Current status of copper corrosion research at SKB

SVENSK KÄRNBRÄNSLEHANTERING

Current status of copper corrosion research at SKB

KYT Copper Corrosion Seminar

Christina Lilja

2017-11-02


• Status in the Swedish Programme

• Corrosion issues

• mentioned by SSM

• from safety assessment

• Ongoing corrosion research

• sulphide corrosion

• oxidising conditions

• stress corrosion cracking

• Recent results

• corrosion in pure water

• Summary

Overview


The Swedish system


• License application for spent fuel repository sent in March

2011

• For 2 facilities

• spent fuel repository in Forsmark (Östhammar)

• encapsulation plant (extension of the existing interim storage, Clab)

in Oskarshamn

• Will be tried according to 2 laws, in parallel

• Nuclear Activities Act – handled by SSM (the Swedish Radiation

Safety Authority)

• Environmental Code – handled by the Environmental Court

Spent fuel management

Oskarshamn

Östhammar


• Environmental Court

• main hearing held September-October 2017

• statement to government, according to the Environmental Act – planned December 20

• SSM Review comments

• ”preliminary evaluation” issued June 2016 (sent to Environmental Court)

• final version will be sent to government around new year 2018

• Also needed: decisions of acceptance from Östhammar and Oskarshamnmunicipalities (spring 2018)

• Construction license

• Government decision 2019(?)

• license to start constructing the repository (= start making tunnels etc)

• according to both Acts at the same time

• SKB to send in PSAR (Preliminary Safety Assessment Report) to SSM and get it approved before start of construction

Updated time schedule


• SSM ”Granskningsrapport”, in Swedish, 683 pages

• a summary included in the decision document (17 pages), also in English

• Summary section focuses on unsaturated conditions [next slide]

• saturation time could be in the range 100 – 6000 years

• Main text touches on many corrosion issues

• probabilistic assessment of pitting

• mechanisms and time for consumtion of entrapped oxygen

• stress corrosion cracking in sulphide solution

• corrosion from high voltage direct current cables

• irradiation effects

• stress corrosion cracking at the inside of the copper

SSM Review comments, June 2016


• Localised corrosion by sulphide under unsaturated buffer conditions

• Larger amounts of sulphide (than calculated for saturated conditions) could come from gaseous H2S (from microbial sulphate reduction) or from a biofilm formed at the canister surface.

• Could maybe cause a passive film, and when broken, lead to pitting.

• SCC under the same conditions

• Formation of a passive film is most probably needed.

• Creep in the copper shell

• The fundamental understanding of the effect of phosphorous on creep ductility is not enough to completely rule out a brittle failure mechanism.

• A possible effect of hydrogen on the deformation properties of the copper need to be considered. The presence of hydrogen in copper in coupled to processes like gamma radiolysis, sulphide corrosion and anoxic corrosion in pure oxygen free water.

• General corrosion at high chloride concentrations together with high sulphide concentrations, under unsaturated condition

SSM list of most urgent issues


• Sulphide corrosion – mass transport limitation?

• Measurements of sulphide depletion as a function of time

• Comparison with estimated flux through a diffusion layer

• gives a limit flux that is still rate determining (at even higher fluxes the kinetics of the

electrochemical reactions can be rate determining)

Issues from SR-Site (I)

• Conclusions for SA

• Also investigations of kinetics of reactions,

and studies of film growth rates

• Comparison with fluxes of sulphide in the

repository (modelling based on ground

water fluxes and sulphide diffusion)

=> the repository fluxes are lower than these

limiting fluxes, and thus sulphide corrosion is

mass transport limited


• SRB activity in buffer and backfill – does it

happen and what is the limiting factor?

• Part of ”Integrated Sulphide Project” – joint

SKB-Posiva project

• WP2 experiments in bentonite

=> most bentonites show a threshold in density

for microbial activity

Issues from SR-Site (II)

• WP 3 modelling of the near-field

• 3 modelling teams (Unibern, Amphos+Claytech, Fraser King)

• Base case and Variant cases

• ongoing, reporting during spring 2018

=> preliminary results indicate the precipitation of FeS(s) is very important and

the limiting process on how much sulphide that reaches the canister


• Sulphide (University of Western Ontario), mostly electrochemical studies (voltammetry, EIS, SEM etc)

• passivity – is it possible and under what conditions?

