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STUDIES ON THE STRESS CORROSION CRACKING (SCC) BEHAVIOR OF
VARIOUS METALS AND ALLOYS USED IN THE DESALINATION AND POWER
PLANTS1
T.L. Prakash, John OHara and Anees U. Malik
Research & Development Center, Saline Water Conversion
Corporation
P.O.Box # 8328, Al-Jubail 31951, Kingdom of Saudi Arabia
SUMMARY
Corrosion problems in desalination plants can increase
substantially the operation and
maintenance cost. The shutdowns resulting from the failures of
components due to
corrosion are extremely expensive. Stress corrosion cracking
(SCC) is one such
corrosion failure commonly encountered due to combined action of
stress and corrosion
medium.
This report describes a study on the Stress Corrosion Cracking
(SCC) behavior of alloys
resulting from the synergistic action of corrodents such as
chlorides, oxidants, H2S, etc.
In this study, the threshold stresses for SCC have been
determined for few generic alloys
namely; carbon steel, 316L, 317L, 904L, 430 and Monel 400 used
in the desalination
plants. The standard Proof Rings and U-Bend samples in NACE and
SHELL solutions
containing H2S are used for the purpose. Electrochemical
polarization measurements
were performed on these alloys in the specified environments to
study the effect of
electrochemical potential on the intergranular SCC.
Fractographic analyses were
conducted by Scanning Electron Microscopy supplemented by Energy
Dispersive
Spectroscopy. The test results showed that the intergranular and
transgranular SCC
fracture of carbon steel and alloy 430 in H2S environment occurs
only in the limited
potential environment, where as, the alloys viz., 316L and 317L
are immune to SCC
under the condition of test performed. The alloy Monel 400 was
also found susceptible
to SCC in presence of H2S.
1 Issued as Technical Report TR 3804/APP 90001 in October 1999.
A paper entitled Studies on the Stress Corrosion Cracking Behavior
of Few Alloys used in the Desalination Plants was presented at the
WSTA 4th Gulf Conference, Bahrain, 13-18 Feb. 1999.
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Fractography of alloy 430 indicated that the failure is
attributed mainly to the sulfide
stress cracking due to synergistic action of sulfide and
chloride that had greatly
enhanced the sensitivity of phases present in the alloy. A
tentative ranking of the alloys
has been established on the basis of the threshold stress values
obtained from the tests
conducted.
1 INTRODUCTION
One of the major factors that control the use of structural
alloys in desalination industry
is its resistance to corrosion in marine environments and other
distillation conditions.
The high chlorinity of seawater associated with its complex salt
composition render it
inherently corrosive to many structural alloys. Its deleterious
effects on ocean
interfacing structures have been documented and the wealth of
information is compiled.
In spite, we continue to experience corrosion related problems
on structures that must
interface with the marine environment. Stress Corrosion Cracking
(SCC) is one such
problem which essentially controls and determines the
suitability of materials from a
wide range of materials as they are very expensive modes of
failures, of particular
relevance to desalination and power plants.
SCC is a stress assisted anodic process as a result of
synergistic action of ions, such as
Cl- , H2S and oxidants like elemental sulfur present in the
solution. The susceptibility to
SCC is influenced by factors like environmental condition,
temperature, hardness of the
material, level of applied stress and microstructure of the
material. The SCC of
materials in acidic solutions containing dissolved hydrogen
sulfide (H2S) has been
termed as sulfide stress cracking (SSC). The failure
characteristics in SSC are most
consistent with a hydrogen embrittlement mechanism where the
fracture modes are
mostly intergranular. The literature available on SCC is quite
vast, hence the present
literature survey is restricted to the following sections
keeping in view of the objectives
of this project.
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In the past it was thought by several investigators that SCC of
a given alloy occurs only
in limited range of specific environments [1]. Subsequently, the
above notion was
diluted when it was found that SCC occurs in wide range of
environments including
pure water [2,3]. A brief account of literature information on
few important categories
of structure alloys where SCC/SSC occurs by environment
interaction is given below.
The carbon steels are prone to SCC in carbonate, bicarbonate,
acetates and phosphate
environments and is identified as the main reason of cracking in
natural gas transmission
lines. In low alloy steels, oxygenated water at high
temperature, NaNO2 - Na 2SO4
solutions, alkaline chloride solutions such as NaCl - Ca (OH)2
under pitting conditions
[4,5], and anhydrous ammonia - methanol solution [6] in the
presence of chloride caused
SCC. Studies on J-55 and N-80 steels have shown that H2S
containing chloride
solutions promote SSC [7]. Similar observation was also made in
AISI 1075 steels and
hardness of steel is also found to influence the SCC [8]. Strong
tendency of SCC in
carbon steels have been noticed in diethanolamine and
manoethanolamine solutions [9],
0.5M NaHCO3 and 0.5M Na2CO3 solutions at 70 oC at high stress
levels [10] and CO2
environment [11]. Synergistic effect of low concentration
chloride in bicarbonate
solutions [12] and low concentration of sulfate [13] causing SCC
in low alloy steels
have also been reported. The effect of sulfide in NACE standard
solution (5% NaCl +
0.5% Acetic acid) was found different from SHELL standard
solution (solution
containing 0.5% Acetic acid) in the promotion of SCC for high
strength low alloy steels
[14].
