Roles of crevice corrosion on stress corrosion cracking for austenitic stainless steels in neutral chloride solutions

Abstract

Corrosion cracks initiated in the potential zones, zone I (active range) and zone III (pitting region), in 1N H2SO4 solution containing 0.5M NaCl at 25°C under pulsating load to promote crack initiation. Based on these facts the following conditions were found to be required for the cracks to initiate : a) cracks must occur at a dissolving surface with {100} faceting, and b) crack growth rate, Ċ, is faster than the dissolution rate, Ih, at the surface. In practical neutral solutions, the dissolving surface specified in the conditon (a) consists of large growing pits or a corroding crevice, depending on the Cl- concentration. The conditions (a) and (b) were also confirmed to apply to SCC initiation from pits under static stress for type 304 steel in 25% MgCl2 solution at 80°C. A circumferential notch, introduced into a round type 310 S steel bar as both corrosion crevice and stress intensifier, extended the SCC range to both less noble potential region and lower concentration range of MgCl2 solutions at 80°C when compared to SCC initiated from pits on a smooth specimen. By decreasing the bulk Cl- concentration, it becomes difficult for cracks to start at pits where Ih markedly increases. However, as Ih in crevice is lower than in pits and the condition (b) is readily satisfied with a lower dissolution rate Ih