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Effects of loading mode on the critical cracking potential of duplex(α+γ) stainless steel in a hot chloride solution
Scripta METALLURGICA et MATERIALIA
Vol. 29, pp. 423-427, 1993 Printed in the U.S.A.
Pergamon Press Ltd. All rights reserved
EFFECTS OF LOADING MODE ON THE CRITICAL CRACKING POTENTIAL OF DUPLEX(a+'/) STAINLESS STEEL IN A HOT CHLORIDE SOLUTION Hyuk-Sang Kwon* *Dept. of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, 373-1 Kusong-dong, Yusung-gu, Taejon, KOREA, 305-701 (Received March 23, 1993) (Revised May 14, 1993) Introduction
Austenlte-ferrite duplex stainless steels, containing 22-26% Cr, 5-7% Ni and 0.15 to 0.25% N, have been used as structural materials for oil and gas production as well as process systems, chemical and power plants where both high resistance to localized and stress corrosion in chlorides and high strength are required(I-3). The high performances of duplex stainless steels have arisen from an ideal combination of austenite and ferrite phases ; the stainless austenite provides good formability, toughness, weldability and high resistance to hydrogen embrittlement, while the stainless ferrite has an excellent resistance to pitting and stress corrosion in chlorides and high strength. Additionally, nitrogen in duplex stainless steels enhances not only mechanical strength, but also the resistance to pitting corrosion(3). One of the common characteristics in stress corrosion cracking(SCC) between austenitic and ferritic, stainless steels in chloride environments is that cracking occurs at potentials noble to a critical value which has been designated as the critical cracking potential, E= (4,5). For austenitic stainless steels, Ecc is insensitive to prior cold work with or without the generation of martensite and has been interpreted as the minimum potential for crack propagation(6). On the other hand, for low interstitial ferritic stainless steels, E~ is extremely sensitive to microstructural variations induced by small amounts of cold work or grain coarsening(5,7). It has been demonstrated(8) that Ecc for the low interstitial ferritic stainless steels, when it is measured at constant load, is that for crack initiation and is determined by the competing rates of generation of a new surface by slip induced film breakdown and repassivation. However, the physical and/or electrochemical meaning for E~ of duplex stainless steels has not yet been studied. It is the purpose of this work to determine ifE¢¢ for duplex stainless steels is a potential for crack initiation or one for crack propagation in a hot chloride environment and to examine the effects of loading modes on the E~ of these alloys. Materials and Exprimentai Procedures
The material used for this study is a commercial alloy designated as Alloy 255(UNS No. $32550), which was provided by Haynes International Co. in the form of mill annealed(solution treated) sheet 1.70mm thick. Chemical compositions of the alloy are presented in Table 1. Mechanical properties and phase distribution of the alloy are shown in Table 2. SCC tests were conducted in uniaxial tension with the constant load type fixtures and environment cell as described previously(9). Tensile specimens used in SC(' tests were cut parallel to the rolling direction of the sheet with a gage section 20 mm long and 2.5 mm wide. The elongation of the specimen due to creep and crack propagation during a SCC test was recorded as a function of time by measurement of the beam deflection. SCC specimens were loaded to 90% of the yield strength. SCC tests were also conducted with a constant extension rate tester(CERT) in which a strain rate of 2.5 x 10"6/s was applied to specimens with the same size as those used in constant load tests. All
423 0956-716X/93 $6.00 + .00 Copyright (c) 1993 Pergamon Press Ltd.
424
EFFECTS
OF L O A D I N G
MODE
Vol.
29, No.
SCC tests were performed in boiling 35% MgCI2 solution deaerated with nitrogen purging, with loads and/or polarization applied after a stabilized corrosion potential had been achieved. All potentials are referred to the saturated calomel electrode(SCE). The selected fractured surfaces were examined with a scanning electron microscope. Table 1. Chemical Compositions of Alloy 255 Duplex Stainless Steel. Fe Bal.