In austenitic stainless steels, SCC was well known since three
decades. The cracking
was mainly due to chloride (which were neutral at high
temperature, acid at low
temperature) and hydroxide solutions [15]. Thiosulphate
environments of weld-
sensitized stainless steels have shown SSC [16]. SCC have been
reported at ambient
temperature [17] and at 90 oC [18] in materials with sensitized
microstructure in chloride
containing aqueous environments and in 0.1M NaCl or synthetic
seawater at 90 oC for
SS 304 and 316 alloys [19]. Alloys SS 304 and 316 was more
susceptible to SCC in HCl
and H2SO4 [0.82 K Mol / m3] solutions [20]. Ferritic stainless
steels (type AISI 405)
were reported to be susceptible to SCC at 288 oC in aqueous
environments [21]. It was
also reported that ferritic steels of type AISI 430 shown lesser
susceptibility to SCC in
chloride solution when compared to sulfate solution [22].
Martensitic stainless steel
1.1 Metal - Environment Interaction
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type AISI 420 (13 Cr SS) was found prone to SCC in H2S
environment and resistant in
CO2 environment. The CO2- H2S - Cl - system inhibited SCC by
favoring the formation
of protective layer [23]. In duplex stainless steels SSC is
severe at 160 oC in 25% NaCl
containing dissolved H2S and also in aerated brine solutions
[24]. SCC was noticed at
ambient temperature in solution of sulfide/3.5 wt % NaCl
containing sulfide [25,26].
The nickel base alloys viz., C-276 and alloy 825 were
susceptible to SSC in HCl
oxidizing solution containing H2S. In chloride containing
solution the SSC has been
observed at temperatures above 204 oC [24]. The copper base
alloys are subjected to
SCC in environments like ammonia, sulfur dioxide, organic
complexing solution like
acetates, tartrates and sulfate solutions [27].
1.2 Threshold Stress for SCC
As the name implies the threshold stress is the stress below
which no SCC occurs. The
main purpose of determining the threshold stress for SCC is to
establish a ranking order
under given condition of metal environment combination,
heat-treated microstructure,
type of stressing and its magnitude. An exact threshold stress
for a given condition is
difficult to define. However, the relative ranking seems quite
obvious.
The material which shows highest SCC resistance for a given
environment may show
susceptibility to SCC when it is heat-treated to different
microstructure. For example,
threshold stress in SCC of carbon and low alloy steels was found
to be influenced by
heat treatment when it is studied using 5% NaCl - 0.5% Acetic
acid solution containing
3000 ppm of H2S [28]. The heat treatment carried out gave
untempered Martensitic
structure which is attacked by H2S and resulted in low threshold
stress values for
cracking. From the result of series of test in Drop Evaporation
Test on highly alloyed
stainless steel and duplex stainless steels as indicated by
their threshold stress values, it
was seen that the highly alloyed stainless steels such as 654
SMO (UNS S 32654) was
most resistant to SCC than the duplex stainless steels viz.,
2205 (UNS S 31803) [29] and
least resistant was 304 (UNS S 32304) [30].
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The fractography in SCC was used mainly for two purposes. First
being failure mode
determination and the other was for the studies of fracture
mechanics. The conventional
metallography and Scanning Electron Microscopy (SEM) were widely
employed for this
purpose. SEM fractography had been used in SCC tested stainless
steel samples to
determine the crystallography of cracking and to determine the
mechanism of fracture.
Normally, transgranular fracture was noticed in SCC [31]. In
this study, the cleavage
nature of transgranular cracking which is typical of SCC was
established.
1.4 Influence of Metallurgy
The metallurgical aspects of the material have profound
influence on SCC. The grain
boundary segregation and phase transformation in steel strongly
affect SCC. It was
found that substitutional elements like Molybdenum in specific
environment typically of
type caustic medium, could affect SCC [32]. But, it is not true
for all elements or in all
solutions. Similarly the phase transformation occurring by aging
process, heat
treatment, cold working, etc. may or may not have beneficial
effects. The example of
beneficial effect to SCC was seen by over-aging of Aluminum-Zinc
alloys, whereas,
such over-aging is not found beneficial in Aluminum Lithium
alloys [33].
1.5 Electrochemical Aspects The SCC in specific environments is
strongly correlated with localized (pit or crevice)
corrosion. The importance of electrochemistry is in the
understanding of kinetics of
SCC in the context of changed local environment. The measurement
of repassivation
potential of localized corrosion would represent the lowest
potential at which special
local environment can be maintained and SCC propagation occurs
in this special
environments. Another factor is the critical potential for SCC.
If these two potentials
are determined and made to coincide by the alterations in the
composition of alloys or
environment (with the help of local chemistry) new SCC resistant
alloys can be
developed or mitigation of SCC could be achieved. Two
outstanding examples of the
electrochemical contribution to SCC are the development of
inexpensive steel [33]
without high nickel content which resist SCC upto 140 oC with
20% NaCl. The other
being the usage of anodic protection from the understanding of
electrochemistry, which
is used worldwide.
1.3 Fractography
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The prevalence of SCC in desalination plant occupies a major
share when compared to
other modes of material failure. In recent years many major SCC
failures have been
reported from Desalination plants. The details of the failure is
briefly described in
Appendices 1 through 4. Although there have been better
understanding of the
corrosion mechanism with the help of environment analysis,
metallography,
fractography, etc., the diversity in the failure modes and the
associated mechanisms are
highly complex and not completely understood, still remain to be
explored.
