Pit Initiation on Stainless Steels in 1 M NaCl With and Without Mechanical Stress

Abstract

The onset of pitting corrosion at MnS inclusions on 304 stainless steel in 1 M NaCl was studied with and without applied mechanical stress with use of microelectrochemical cells. Polarization curves of areas (100 μm diam) without inclusion showed no pitting at potentials below that of oxidation evolution; stress had no effect on the corrosion behavior. Areas containing five round inclusions of about 4 μm in size showed stable pitting at about 400 mV; the effect of stress shifted the pitting potential to values that were 150 mV more negative. Polarization curves measured on large deep MnS inclusions showed active pitting. Curves of areas with single large, shallow MnS inclusions showed multiple current transients during dissolution of the inclusion without stress, but the metastable events did not initiate stable pitting. The dissolution of shallow MnS inclusion did not form a deep microcrevice between the MnS and stainless steel matrix. However, under applied stress, cracks were formed within the shallow MnS inclusion and active pitting occurred. To explore whether such cracks might serve to generate locally high concentrations of aggressive species, the pH and chloride concentration inside a crack were simulated using a finite difference model. For experimental conditions where stable pitting was observed, the simulations predicted that the solution composition at the base of a typical 13 μm deep crack correspond to a pH of around 2, and a chloride concentration of about 6 M led to stable pitting. © 2001 The Electrochemical Society. All rights reserved.