INITIATION OF C1SCC CRACKS FROM CREVICES RATHER THAN FROM PITS

摘要

SCC tests were conducted for various austenitic Fe-Cr-Ni stainless steels in neutral chloride solutions to examine ranges for SCC in electrode potential, Cl~- concentration and temperature. The lower limiting potential for SCC is the repassivation potential, E_R, for pits or crevices above which steels can provide dissolving surface for cracks to initiate. The upper one is the potential E_U below which dissolution rate, V, at local anodes is lower than crack growth rate, C. The latter condition V< C has a great role to distinguish crevices from pits as initiation sites for SCC cracks. Crack development from pits is limited only in highly concentrated Cl~-solutions, while that from crevices can be widely observed even in as low as 10 ppm Cl~-solutions. These facts could be attributed to lower rates of dissolution inside crevices contrasted to higher ones at pits, which is well explained based on mass transfer analysis.