Passivity breakdown on AISI Type 403 stainless steel in chloride-containing borate buffer solution

Passivity breakdown on AISI Type 403 stainless steel (SS), a commonly employed blade alloy in low pressure steam turbines, has been studied and the data are interpreted in terms of the point defect model (PDM). The near normal distribution in breakdown potential measured in deaerated borate buffer solution (pH = 8.1 ± 0.1) with different chloride concentrations is in satisfactory agreement with the quantitative characterization of the breakdown potential distribution using the PDM. The linear dependence of breakdown potential on the square root of potential scan rate (υ1/2), as predicted by the PDM, yields an estimate of the critical areal concentration of condensed vacancies at the metal/film interface (ξ < 7.0 × 1014 cm−2) that leads to passivity breakdown. This is in excellent agreement with that calculated from the unit cell dimensions of the substrate Fe–Cr alloy (ξ ≈ 1015 cm−2) and the barrier layer oxide (Cr2O3) (ξ ≈ 1014 cm−2) for vacancy condensation on the alloy lattice or on the cation sublattice, respectively, of the film. These provide convincing evidences for the validity of the PDM for modeling passivity breakdown on Type 403 SS.