The role of molybdenum additions to austenitic stainless steels in the inhibition of pitting in acid chloride solutions

In order to clarify the mechanism for increased resistance to pitting in acid chloride solutions by addition of Me to CrNi stainless steels, the anodic polarization curves, a.c. electrode impedances, ellipsometric parameters and X-ray photo-electron spectra have been measured on the Mo-containing steels passivated in 1N HCl. The results showed that the presence of an adequate amount of Cr is indispensable for the improvement of pitting resistance by the Mo addition. The passive films of the Mo-containing steels were found to be composed of a complex oxyhydroxide containing Cr3+ Fe2+, Ni2+, Mo6+ and Cl− and showed a rather higher d.c. resistance in HCl solution than in H2SO4 solution. The thickness of the passive film increases with increase in Mo content.