C 0.02
N 0.18
Cr 25.2
Ni 5.9
Mo 3.2
Cu 1.8
Mn 0.90
Si 0.31
Table 2. Mechanical Properties and Phase Volume Ratio of Alloy 255.
Condition
0.2% Yield Strensth (MPa)
Tensile Strength ~,lPa)
Elongation (%)
AusteniteVolume Percent
Mill Annealed
701
847
27
48
Results and Discussion The influence of applied potential on time to failure has been employed to identify the E~, and characterize the susceptibility to SCC. It is evident in Figure 1 that Alloy 255 is immune to SCC at the corrosion potential(-470 mV, E ~ or under freely corroding conditions but becomes susceptible when anodicaily polarized, exhibiting a critical cracking potential 135 mV noble to the E0¢(-470 mV). Thus, the E~ for the mill annealed alloy is determined to be 335 mV. The large difference in the corrosion and critical cracking potential indicates that Alloy 255 possesses an exceptional resistance to SCC in hot chloride environments. Elongations of specimens undergoing stress corrosion were measured to identify the induction and propagation period of crack. Figure 2 shows an elongation-time curve for mill annealed Alloy 255 polarized to a potential 5 mV noble to the E~ (-335 mV, SCE) in boiling 35% MgCI: and loaded to 90% of the yield strength. Evidently, the elongation initially obeys a logarithmic creep law. The transition of the induction period to the propagation period is revealed by the deviation of the elongation from the logarithmic relationship. As confirmed by metallographic observations in Figure 2, corrosion trenches were formed during the induction period and cracks, developed from the corrosion trenches, propagated to failure at an increasing rate. The formation of corrosion trenches during the induction period results from a localized corrosion that is associated with film breakdown. The film breakdown of stainless steels in a hot chloride solution occurs either by a chemical attack by chloride ions or by mechanical means such as creep or emergence of slip steps. Samples of Alloy 255 exposed to boiling 35% MgCI2 for 50 hours at open circuit or at potential 5 mV noble to E¢~ did not reveal any localized corrosion, indicating that the protective film of Alloy 255 is essentially free of defects capable of functioning as sites for the chemical breakdown due to chloride ion attack. Actually, the film breakdown potential of Alloy255 was measured potentiodynamieally to be -270 mV in deaerated and boiling 35% MgCI2 solution. Thus, the corrosion trenches appear to have been formed by localized corrosion due to slip induced film breakdown. Corrosion within the corrosion trenches is a type of occluded cell corrosion characterized by the well known acidification and chloride ion concentration. It was reported previously(10,11) that the critical value of pH and chloride concentration should be maintained within the occluded cell for pit growth or crack propagation. As the degree of occlusion of a corrosion trench increases due to anodic dissolution at active sites, the acidity and chloride concentration will attain a critical value at which cracks can be initiated from the bottom of the trenches. In this regard, the induction period in Figure 2 seems to be the time taken for the trenches to achieve a critical degree of occlusion for crack initiation and propagation. Cracks, initiated from the bottom of the trenches, propagated in a transgranularquasi cleavage mode across both the ferrite and austenite phases without keying action by either phase, as shown in Figure 3. This agrees well with the previous results(12), demonstrating that, at high applied stress, the stress corrosion cracks in duplex stainless steels propagated transgranularly through both phases.
3
Vol.
29, No.
