Pitting of stainless steel in bromide, chloride and bromide/chloride solutions

The pitting and localized dissolution characteristics of a high purity Fe-18%Cr-12%Ni stainless steel were studied in bromide, chloride and mixed solutions. The mass transfer rates of dissolution products within artificial pits were measured when a salt layer covered the steel. The mass transfer controlled dissolution was more rapid in the bromide than in the chloride. The overvoltage for active dissolution of the steel was measured in solutions saturated with its dissolved products. The overvoltage was lower in chloride than in bromide solutions by about 180 mV. The concentration at which the active surface repassivated was measured as the concentrated solution adjacent to the surface diffused away. The measured pitting potential increased with an increased ratio of bromide to chloride for the freely exposed steel surface. The lower pitting potential in chloride solutions was attributable to higher rates of dissolution of the steel in concentrated pit solutions, lower mass transfer rates from pits, and lower concentrations at which the pits repassivated. (C) 1999 Elsevier Science Ltd. All rights reserved.