Microelectrochemical Measurements of the Dissolution of Single MnS Inclusions, and the Prediction of the Critical Conditions for Pit Initiation on Stainless Steel

A combination of mathematical modeling and experiments on single MnS inclusions was used to investigate the role of MnS inclusions on the initiation of pitting corrosion of 304 stainless steel. Isolation of single MnS inclusions in chloride-containing solutions with use of microcapillaries as electrochemical cells showed that the orientation of the MnS inclusion played a significant role in pit initiation. Large, shallow MnS inclusions failed to initiate pitting corrosion, while the same inclusions, oriented narrow and deep, consistently exhibited the onset of localized corrosion. The formation of a microcrevice was observed between the dissolving MnS inclusion and stainless steel. The microcrevice resulted in a locally occluded region where aggressive ions can accumulate. A mathematical model was developed to explore the local chemistry within a one-dimensional microcrevice of which one wall was MnS and the other was stainless steel. The results of the simulations supported the view of a critical solution chemistry of sulfur species, in a chloride environment, as a possible trigger mechanism for localized corrosion. A critical microcrevice geometry of approximately 1 μm was predicted to be sufficiently large to generate stable pitting in the system under study.