3
EFFECTS OF LOADING MODE
425
The critical cracking potential of austenitic stainless steels was originally proposed as that for the initiation of stress corrosion cracks(13) ; however, it also was demonstrated that growth of a crack that had been initiated at a potential noble to E¢¢ also was stopped by cathodic polarization to a potential within 5 mV of Etc. Thus, the Ecc of austenitic stainless steels appears, in fact, to be the potential for crack propagation. In contrast to this, the Ec~ of low interstitial ferritic stainless steels was demonstrated to be that for crack initiation and to be more noble than that for crack propagation(8) ; for example, the E,~ for 26Cr-IMo ferritic stainless steel stressed to 90% of the yield strength in boiling 42% LiCl is -485 mV, whereas the minimum potential for crack propagation is -625 mV. If the critical cracking potential of Alloy 255 is that for crack initiation, it follows that cracks initiated by polarization to a potential noble to the E~ should continue to propagate when the applied potential is shifted to a value active to E~. In order to examine this concept, cracks were initiated by polarization to -330 mV, 5 mV noble to E¢c, to establish a well occluded propagating crack. Then, the applied potential was shifted in the active direction to determine the potential arresting crack propagation, and the results were presented in Table 3 and Figure 4. The data in Table 3 demonstrate that cracks initiated at -330 mV continue to propagate to failure at all potentials more noble than approximately -380 mV. Figure 4 shows that growth of cracks was arrested rapidly by cathodic polarization to -380 mV. Thus, the potential, -380 mV, is that for crack propagation or the minimum potential permitting crack propagation for Alloy 255 loaded 90% of the yield strength. The large difference between the Ec~ (-335 mV) and the minimm potential(-380 mV) for crack propagation clearly demonstrated that the E¢~ for duplex stainless steel is that for crack initiation. In this regard, Alloy 255 behaves in the same manner as that of the low interslitial ferritic stainless steels. Table 4. Influence of Potential Shift on Growth of Cracks in Mill Annealed Alloy 255 loaded to 90% of the Yield Strength and Polarized Initially at -330 mV in Boiling 35% MgCI 2 Solution.
Potential Shift (mY, SCE)
Results
-330 -330 ~ -360 -330 --~ -370 -330 ~ -380 -330 --~ -400
Failure Failure Failure No Failure No Failure
The critical cracking potentials mentioned above were measured under a condition of constant load in which the exposure rate of unfilmed surface decreases with time according to the logarithmic creep law. Since the Ecc for mill annealed Alloy 255 is that for crack initiation, it will depend on loading conditions. Figure 5 illustrates the effects of applied potential on the ratio of strain-to-failure of Alloy 255 in boiling 35% MgCl 2 to that in silicone oil measured at a constant strain rate of 2.5xl0"Ns. It is evident from Figure 5 that the strain-to-failure ratio has low values below 0.4 at potentials noble to about -380mV and increases rapidly to a value above 0.8 as the applied potential approaches the minimum potential for crack propagation(-380 mV) that is determined in constant loading tests. Specifically, it was observed that stress corrosion cracks were formed on samples polarized to potentials noble to -380 inV. On the other hand, samples polarized to potentials active to -380 mV were observed to fail in a ductile fracture mode, indicating no occurrence of stress corrosion cracks. Thus, the critical cracking potential of Alloy 255 measured at a constant strain rate is, in fact, -380 mV and corresponds to the minimum potential for crack growth at constant load. From the above discussion, the minimum potential permitting crack growth in duplex alloy is constant at -380 mV irrespective of loading mode. This potential is indeed a repassivation potential for growing cracks.
Conclusions Mill annealed Alloy 255 loaded to 90% of the yield strength in boiling 35% MgCI 2 solution is immune to SCC at the open circuit corrosion potential(Eo¢, -470 mV); however, susceptibility is induced by anodic polarization exhibiting a critical cracking potential(Ec¢, -335 mV) that is 135 mV noble to the corrosion potential. The elongation-time curve of Alloy 255 undergoing stress corrosion clearly exhibits the induction and propagation periods of cracks. The induction period is regarded as a time for localized corrosion cells, activated by slip, to achieve a critical degree of occlusion for crack initiation. E¢¢ for Alloy 255 in a hot chloride solution depends on loading mode. E~(-335 mV) for
426
EFFECTS OF LOADING M O D E
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29, No.
