Role of Copper on Repassivation of Stainless Steel Pits

The effect of copper on the repassivation of pits in stainless steels was examined through potentiodynamic polarization of one-dimensional (1D) pits at two different downward scan rates for 17-4 PH stainless steel. Post-mortem characterization of tested 1D pits revealed that copper enriches on the pit surface, which most likely occurs through reduction of CuCl3 (2-) inside the pit during the downward potential scan. 1D diffusion analysis revealed significant amounts of copper replating can occur before the attainment of critical pit chemistry for repassivation when scanning at a high rate, which complicates repassivation potential (E (rp)) measurements. Copper replating may lead to higher value of measured E (rp) by, (i) reducing the measured net anodic current density, (ii) blocking the dissolution of stainless steel underneath the copper deposits, (iii) enhancing local hydrogen evolution kinetics to raise the pH of the pit bottom. The relative strength of the copper replating effect is controlled by scan rate or more generally, pit growth conditions and local pit chemistry. Implications of the copper replating effect on measuring a lower-bound value of repassivation potentials are discussed in light of existing repassivation potential measurement techniques. The pit growth conditions that could lead to copper replating in real pits are discussed.