Pit initiation on stainless steels in 1 M NaCl with and without mechanicalstress

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 mum diam) wi thout inclusion showed no pitting at potentials below that of oxidation evo lution; stress had no effect on the corrosion behavior. Areas containing fi ve round inclusions of about 4 mum in size showed stable pitting at about 4 00 mV; the effect of stress shifted the pitting potential to values that we re 150 mV more negative. Polarization curves measured on large deep MnS inc lusions showed active pitting. Curves of areas with single large, shallow M nS 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 micr ocrevice between the MnS and stainless steel matrix. However, under applied stress, cracks were formed within the shallow MnS inclusion and active pit ting occurred. To explore whether such cracks might serve to generate local ly high concentrations of aggressive species, the pH and chloride concentra tion inside a crack were simulated using a finite difference model. For exp erimental conditions where stable pitting was observed, the simulations pre dicted that the solution composition at the base of a typical 13 mum deep c rack correspond to a pH of around 2, and a chloride concentration of about 6 M led to stable pitting. (C) 2001 The Electrochemical Society.