Consideration of Repassivation Process Based on the Measurement pH in Crevice and Metal Dissolution Rate of SUS304 Stainless Steel in Artificial Seawater

Abstract

The crevice corrosion test by two-step potentiostatic polarization and the pH in crevice （pH_c） with a semiconductor chemical sensor were simultaneously measured, and the repassivation mechanism for SUS304 stainless steel were investigated.

The total metal dissolution current density （i_metal） of crevice corrosion is shown as the sum of the current density detected externally （i_out） and inner crevice corrosion caused by hydrogen generation reaction（i_in）. When crevice corrosion has progressed sufficiently, there is nice correlation between i_out and pH_c. If there is a lot of i_in, the consumption of H⁺ will increase and it will be easier to repassivate. The repassivation pH was 2.5 to 3.0 in both ASW and 1/10ASW. Current density threshold （10^－5 A・cm^－2） and relationship between pH_c and potential the immobilization potential （ER） obtained from was was 80 mV（SHE） in ASW and 120 mV（SHE） in 1/10ASW, the lower Cl^－ concentration in the external environment, E_R became noble. This is because the ratio of i_in in i_metal is high, the consumption of H⁺ increases, and the effect of diffusion and dilution of anolyte makes it easy to repassivation.