Environmentally assisted cracking behavior of duplex stainless steel in concentrated sodium chloride solution

Corrosion Science 42 (2000) 1741±1762 www.elsevier.com/locate/corsci

Environmentally assisted cracking behavior of duplex stainless steel in concentrated sodium chloride solution Wen-Ta Tsai*, Shyan-Liang Chou Department of Materials Science and Engineering, National Cheng Kung University, Tainan, Taiwan Received 7 June 1999; accepted 20 January 2000

Abstract The e€ect of applied potential on the environmentally assisted cracking in 2205 duplex stainless steels (DSSs) in 26 wt% NaCl of pH 2 was investigated. Slow strain rate testing (SSRT) technique was employed. The results showed that the reduction in ultimate tensile strength (UTS), the uniform elongation (UEL) and the reduction of area (RA) varied with the applied potential. A complicated function of potential for the environmentally assisted cracking was found. Stress corrosion cracking occurred in a narrow potential range (ÿ380 to ÿ500 mV) near to corrosion potential. Hydrogen assisted cracking, on the other hand, participated in the fracture process of 2205 DSS under cathodic polarization in this acidic concentrated chloride solution. Fractographical examinations showed that ferrite phase, as compared with the austenite phase, was less resistant to environmentally assisted cracking. 7 2000 Elsevier Science Ltd. All rights reserved. Keywords: 2205 Duplex stainless steel; Environmentally assisted cracking; Stress corrosion cracking; Hydrogen assisted cracking; Applied potential; Slow strain rate testing

1. Introduction Duplex stainless steels (DSSs) containing both ferrite and austenite phases have * Corresponding author. Fax: +88-66-275-4395. E-mail address: [email protected] (W.-T. Tsai). 0010-938X/00/$ - see front matter 7 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 0 - 9 3 8 X ( 0 0 ) 0 0 0 2 9 - 9

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better stress corrosion cracking (SCC) resistance than single phase austenitic stainless steels (SSs) in chloride (Clÿ) containing solution. Generally speaking, the resistance of duplex stainless steel to SCC in chloride solution increases with increasing amount of ferrite phase in the material. This is intimately related to the fact that the yield strength of the ferrite component often exceeds the austenite phase which often re¯ects as a blocking for transgranular cracks in ferrite grains [1,2]. The newly developed duplex stainless steels always contain nitrogen (N) element. It is generally recognized that nitrogen has beni®cal e€ects on improving the mechanical properties and corrosion resistances of stainless steels [3]. Since nitrogen is mainly dissolved in the austenite phase, its presence not only leads to an increase in the stability but also causes the improvement in the strength of the austenite phase [4]. The partitioning of alloying elements in each phase of a duplex stainless steel induces a di€erence in the electrochemical potential between the ferrite and austenite phases. Galvanic corrosion or selective dissolution in one vulnerable phase may thus occur. Yau et al. [5] studied the galvanic corrosion of duplex FeCr±10%Ni alloys in reducing acids and found that preferential attack occurred on the ferrite phase. Fourie et al. [6] observed that the austenite of Fe±21.96Cr± 5.58Ni±2.95Mo±0.15N duplex stainless steel was 20 mV more noble than the ferrite in 1 M NaCl + 1 M H2SO4 solution. Therefore, austenite phase was cathodically protected by the ferrite phase. On the other hand, Sridhar and Kolts [7] found that preferential corrosion of the austenite occurred for the low-N duplex stainless steel (Fe±25.2Cr±11.8Ni±4.01Mo±0.006N). But in high-N duplex stainless steel (Fe±25.6Cr±5.7Ni±3.4Mo±0.17N), they found that corrosion occurred preferentially in the austenite in some environments such as sulfuric and phosphoric acids, while the ferrite was prone to corrosion in other environments such as hydrochloric acid and oxidizing chloride solutions. Whether the di€erence in electrochemical behavior between the two constituent phases a€ects the environmentally assisted cracking (EAC) is of interest. The susceptibility of duplex stainless steels to SCC has been widely investigated recently. Laitinen et al. [8] investigated the SCC of duplex stainless steel (UNS S31803, Fe±22.5Cr±6Ni±2.85Mo±0.17N) in 50 wt% CaCl2 at 1008C. They found that crack growth was mainly through the austenite phase. On the contrary, Jargelius et al. [9] found that austenite rather than ferrite was the crack arrester in duplex stainless steel (Fe±22Cr±5.5Ni±3Mo±0.15N) in various boiling concentrated chloride solutions, namely 43.5% MgCl2, 63% CaCl2 and 50% LiCl solutions. For duplex stainless steel (Fe±25.2Cr±5.9Ni±3.2Mo±0.18N), cracks propagated in a transgranular and quasi-cleavage mode across both the ferrite and austenite phase after constant extension rate test in boiling 35% MgCl2 [10]. These informations indicate that there exist some controversies concerning the e€ect of microstructure on the cracking process in duplex stainless steels. Hence, further investigation on the environmentally assisted cracking behavior of duplex stainless steel is needed. Furthermore, duplex stainless steels may ®nd applications in reducing aqueous environments or be used under cathodic polarization conditions. Under such

