The Dissolution and Passivation of a Single-Crystal Ni50Fe50 Alloy and the Influence of Sulfur Studied by Electron Spectroscopy for Chemical Analysis

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Abstract
Surface composition changes that occur during the dissolution and passivation of a single-crystal Ni50Fe50 (100) alloy were investigated by electron spectroscopy for chemical analysis (ESCA). When the alloy is exposed to sulfuric acid (0.05 M), the nickel concentration on the surface increases from 50 to 85%. This enrichment of nickel on the surface is also found after dissolution of the alloy in the active region. The passive film is formed by a nickel oxide layer on the alloy surface, with nickel and iron hydroxides on the oxide surface. Alloys containing sulfur in solid solution exhibit a completely different electrochemical and corrosion behavior. Sulfur segregates on the surface during dissolution of the alloy. A monolayer of sulfur present on the surface hinders the formation of the passive film. Further accumulation of sulfur on the surface causes the formation of nickel sulfide islands. These sulfide islands grow and cover ∼ 30% of the surface, and the rest of the surface is covered by adsorbed sulfur. The passive film cannot be formed on this surface, and the alloy is actively corroded in the potential range where a sulfur-free alloy is normally passivated.

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