material likely

contributed to the EDS measurements. Therefore, the main difficulty of EDS lies in the inability

to know that there is separation between the signal of the adsorbed species from the base

material.

ICP-OES, a highly sensitive and accurate aqueous technique for metal analysis, was a

technique utilized to complement SEM-EDS in these studies. It was indicated from EDS that

distinct metal iodides had formed (Figure 2-6), but the separation of the base material from the

adsorbed species was important for verification. A water leach of the samples allowed for

107

selective leaching of the water-soluble metal iodides. All of the metals in SS304L and SS316L

can form water soluble metal iodides (Table S1-1). The leachate was then analyzed by ICP-OES.

A comparison of molar ratios between iodine and the metals allowed for understanding of the

metal iodides formed on the materials. This data supported what was shown with EDS analyses

that metal iodides formed with Fe as well as with the minor alloying components (e.g., Ni, Cr).

For metals at very low levels in the alloy (e.g., Mn at ~1-2 mass%), the formation of metal

iodides is minimal compared to Fe and for the purpose of models [6], the exclusion likely causes

little issue. Nickel is the third most prevalent metal after Fe and Cr (ranges from 8-14% in SS304

and SS316) and a very favorable interaction between Ni and I was observed where NiI2 analyzed

was beyond what would be expected based on the fraction of Ni in the alloy. This is

demonstrated in Figure 2-9, which shows the Fe/Ni molar ratio was decreased by two in the

leachate and the EDS of the corrosion product due to an increase in Ni. Therefore, neglecting the

interaction with Ni could lead to substantial errors in kinetic models and is likely the reason for

the necessity of “impurities” in Wren’s model [6].

X-ray diffraction (XRD) analysis was another technique that was used in the current

study. Experiments with metal particles gave very little diffraction information, which is likely

due to the deliquescence of the sample resulting in amorphous solids. XRD analysis was used

mostly for Ni-200, or pure nickel. With the XRD analysis on Ni-200, diffraction patterns of the

corrosion products were matched to NiI2*6H2O. This gave direct evidence of the formation of

the metal iodide hydrate, however other species may be on the surface but not diffract or not

have peaks that can be matched to the crystallography databases.

X-ray photoelectron spectroscopy (XPS) was another analytical technique used. Due to

sample analysis cost and availability of the instrument, XPS was used sparingly for a subset of

108

samples. One benefit of XPS is that it is sensitive to only a few monolayers, so it was less likely

to have interference from the base material compared to EDS analysis. The other benefit is that

XPS gives the binding energy, which is specific to the compound and therefore gives oxidation

state information for many species. For example, XPS identified the binding energy of iodine in

the Ni particles at 619.0 eV, which is consistent with I– (-1) in a metal iodide. The drawback of

using XPS for the stainless-steel samples is that metal iodides of transition metals have similar

binding energies and can be indistinguishable from one another.

Chapter 4 focused less on characterization and more on understanding some of the

variables that can change adsorption, such as humidity and oxygen but also surface type. For

those studies, four alloys (i.e., SS304, SS316, Inconel-625, Inconel-718) and one metal (i.e., Ni)

were tested in the form of coupons. It was thought from the high interaction between I and Ni in

the stainless steels that nickel-based alloys would be highly adsorbing; however, this was not the

case. The low adsorption of iodine on the nickel-based alloys is likely due to the minor metals in

these alloys, specifically Cr and Mo [7]. For example, SS316 has Mo added and that is likely

why it is less reactive than SS304, but Mo is present at lower concentrations than Inconel alloys

625 and 718. It was thought that oxygen played a critical role in iodine adsorption [8], but the

similar results between humid argon (oxygen-free) and humid air in Chapter 4 refuted that.

Humidity was found to be the key parameter for adsorption, both for initiating adsorption but

also decreasing adsorption when humidity levels were >67%. It was outside of the scope of this

work to delve fully into the mechanism by which humidity is facilitating adsorption.

