Effect of Molybdenum on Pitting Corrosion Resistance of Stainless Steels in Chloride and Bromide Solutions

Abstract

The effect of Mo on the pitting potential, dissolution kinetics, and repassivation behavior of high purity ferritic, Fe-18%Cr-x%Mo, and austenitic, Fe-18%Cr-12-15%Ni-x%Mo, stainless steels was studied in bromide and chloride solutions. Large increases in pitting potential of Fe-18%Cr-x%Mo in chloride solution were found with increased Mo content, compared with distinctly smaller increases in bromide solution. The difference in pitting potentials of ferritic alloys between Fe-18%Cr-2% and 5% Mo in chloride and bromide solutions were in good agreement with the difference in the increased dissolution overvoltages in the solution saturated with dissolved products within artificial pits.
Austenitic stainless steels also showed larger increases in pitting potential in chloride solution than in bromide solution with increased Mo content. In the case of the austenitic steels, the difference in the pitting potentials between in bromide and chloride solutions, however, was not attributable to the difference in dissolution rates or repassivation characteristic in the solution saturated with dissolved products. It is supposed that initiation processes may play an important role in the pitting of austenitic stainless steels.