Effect of Molybdenum on the Pitting Potential of High Purity 18% Cr Ferritic Stainless Steels

The anodic behavior of high purity 18% Cr ferritic stainless steels, with a content from 0 to 5% molybdenum, was studied in 1N NaCl and in 1N HCl solutions. The studies were complemented by Auger electron spectroscopy analysis of the films in HCl solutions. It was found that the effect of molybdenum on the pitting potential of these alloys in NaCl solutions could be rationalized assuming that pit initiation is related to localized acidification at the metal-solution interface. The Auger analysis showed a high concentration of chloride ions in the films formed in HCl solutions. No molybdenum enrichment was found in the films. The anodic behavior of these alloys in HCl solutions was related to the formation of a slowly dissolving salt layer of CrCl3 on the alloy surface. The results suggest that molybdenum exerts its influence by reducing the dissolution rate of the salt layer.