• important to make scan in ”backward” direction to distinguish passivity from onset of general corrosion

• all evidence points to passivity requires sulphide fluxes higher than in repository

• nature of cathodic reaction – is it HS- (H2S) or H+ (H2O) that is reduced?

2Cu + HS- + H2O Cu2S + H2 + OH-

will also be investigated with DFT (density functional theory) modelling at KTH

Ongoing – sulphide (I)

5.0x10-5 M Na2S and 25 Hz 2.0x10-3 M Na2S and 25 Hz


• Stress corrosion cracking in sulphide (Swerea Kimab)

• attempt to repeat the Japanese study from 2008 (Tanaguchi and Kawasaki)

• only ductile fractures

• maybe some superficial cracks – but the same as in creep experiments

without corrosion?

• report under review

• internally SKB – calculations of tensile stresses in the canister (never going

through all the thickness), studies of possibilities for crack growth

• Localised corrosion (Swerea Kimab, Micans), exposure of copper to

• gaseous H2S, 10 and 104 ppm

• solution with microbs (10and 30 days, different amounts of lactate and

sulphate)

• will look for pits with optical microscopy

Ongoing – sulphide (II)


• Localised corrosion under oxidising conditions

• SKB co-operation with NWMO

• measurements of active-passive regions for pH, Cl-, CO32-, SO4

2- (done at

UWO)

• also work with multi-array electrodes, modelling etc

Ongoing – localised corrosion


• Long-term experiments at Äspö Hard Rock Laboratory:

Minican, LOT, Prototype etc

• not always designed to be corrosion experiments – more

difficult to interpret results concerning corrosion

• Some general experiences, example Minican

• first period, 1 month, of corrosion by oxygen

(rate 1.5 µm/y)

• after that sulphide corrosion, 8.4 years

(rate 0.006 µm/y)

• misleading to use one average corrosion rate!

Recent research – Äspö HRL

O:S ~ 70:30

Prototyp

Rör T58• pits and surface defects => from

manufacturing as the same in material

stored at the shelf have similar

appearance


• Hydrogen from corrosion: irradiation, sulphide

corrosion, (corrosion by pure water)

• intrusion in the copper?

• to be able to measure any difference at exposure

- need to check

• accuracy in methods

• reproducibility in methods

• example from fusion analysis, SKB R-17-15

• Round-robin test

• large spread, also within some laboratories

=> very sensitive to surface treatment

Ongoing – hydrogen

freshly prepared by lab

prepared by Kimab


• Based mainly on experiments by Gunnar Hultquist, a group at KTH

• results published 1986, and 2007-2015

• claiming a reaction of copper with pure water

• driving force, a new, hypothetical, stable phase: Cu + yH2O ⇄ HxCuOy + (y-x/2) H2

• with equilibrium pressure: pH2=1 mbar (at around 45 °C)

• Could be compared to established thermodynamic data

• the dominating reaction: Cu + H+ ⇄ Cu+ + ½ H2

• equilibrium calculation including all known phases and species of Cu-O-H gives: pH2=2.5×10-6 mbar, [Cu]=4×10-12 M (at 25 °C)

• If this new driving force exists, the long-term durability of e.g. copper canisters in a KBS-3 repository would possibly be affected as the availability of water is more or less unlimited

=> SKB used several different approaches to investigate this issue

Corrosion in pure water - the issue


• Studies of hydrogen evolution in metal (stainless steel)

contained system (similar to Gunnar Hultquist’s set-up)

• Also studies of formation of copper corrosion products

• in separate set-ups of lower chambers sealed with Pd

membrane in glove box

Uppsala University – set-up


• first phase of experiments (2011-2013):

hydrogen was detected – but the same

amount in exp. without copper!

• second phase (2014-2016), with

redesigned equipment:

no hydrogen pressure above

background

• separate chambers, exposed 0-29 months

• surfaces investigated with XPS:

no oxide layer formed

• water analysed with ICP-MS:

not increasing with time

Uppsala University - results

blue – background with water and glass

red – with copper in water


• Studies of hydrogen evolution in glass test tubes

• prepared in glove box, stored in N2 atmosphere, at 70 °C

• gas phase

• sampled with syringe through the rubber stopper

• analysed with GC

• different copper qualities and surfaces treatments used

Micans – set-up

Series N8 after 806 days, no visual change of surface


• Results with Cu-OFP 99,95% (SKB canister copper), samples 100×10×2 mm3

• yields hydrogen gas, measured up to 800 days

• evacuated several times (new N2 gas) – new hydrogen is measured

• hydrogen also without water => out gassing from the material!

