Investigation of stress corrosion susceptibility of duplex (α + γ) stainless steel in a hot chloride solution

Abstract

The stress corrosion susceptibility of 25Cr–6Ni–3Mo duplex stainless steel (alloy 255, UNS S32550) in boiling 35% MgCl2 solution has been examined in terms of the relationship between the critical cracking potential Ecc for duplex stainless steel. Mill-annealed alloy 255 loaded to 90% of the yield strength in a boiling MgCl2 solution is immune to stress corrosion cracking (SCC) at the open circuit corrosion potential (Ecc = − 470 mV vs. standard colomel electrode (SCE)); however, the susceptibility is induced by anodic polarization, exhibiting a critical potential (Ecc = − 335 mV vs. SCE) that is 135 mV more positive than the corrosion potential. A small amount of cold work significantly increases the susceptibility to SCC by shifting Ecc in the negative direction without any effect on Eoc. Ecc for mill-annealed alloy 255 is shown to be a potential for crack initiation and varies from −335 to −380 mV vs. SCE, depending on the microstructural and loading conditions. For alloy 255, the minimum potential for crack propagation is constant at −380 mV, irrespective of any cold work and the loading mode. The potential (−380 mV) is indeed a critical potential, capable of passivating growing cracks completely, and is found to correspond to the repassivation potential for growing pits that is determined from the cyclic polarization curve measured at a slow scan rate in the unstressed state.