Surface Composition of Stainless Steels during Anodic Dissolution and Passivation Studied by ESCA

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Abstract
A molybdenum containing austenitic steel was exposed in hydrochloric acid at various potentials in the active and passive ranges of the alloys. The surface compositions were analyzed by the ESCA technique. The influence of pretreatment by ion bombardment and mechanical polishing on the passivation behavior was investigated. The passive film formed on the surface consists mainly of a mixed Fe‐Cr‐Mo oxide. The average content of Cr3+ in the oxide is about 70%. The inner layers of the oxide product consist mainly of Cr oxide. The Ni content in the oxide is low. The concentration and the chemical state of Mo is potential dependent. At low potentials in the passive range, the four‐valency state is predominant, while at high potentials Mo exists mainly in its six‐valency state. On the surface of the oxide, a layer of hydroxide is present. Chloride ions are incorporated into the passive film. The thickness of the passive film increases with the potential in the passive range from 10 to 15Å. The composition of the metal phase changes during active dissolution. Thus, the alloying elements are enriched on the surface and thereby control the dissolution rate, control overpotentials, and provoke passivation of the alloy.

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