the alloy, measured at constant load, is a potential for crack initiation. In contrast to this, the critical cracking potential measured at constant strain rate is more active than that measured at constant load and corresponds to the minimum potential(-380 mV) for crack propagation at constant load. The potential, -380mV, is indeed a critical potential for the repassivation of growing cracks irrespective of loading mode. Acknowledeements
The author thanks the Korea Science & Engineering Foundation for the financial support under the grant (8810605-003-2). The Alloy 255 was kindly supplied by the Haynes International Corporation. References
1. M. A. Streicher, Metal Progress 128, 29 (1985). 2. James D. Redmond, Chemical Engineering 27, 153 (1986). 3. N. Sridhar, J. Kolts, S. K. Srivastava and A. I. Asphahani, Duplex Stainless Steels, ASM, MetalsPark, OH, 481 (1983). 4. H. H Uhlig, Stress Corrosion Cracking and Hydrogen Embfittlement of Iron Base Alloys, R. W. Staehle, J. Hochmann, R. D. McCright, and J. E. Slater, eds., NACE, Houston, TX, p. 174, (1977). 5. T. W. Mohr, A. R. Troiano and R. F. Hehemann, Corrosion 37, 199 (1981) 6. F. P. Vacarro, R. F. Hehemann, and A. R. Troiano, Corrosion 38, 549 (1982). 7. I. E. Locci, H. S. Kwon, A. R. Troiano, and R. F. Hehemann, Corrosion 43, 465 (1987). 8. H. S. Kwon, R. F. Hehemann and A. R. Troiano, Corrosion 48, 838 (1992). 9. M. Smialowski, and M. Rychik, Corrosion 23, 218 (1967). 10. M. Marek, and R. F. Hochmann, Corrosion 26, 5 (1970). 11. J. C. Scully, Corrosion Sci., 26, 997 (1980). 12. S. Shimodaira, M. Takano, Y. Takizawa, and H. Kamide, Stress Corrosion Cracking and Hydrogen Embrittlement of Iron Base Alloys, R. W. Staehle, J. Hochmann, R. D. McCright, and J. E Slater, eds., NACE, Houston, TX, p. 1003, (1977). 13. H. H. Uhlig, J. Electrochem Soc., 116, 173 (1969).
-250
A -300 LLI O
~ -350,
MgCI2, 125"C 90% Y.S. ill anneal~ o_.~
~
.,,-Ecc
0---4,-
~ -400. w 0IL -450-
o.-a.
--Eo~ -500 10
102
103
104
TIME TO FAILURE (MIN.)
Figurel. Influence of applied potential on time to failure of mill annealed Alloy 255 loaded 90% of the yield strength in boiling 35% MgCI2 at 125°C.
Figure3. SEM fractograph of Alloy 255 loaded 90% of the yield strength and polarized to -330 mV in boiling 35% MgCIr
3
Vol.
29, No.
3
EFFECTS OF LOADING MODE
427
300
, ~ "
Polarization to -380mV
~" 200 Z
O
O
..I
uJ 100
1Annealed CI2, 125°C Y.S. I
I
I
IIIII
I
I
I
I IIIll
10
I
I
I
I
I Ill
10 2
10 .3
T I M E (MIN.)
Figure 2. Elongation - time curve for Alloy 255 stressed 90% of the yield strength in boiling 35% MgC12 at -330mV. Each cross sectional metallograph was obtained from the specimen that was removed from the environment at the stage indicated by an arrow on the curve.
Figure 4. Effect of potential shift to -400 mV on crack propagation of Alloy 255 initially polarized at -330 mV and loaded 90% of the yield strength in boiling 35% MgCI 2 solution.
1.0-
_
Mill Annealed Alloy 255 MgCl 2, 125°C
II
0.8
< 0.s
~Z 0.4
9
1-
0.2-
-3 0
-3 5
-3 o
.3;s
APPLIED POTENTIAL (mV, SCE)
Figure 5. Effects of applied potential on the ratio of strain-to-failure of Alloy 255 in boiling 35% MgCI: to that in silicone oil ; tested at the constant rate of 2.5 × 10-6/s.
