Cyclic stress effect on stress corrosion cracking of duplex stainless steel in chloride and caustic solutions
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Duplex stainless steel (DSS) is a dual-phase material with approximately equal volume amount of austenite and ferrite. It has both great mechanical properties (good ductility and high tensile/fatigue strength) and excellent corrosion resistance due to the mixture of the two phases. Cyclic loadings with high stress level and low frequency are experienced by many structures. However, the existing study on corrosion fatigue (CF) study of various metallic materials has mainly concentrated on relatively high frequency range. No systematic study has been done to understand the ultra-low frequency (10-5 Hz) cyclic loading effect on stress corrosion cracking (SCC) of DSSs. In this study, the ultra-low frequency cyclic loading effect on SCC of DSS 2205 was studied in acidified sodium chloride and caustic white liquor (WL) solutions. The research work focused on the environmental effect on SCC of DSS 2205, the cyclic stress effect on strain accumulation behavior of DSS 2205, and the combined environmental and cyclic stress effect on the stress corrosion crack initiation of DSS 2205 in the above environments. Potentiodynamic polarization tests were performed to investigate the electrochemical behavior of DSS 2205 in acidic NaCl solution. Series of slow strain rate tests (SSRTs) at different applied potential values were conducted to reveal the optimum applied potential value for SCC to happen. Room temperature static and cyclic creep tests were performed in air to illustrate the strain accumulation effect of cyclic stresses. Test results showed that cyclic loading could enhance strain accumulation in DSS 2205 compared to static loading. Moreover, the strain accumulation behavior of DSS 2205 was found to be controlled by the two phases of DSS 2205 with different crystal structures. The B.C.C. ferrite phase enhanced strain accumulation due to extensive cross-slips of the dislocations, whereas the F.C.C. austenite phase resisted strain accumulation due to cyclic strain hardening. Cyclic SSRTs were performed under the conditions that SCC occurs in sodium chloride and WL solutions. Test results show that cyclic stress facilitated crack initiations in DSS 2205. Stress corrosion cracks initiated from the intermetallic precipitates in acidic chloride environment, and the cracks initiated from austenite phase in WL environment. Cold-working has been found to retard the crack initiations induced by cyclic stresses.