It is clear from the above literature review that till to date
no data is available to
determine the susceptibility of various metals and alloys to SCC
resulting from
synergistic action of corrodents which are normally encountered
in Desalination and
Power Plants. The present investigation, although less
comprehensive, is aimed to carry
out a systematic study of such phenomenon and to understand the
nature and mechanism
so that occurrence of SCC can be minimized.
2. OBJECTIVES The objectives of the proposed work are the
following :
(i) To investigate the susceptibility of materials viz.,
stainless steels of grade AISI
316L, AISI 317L, AISI 430 and 904, Monel 400 and Carbon steel to
SCC in the
standard NACE and SHELL solutions (i) containing saturated .H2S
gas and (ii)
containing 0.1M Na2S.
(ii) To establish a ranking order with regard to SCC resistance
for the above alloys
by determining the threshold stress.
(iii) To carry out fractography on the SCC failed specimen using
Scanning Electron
Microscope to understand the mechanism of cracking.
(iv) To assess the effect of electrochemical potential on the
alloy passivity to
corrodent species by performing the electrochemical polarization
measurements
in the specified environments.
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3. EXPERIMENTAL DETAILS The materials selected for this study
are CS (Carbon Steel), AISI 316L& 317L
(austenitic stainless steel), 430(ferritic stainless steel) and
904 (super austenitic stainless
steel) and Monel 400 (nickel base alloy). The chemical
composition and the mechanical
properties of these alloys are shown in Table 1. The materials
selected are typical
commercial alloys normally used in desalination and power
plants.
3.1 Stress Corrosion Cracking Tests 3.1.1 Round and flat tensile
samples: Round and flat tensile samples of CS, 316L, 317L, 430 and
Monel 400 were machined
from rod/sheet material stocks. All the materials selected were
of mill finished
commercial grades. The schematic drawing of round test sample is
shown in Figure 1,
the photograph of sheet sample is shown in Figure 2. The tests
were carried out in
Cortest Proof Rings [34] with corrosion testing environment
chamber. An hour meter
and H2S gas manifold were used to measure the time of failure of
specimen and H2S gas
monitoring during test respectively. The photographs of the
Cortest Proof Ring and
Cortest Proof Rings Battery with hour meter and manifold are
shown in Figure 3.
The media employed for the tests were (i) NACE solution (having
composition 5%
NaCl + 0.5% CH3COOH) prepared from distilled water and
continuously bubbled with
H2S to maintain H2S saturation in solution. (ii) SHELL solution
(having composition
0.5% CH3COOH) prepared from distilled water and continuously
bubbled with H2S to
maintain H2S saturation as in (i).
The samples were tested in ambient temperature with the Cortest
Proof Ring at 70, 80,
85 and 90% of their respective 0.2% yield stress (YS) with the
help of loading nut and
calibration charts. During the test H2S was continuously bubbled
in the solution. The
time to rupture of the samples were recorded. The samples those
have crossed 500 hours
without rupture were withdrawn from the test. During the test,
samples were periodically
withdrawn for examination of any initiation of cracks or
corrosion pit development.
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3.1.2 U-Bend Samples A U-bend specimen is prepared generally
through a rectangular strip that is bent 180
degrees around a predetermined radius and maintained in the
resulting constant strain
condition during stress corrosion testing. The specimens are
most easily be made from
sheet or strip. The main advantage of U-Bend specimen is that it
is simple and most
useful for detecting large differences between SCC resistances
of different alloys in the
same environment or one alloy under different metallurgical
conditions or one alloy in
several environments.
The U-Bend specimen is stressed by bending the specimen to
U-shape in a fixture either
manually or through Universal Testing Machine (UTM) and
maintaining it in the same
shape by means of bolts and nuts. When U-Bend sample is stressed
the material in the
outer fibers of the bend is strained into the plastic region.
The total strain on the
outside of the bend is given by the following equation:
T = ---------- When T
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The samples were tested in ambient temperature. These samples
outer fiber are stressed
to approximately 70, 80, 85 and 90% of their respective 0.2%
yield stress (YS) by
bending them to corresponding diameter with help of rollers as
detailed above. The
samples outer fiber surface was critically examined with help of
magnifying glass for
crack freeness before they are immersed in the media selected.
The samples are
withdrawn periodically for the purpose of inspection (every
week) till they have crossed
500 hours without any appearance of cracks at the outer most
fiber surface. The samples
those crossed 500 hours without crack appearance were continued
up to 2000 hours.
The data obtained from U-Bend samples are quantitative and
procedure allows for
multiple and field-testing. The limitation of their test is that
actual volume of material
tested is relatively small (only small portion of the bend
radius, i.e., outer most surface
experiences the highest stress) [35]. Hence very accurate
results are difficult to obtain in
this test.
3.2 Electrochemical Tests The electrochemical polarization
techniques were performed to measure the absolute
corrosion rates. Tafel plots were generated for this purpose on
samples by polarizing the
specimen about 300 mV anodically (positive- going potential) and
cathodically
(negative-going potential) from the corrosion potential, Ecorr.
The potential is stepped in
staircase waveform. The resulting current is plotted on a
logarithmic scale. The
corrosion current Icorr is obtained from Tafel plot by
extrapolating the linear portion of
the curve to Ecorr. The corrosion rate was calculated from the
Icorr.
Experiments were carried out on the samples using EG&G model
273 Potentiostat with
Softcorr Corrosion Software M342. A saturated calomel electrode
was used as reference
electrode. Photograph of the potentiostat along with cell is
shown in Figure 6.
