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 −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. β‐MoCl2 is identified in the film on the Mo‐containing steels. Chloride is present in both the oxide and hydroxide layers. The film formed at −75 mV(SCE) on the Mo‐free alloy contains over 50% more Cl− 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− in the film and makes the steel more resistant to pitting. © 1999 The Electrochemical Society. All rights reserved.

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A key question arising from studies on the corrosion behavior of molybdenum has been whether the passivation of molybdenum originates from a three-dimensional oxide film or from chemisorbed oxygen. In the present paper an alternative explanation for the apparent passivation behavior of molybdenum is given based on XPS measurements in combination with electrochemical techniques. Below the transpassive potential region, anodic current is obtained only if the sample has been cathodically activated and not after activation by ion etching. The experiments also show that the commonly used cathodic activation is insufficient to reduce the air-formed film. Thus, for such samples the anodic current in the apparent passive region could be due to oxidation of low valent oxides on the surface. However, a transient anodic current is observed also for ion-etched samples that have been subjected to cathodic polarization. In addition, impedance measurements show an unexpectedly large capacitance in the apparent passive region, indicating some kind of adsorption equilibrium at the metal-solution interface. This leads to the suggestion that hydrogen formed in the reduction process can diffuse into the molybdenum metal and cause a limiting oxidation of hydrogen upon positive polarization. Thus, in the apparent passive region the current is mainly due to oxidation of hydrogen and an extension of the transpassive corrosion yields a very low corrosion current for molybdenum. This is in agreement with the XPS experiments, where no more than a monolayer of oxidized molybdenum can be detected in the apparent passive region.

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