109

5.2 Ongoing (Unpublished) Work

Based on the possibility of photolytic induced dissociation of iodine occurring during

reaction, experiments were carried out with no light and with ambient interior light. SEM

micrographs of these coupons are presented in Figure 5-1

Figure 5-1. Ni-200 coupons exposed to iodine at 27% RH run in (left) no light or (right) with light.

Although, the coupon run in the dark appeared to be heavily corroded, the actual mass

gain was much lower, as seen in Table 5-1, even though the iodine concentration was very

similar. This suggests that photolytic reactions do impact the overall extent of iodine adsorption

and degree of reaction/corrosion.

Table 5-1. Summary of mass uptake for Ni-200 coupons exposed to similar amounts of iodine with light and without light. Relative humidity was 25-29% for both samples.

Material type Conditions Mass uptake (mg/min)

Iodine Concentration (mol/cm3)

Ni-200 No light 0.19 5.12×10-9 Ni-200 Light 0.38 5.13×10-9

110

The SEM-EDS analysis of the coupons show very different metal distributions (see Table

5-2). One difference being the oxygen content which is much higher in the experiments done in

light. However, as mentioned previously EDS does not quantify oxygen very well so only the

approximate magnitude can be compared. The experiments in light also have a higher surface

variation with large ranges in the iodine and nickel concentrations as well as larger corrosion

product particles. The Ni-200 exposed without light had very low and consistent oxygen content

as well as consistent amounts of Ni and I ranging from a 1:1 to 1:2 molar ratio across the coupon.

The 1:1 molar ratio seen in areas is likely due to the depth of the adsorbed layer which may have

been thin, resulting in contributions of X-rays from the base material.

Table 5-2. SEM-EDS data of Ni-200 coupons exposed with and without light.

Sample O (at%) Ni (at%) I (at%)

Ni-200 (no light) 2.6 ± 0.3 39 ± 6 58 ± 6

Ni-200 (light) 12 ± 5 49 ± 20 39 ± 20

5.3 Future Work

Future work should continue to delve into the role of humidity. This is likely best

accomplished through a combination of modelling and experimental efforts. Experimental tests

could be used to determine the critical humidity levels for materials other than Ni-200.

Modelling the possible surface reactions that could take place between I2 and water on metal

surfaces could assist in narrowing the list of possible species to form under these conditions.

Based on the species determined through modelling and the potential vibrational signatures, in

situ surface analysis using Raman or Fourier transform infrared spectroscopy could potentially

measure the intermediate iodine species being formed. Experiments could also be conducted to

111

monitor the species of iodine going to the outlet through mass spectrometry. If hydrolysis or

photolytic effects are changing the iodine species, then the change in species could be detected

by a decrease in I2 or potentially detection of masses corresponding to other iodine species.

112

5.4 References

1. Haefner D, Tranter T. Interaction of Radioactive Iodine Gaseous Species with Gaseous

Species with Nuclear-Grade Activated Carbons. Technical Report. Idaho Falls, ID: Idaho

National Laboratory; 2007 February 2007. Contract No.: INL/EXT-07-12299

2. Wren JC, Ball JM, Glowa GA. The interaction of iodine with organic material in

containment. Nucl Technol. 1999;125(3):337-62.

3. Simondi-Teisseire B, Girault N, Payot F, Clément B. Iodine behaviour in the containment

in Phébus FP tests. Annals of Nuclear Energy. 2013;61:157-69.



5. Jones RG. Halogen adsorption on solid surfaces. Progress in Surface Science. 1988;27(1-

2):25-160.



7. Sugimoto K, Sawada Y. The role of molybdenum additions to austenitic stainless steels

in the inhibition of pitting in acid chloride solutions. Corrosion Science. 1977;17(5):425-45.




113

APPENDIX

6

6.1 Chapter 2 Supporting Information

In addition to the figures and tables included in the main article of CHAPTER TWO, this

supplementary information is included as an additional resource. It contains specific locations

used for EDS spot analysis, low resolution images of the samples as well as tables for reference.