• this hypothesis corroborated by first heating copper and then exposing – no hydrogen in

the tubes

Micans – results 2011-2013

0 100 200 300 400 500 600 700 800 900

Time (days)

0

1

2

3

4

5

6

7

H2 (

mb

ar)

N8:1 N8:2 N8:3 N8:4 N8:5 N8:6 N8:7 N8:8 N8:9 N8:10 N8:11 N8:12 N8:13 N8:14 N8 K:1 N8 K:2 N8 K:3 N8 K:4 N8 K:5

also polished


• Results with Cu-OF 99.95% (Goodfellow), foils 100×10×0.1 mm3

• the same Cu as Gunnar Hultquist used (publications 2009, 2013, 2015)

• Conclusion: no hydrogen above background

Micans – results 2014-2015

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 50 100 150 200 250 300 350

Hyd

roge

n p

art

ial p

ress

ure

[m

bar]

Time [days]

99.95% Cu-OF; Goodfellow

As received

Electropolishing + reduction + heating to 400 °C (Method I)

Heating to 400 °C only

Water only; no copper

Hultquist et al. [7] Data from Hultquist et al. 2015


Other results – looking for new phase

Cu+

O2-

H+

Cu+

O2-

H+

• KTH: Pavel Korzhavyi, Yunguo Li

• DFT calculations of possible configurations of CuOH

• structure of ”CuOH” resembles structure of cuprite

and cubic ice (ice-VII)

• calculation of ∆G° (and ∆H°): less stable than Cu2O

• KTH: Inna Soroka

• synthesis and characterisation of ”CuOH”

• recipe from 1955

• reduction of Cu(II) with Fe(II) EDTA solution

• studied with XRD, FTIR, XPS, SEM

• results: a hydroxide exists: CuOH×H2O, but it is less

stable than Cu2O

• Conclusion: no new driving force for corrosion


• KTH: Joakim Halldin Stenlid, Claudio Lousada

• DFT calculations of H2O on Cu and Cu2O surfaces

• ideal surfaces, and with defects

• Results

• ~½ monolayer OH can form (∆G◦<0) from 1 mono-layer

H2O, but not further

• H2 dissociates from Cu; less clear for Cu2O

• corresponds to some ng H2 per cm2

Conclusion: surface reactivity can not explain the

amount of H2 in Hultquist’s experiment

(ca 100 ng H2 per cm2)

Other results – surface reactions


• SKB cannot find any support for the existence of a sustained

corrosion of copper in pure, O2-free water above the very limited

extent predicted by established thermodyamic data

• This extent described by thermodynamic data is completely

negligible compared to other corrosion phenomena in a typical

repository environment.

Corrosion in pure water - conclusion


• Focus on unsaturated conditions

• SSM questions to be handled to PSAR

• Sulphide most important long-term corrodant

• prerequisites for formation of a passive film?

• stress corrosion cracking

• occurrence (stresses, material, environment)?

• mechanism?

• Corrosion i pure oxygen-free water

• SKB cannot find any support for a sustained corrosion process extending

what established thermodynamics predict

Summary – current corrosion research


• SKB reports, examples

• Minican: TR-16-12, TR-16-13

• Surface studies of copper: P-13-50, P-17-11

• Hydrogen Round robin: R-17-15

• Cu in pure water: TR-15-03, TR-16-01, TR-16-03

• Many papers

• recent examples from many of the areas may be found in the proceedings

from the 6th Int. Workshop on Long-term Prediction of Corrosion Damage in

Nuclear Waste Systems, published in Corrosion, Engineering, Science and

Technology, 2017, Vol 52, No 51 (open access)

http://www.tandfonline.com/toc/ycst20/52/sup1?nav=tocList

Read more

http://www.tandfonline.com/toc/ycst20/52/sup1?nav=tocList

Thank you for your

attention!

Current status of copper corrosion research at SKB

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