The Role of Micro Pits in the Initiation Process of Crevice Corrosion of SUS304 Stainless Steel in an Aqueous Chloride Solution

Abstract

On free surfaces without crevice structures, micro pits of several micrometers in diameter that are repassivated immediately are called metastable pits. The role of the micro pit as an initiation site of crevice corrosion was examined by comparing the metastable pitting corrosion potential (VAcMS) of SUS304 stainless steel measured on the free surface with the critical potential for crevice corrosion (VCREV). VCREV presented the similar value to VAcMS. It was also revealed that micro pits, as well as SUS304 stainless steel surface, were hard to be repassivated 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 VAcMS and VCREV less noble, will directly contribute to a crevice corrosion initiation by facilitating micro pit initiations. pH decrease accelerating passive dissolution would promote a crevice corrosion initiation indirectly by increasing Cl migration into a crevice structure.