Vol. 29, pp. 423-427, 1993 Printed in the U.S.A.
Pergamon Press Ltd. All rights reserved
EFFECTS OF LOADING MODE ON THE CRITICAL CRACKING POTENTIAL OF DUPLEX(a+'/) STAINLESS STEEL IN A HOT CHLORIDE SOLUTION Hyuk-Sang Kwon* *Dept. of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, 373-1 Kusong-dong, Yusung-gu, Taejon, KOREA, 305-701 (Received March 23, 1993) (Revised May 14, 1993) Introduction
Austenlte-ferrite duplex stainless steels, containing 22-26% Cr, 5-7% Ni and 0.15 to 0.25% N, have been used as structural materials for oil and gas production as well as process systems, chemical and power plants where both high resistance to localized and stress corrosion in chlorides and high strength are required(I-3). The high performances of duplex stainless steels have arisen from an ideal combination of austenite and ferrite phases ; the stainless austenite provides good formability, toughness, weldability and high resistance to hydrogen embrittlement, while the stainless ferrite has an excellent resistance to pitting and stress corrosion in chlorides and high strength. Additionally, nitrogen in duplex stainless steels enhances not only mechanical strength, but also the resistance to pitting corrosion(3). One of the common characteristics in stress corrosion cracking(SCC) between austenitic and ferritic, stainless steels in chloride environments is that cracking occurs at potentials noble to a critical value which has been designated as the critical cracking potential, E= (4,5). For austenitic stainless steels, Ecc is insensitive to prior cold work with or without the generation of martensite and has been interpreted as the minimum potential for crack propagation(6). On the other hand, for low interstitial ferritic stainless steels, E~ is extremely sensitive to microstructural variations induced by small amounts of cold work or grain coarsening(5,7). It has been demonstrated(8) that Ecc for the low interstitial ferritic stainless steels, when it is measured at constant load, is that for crack initiation and is determined by the competing rates of generation of a new surface by slip induced film breakdown and repassivation. However, the physical and/or electrochemical meaning for E~ of duplex stainless steels has not yet been studied. It is the purpose of this work to determine ifE¢¢ for duplex stainless steels is a potential for crack initiation or one for crack propagation in a hot chloride environment and to examine the effects of loading modes on the E~ of these alloys. Materials and Exprimentai Procedures
The material used for this study is a commercial alloy designated as Alloy 255(UNS No. $32550), which was provided by Haynes International Co. in the form of mill annealed(solution treated) sheet 1.70mm thick. Chemical compositions of the alloy are presented in Table 1. Mechanical properties and phase distribution of the alloy are shown in Table 2. SCC tests were conducted in uniaxial tension with the constant load type fixtures and environment cell as described previously(9). Tensile specimens used in SC(' tests were cut parallel to the rolling direction of the sheet with a gage section 20 mm long and 2.5 mm wide. The elongation of the specimen due to creep and crack propagation during a SCC test was recorded as a function of time by measurement of the beam deflection. SCC specimens were loaded to 90% of the yield strength. SCC tests were also conducted with a constant extension rate tester(CERT) in which a strain rate of 2.5 x 10"6/s was applied to specimens with the same size as those used in constant load tests. All
423 0956-716X/93 $6.00 + .00 Copyright (c) 1993 Pergamon Press Ltd.
424
EFFECTS
OF L O A D I N G
MODE
Vol.
29, No.
SCC tests were performed in boiling 35% MgCI2 solution deaerated with nitrogen purging, with loads and/or polarization applied after a stabilized corrosion potential had been achieved. All potentials are referred to the saturated calomel electrode(SCE). The selected fractured surfaces were examined with a scanning electron microscope. Table 1. Chemical Compositions of Alloy 255 Duplex Stainless Steel. Fe Bal.