Graphite electrodes were used as auxiliary electrodes. Button
samples of 14 mm dia and
2 mm thick were machined from rod/ sheet stock of sample
material. They were
polished at one side to 600 # grade paper. The media employed in
the electrochemical
tests are i) NACE solution ii) Natural seawater iii) Natural
seawater containing varied
amounts of sulfide ion concentration obtained by dissolving
known quantities of sodium
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disulfide crystal iv) SHELL solution and v) SHELL solution
containing known amount
of sulfide ion concentration.
4. RESULTS AND DISCUSSION The results of the SCC tests carried
out on round and flat tensile samples are shown in
Figure 7 & 8. The results revealed that the SCC occurred
more readily in CS samples
in NACE solution and SHELL solution saturated with H2S at stress
ranges of 70, 80, 85
and 90% YS. The alloys 430 and Monel 400 are also found
susceptible to SCC only
when they are stressed to 90% YS in NACE solution containing
H2S. The alloys 316L
and 317L were found immune to SCC in all the condition of tests
in NACE or SHELL
solutions containing H2S.
The results of the SCC tests carried out on U-bend samples are
shown in Tables 2
through 9. The results obtained were almost identical to that of
round tensile samples
except that the time taken was quite considerable for SCC onset
due to nature of sulfide
ion present in the medium. SCC occurred in the alloy 430 in NACE
+ 0.1M Na2S
media stressed to 90% YS. The first appearance of crack was
noticed after 1344 hrs in
samples stressed to 90% YS, whereas first appearance of cracking
was observed after
1920 hours of testing in sample stressed to 85% YS (Table 4).
The intensity of
cracking/pitting was however less in medium of SHELL + 0.1M Na2S
(Table 9) when
compared to cracking in NACE +0.1 M Na2S medium. The 316L, 317L
and 904L
samples were however free from SCC was observed upto 2000 hours
of exposure. The
photographs of typical alloy 316L samples after exposure are
shown in Figures 9 &10
The photographs of samples of alloy 430 when exposed up to 1344
hours (70, 80,
85 & 90% YS) in NACE + 0.1M Na2S and close up view of cracks
associated with
pitting are shown in Figures 11 & 12, respectively. The
photograph of alloy 430
samples exposed up to 1920 hrs (70, 80, 85 & 90% YS) in
SHELL + 0.1M Na2S
solution is shown in Figure 13. Due to some limitation of U-bend
test, as explained in
earlier section, the results are not discussed in detail.
The fracture of SCC tested round and flat samples of CS and
alloy 430 were analyzed in
Scanning Electron Microscopy (SEM). The fractures revealed
intergranular as well as
transgranular mode of crack propagation (Figures 14 & 15).
Branching of secondary
cracks from the primary cracks, which is typical of SCC failure
mode were noticed. The
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Energy Dispersive Spectrum obtained during SEM fractography at
fracture crack tips
containing corrosion products showed sulfur rich regions
(Figures 14b & 15b). This also
confirms the onset of SCC due to sulfide activity.
The threshold stress for CS in NACE and SHELL solution is 75%
YS, whereas it is
85% for 430 and Monel 400 alloys in NACE solution. Such
threshold stress was not
found to exist for the 430 and Monel 400 alloys in SHELL
solution at all the stress
levels. The alloys 316L and 317L, however, did not show any
threshold limits up to
90% YS either in NACE or SHELL solutions containing H2S. It is
possible that
threshold stress might be greater than the YS of these
materials. CS has shown greater
susceptibility to SCC in the tested medium when compared to
other alloys (Figures 7 & 8)
Sulfides play a dominant role in the structural steels
particularly in their resistance to
sulfide stress cracking. The water present would obviously
assist the corrosion
mechanism. The reaction will be of the type
H2S + Fe FeS + 2 H
The nascent hydrogen is then expected to embrittle the alloy.
The presence of chloride
all the more aggravate corrosion leading to early failure of
steel. The instances of
Sulfide Stress Cracking of stainless steels have been reported
[36] wherein failure of
specimens have been promoted in high chloride environment
(>25% NaCl) at elevated
temperature and pressure saturated with H2S. The chloride
content used in some of the
test being 5% , it is conceivable that the stainless steel of
type 316L and 317L are less
likely to be affected by H2S as evidenced in the experiment. The
effect of H2S in
SHELL solution suggest that except CS, other alloys were immune
to SCC. CS was
found prone to SCC > 75% YS. The synergistic effect of
chloride in presence of oxidant
(CH3COOH) and H2S to promote SCC in alloys 430 and Monel 400 at
stresses > 85%
YS was clearly demonstrated as seen from the results of NACE
solution experiment
(Figure 7).
The presence of H2S seems to have exerted a strong influence on
the repassivation
which is manifested by cracking in alloy 430. The hydrogen
embrittlement is well
known in alloy 430 particularly when cathodically protected. It
is possible that
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embrittlement is brought about by ferrite phase of the alloy
much more than austenite
[37]. It is also known that cracking of ferrite taken place by
mechanical twinning [38],
in this respect, hydrogen embrittlement could greatly enhance
the sensitivity of ferrite to
cracking. This point is very important at low temperature and
indeed evidenced in the
fractography performed on the failed samples of alloy 430
(Figures 15a & b). Cracked
regions had contained products rich in sulfides as determined in
EDAX. Reports have
been published elsewhere that high ferrite duplex stainless
steels (70% ferrite) is
inferior to that of low ferrite duplex stainless steels (50%
ferrite) for hydrogen
embrittlement [39].
