STRESS-CORROSION CRACKING AND ANODIC-DISSOLUTION OF 316L STAINLESS-STEEL IN HOT LITHIUM HYDROXIDE

The stress corrosion cracking (SCC) of AISI 316L stainless steel has been investigated in a concentrated LiOH solution at 95-degrees-C as a function of the electrochemical corrosion potential (ECP). These data were combined with electrochemical and surface analytical studies of the anodic dissolution of the alloy and its individual constituents (Fe, Cr, Ni, Mo) in the same environment. The results show that the occurrence of SCC as well as the cracking mode are strongly potential-dependent. Type 316L stainless steel was found to be immune to SCC as long as its ECP is in the "primary" passive range. However, SCC does occur when the potential is raised to higher values. The potential ranges in which type 316L stainless steel tends to be susceptible to SCC are associated with a "secondary" passive range and an intermediate secondary active-passive transition zone. The cracking mode also transforms from transgranular SCC (TGSCC) to intergranular SCC (IGSCC) with increasing potential. Electrochemical studies of the anodic dissolution behavior of the pure alloying elements Fe, Cr, Ni, and Mo in the hot LiOH solution, combined with Auger electron spectroscopy (AES) analyses of the passive films formed on type 316L stainless steel in the primary and secondary passive potential range, reveal that selective dissolution of chromium should mainly be responsible for the occurrence of SCC. Depending on the electrochemical potential, the specific anodic dissolution of chromium is thought to initiate primarily at slip steps or at grain boundaries, which directly fixes the crack path.