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

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

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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

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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