Effects of molybdenum on the pitting of ferritic- and austenitic-stainless steels in bromide and chloride solutions

Abstract

The effect of Mo on the pitting potential, dissolution kinetics, and repassivation behavior of high purity ferritic stainless steels, Fe–18%Cr– x%Mo, and austenitic stainless steels, Fe–18%Cr–12–15%Ni– x%Mo, was studied in solutions of bromide and of chloride. Large increases in the pitting potential of Fe–18%Cr– x%Mo in chloride solution were found with increasing Mo content, compared with the distinctly smaller increases in bromide solution. There was an excellent correlation between the increase in pitting potentials of ferritic alloys between Fe–18%Cr–2% and 5%Mo in the chloride and bromide solutions with the increased dissolution overvoltages in the saturated solution of dissolved products within artificial pits. Austenitic stainless steels also showed larger increases in pitting potential in chloride solution than in bromide solution. Higher pitting potentials were recorded for the austenitic than the ferrite steels with the same Mo content. For austenitic steels, the pitting potentials in bromide and chloride solutions were not attributable solely to the difference in dissolution rates or repassivation characteristics in saturated solution of the dissolved products. It was concluded that initiation processes play an important role in the pitting of austenitic stainless steels.