INVESTIGATION OF STRESS-CORROSION SUSCEPTIBILITY OF DUPLEX (ALPHA+GAMMA) 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 E(cc) and the corrosion potential. Also, various factors affecting E(cc) have been investigated to clarify the concept of E(cc) 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 (E(cc) = -470 mV vs. standard colomel electrode (SCE)); however, the susceptibility is induced by anodic polarization, exhibiting a critical cracking potential (E(cc) = -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 E(cc) in the negative direction without any effect on E(oc), E(cc) 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.