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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 O’Hara 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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1.1 Metal - Environment Interaction 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

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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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1.3

Fractography

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.

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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 “ε” on the outside of the bend is given by the following equation: T ε = ---------- When T 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.

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It is possible that

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

No.

UNS No.

1

Composition (%) Fe

Mild Steel

J2503 Bal

2

316L

3

Cr

0.5

C

Cu

Mn

Si

0.3

1.2

Max Max Max

Max

Max

Max

S31603 Bal

16.0 3.0

11.0

0.02

0.2

1.0

317L

S31703

Bal.

18.5 3.2

13.5

0.02

-

1.0

-

0.08 N

4

904L

N08904

Bal.

20

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

2.5

-

66.5

0.3

Bal.

-

0.5

Monel N4000 400

0.2

Ni

0.25

6

0.5

Mo

Others

4.74 24.5 -

0.6 0.04P,0.04S 1.0 0.04P,0.02S

0.024S

B. Mechanical Properties (Room Temperature) S.No.

Alloy

UNS No.

0.2% Yield Stress (Mpa)

UTS

Elongation(%)

(Mpa)

1 2 3 4

Carbon steel 316L 317L 430

J2503 S31603 S31703 S43000

179 170 216 205

324 485 525 450

30 35 40 28

5

904L

N08904

220

490

35

N08904

172

480

30

6

Monel 400

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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)

1 2 3 4 5

70 70 75 75 85

NFC NFC NFC NFC NFC

6 7 8

85 90 90

NFC NFC NFC

Appearance of surface/cross section NC NC NC NC Brown coloration over the bent portion --DO---DO---DO--

NFC-indicates no first crack beyond 2000 hours.

Remarks

No cracking

--DO---DO---DO--

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

1 2 3 4 5 6 7

70 70 75 75 85 85 90

Time of first appearance of crack (Hours) NFC NFC NFC NFC NFC NFC NFC

8

90

NFC

( % of YS)


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Appearance of surface/cross section Brown coloration over the bent portion --DO

Remarks

-

-


Table 4. U-Bend Specimen Testing of AISI 430 Exposed to NACE Solution Containing 0.1M Na2S S.No.



Appearance of surface/cross section

Remarks

NC NC NC NC

-

1 2 3 4 5

70 70 75 75 85

NFC NFC NFC NFC 1920

6 7

85 90

1920 1344

8

90

1344

Few pits at outer radius --DO--Pitting at few places over the bent radius Moderate pitting at few places over the bent radius


Cracking is prominently associated with pitting over the bent radius when test contd. Beyond 1920 hrs. --DO---





1 2 3 4 5 6 7

70 70 75 75 85 85 90

NFC NFC NFC NFC NFC NFC NFC

8

90

NFC


2299

Appearance of surface/cross section NC NC NC NC NC NC Faint brown coloration over the bent radius --DO--

Remarks

No cracking

--DO--


Table 6. U-Bend Specimen Testing of AISI 316L Exposed to SHELL Solution Containing 0.1M Na2S S.No.

1 2 3 4 5

6 7 8



70 70 75 75 85

NFC NFC NFC NFC NFC

85 NFC 90 NFC 90 NFC NFC-indicates no first crack beyond 2000 hours.


Remarks

NC NC NC NC Faint brown No cracking coloration over the bent radius --DO---DO---DO---DO---DO---DO-NC - indicates No Change




1 2 3 4 5 6 7

70 70 75 75 85 85 90

NFC NFC NFC NFC NFC NFC NFC

8

90

NFC


2300

Appearance of surface/cross section NC NC NC NC NC NC Brown coloration over the bent radius --DO--

Remarks

No cracking --DO--



Applied Stress


Remarks

( % of YS)


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

-



Table 9. U-Bend Specimen Testing of AISI 430L Exposed to SHELL Solution Containing 0.1M Na2S S.No.

Applied Stress


Remarks

( % of YS)


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.

8

90

1920

--DO--


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Hair line cracking distributed all along the bent radius --DO--


Table 10. Potentiostatic Polarization data from NACE Solution and Natural Seawater (NSW). S.No.