The Tafel plots generated from potentiostatic polarization
experiments are shown in
Figures 16 through 23. The data obtained from the
electrochemical experiments are
shown in Tables 10 & 11. The results obtained from NACE
solution and natural
seawater containing 0.1M of sulfide indicated that alloys 316L
and 317L showed higher
current densities relative to the Monel 400. The current density
in natural seawater
solution for alloy 430 with 0.1M sulfides was lowest can be
attributed to the
development of a stable passive film over the surface of the
alloy.
The electrochemical data from the SHELL solution revealed that
lowest current
densities for 317L alloy in 0.1M sulfide solution, while the
highest was observed for
alloy Monel 400 except CS. In general, for all the alloys
studied, high sulfide content
moved the corrosion potential to active direction thus enhancing
localized corrosion.
Lowest current densities exhibited by alloys 316L and 317L
indicated that they are least
susceptible to corrosion in presence of sulfide.
From the results of electrochemical tests it is seen that the
synergistic effect of chloride
and sulfide on the corrosion behavior were prominent
particularly for alloys 316L and
317L. For alloys 430 and Monel 400 such effect were not noticed.
However, under the
influence of stress as noticed from the SCC test results, the
trend was reverse. It is
plausible that the passive films formed over the alloy 430 and
Monel 400 was less stable
and get disrupted easily, leading to SCC. The data generated in
this investigation suggests a tentative ranking of alloys could
be
made with respect to their susceptibility to SCC. On the whole,
at ambient temperature,
austenitic steels (alloys 316L, 317L and 904L) were better
resistant than the Monel 400
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and alloy 430 in solutions containing chloride and sulfide ions
when stressed beyond
80% YS. The tentative ranking can be expressed as (in order of
most resistant to SCC):
316L, 317L & 904L > Monel 400 > 430 > CS 5.
CONCLUSIONS The following conclusions are drawn on the basis of
investigations carried out. (i) In solutions containing sulfide,
the chlorides demonstrated the synergistic effect
promote SCC in alloys 430 and Monel 400 at stress levels >
85% YS.
(ii) Alloys 316L and 317L were found SCC resistant under all
conditions of
the tests performed.
(iii) Alloys 316L and 317L had shown higher current densities
relative to the other
alloys in presence of specified oxidant, chloride and sulfide
ionic species. Under
the influence of stress, they were least susceptible to
corrosion.
(iv) CS was found prone to SCC at stress levels > 75% YS in
solutions containing
specified amounts of sulfide, chloride and oxidants.
(v) A tentative ranking of the alloys have been established on
the basis of threshold
stress value in solutions containing chloride and sulfide ions
(in the order of
increasing resistance to SCC) 316L, 317L, 904L > Monel 400
> 430 > CS.
(vi) The failure of alloy 430 is mainly attributed to sulfide
stress cracking as sulfides
greatly enhanced the sensitivity of phases present in the alloy
to cracking as
evidenced from fractography.
(vii) Sulfide ion displaces the corrosion potential in active
direction thereby
increasing the risk for localized corrosion for all the alloys
studied.
6. RECOMMENDATIONS (1) From the investigation carried out it is
apparent that austenitic stainless steels of
type AISI 316L, 317L and high alloy 904L are the alloys of
choice in
desalination plant environments containing high chloride. In
these steels if any
stresses arising from fabrication, fit-up, welding and
differential heating could
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increase the susceptibility of these alloys to SCC and hence
these stresses should
be avoided in practice.
(2) Chlorides and sulfides do cause SCC in carbon steel and
alloy 430. Although it
is not possible to eliminate chlorides in desalination plants,
meticulous care
should be exercised to minimize their introduction as an
effective and essential
alloy. Satisfactory use of these alloys could be permitted by
minimizing the
fabrication stress and cold work avoiding thermal insulation and
gasket material
high in chloride, avoiding elastomers, lubricants, sealants and
other material
containing halogens.
(3) Alloy Monel 400 is deemed to have moderate susceptibility to
SCC in
desalination plant environment containing chlorides. However,
its successful
use could be made by decisively controlling the stress levels,
water chemistry,
design parameters, thermo-hydraulic characteristics, presence
and absence of
crevices and biological activity as evidenced from the reported
Bio-Corrosion of
Monel 400 bolts by sulfur reducing bacteria [40] in Al-Jubail
intake system.
7. FURTHER SCOPE OF SCIENTIFIC WORK
(1) The susceptibility of alloys to SCC is significantly
affected by the synergistic
action of chloride in presence of sulfide. Hence further testing
is therefore
required to determine sources and levels of sulfide and possible
prevention
approach in desalination process.
(2) Temperature plays dominant role in the repassivation and
hydrogen
embrittlement of alloy 430. Hence the response of ferrite phase
to temperature
changes should obviously be further investigated. It is likely
that at high
temperature hydrogen embrittlement (cathodic cracking) decreases
while
repassivation (anodic cracking) is very much accelerated which
are not only
important from metallurgical and scientific view point, it is
also of practical
interest since alloy 430 is one of the major material of
construction in many
pumps used in Line 3 (water transmission system of SWCC).
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(3) Due to lack of standardized test method for particular
application, more test
results should be obtained on enlarged list of commercial alloys
from various
laboratories and industries which can be realistically compared
and used by
design engineers to select materials which will ensure reliable
operation in
environments where stress corrosion cracking or sulfide stress
cracking could be
a problem.