Figure S2-1. Pictures of particle samples after triplicate iodine exposures including (left) SS304L+I and (right) SS316L+I.

114

Figure S2-2. Enlarged view of Figure 2-6 from the text.

Figure S2-3. SEM-EDS regions for SS304L+I-2 taken at 350×.

115

Figure S2-4. SEM-EDS summary of SS316L+I following iodine loading.

Figure S2.5. XRD data for SS304L+I (replicate 1).

116

Figure S2-6. XRD data for SS316L+I (replicate 1).

117

Figure S2-7. Summary of elemental distributions for SS403L material including the base material, SEM-EDS data and ICP-OES data.

Table S2-1. Physical and thermodynamic (at 298.15 K) properties of possible metal-iodide, metal-iodide-hydrate, and metal-oxide compounds that are possible from this study.

Properties include melting temperature (Tm), density (ρ), heat capacity (cp), enthalpy of formation (∆𝑯𝑯𝒇𝒇

° ), and Gibb’s free energy of formation (∆𝑮𝑮𝒇𝒇° ) [1-4].

Compound Physical form (crystal) Solubility in H2O Hygroscopic Tm ∆𝑯𝑯𝒇𝒇

° ∆𝑮𝑮𝒇𝒇° (°C) (kJ/mol) (kJ/mol)

CrI2 Red/brown Soluble Yes 867 -156.9 – CrI3 Dark green hexagonal Slightly soluble No 500(d) -205.0 -202.5 FeI2 Reddish violet Soluble Yes 594 -104.60 -111.74 FeI2*4H2O Black leaflets Soluble Yes 90(d) – – NiI2 Black hexagonal Soluble Yes 800(s) -78.241 -76.061 NiI2*6H2O Green monolithic Soluble Maybe 43(lw) – – MnI2 White hexagonal Soluble Yes 80(d) – – MnI2*4H2O Red Very soluble – – – – Cr2O3 Green hexagonal Insoluble No 2320 -1141 1058.966 FeO Black cubic Insoluble No 1377 -271.96 -251.4 Fe2O3 Red-brown hexagonal Insoluble No 1539 -824.25 -742.342 Fe3O4 Black cubic Insoluble No 1597 -1118 -1015.29 NiO Green cubic Insoluble No 1957 -239.74 -211.60 MnO Green cubic Insoluble No 1842 -385.2 -362.91

0

10

20

30

40

50

60

70

Fe Cr Ni Mn I

Atom

ic%

Comparison of EDS and ICP-OES of 304L

Base Material 100X

Exposed Base Material 304L+I 850X

Exposed iodine layerRep2 (350X)

ICP-OES Leach (Average of Replicates)

118

Table S2-2. Summary of metal-iodide compounds from the ICDD (International Centre for Diffraction Data; PDF# = powder diffraction file#) and the Inorganic Crystal Structure Database (ICSD). Here, a, b, and c denote unit cell parameters; V is the unit cell volume,

and ρ is the cell density.

Compound SG SG# a b c V ρ ICDD (PDF#) ICSD# CrI2 C12/m1 12 7.545 3.929 7.505 200.8 5.06 01-073-6156 23892 CrI3 P3121 152 6.86 6.86 19.88 810.2 5.32 00-006-0446 – FeI2 P3�m1 164 4.05 4.05 6.76 96.03 5.35 01-071-3837 52369 MnI2 P3�m1 164 4.146 4.146 6.829 101.7 5.04 01-076-0174 33673 NiI2 R3�mH 166 3.903 3.903 19.67 259.5 6 01-020-0785 22108

Figure S2-8. SEM micrographs used for EDS data in Figure S8 including (left) 100× of SS304L base material, (middle) 850× of SS304L-1+I sample, and (right) 350× of SS304L-

2+I corrosion region.

Table S2-3. Compositions for stainless steels (from the vendor) in mass%.

