Coupling In-Situ Transmission Electron Microscopy and Electrochemical Measurements to Study Pitting Corrosion Initiation Resulting from MnS Dissolution

Abstract

Over the past several decades, intensive research on localized corrosion has allowed a deeper understanding of the origin of pit nucleation and growth in stainless steel exposed to atmospheric conditions. However, the problem of pitting corrosion triggered by dissolution of non-metallic particles (e.g., manganese sulfide “MnS”) within the steel matrix is not fully resolved and requires further study. A remaining question is whether pit formation is controlled by Cr depletion in the surrounding matrix, sulfur contamination of the surrounding matrix, or differences in dissolution rates among MnS particles due to variations in composition. In the present work, we observe MnS dissolution in 304 SS exposed to aqueous solutions with NaCl concentrations from 0.5 M to 3 M. Using real-time in-situ transmission electron microscopy (TEM), coupled with an integrated liquid cell for electrochemical measurements, the mechanisms of MnS and 304 SS matrix dissolution, as well as corrosion product formation during exposure were investigated and explained. An atomistically informed mechanistic model for the reaction of alloy surfaces is developed based on the pit chemistry affected by a dissolved MnS inclusion.