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Table 1. Chemical Composition and Mechanical Properties of
Alloys A. Chemical Composition:
S. Alloy UNS Composition (%) Others
No. No. Fe Cr Mo Ni C Cu Mn Si
1 Mild Steel
J2503 Bal 0.5
Max
0.2
Max
0.5
Max
0.25
Max
0.3
Max
1.2
Max
0.6 0.04P,0.04S
2 316L S31603 Bal 16.0 3.0 11.0 0.02 0.2 1.0 1.0 0.04P,0.02S
3 317L S31703 Bal. 18.5 3.2 13.5 0.02 - 1.0 - 0.08 N
4 904L N08904 Bal. 20 4.74 24.5 0.017 1.4 1.5 - 1.4Cu,1.0Si
5 430 S43000 Bal. 18.0 - - 0.12 - 1.0 1.0 0.04P,0.03S
6 Monel 400
N4000 2.5 - - 66.5 0.3 Bal. - 0.5 0.024S
B. Mechanical Properties (Room Temperature)
S.No. Alloy UNS
No.
0.2% Yield Stress (Mpa)
UTS
(Mpa)
Elongation(%)
1 Carbon steel J2503 179 324 30 2 316L S31603 170 485 35 3 317L
S31703 216 525 40 4 430 S43000 205 450 28
5 904L N08904 220 490 35
6 Monel 400 N08904 172 480 30
2 2 9 7
-
Table 2. U-Bend Specimen Testing of AISI 316L Exposed to NACE
Solution Containing 0.1M Na2S
S.No. Applied Stress
( % of YS)
Time of first appearance of crack (Hours)
Appearance of surface/cross
section
Remarks
1 70 NFC NC - 2 70 NFC NC - 3 75 NFC NC - 4 75 NFC NC - 5 85 NFC
Brown coloration
over the bent portion
No cracking
6 85 NFC --DO-- --DO-- 7 90 NFC --DO-- --DO-- 8 90 NFC --DO--
--DO--
NFC-indicates no first crack beyond 2000 hours. NC - indicates
No Change Table 3. U-Bend Specimen Testing of AISI 317L Exposed to
NACE Solution Containing 0.1M Na2 S
S.No. Applied Stress ( % of YS)
Time of first appearance of crack (Hours)
Appearance of surface/cross
section
Remarks
1 70 NFC - - 2 70 NFC - - 3 75 NFC - - 4 75 NFC - - 5 85 NFC - -
6 85 NFC - - 7 90 NFC Brown coloration
over the bent portion
-
8 90 NFC --DO - NFC-indicates no first crack beyond 2000 hours.
NC - indicates No Change
2 2 9 8
-
Table 4. U-Bend Specimen Testing of AISI 430 Exposed to NACE
Solution Containing 0.1M Na2S
S.No. Applied Stress
( % of YS)
Time of first appearance of crack (Hours)
Appearance of surface/cross
section
Remarks
1 70 NFC NC - 2 70 NFC NC - 3 75 NFC NC - 4 75 NFC NC - 5 85
1920 Few pits at outer
radius -
6 85 1920 --DO--- - 7 90 1344 Pitting at few
places over the bent radius
Cracking is prominently associated with pitting over the bent
radius when test contd. Beyond 1920 hrs.
8 90 1344 Moderate pitting at few places over the bent
radius
--DO---
NFC-indicates no first crack beyond 2000 hours. NC - indicates
No Change Table 5. U-Bend Specimen Testing of AISI 904L Exposed to
NACE Solution Containing 0.1M Na2S
S.No. Applied Stress
( % of YS)
Time of first appearance of crack (Hours)
Appearance of surface/cross
section
Remarks
1 70 NFC NC - 2 70 NFC NC - 3 75 NFC NC - 4 75 NFC NC - 5 85 NFC
NC - 6 85 NFC NC - 7 90 NFC Faint brown
coloration over the bent radius
No cracking
8 90 NFC --DO-- --DO-- NFC-indicates no first crack beyond 2000
hours. NC - indicates No Change
2 2 9 9
-
Table 6. U-Bend Specimen Testing of AISI 316L Exposed to SHELL
Solution Containing 0.1M Na2S
S.No. Applied Stress
( % of YS)
Time of first appearance of crack (Hours)
Appearance of surface/cross
section
Remarks
1 70 NFC NC - 2 70 NFC NC - 3 75 NFC NC - 4 75 NFC NC - 5 85 NFC
Faint brown
coloration over the bent radius
No cracking
6 85 NFC --DO-- --DO-- 7 90 NFC --DO-- --DO-- 8 90 NFC --DO--
--DO--
NFC-indicates no first crack beyond 2000 hours. NC - indicates
No Change Table 7. U-Bend Specimen Testing of AISI 317L Exposed to
NACE Solution Containing 0.1M Na2S
S.No. Applied Stress
( % of YS)
Time of first appearance of crack (Hours)
Appearance of surface/cross
section
Remarks
1 70 NFC NC - 2 70 NFC NC - 3 75 NFC NC - 4 75 NFC NC - 5 85 NFC
NC - 6 85 NFC NC - 7 90 NFC Brown coloration
over the bent radius No cracking
8 90 NFC --DO-- --DO-- NFC-indicates no first crack beyond 2000
hours. NC - indicates No Change
2 3 0 0
-
Table 8. U-Bend Specimen Testing of AISI 904L Exposed to NACE
Solution Containing 0.1M Na2S
S.No. Applied Stress
( % of YS)
Time of first appearance of crack (Hours)
Appearance of surface/cross
section
Remarks
1 70 NFC NC -
2 70 NFC NC -
3 75 NFC NC -
4 75 NFC NC -
5 85 NFC NC -
6 85 NFC NC -
7 90 NFC NC -
8 90 NFC NC - NFC-indicates no first crack beyond 2000 hours. NC
- indicates No Change Table 9. U-Bend Specimen Testing of AISI 430L
Exposed to SHELL Solution Containing 0.1M Na2S
S.No. Applied Stress
( % of YS)
Time of first appearance of crack (Hours)
Appearance of surface/cross
section
Remarks
1 70 NFC NC -
2 70 NFC NC -
3 75 NFC NC -
4 75 NFC NC -
5 85 NFC NC -
6 85 NFC NC -
7 90 1920 Small pits and cracks are seen at outer radius.