Mass% Cr Mn Fe Ni Mo SS304L 18-20 <2 Balance 8-12 0 SS316L 16-18 <2 Balance 10-14 2-3

119

Figure S2-9. TGA data for SS304L+I and SS316L+I.

Table S2-4. Minimum and maximum at% for EDS data in Figure 2-6.

Locations O Cr Fe Ni I Minimum of 1-3 1.2 1.5 31.4 0.7 60.5 Maximum of 1-3 3.3 3.9 33.2 0.9 64.3 Minimum of 4-8 1.5 1.7 8.9 16.8 53.0 Maximum of 4-8 9.8 8.9 16.7 18.5 62.0

Table S2-5. Additional data for Figure 2-9 including replicate data for ICP-OES and minimum and maximum values for EDS analysis.

Mol% in Leachate with Iodine Fe/I Ni/I Cr/I M/I Fe/Ni Fe/Cr 304L-1 (OES) 0.35 0.08 0.09 5.31E-01 4.39 3.67 304L-2 (OES) 0.37 0.12 0.06 5.56E-01 3.16 6.02 304L-3 (OES) 0.38 0.10 0.05 5.43E-01 3.69 8.43 EDS 304L+I-2 (350X) Low 0.31 0.09 0.08 0.53 2.36 3.23 EDS 304L+I-2 (350X) High 0.35 0.14 0.10 0.59 3.61 3.87 EDS Base Material Low N/A N/A N/A N/A 6.22 2.72 EDS Base Material High N/A N/A N/A N/A 7.55 2.80

120

6.2 Chapter 2 Supporting Information References







2007;7(5):1381-93.


& Sons, Inc.; 2014.

121


In addition to the figures and tables included in the main article, this supplementary

information is included as an additional resource for Chapter 3. It contains specific locations

used for EDS spot analysis, low resolution images of the samples, TGA spectra as well as tables

for reference.

Figure S3-1. Pictures of the metal particles (a) before exposure and (b) after 32-day room-temperature exposure to I2(g) in air.

Figure S3-2. Fe granules exposed to I2(g) in (a) N2(g) and (b) air.

122

Figure S3-3. Fe granule exposed in air at 60°C. Surface shows complete oxide layer and localized regions of high iodine. Green boxes shown are were EDS data were collected.

Figure S3-4. EDS locations for Fe+I sample at different magnifications.

123

Figure S3-5. DTA-TGA of Fe granule exposed in air at 60ºC. TGA is shown in green, DTA is shown in blue.

124

Figure S3-6. SS16L+I at RT showing corrosion layer around the particle.

125

Figure S3-7. Phase map of (FEGN60) exposed in N2(g) at 60ºC (see Figure 6e-j in main paper).

126

Table S3-1. Physical and thermodynamic (at 298.15 K) properties of possible metal-iodide, metal-iodide-hydrate, and metal-oxide compounds that are possible from this study.

Properties include melting temperature (Tm), enthalpy of formation (∆𝑯𝑯𝒇𝒇° ), and Gibb’s free

energy of formation (∆𝑮𝑮𝒇𝒇° ) [1-3].

Compound Physical form (crystal) Solubility in H2O Hygroscopic Tm ∆𝑯𝑯𝒇𝒇

° ∆𝑮𝑮𝒇𝒇° (°C)(a) (kJ/mol) (kJ/mol)

CrI2 Red/brown Soluble Yes 867 -156.9 – CrI3 Dark green hexagonal Slightly soluble No 500(d) -205.0 -202.5 FeI2 Reddish violet Soluble Yes 594 -104.60 -111.74 FeI2·4H2O Black leaflets Soluble Yes 90(d) – – NiI2 Black hexagonal Soluble Yes 800(s) -78.241 -76.061 NiI2·6H2O Green monolithic Soluble Maybe 43(lw) – – MnI2 White hexagonal Soluble Yes 80(d) – – MnI2·4H2O Red Very soluble – – – – Cr2O3 Green hexagonal Insoluble No 2320 -1141 -1058.966 FeO Black cubic Insoluble No 1377 -271.96 -251.4 Fe2O3 Red-brown hexagonal Insoluble No 1539 -824.25 -742.342 Fe3O4 Black cubic Insoluble No 1597 -1118 -1015.29 NiO Green cubic Insoluble No 1957 -239.74 -211.60 MnO Green cubic Insoluble No 1842 -385.2 -362.91

(a) “d” denotes “decomposes”, “s” denotes “sublimes”

Table S3-2. Laser diffractometer particle size (sp) analysis results of five replicate measurements of the received materials ± 1standard deviation.

