Passivation of Stainless Steels in Hydrochloric Acid

High alloyed stainless steels (Fe20Cr20Ni, Fe20Cr20Ni6Mo, and Fe20Cr20Ni6Mo0.2N) were polarized in 0.1 M HCl + 0.4 M NaCl solution for 10 min and 2 h between {minus}75 and 800 mV vs. saturated calomel electrode (SCE). The composition of the passive films was analyzed by angle-resolved X-ray photoelectron spectroscopy, and showed that oxide particles are formed during the initial stage of the passivation process. It is suggested that they are formed by deprotonation of hydroxide. For prolonged exposure, a uniform oxide film is developed under the hydroxide layer. The composition of the film is strongly dependent on the potential. Molybdenum decreases while Cr, Fe, and Ni increase with the potential. The content of Cr in the hydroxide layer decreases with the potential. {beta}-MoCl{sub 2} is identified in the film on the Mo-containing steels. Chloride is present in both the oxide and hydroxide layers. The film forced at {minus}75 mV(SCE) on the Mo-free alloy contains over 50% more Cl{sup {minus}} compared to the film formed on Mo-containing steels. It is suggested that one of the beneficial effects of Mo in stainless steels is that it forms soluble oxochloro complexes, which thereby lower the amount of Cl{sup {minus}} in the film and makesmore » the steel more resistant to pitting.« less