The role of micro pit in the initiaion process of crevice corrosion of SUS304 stainless steel in aqueous chloride solution

On a free surface not having crevice structures, micro pits on the order of several μm in diameter that are repassivated immediately are called meta-stable pits. The role of the micro pit as an initiation site of crevice corrosion was examined by comparing the meta-stable pitting corrosion potential（VcMS）of SUS304 steel measured on the free surface with the critical potential for crevice corrosion（VCREV）. VCREV presented the similar value to VcMS. It was also revealed that micro pits, as well as SUS304 stainless steel surface, were hard to be repasivated at pH≥2.0, meaning that micro pits can be initiation sites of crevice corrosions. Investigating Cl− concentration and pH dependencies on each potential, the influences of these factors were also examined in terms of the contribution to a crevice corrosion initiation. Cl− ions, which have VcMS and VCREV less noble, will directly contribute to a crevice corrosion initiation by facilitating micro pit initiations. pH decrease accelerating a passivation dissolution would promote a crevice corrosion initiation indirectly by increasing a Cl− migration into a crevice structure. © 2021 Japan Society of Corrosion Engineering. All rights reserved.