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circumstances, the susceptibility to hydrgen assisted cracking (HAC) under straining should be considered. The e€ect of cathodic charging on the change of mechanical properties of duplex stainless steels has been investigated [11±15]. The relative resistance of each phase to HAC, however, has not yet been thoroughly investigated. In this study, the cracking behavior of duplex stainless steel in concentrated 26 wt% NaCl solution of pH 2 under anodic or cathodic polarization is explored. Whether there exist distinct potential ranges for the occurrence of SCC and HAC is investigated. The role of each constituent phase (austenite or ferrite) on the cracking process is also studied. 2. Experimental The material used in this investigation was 2205 duplex stainless steel with its chemical composition shown in Table 1. This material was heat treated at 1373 K (11008C) for 0.5 h then water quenched. The respective microstructures in the transverse and the longitudinal orientations after heat treatmented are shown in Fig. 1(a) and (b), respectively. As can be seen in these micrographs, the austenite phase (bright area) was embedded in the semi-continuous ferrite matrix (grey area). The volume fractions of ferrite and austenite, determined by image analyzer, were 58% and 42%, respectively. Potentiodynamic polarization curve was determined in 26 wt% NaCl solution of pH 2, adjusted by adding dilute HCl solution. The specimen was cut into a square shape and mounted in epoxy resin, with copper wire connected to the rear surface of the specimen. The specimen was then polished with SiC paper to a grit of 600 ®nish, washed in distilled water and rinsed in acetone before testing. The testing solution used was deaerated with nitrogen gas before and during the polarization curve measurement. A saturated calomel electrode (SCE) was used as the reference electrode and a pair of platinum wires were used as the counter electrode. An EG&G Princeton Applied Research Potentiostat/Galvanostat model 273 was used to perform the polarization measurement. The potentiodynamic polarization curve was determined from the cathodic potential towards the anodic direction at a potential scan rate of 1 mV sÿ1. For comparison, the polarization curve of AISI Type 304 stainless steel with its chemical composition listed in Table 1 was also determined. Slow strain rate testing (SSRT) technique was employed to conduct tensile test Table 1 Chemical compositions of 2205 duplex stainless steels and AISI type 304 stainless steel used (wt%)

2205 DSS 304 SS

Fe

Cr

Ni

Mo

C

Si

Mn

Cu

P

S

N

bal. bal.

22.40 18.20

5.42 8.05

3.24 0.10

0.014 0.041

0.41 0.43

1.43 1.54

0.21 0.08

0.025 0.034

0.004 0.005

0.198 0.055

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in deaerated 26 wt% NaCl solution of pH 2 at room temperature. Round bar specimen with a gage length of 24 mm and a gage diameter of 4.0 mm was used. The specimen was ground with SiC paper to 600-grit in longitudinal direction before tensile test. SSRTs were conducted at open circuit potential (OCP) and under various applied potentials condition (from ÿ245 to ÿ1500 mVSCE), at a constant strain rate of 1  10ÿ6 sÿ1. In each test, the solution was bubbled with nitrogen gas before and during the test. At least two tests were conducted for each potential controlled SSRT. Ultimate tensile strength (UTS) and uniform

Fig. 1. Microstructure of 2205 duplex stainless steel. (a) Transverse, (b) longitudinal orientation.