Hair line cracking distributed all along the bent radius
8 90 1920 --DO-- --DO-- NFC-indicates no first crack beyond 2000
hours. NC - indicates No Change
2 3 0 1
-
Table 10. Potentiostatic Polarization data from NACE Solution
and Natural
Seawater (NSW).
S.No. Material Electrolyte E corr (mv)
I corr (A/cm2)
CR (mpy)
1 316L i) NACE Solution
ii) NSW
iii) NSW + 0.064 M Sulfide iv) NSW + 0.1 M Sulfide
-59.6
-216
-348 -389
1.53
0.32
0.31 24.64
0.67
0.13
0.137 10.84
2 317L i) NACE Solution ii) NSW iii) NSW + 0.064 M Sulfide iv)
NSW + 0.1 M Sulfide
-212 -261 -383 -392
0.58 0.31 0.39 15.72
0.26 0.13 0.17 8.17
3 430 i) NACE Solution ii) NSW iii) NSW + 0.064 M Sulfide iv)
NSW + 0.1 M Sulfide
-332 -65 -480 -503
0.47 0.16 16.72 1.62
0.21 0.06 7.35 7.12
4 Monel 400
i) NACE Solution
ii) NSW
iii) NSW + 0.064 M Sulfide
iv) NSW + 0.1 M Sulfide
-207
-244
-315
-601
9.53
8.85
1.61
7.98
3.7
3.45
0.63
3.11
Table 11. Potentiostatic Polarization data from SHELL
Solution
S.No. Material Electrolyte E corr (mv)
I corr (A/cm2)
CR (mpy)
1 Mild Steel
i) SHELL Solution ii) SHELL Solution + 0.1 M Sulfide
-705 -715
57.88 37.71
25.46 16.59
2 316L i) SHELL Solution ii) SHELL Solution + 0.1 M Sulfide
-100 -72
1.31 4.81
0.57 2.11
3 317L i) SHELL Solution ii) SHELL Solution + 0.1 M Sulfide
-62 -125
0.34 3.87
0.15 1.7
4 430 i) SHELL Solution ii) SHELL Solution + 0.1 M Sulfide
-150 -215
0.93 1.6
0.41 0.7
5 Monel 400
i) SHELL Solution ii) SHELL Solution + 0.1 M Sulfide
-492 -450
9.96 11.89
3.88 4.64
2 3 0 2
-
Figure 1. Schematic Drawing of Round Tensile SCC Test Sample
Figure 2. Photograph of sheet tensile samples
2 3 0 3
-
Figure 3. Photographs of Cortest Proof Ring (a) Sample set up,
(b) Battery of
Proof Ring under test.
2 3 0 4
-
(4) U-Bend Sample With Bolt and Nut
(5) Final U-Bend Sample
(1) Flat Strip of Sample Piece
(2) Sample Between Roller Fixture and Ram
(3) U-Bend Sample Formation Over Ram
Figure 4. Schematic Diagram of U-Bend Sample Preparation
Stages
2 3 0 5
-
Figure 5. Photographs showing (a) making of a U-Bend sample
through fixture In an Universal Testing Machine, (b) Universal
Testing Machine.
2 3 0 6
-
Figure 6. Photographs of (a) EG&G Potentiostat assembly (b)
Corrosion Cell
2 3 0 7
-
F ig u r e 7 . S C C o f a l lo y s in N A C E S o ln . C o n ta
in in g H y d r o g e n S u lf id e
7 5 8 0 8 5 9 03 0 0
4 0 0
5 0 0
6 0 0
T im e to F a ilu re (H rs .)
0 .2 O f fs e t Y ie ld S tr e n g th %
316, 317, 430, 400, MS 316, 317, 430, 400 316, 317, 430, 400
316, 317
Indicates no failure
400MS 430
2 3 0 8
-
Figure 8. SCC of Alloys in SHELL Solution Containing Hydrogen
Sulfide
75 80 85 90300
400
500
600
0.2 Offset Yield Strength %
Time to Failure (Hrs.)
316, 317, 430, 400, MS 316, 317, 430, 400 316, 317, 430, 400
316, 317, 430, 400
CS
Indicates no failure
2 3 0 9
-
Figure 9. Photograph of U-Bend samples (alloy 316L) stressed to
70%, 80%, 85% & 90% YS exposed to NACE solution containing 0.1
M Na2S. Exposure time 1344 hrs
Figure 10. Photograph of U-Bend samples (alloy 316L) stressed to
70%, 80%, 85% & 90% YS exposed to SHELL solution
containing 0.1 M Na2S. Exposure time 1344 hrs
2 3 1 0
-
Figure 11. Photograph of U-Bend samples (alloy 430) stressed to
70%, 80%, 85% & 90% YS exposed to NACE solution containing 0.1
M Na2S. Exposure time 1344 hrs
2 3 1 1
-
Figure 12. Photograph of U-Bend (alloy 430) stressed to 90% YS
exposed to NACE solution containing 0.1 M Na2S. Exposure time 1344
hrs. (a) Cross section view (b) End view showing pits and
cracks.