C 0.02
N 0.18
Cr 25.2
Ni 5.9
Mo 3.2
Cu 1.8
Mn 0.90
Si 0.31
Table 2. Mechanical Properties and Phase Volume Ratio of Alloy 255.
Condition
0.2% Yield Strensth (MPa)
Tensile Strength ~,lPa)
Elongation (%)
AusteniteVolume Percent
Mill Annealed
701
847
27
48
Results and Discussion The influence of applied potential on time to failure has been employed to identify the E~, and characterize the susceptibility to SCC. It is evident in Figure 1 that Alloy 255 is immune to SCC at the corrosion potential(-470 mV, E ~ or under freely corroding conditions but becomes susceptible when anodicaily polarized, exhibiting a critical cracking potential 135 mV noble to the E0¢(-470 mV). Thus, the E~ for the mill annealed alloy is determined to be 335 mV. The large difference in the corrosion and critical cracking potential indicates that Alloy 255 possesses an exceptional resistance to SCC in hot chloride environments. Elongations of specimens undergoing stress corrosion were measured to identify the induction and propagation period of crack. Figure 2 shows an elongation-time curve for mill annealed Alloy 255 polarized to a potential 5 mV noble to the E~ (-335 mV, SCE) in boiling 35% MgCI: and loaded to 90% of the yield strength. Evidently, the elongation initially obeys a logarithmic creep law. The transition of the induction period to the propagation period is revealed by the deviation of the elongation from the logarithmic relationship. As confirmed by metallographic observations in Figure 2, corrosion trenches were formed during the induction period and cracks, developed from the corrosion trenches, propagated to failure at an increasing rate. The formation of corrosion trenches during the induction period results from a localized corrosion that is associated with film breakdown. The film breakdown of stainless steels in a hot chloride solution occurs either by a chemical attack by chloride ions or by mechanical means such as creep or emergence of slip steps. Samples of Alloy 255 exposed to boiling 35% MgCI2 for 50 hours at open circuit or at potential 5 mV noble to E¢~ did not reveal any localized corrosion, indicating that the protective film of Alloy 255 is essentially free of defects capable of functioning as sites for the chemical breakdown due to chloride ion attack. Actually, the film breakdown potential of Alloy255 was measured potentiodynamieally to be -270 mV in deaerated and boiling 35% MgCI2 solution. Thus, the corrosion trenches appear to have been formed by localized corrosion due to slip induced film breakdown. Corrosion within the corrosion trenches is a type of occluded cell corrosion characterized by the well known acidification and chloride ion concentration. It was reported previously(10,11) that the critical value of pH and chloride concentration should be maintained within the occluded cell for pit growth or crack propagation. As the degree of occlusion of a corrosion trench increases due to anodic dissolution at active sites, the acidity and chloride concentration will attain a critical value at which cracks can be initiated from the bottom of the trenches. In this regard, the induction period in Figure 2 seems to be the time taken for the trenches to achieve a critical degree of occlusion for crack initiation and propagation. Cracks, initiated from the bottom of the trenches, propagated in a transgranularquasi cleavage mode across both the ferrite and austenite phases without keying action by either phase, as shown in Figure 3. This agrees well with the previous results(12), demonstrating that, at high applied stress, the stress corrosion cracks in duplex stainless steels propagated transgranularly through both phases.
3
Vol.
29, No.
