The behavior of chromium and molybdenum in the propagation process of localized corrosion of steels

In order to clarify the effect of Cr and Mo on the propagation of localized corrosion of steels, the electrochemical behavior of pure chromium and molybdenum and of some stainless steels was studied under conditions similar to those existing inside occluded corrosion cells of steels. Oxygen-free FeCl₂ and/or CrCl₃ solutions were used for experiments. The surface films formed in these conditions were analyzed by means of AES and XPS. Cr is passive if the pH value >ca. 1.8 and Cr may retard the propagation of localized corrosion of steels in its early stage. The hydrolysis of Cr³⁺ may change the pH to even lower values. At such low pH and high Cl^? concentrations, Cr is active in the potential range of interest for localized corrosion of steels. Mo is passive under these more acidic conditions and the passivity of Mo in this potential range is attributed to the formation of a film of MoO₂ (or hydrated oxide), which is extremely thin but very protective. Mo is thus effective to retard the propagation of localized corrosion of steels in its more advanced stage.