Micro-Electrochemical Aspects of the Effects of Temperature on Pit Initiation at MnS Inclusion in Type 304 Stainless Steel

Abstract

In 1 M NaCl, the pitting potentials of resulfurized Type 304 stainless steel decreased as the temperature increased from 25 °C to 75 °C. The dissolution potential of MnS in 0.1 M Na2SO4 also decreased with increase in temperature. As the macroscale pitting potentials at all temperatures ranged within the dissolution potential range of MnS, the decrease in MnS dissolution potential was critical in determining the decrease in pitting potentials at high temperatures. However, in 1 M NaCl, the microscale pitting potentials exceeded those of the macroscale pitting: pits were generated at the boundaries of the MnS and steel matrix at 25 and 50 °C, but at 75 °C, pitting occurred in a deeper portion of the MnS inclusion. The formation of an occluded geometry is essential for pitting; however, because the dissolution rate and potential of MnS vary with temperature, the shape and location of the occluded geometry at inclusions as well are expected to vary with temperature. The depassivation pH of Type 304 stainless steel in 1 M NaCl–10 mM Na2S2O3 increased from 2 to approximately 3 at temperatures from 25 °C to 75 °C. The dissolved MnS species appeared to significantly influence the decrease in pitting potential.