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elongation (UEL) were recorded in each test. After tensile tests, all the specimens were examined under an optical microscope (OM) and a scanning electron microscope (SEM). 3. Results and discussion 3.1. Electrochemical polarization behavior The potentiodynamic polarization curves of both AISI Type 304 stainless steel and 2205 duplex stainless steel in 26 wt% NaCl solution of pH 2 are shown in Fig. 2. It can be clearly seen from this ®gure that the corrosion potential (Ecorr) of

Fig. 2. Potentiodynamic polarization curves for 2205 duplex stainless steel and AISI type 304 stainless steel in 26 wt% NaCl solution of pH 2 at room temperature.

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2205 duplex stainless steel (ÿ395 mVSCE) was slightly higher than that of AISI Type 304 stainless steel (ÿ470 mVSCE). Furthermore, the passive range of the former was much wider than that of the latter in such acidic concentrated chloride solution. It had been reported that increasing N content in the 18Cr±8Ni austenitic stainless steel would result in an expansion in the passive range and a decrease in the passive current density in 1 N H2SO4 + 0.5 M NaCl solution [16]. Besides, the co-existence of Mo and N exerts a synergistic e€ect on improving the pitting corrosion resistance [17,18]. The signi®cant di€erence in the polarization behavior between these two materials, as revealed in Fig. 2, is mainly due to the high N (0.2 wt%) and Mo (3.36 wt%) contents, accompanied by Cr, of the 2205 duplex stainless steel. Besides the superior passivity, the much higher pitting corrosion potential of 2205 duplex stainless steel also indicated that it was more resistant to pitting corrosion than AISI Type 304 stainless steel in the acidic chloride solution. 3.2. Slow strain rate testing The stress vs. percentage of elongation curves of duplex stainless steel obtained in air and in 26 wt% NaCl solution of pH 2 at open circuit potential (OCP) are shown in Fig. 3. The close similarity in the tensile test curves indicate that 2205 duplex stainless steel was resistant to environmentally-assisted cracking even in such concentrated and acidic chloride solution. During tensile test in 26 wt% NaCl solution, the change of potential was monitored. The variation of potential with time (in terms of elongation during SSRT) is also shown in Fig. 3. The result showed that during elastic straining, the open circuit potential was about ÿ175 mVSCE and remained unchanged until the onset of yield. As can be seen in Fig. 3, a sharp decrease in potential occurred when the specimen started to deform plastically. In the testing solution at OCP (ÿ175 mVSCE), passive ®lm would be formed on the specimen surface. In the elastic region, the strain di€erence between the metal and the passive ®lm might be small so that intimate contact was attained. Beyond the yield point of 2205 duplex stainless steel, however, the initially existing passive ®lm began to be broken due to the straining. Hence, a fresh metal surface was created and exposed to the solution. Consequently, a decrease in potential was seen. A plateau potential in the range from ÿ250 to ÿ270 mV was then reached and remained until necking was commenced. The existence of plateau potential was resulted from the compromise between surface passivation and ®lm breakdown. However, since no evidence of environmentally-assisted cracking was observed, the competition between repassivation and ®lm breakdown under plastic deformation condition did not favor crack initiation. As the stress level reached the ultimate tensile strength, the extensive and high plastic deformation rate made the repassivation to become less favorable and slower than the ®lm breakdown rate. As a result, the potential was decreased toward more negative (active) direction till ÿ315 mVSCE at fracture. The e€ect of applied potential on the stress vs. elongation curves of 2205 duplex