2 3 1 2
-
Figure 13. Photograph of U-Bend (alloy 430) stressed to 70%,
80%, 85% & 90%
YS exposed to SHELL solution containing 0.1 M Na2S. Exposure
time 1344 hrs.
2 3 1 3
-
0 5 10Energy (keV)
0
1000
2000
3000
Counts
CO
Fe
PS
Cl Ca Cr Mn
Fe
Fe
Cu
Figure 14. SEM Fractrograph of SCC tested CS sample. a)
Fractrograph
showing intergranular and intragranular fracture modes. b) EDAX
spectrum taken at crack tip.
2 3 1 4
-
0 5 10
Energy (keV)
0
500
1000
Counts
O
S
Cr
Cr
Fe
FeNi
Cu
Cu
Figure 15. SEM Fractrograph of SCC tested 430 alloy sample. (a)
Fractrograph showing intergranular and intragranular fracture
modes, (b) EDAX spectrum taken at a crack tip.
2 3 1 5
-
Figure 16. Potential Polarization Curves (Tafel Plots) Showing
the
Effect of Varied Sulfide Content on 316L. 1-NACE Solution,
2-Natural Seawater, 3-Natural Seawater + 0.06M Sulfide and 4 -
Natural Seawater + 0.1M sulfide.
2 3 1 6
-
Figure 17. Potential Polarization Curves (Tafel Plots) Showing
the Effect of
Varied Sulfide Content on 317L. 1-NACE Solution, 2-Natural
Seawater, 3-Natural Seawater + 0.06M Sulfide and 4 - Natural
Seawater + 0.1M sulfide.
2 3 1 7
-
Figure 18. Potential Polarization Curves (Tafel Plots) Showing
the
Effect of Varied Sulfide Content on 430 alloy. 1-NACE Solution,
2-Natural Seawater, 3-Natural Seawater + 0.06M
Sulfide and 4 - Natural Seawater + 0.1M sulfide.
2 3 1 8
-
Figure 19. Potential Polarization Curves (Tafel Plots) Showing
the Effect of Varied Sulfide Content on Monel 400 alloy. 1-NACE
Solution, 2-Natural Seawater, 3-Natural Seawater + 0.06M Sulfide
and 4 - Natural Seawater + 0.1M sulfide.
2 3 1 9
-
Figure 20. Potential Polarization Curves (Tafel Plots) Showing
the
Effect of Sulfide Content on 316L. 1-SHELL Solution and 2- SHELL
Solution + 0.1M sulfide.
2 3 2 0
-
Figure 21. Potential Polarization Curves (Tafel Plots) Showing
the Effect of
Sulfide Content on 317L. 1-SHELL Solution and 2- SHELL Solution
+ 0.1M sulfide.
2 3 2 1
-
Figure 22. Potential Polarization Curves (Tafel Plots) Showing
the
Effect of Sulfide Content on 430 Alloy. 1-SHELL Solution and 2-
SHELL Solution + 0.1M sulfide.
2 3 2 2
-
Figure 23. Potential Polarization Curves (Tafel Plots) Showing
the Effect of
Sulfide Content on 400 Alloy. 1-SHELL Solution and 2- SHELL
Solution + 0.1M sulfide.
2 3 2 3
-
APPENDIX- 1
SCC FAILURE OF INTERMEDIATE BEARING SUPPORT LOCATION : Main
Seawater Pump, Assir Plant CAUSE : Residual stresses at rim and arm
joint due to improper
manufacturing practice combined with local seawater corrosion.
MATERIAL : Ni-Resist Cast Iron (ASTM - A 493 D2).
Figure 24. SCC Failure Photograph of Intermediate Bearing
Support
2 3 2 4
-
APPENDIX - 2
SCC FAILURE OF SEAWATER INTAKE PIPE COLUMN LOCATION : Seawater
intake system, Shoaiba, Plant Phase-1 CAUSE : Cumulative buildup of
residual stresses at the column inner
surface due to water hammering effect during operation combined
with local seawater corrosion
MATERIAL : Ni-Resist Cast Iron
Figure 25. SCC Failure Photograph of Seawater Intake Pipe
2 3 2 5
-
APPENDIX - 3
SCC FAILURE OF STEAM TURBINE BLADES LOCATION : C-8, Turbine # 81
Blade, Al-Jubail Plant CAUSE : High stress at the pits of the
trailing edges. MATERIAL : 17- 4 PH Stainless Steel a) Photograph
showing pits at trailing edges of the blade. b) Microphotograph
showing transgranular and intergranular failure mode, X 400 Figure
26. SCC Failure Photographs of Steam Turbine Blades
2 3 2 6
-
APPENDIX - 4
SCC FAILURE OF BRINE RECIRCULATING COLUMN LOCATION : Al-Jubail
Plant, Phase-1 CAUSE : Presence of residual stresses due to
improper heat treatment
during fabrication of column pipe. MATERIAL : Ni-Resist Cast
Iron
Figure 27. SCC Failure Photograph of Brine Re-Circulating Column
Pipe
2 3 2 7
-
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2 3 3 0