3
EFFECTS OF LOADING MODE
425
The critical cracking potential of austenitic stainless steels was originally proposed as that for the initiation of stress corrosion cracks(13) ; however, it also was demonstrated that growth of a crack that had been initiated at a potential noble to E¢¢ also was stopped by cathodic polarization to a potential within 5 mV of Etc. Thus, the Ecc of austenitic stainless steels appears, in fact, to be the potential for crack propagation. In contrast to this, the Ec~ of low interstitial ferritic stainless steels was demonstrated to be that for crack initiation and to be more noble than that for crack propagation(8) ; for example, the E,~ for 26Cr-IMo ferritic stainless steel stressed to 90% of the yield strength in boiling 42% LiCl is -485 mV, whereas the minimum potential for crack propagation is -625 mV. If the critical cracking potential of Alloy 255 is that for crack initiation, it follows that cracks initiated by polarization to a potential noble to the E~ should continue to propagate when the applied potential is shifted to a value active to E~. In order to examine this concept, cracks were initiated by polarization to -330 mV, 5 mV noble to E¢c, to establish a well occluded propagating crack. Then, the applied potential was shifted in the active direction to determine the potential arresting crack propagation, and the results were presented in Table 3 and Figure 4. The data in Table 3 demonstrate that cracks initiated at -330 mV continue to propagate to failure at all potentials more noble than approximately -380 mV. Figure 4 shows that growth of cracks was arrested rapidly by cathodic polarization to -380 mV. Thus, the potential, -380 mV, is that for crack propagation or the minimum potential permitting crack propagation for Alloy 255 loaded 90% of the yield strength. The large difference between the Ec~ (-335 mV) and the minimm potential(-380 mV) for crack propagation clearly demonstrated that the E¢~ for duplex stainless steel is that for crack initiation. In this regard, Alloy 255 behaves in the same manner as that of the low interslitial ferritic stainless steels. Table 4. Influence of Potential Shift on Growth of Cracks in Mill Annealed Alloy 255 loaded to 90% of the Yield Strength and Polarized Initially at -330 mV in Boiling 35% MgCI 2 Solution.
Potential Shift (mY, SCE)
Results
-330 -330 ~ -360 -330 --~ -370 -330 ~ -380 -330 --~ -400
Failure Failure Failure No Failure No Failure
The critical cracking potentials mentioned above were measured under a condition of constant load in which the exposure rate of unfilmed surface decreases with time according to the logarithmic creep law. Since the Ecc for mill annealed Alloy 255 is that for crack initiation, it will depend on loading conditions. Figure 5 illustrates the effects of applied potential on the ratio of strain-to-failure of Alloy 255 in boiling 35% MgCl 2 to that in silicone oil measured at a constant strain rate of 2.5xl0"Ns. It is evident from Figure 5 that the strain-to-failure ratio has low values below 0.4 at potentials noble to about -380mV and increases rapidly to a value above 0.8 as the applied potential approaches the minimum potential for crack propagation(-380 mV) that is determined in constant loading tests. Specifically, it was observed that stress corrosion cracks were formed on samples polarized to potentials noble to -380 inV. On the other hand, samples polarized to potentials active to -380 mV were observed to fail in a ductile fracture mode, indicating no occurrence of stress corrosion cracks. Thus, the critical cracking potential of Alloy 255 measured at a constant strain rate is, in fact, -380 mV and corresponds to the minimum potential for crack growth at constant load. From the above discussion, the minimum potential permitting crack growth in duplex alloy is constant at -380 mV irrespective of loading mode. This potential is indeed a repassivation potential for growing cracks.
Conclusions Mill annealed Alloy 255 loaded to 90% of the yield strength in boiling 35% MgCI 2 solution is immune to SCC at the open circuit corrosion potential(Eo¢, -470 mV); however, susceptibility is induced by anodic polarization exhibiting a critical cracking potential(Ec¢, -335 mV) that is 135 mV noble to the corrosion potential. The elongation-time curve of Alloy 255 undergoing stress corrosion clearly exhibits the induction and propagation periods of cracks. The induction period is regarded as a time for localized corrosion cells, activated by slip, to achieve a critical degree of occlusion for crack initiation. E¢¢ for Alloy 255 in a hot chloride solution depends on loading mode. E~(-335 mV) for
426
EFFECTS OF LOADING M O D E
Vol.