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stainless steel in 26 wt% NaCl solution of pH 2 is demonstrated in Fig. 4. The SSRT results indicated that the tensile behavior of 2205 duplex stainless steel was a strong function of the applied potential. Environmentally assisted cracking was found at certain applied potentials. The variations of the ultimate tensile strength (UTS) and the percentage of uniform elongation (UEL), obtained from SSRTs, with applied potential are shown in Fig. 5. At ÿ245 mVSCE, the values of UTS and UEL were 732 MPa and 28%, respectively, very close to those obtained from the air and OCP tests. As can be seen in Fig. 5, a drop in UTS was observed as the potential decreased from ÿ380 to ÿ420 mVSCE. Then the value of UTS raised to the air level as the potential was varied until ÿ750 mVSCE. Below this potential, the value of UTS decreased again with decreasing potential. The potential dependence of UEL was similar to that found for UTS (Fig. 5). The optical macrographs showing the appearances of the fractured specimens after SSRT in air and in 26 wt% NaCl solution of pH 2 at various applied potentials are manifested in Fig. 6. Besides the air test, ductile fractures with notable neckings were found at OCP and at applied potentials of ÿ245, ÿ600 and

Fig. 3. Stress±elongation curves of 2205 duplex stainless steel and the change of open circuit potential during SSRT in 26 wt% NaCl solution of pH 2.

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ÿ750 mVSCE. On the other hand, less amount of necking was found if the applied potential of the specimen was held either in the range from ÿ380 to ÿ500 mVSCE or ÿ900 to ÿ1500 mVSCE. Secondary cracks and indications of environmentally assisted cracking were also seen on the specimen surfaces close to the fractured sites. These macrographical results indicated that 2205 duplex stainless steel was susceptible to environmentally assisted cracking in the acidic 26 wt% NaCl solution, which was potential dependent. The results shown in Figs. 5 and 6 agreed well with each other. As just mentioned, the potential dependence of cracking behavior of 2205 duplex stainless steel was very complicated. This steel was immue to environmentally assisted cracking in the acidic 26 wt% NaCl solution in the passive potential region such as ÿ245 mVSCE. Below this potential till ÿ500 mVSCE, 2205 duplex stainless steel was susceptible to stress corrosion cracking. As the potential fell in the range from ÿ500 to ÿ750 mVSCE, ductile failure without the occurrence of environmentally assisted cracking was again found. However, as

Fig. 4. SSRT stress±elongation curves for 2205 duplex stainless steel in 26 wt% NaCl solution of pH 2 at various applied potentials.

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the potential was further moved toward the cathodic direction (<ÿ900 mVSCE), hydrogen-assisted cracking was observed. The susceptibility of 2205 duplex stainless steel to environmentally assisted cracking with applied potential in 26 wt% NaCl solution of pH 2, as manifested by the change of reduction of area (RA) after SSRT tests, is shown in Fig. 7. The loss of ductility as indicated by a decrease in reduction of area of the specimen (compared with that in air test) was associated with the commencement and propagation of crack. Depending on the potential, however, the forms and causes for the occurrence of environmentally assisted cracking, namely SCC vs. HAC, were di€erent. The e€ect of applied potential of the changes of UTS, UEL and RA values after SSRTs are summarized in Table 2.

Fig. 5. E€ect of applied potential on the changes of UTS and UEL of 2205 duplex stainless steel after SSRT in 26 wt% NaCl solution of pH 2.

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3.3. Fractography Depending on the applied potentials, 2205 duplex stainless steel might be failed in ductile, SCC or HAC mode after SSRT tests in 26 wt% NaCl solution of pH 2 at room temperature. In the following, the occurrence of di€erent form of fracture morphology is described and discussed.

Fig. 6. Macrographs of the fractured specimens of 2205 duplex stainless steel after SSRT in 26 wt% NaCl solution of pH 2 failed at di€erent applied potentials.

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3.3.1. Ductile fracture Ð no environmentally assisted cracking In the passive potential range, 2205 duplex stainless steel was resistant to environmentally assisted cracking in 26 wt% NaCl solution of pH 2 at room temperature. The SEM fractograph for the specimen fractured at OCP is given in Fig. 8. Ductile failure with extensive necking (Fig. 8(a)) was observed. The fractured surface showed that only dimples (Fig. 8(b)) were formed with no evidence of environmental e€ect involved in the fracture process. Similar result was also found at an applied potential of ÿ245 mVSCE. The resistance of 2205 duplex stainless steel to environmentally assisted cracking was attributed to its high repassivation rate when the potential was in the passive region. Even at a high stress state while extensive plastic deformation occurred, the broken passive

Fig. 7. E€ect of applied potential on the change of RA of 2205 duplex stainless steel after SSRT in 26 wt% NaCl solution of pH 2.