E corr

I corr

CR

(mv)

(µ µA/cm2)

(mpy)

i) NACE Solution

-59.6

1.53

0.67

ii) NSW

-216

0.32

0.13

iii) NSW + 0.064 M Sulfide

-348

0.31

0.137

iv) NSW + 0.1 M Sulfide i) NACE Solution ii) NSW iii) NSW + 0.064 M Sulfide iv) NSW + 0.1 M Sulfide i) NACE Solution ii) NSW iii) NSW + 0.064 M Sulfide iv) NSW + 0.1 M Sulfide i) NACE Solution

-389 -212 -261 -383 -392 -332 -65 -480 -503 -207

24.64 0.58 0.31 0.39 15.72 0.47 0.16 16.72 1.62 9.53

10.84 0.26 0.13 0.17 8.17 0.21 0.06 7.35 7.12 3.7

ii) NSW

-244

8.85

3.45

iii) NSW + 0.064 M Sulfide

-315

1.61

0.63

iv) NSW + 0.1 M Sulfide

-601

7.98

3.11

Material

1

316L

2

317L

3

430

4

Monel 400

Electrolyte

Table 11. Potentiostatic Polarization data from SHELL Solution S.No.

Material

1

Mild Steel

2

3

4

5

316L

317L

430

Monel 400

Electrolyte i) SHELL Solution ii) SHELL Solution Sulfide i) SHELL Solution ii) SHELL Solution Sulfide i) SHELL Solution ii) SHELL Solution Sulfide i) SHELL Solution ii) SHELL Solution Sulfide i) SHELL Solution ii) SHELL Solution Sulfide

+ 0.1 M

E corr (mv) -705 -715

I corr (µ µA/cm2) 57.88 37.71

CR (mpy) 25.46 16.59

+ 0.1 M

-100 -72

1.31 4.81

0.57 2.11

+ 0.1 M

-62 -125

0.34 3.87

0.15 1.7

+ 0.1 M

-150 -215

0.93 1.6

0.41 0.7

+ 0.1 M

-492 -450

9.96 11.89

3.88 4.64

2302

Figure 1. Schematic Drawing of Round Tensile SCC Test Sample

Figure 2. Photograph of sheet tensile samples

2303

Figure 3. Photographs of Cortest Proof Ring (a) Sample set up, (b) Battery of Proof Ring under test.

2304

(1) Flat Strip of Sample Piece

(2) Sample Between Roller Fixture and Ram

(3) U-Bend Sample Formation Over Ram

(4) U-Bend Sample With Bolt and Nut

(5) Final U-Bend Sample

Figure 4. Schematic Diagram of U-Bend Sample Preparation Stages

2305

Figure 5. Photographs showing (a) making of a U-Bend sample through fixture In an Universal Testing Machine, (b) Universal Testing Machine.

2306

Figure 6. Photographs of (a) EG&G Potentiostat assembly (b) Corrosion Cell

2307

T im e t o F a ilu r e ( H r s .) 600

316, 317, 430, 400, MS

316, 317, 430, 400

316, 317

316, 317, 430, 400

500

400

400

MS 430 Indicates no failure 300 75

80

85

90

0 .2 O f f s e t Y ie ld S tr e n g th %

F ig u r e 7 . S C C o f a llo y s in N A C E S o ln . C o n t a in in g H y d r o g e n S u lf id e

2308

Time to Failure (Hrs.) 600

316, 317, 430, 400, MS

316, 317, 430, 400

316, 317, 430, 400

316, 317, 430, 400

500

CS

400

Indicates no failure

300 75

80

85

90

0.2 Offset Yield Strength % Figure 8. SCC of Alloys in SHELL Solution Containing Hydrogen Sulfide

2309

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

2310

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

2311

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.

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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.

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Counts Fe

3000

2000

Fe

1000

O C Fe P

S Cl

Ca

Cr Mn

0 0

5

Cu 10 Energy (keV)

Figure 14. SEM Fractrograph of SCC tested CS sample. a) Fractrograph showing intergranular and intragranular fracture modes. b) EDAX spectrum taken at crack tip.

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Counts

Fe

1000

S

500

Fe Cr O

Cr

Ni Cu Cu

0 0

5

10 Energy (keV)

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.

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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.

2316

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.

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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.

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Figure 19. Potential Polarization Curves (Tafel Plots) Showing the Effect of Varied Sulfide Content on Monel 400 alloy. 1NACE Solution, 2-Natural Seawater, 3-Natural Seawater + 0.06M Sulfide and 4 - Natural Seawater + 0.1M sulfide.

2319

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.

2320

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.

2321

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.

2322

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.

2323

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

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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

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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

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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

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