Sample sp (µm) Ni 17.8 ± 1.0 Fe 51.0 ± 1.6 Cr 24.9 ± 1.1 304L 21.2 ± 1.0 316L 59.1 ± 1.5

Table S3-3. XPS data for as received nickel particles and exposed nickel particles. Two replicates of each were measured and the values are listed in the table.

Element Ni Particles as received (at%) Ni+I (at%) C (1s) 19.99, 20.16 21.64, 25.93 O (1s) 39.68, 39.95 35.54, 37.70 Ni (2p) 39.78, 40.24 36.69, 39.04 I (4d) 0.08, 0.11 1.62, 1.84

127

Table S3-4: Gravimetric iodine mass uptake (m%I,g; mass adsorbed iodine per mass of starting material) after 32 days for each sample type.

Sample m%I,g Fe+I <0.1% Cr+I <0.1% Ni+I <0.1% 304L+I 114.6% 316L+I 8.2%

Figure S3-8. Summary of Monte Carlo simulations performed with CASINO (v2.48.1) software for (a) Fe/Fe2O3 (Fe2O3 layer is 5 nm), (b) Ni/NiO (NiO layer is 2.5 nm), and (c) Cr/Cr2O3 (Cr2O3 layer is 2.5 nm). Densities used for Fe, Ni, Cr, Fe2O3, NiO, and Cr2O3

were 7.874, 8.908, 7.19, 5.25, 6.67, and 5.22 g cm-3, respectively.

128








2007;7(5):1381-93.

129


In addition to the figures and tables included in the main article, this supplementary

information is included as an additional resource for Chapter 4. It contains specific locations

used for EDS analysis of received materials, low resolution images of the samples, TGA spectra

as well as tables for reference.

Figure S6-1. SEM-EDS analysis of unexposed materials.

Table S6-1. Metal constituents of the metal and alloys used based on information from the supplier.

Material Fe Cr Ni Mn Mo Nb/Ta Ti Al Ni-200 <0.25 <0.25 >99.5 – – – – – Inconel 625 <5 20-23 >58 <0.5 8-10 3.15-4.15 – – Inconel 718 balance 17-21 50-55 <0.35 2.8-3.3 4.75-5.5 – – Inconel 718 (cert form) 18.35 – 53.42 – 2.98 5.04 0.92 0.55

SS304 balance 18-20 8-10.5 <2 – – – – SS316 balance 16-18 10-14 <2 2-3 – – –

130

Figure S6-1. SEM micrographs of surface treatments for Ni-200 at 500× of (left) untreated and (right) electropolished coupons.

Figure S6-3. Micrograph of powder from Ni 200 coupon exposure in flow through system. EDS spots are shown in yellow with data tabulated in Table S4-2.

131

Table S6-2. SEM-EDS of locations shown in Figure S4-3 for corrosion product of Ni-200 exposed to I2(g).

Sample O Ni I Ni-200 2.2 ± 0.3 35 ± 1 63 ± 1

Figure S6-4. Micrograph of SS316 + I2 post leaching. Boxes show regions analyzed by EDS.

132

Figure S6-5. SEM-EDS of Inconel 625 precipitant at 700×. Yellow boxes indicate regions sampled by EDS.

Figure S6-6. SEM-EDS of the Inconel 718 coupon exposed to 87% relative humidity and I2.

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Table S6-3. EDS spot data from Figure S4-6 (values in at%).