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®lm could be repaired at a fast rate. As a consequence, the formation of a crack nucleus was not favored and the environmentally assisted cracking was prohibited. At ÿ600 and ÿ750 mVSCE, the fractured surfaces of 2205 duplex stainless steel after SSRT tests in 26 wt% NaCl solution also revealed ductile mode of failure. As can be seen from the polarization curve (Fig. 2), the specimens were in the cathodic region at these potentials. The absence of environmentally assisted cracking at potentials about 200 mV below the corrosion potential (ÿ395 mVSCE) was not only due to the absence of passive ®lm formation but also due to the low metal dissolution rate in the testing environment. At cathodic potential, hydrogen entry into the steel might happen. But at low cathodic polarization, the amount of hydrogen pick-up was insigni®cant so that hydrogen assisted cracking was not observed during SSRT. 3.3.2. Stress corrosion cracking Fig. 9 shows the SEM fractographs of 2205 duplex stainless steel after SSRT test in 26 wt% NaCl solution of pH 2 at an applied potential of ÿ400 mVSCE. Surprisingly, it was found that the specimen failed in brittle manner with a small amount of area reduction in the fractured site. As shown in Fig. 9(a), secondary cracks were found on the periphery of the specimen. On the fractured surface, two distinct features as represented by the areas marked as A and B were seen on the SEM micrograph. The areas marked as A revealed the typical appearance showing the characteristics of stress corrosion cracking. In other words, these areas are the locations where cracks initiated and propagated. At a high magni®cation, transgranular mode of fractured surface resulting from stress corrosion cracking was seen for area A (see Fig. 9(b)). In area B, on the other hand, dimple mode of ductile failure was observed (Fig. 9(c)), indicating the ®nal fracture without Table 2 E€ect of applied potential on the variations of UTS, UEL and RA after SSRT in 26 wt% NaCl solution (pH 2) at 1  10ÿ6 sÿ1a Potential (mV)

UTS (MPa)

UEL (%)

RA (%)

Remark

Air OCP ÿ245 ÿ380 ÿ400 ÿ420 ÿ500 ÿ600 ÿ750 ÿ900 ÿ1200 ÿ1500

737 732 731 735 729 724 733 736 733 726 687 658

28.9 28.4 28.5 28.4 28.0 25.5 26.4 29.0 27.3 23.2 13.6 7.9

83.0 82.4 82.0 71.2 51.0 60.2 63.2 79.9 68.4 39.2 3.5 3.5

± No EAC No EAC SCC SCC SCC SCC No EAC No EAC HAC HAC HAC

a EAC: environmentally assisted cracking; SCC: stress corrosion cracking; HAC: hydrogen assisted cracking.

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Fig. 8. SEM fractographs of the fractured specimen of 2205 duplex stainless steel after SSRT in 26 wt% NaCl solution of pH 2 at OCP. (a) Macrograph, (b) high magni®cation of (a).

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Fig. 9. SEM fractographs of the fractured specimen of duplex stainless steel after SSRT in 26 wt% NaCl solution of pH 2 and at ÿ400 mV. (a) Macrograph, (b) high magni®cation for area A in (a), (c) high magni®cation for area B in (a).