29, No.
the alloy, measured at constant load, is a potential for crack initiation. In contrast to this, the critical cracking potential measured at constant strain rate is more active than that measured at constant load and corresponds to the minimum potential(-380 mV) for crack propagation at constant load. The potential, -380mV, is indeed a critical potential for the repassivation of growing cracks irrespective of loading mode. Acknowledeements
The author thanks the Korea Science & Engineering Foundation for the financial support under the grant (8810605-003-2). The Alloy 255 was kindly supplied by the Haynes International Corporation. References
1. M. A. Streicher, Metal Progress 128, 29 (1985). 2. James D. Redmond, Chemical Engineering 27, 153 (1986). 3. N. Sridhar, J. Kolts, S. K. Srivastava and A. I. Asphahani, Duplex Stainless Steels, ASM, MetalsPark, OH, 481 (1983). 4. H. H Uhlig, Stress Corrosion Cracking and Hydrogen Embfittlement of Iron Base Alloys, R. W. Staehle, J. Hochmann, R. D. McCright, and J. E. Slater, eds., NACE, Houston, TX, p. 174, (1977). 5. T. W. Mohr, A. R. Troiano and R. F. Hehemann, Corrosion 37, 199 (1981) 6. F. P. Vacarro, R. F. Hehemann, and A. R. Troiano, Corrosion 38, 549 (1982). 7. I. E. Locci, H. S. Kwon, A. R. Troiano, and R. F. Hehemann, Corrosion 43, 465 (1987). 8. H. S. Kwon, R. F. Hehemann and A. R. Troiano, Corrosion 48, 838 (1992). 9. M. Smialowski, and M. Rychik, Corrosion 23, 218 (1967). 10. M. Marek, and R. F. Hochmann, Corrosion 26, 5 (1970). 11. J. C. Scully, Corrosion Sci., 26, 997 (1980). 12. S. Shimodaira, M. Takano, Y. Takizawa, and H. Kamide, Stress Corrosion Cracking and Hydrogen Embrittlement of Iron Base Alloys, R. W. Staehle, J. Hochmann, R. D. McCright, and J. E Slater, eds., NACE, Houston, TX, p. 1003, (1977). 13. H. H. Uhlig, J. Electrochem Soc., 116, 173 (1969).
-250
A -300 LLI O
~ -350,
MgCI2, 125"C 90% Y.S. ill anneal~ o_.~
~
.,,-Ecc
0---4,-
~ -400. w 0IL -450-
o.-a.
--Eo~ -500 10
102
103
104
TIME TO FAILURE (MIN.)
Figurel. Influence of applied potential on time to failure of mill annealed Alloy 255 loaded 90% of the yield strength in boiling 35% MgCI2 at 125°C.
Figure3. SEM fractograph of Alloy 255 loaded 90% of the yield strength and polarized to -330 mV in boiling 35% MgCIr
3
Vol.
29, No.
3
EFFECTS OF LOADING MODE
427
300
, ~ "
Polarization to -380mV
~" 200 Z
O
O
..I
uJ 100
1Annealed CI2, 125°C Y.S. I
I
I
IIIII
I
I
I
I IIIll
10
I
I
I
I
I Ill
10 2
10 .3
T I M E (MIN.)
Figure 2. Elongation - time curve for Alloy 255 stressed 90% of the yield strength in boiling 35% MgC12 at -330mV. Each cross sectional metallograph was obtained from the specimen that was removed from the environment at the stage indicated by an arrow on the curve.
Figure 4. Effect of potential shift to -400 mV on crack propagation of Alloy 255 initially polarized at -330 mV and loaded 90% of the yield strength in boiling 35% MgCI 2 solution.
1.0-
_
Mill Annealed Alloy 255 MgCl 2, 125°C
II
0.8
< 0.s
~Z 0.4
9
1-
0.2-
-3 0
-3 5
-3 o
.3;s
APPLIED POTENTIAL (mV, SCE)
Figure 5. Effects of applied potential on the ratio of strain-to-failure of Alloy 255 in boiling 35% MgCI: to that in silicone oil ; tested at the constant rate of 2.5 × 10-6/s.