Regions Ni/Fe Ni/Cr I

Spots 1-3 2.48 ± 0.03 2.5 ± 0.1 0.5 ± 0.3

Spots 4-6 2.4 ± 0.2 3.9 ± 0.4 26 ± 1 Spots 7-12 2.8 ± 0.2 8 ± 4 32 ± 5

Table S6-4. Humidity tests for flow-through system.

Flow Rate of Bubbler (scfh)

Flow Rate of Dry Air (scfh)

RH (ave ± SD)

0.10 0.90 14.5 ± 2.5% 0.25 0.75 29 ± 2.5% 0.50 0.50 45 ± 2.5% 0.75 0.25 67 ± 2.5% 1.00 0.00 89 ± 2.5%

Figure S6-7. Atomic ratios of Ni to some of the alloying metals in Inconel 625, in the untreated coupon and post leaching (see Figure S8) as well as the precipitate see Figure S4-

5.

0

5

10

15

20

25

30

Ni/Fe Ni/Cr Ni/Mo Ni/Nb

Untreated Coupon High Iodine Leachate Low Iodine post leaching Precipitant

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Figure S6-8. SEM-EDS of leached EP Inconel 625 coupon. Dark regions are low iodine and bright regions are high iodine. Relevant ratios are shown in Figure S4-7.

Table S6-5. Physical properties of some relevant metal iodides [1].

Compound Physical form (crystal) Solubility in H2O

CrI2 Red/brown Soluble CrI3 Dark green hexagonal Slightly soluble CuI Insoluble FeI2 Reddish violet Soluble FeI2*4H2O Black leaflets Soluble MoI2 Back crystal Insoluble MoI3 Black solid Insoluble MoI4 Black crystal Insoluble NbI3 Black solid No data

NbI4 Gray orthogonal crystal Reacts with water

NbI5 Yellow black monoclinic Reacts with water

NiI2 Black hexagonal Soluble NiI2*6H2O Green monoclinic Soluble MnI2 White hexagonal Soluble MnI2*4H2O Red Very soluble

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Figure S6-9. XRD data from Ni-200 sample showing diffraction peak location fits for NiI2•6H2O. Some minor peaks remained unidentified and peak height differences could be due to perferred orientation.

Figure S6-2 SEM-EDS of SS316 in air (a) and argon (b) with the corresponding EDS for air (c) and argon (d).

136

Figure S6-11 SEM-EDS of Inconel 718. EDS spots are shown in yellow boxes and the tabulated data is in Table S4-6.

Table S6-6 EDS data of Inconel 718 coupon (at%). Spot EDS locations shown in Figure S11.

Region O Cr Fe Ni Cu Zn Nb Mo I

Average Spectrum 1-3 11.2 6.5 11.0 16.2 0.7 8.3 0.5 0.7 45

Standard Deviation 0.4 0.6 0.4 0.7 0.2 0.6 0.2 0.1 1

Average Spectrum 4-11 9.4 16.3 17.1 37 0.4 2.6 1.9 1.4 13

Standard Deviation 0.6 0.8 0.4 2 0.2 0.8 0.3 0.3 3

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Table S6-7 EDS of Figure s12, Inconel 625 edge and top showing that high iodine corresponds to high nickel on the edges of the coupon.

Location Edge Top

Element Atom [%]

abs. error [%] (1 sigma)

Atom [%] Uncertainty

Nickel 35.2 0.8 57.0 0.99 Oxygen 21.4 0.7 3.4 0.92 Chromium 19.5 0.4 27.6 0.67 Iodine 9.6 0.4 0.1 0.15 Molybdenum 3.9 0.2 5.2 0.49 Copper 3.7 0.1 - - Niobium 2.5 0.1 2.1 0.23 Iron 2.2 0.1 4.6 0.26 Zinc 2.0 0.1 - -

Figure S6-12. Inconel 625 (left) edge and (right) top view. EDS data is tabulated in Table S4-7.



& Sons, Inc.; 2014.

characterization of the adsorbed species of molecular iodine ...

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