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environmental e€ect involved. Similar results were found for the specimens tested at applied potentials very close to Ecorr, i.e., in the range from ÿ380 to ÿ500 mVSCE. In this potential range, 2205 duplex stainless steel was susceptible to stress corrosion cracking in acidic 26 wt% NaCl solution. The decreases in UTS, UEL and RA values as depicted in Figs. 5 and 7 were resulted from the occurrence of stress corrosion cracking, in good agreement with the SEM examinations. Indeed, it was surprised to ®nd that stress corrosion cracking occurred in 2205 duplex stainless steel in 26 wt% NaCl solution at room temperature in such a narrow potential range in the vicinity of corrosion potential. The e€ect of applied current on the stress corrosion cracking behavior of a duplex stainless steel (Fe± 25Cr±6.5Ni±3.3Mo±0.5W±0.1N) in NACE solution (5% NaCl + 0.5% acetic acid/1 bar H2S) at room temperature had been investigated by Rhodes et al. [19]. In the current densities range from ÿ200 to + 260 mA/cm2, they found that only the specimens cathodically polarized to ÿ10 and ÿ50 mA/cm2 showed transgranular stress corrosion cracking. At high cathodic current densities and under anodic polarization, no SCC was found. They explained that the detrimental e€ect of mild cathodic polarization was a result of depassivation and subsequent selective corrosion rather than absorption of cathodically charged hydrogen. In this investigation, however, SCC was not only found at mild cathodic but also at small anodic polarizations. It has been recognized that there exists a potential di€erence between ferrite and austenite phases in a duplex stainless steel. [6,20]. For duplex stainless steel (Fe± 21.96Cr±5.58Ni±2.95Mo±0.15N), Fourie et al. [6] found that austenite phase was about 20 mV more noble than ferrite in 1 M NaCl + 1 M H2SO4 solution. Symniotis [20] also found that austenite phase was noble than ferrite for duplex stainless steel Fe±21.9Cr±5.54Ni±3.04Mo±0.14N in 2 M H2SO4+ 0.1 M HCl solution. Therefore, ferrite phase cathodically protected austenite phase in acid chloride containing environments at corrosion potential. Under loading condition and at a potential near Ecorr, preferential dissolution of the less noble phase (ferrite in this case) might occur. This localized preferential dissolution might cause stress concentration and assist crack initiation in the ferrite. Eventually, SCC was motivated and led to ®nal fracture. Moreover, considering the crack propagation behavior, Newman [21] pointed out that the potential-dependent cracking velocity of each phase was di€erent in duplex stainless steel. And since there could be only one electrode potential at the crack tip, the crack propagation was then determined by the most sensitive phase if the potential fell in its optimal ranges for SCC. From the crack initiation point of view, it is thus suggested that there may also exist a prevailing potential range while crack initiation is favored in one speci®c phase of the duplex stainless steel. Fig. 10 demonstrates the cross section micrographs of 2205 duplex stainless steel, taken from the location just beneath the primary fractured surface, after SSRT at ÿ400 mVSCE. Both the micrographs of the transverse (Fig. 10(a)) and longitudinal (Fig. 10(b)) orientations revealed that cracks propagated in the ferrite phase with austenite exhibited a better resistance to cracking. Similar observations were found for the specimens tested at the potential in the range from ÿ380 to

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Fig. 10. Cross section micrograph just below the primary fractured surface, showing the cracks in 2205 duplex stainless steel after SSRT in 26 wt% NaCl solution of pH 2 and at ÿ400 mV. (a) Transverse, (b) longitudinal orientation.

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Fig. 11. SEM fractographs of the fractured specimen of duplex stainless steel after SSRT in 26 wt% NaCl solution of pH 2 and at ÿ900 mV. (a) Macrograph, (b) high magni®cation for area A in (a), (c) high magni®cation for area B in (a).

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ÿ500 mVSCE. These results suggested that ferrite was favorable to crack initiation and propagation in this potential range. 3.3.3. Hydrogen assisted cracking SEM fractographs of the fracture specimen of duplex stainless steel after SSRT in 26 wt% NaCl at ÿ900 mVSCE are shown in Fig. 11. As can be seen in Fig. 11, secondary cracks were observed. At higher magni®cations, brittle cleavage was found on the fractured surface at area A (Fig. 11(b)), while dimples were seen on the surface at area B (Fig. 11(c)). Further examination showed that cracks might also be formed in the interior of the duplex stainless steel specimen. Longitudinal cross section micrograph taking from the fractured specimen after SSRT in 26 wt% NaCl solution at an applied potential of ÿ1500 mV is shown in Fig. 12. Cracks perpendicular to the loading direction were seen inside and close the surface of the specimen. It is important to note that almost all the interior cracks were formed in the ferrite phase. The appearance of the fractured surface under cathodic polarization condition was similar to that found for stress corrosion cracking as depicted in Fig. 9. However, since under deep cathodic polarization condition, hydrogen pick-up became inevitable, thus the fracture was mainly a result of hydrogen assisted cracking. Similar results have been found elsewhere [12,22]. It is generally known that the strength and the modulus of elasticity of ferrite are higher than those of austenite. The resistance of ferrite to hydrogen embrittlement is, on the contrary, inferior to austenite. Therefore, under cathodic polarization in acidic solution, the ferrite would become even more brittle due to hydrogen absorption. In SSRT, although the strains for both ferrite and austenite

