Environmental Factors Affecting the Critical Potential for Pitting in 18–8 Stainless Steel

TLDR

A critical potential below which pitting of 18–8 stainless steel does not occur in aqueous chloride media has been confirmed. The study examines how inhibition and measurement reproducibility affect the critical potential for pitting. Pitting initiates when chloride concentration locally displaces oxygen from the passive film, breaking passivity. Increasing chloride concentration lowers the critical potential, while other anions, lower temperature, and alkaline pH raise it, indicating competitive adsorption controls the pitting threshold.

Abstract

The concept of a critical potential below which pitting of 18–8 and other passive alloys does not occur in aqueous Cl− media is affirmed. Increasing Cl− concentration shifts the critical potential to more active values. The potential is shifted to more noble values by presence of other anions, e.g., , , , , sufficient concentrations of which act as pitting inhibitors. Lowering of temperature similarly enobles the critical potential. The shift at 0°C exceeds the oxidation‐reduction potential for accounting for resistance of 18–8 to pitting in solutions at ice temperature but not at room temperature. The critical potential is not affected appreciably in the acid pH range; it moves markedly in the noble direction in the alkaline range corresponding to observed resistance to pitting in alkaline Cl− media. These results are interpreted in terms of competitive adsorption of Cl− and other anions for sites on the alloy surface. Only at a sufficiently high surface concentration of Cl− is oxygen, making up the passive film, displaced locally, and passivity thereby destroyed resulting in a pit. The special behavior of inhibition and factors affecting reproducibility of measurements are discussed.