Fig. 12. Longitudinal cross section micrograph showing cracks in ferrite phase of 2205 duplex stainless steel after SSRT in 26 wt% NaCl solution of pH 2 and at ÿ1500 mV.

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Fig. 13. Schematic diagram showing the preferential fracture of ferrite in a duplex stainless steel at cathodic polarization condition under SSRT.

in 2205 duplex stainless steel were the same, the stress incurred in ferrite would be much greater than in austenite. As a result, brittle fracture could occur in the hydrogen embrittled ferrite phase if a critical strain was reached. A schematic diagram showing the preferential fracture of ferrite in a duplex stainless steel under cathodic polarization, or hydrogen charging condition is depicted in Fig. 13. In this ®gure, the dashed lines represent the respective stress vs strain curves for the ferrite and austenite phases before hydrogen entry. While the solid lines are for those under cathodic polarization, represented as aH and gH for ferrite and austenite, respectively, revealing a loss of ductility in ferrite and an increase in strength of austenite due to hydrogen solid solution strengthening. Since the strains for both phases are the same during SSRT, the stresses exerted on each phase are di€erent. Once the strain reaches a critical value (ec) where the ultimate tensile stress is exceeded for ferrite phase (with a higher modulus of elasticity), preferential fracture is certainly to take place (see Fig. 12). The loss of ductility of

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2205 duplex stainless steel under high cathodic polarization condition, as revealed in Fig. 7, may also thus be explained. 3.3.4. Potential dependence of environmentally assisted cracking Previous investigation [23] has found that in near neutral 26 wt% NaCl solution at 908C, 2205 duplex stainless steel was immue to stress corrosion cracking with the potential held in the passive region. However, when the potential was held above the pitting corrosion potential in the above solution, pitting assisted stress corrosion cracking would occur. Based on these observations, a schematic diagram illustrating the e€ect of applied potential on the environmentally assisted cracking for 2205 duplex stainless steel is shown in Fig. 14. As indicated in this diagram, 2205 duplex stainless steel is susceptible to environmentally assisted cracking which is strongly dependent on the potential applied. At high anodic polarization condition, pitting corrosion assists the initiation of stress corrosion

Fig. 14. Schematic diagram illustrating the e€ect of applied potential on the environmentally assisted cracking for 2205 duplex stainless steel.

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cracking in chloride solution. On the other hand, the pick-up of hydrogen promotes hydrogen-assisted cracking under deep cathodic polarization condition. Most interestingly, there exists a potential range near corrosion potential where stress corrosion cracking initiation in one vulnerable phase becomes possible. The potenial di€erence between the two consitituent phases may also exert some e€ect on the stress corrosion cracking process. 4. Conclusions The susceptibility to environmentally assisted cracking of 2205 duplex stainless steel in acidic 26 wt% NaCl solution at room temperature was strongly dependent on the applied potential. In the anodic passive region and at OCP, 2205 duplex stainless steel was immue to cracking in the above solution. In the potential range from ÿ380 to ÿ500 mVSCE (near Ecorr), stress corrosion cracking occurred. At applied potentials of ÿ600 and ÿ750 mVSCE (below Ecorr), the material tested was again resistant to environmentally assisted cracking. But below ÿ900 mVSCE, hydrogen assisted cracking was observed. In the situations where stress corrosion cracking or hydrogen assisted cracking took place, ferrite was found more favorable for crack initiation and propagation in 2205 duplex stainless steel. Acknowledgements Financial support by the National Science Council of the Republic of China under Contract No. NSC 85-2216-E-006-026 is gratefully acknowledged. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]

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