Localized Corrosion of Iron in Alkaline Sulfide Solutions: Iron Sulfide Formation and the Breakdown of Passivity

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Abstract
The breakdown of passivity of iron in alkaline sulfide solution is a two‐step process. The first step involves the nucleation of mackinawite patches at faults in the oxide film and their subsequent growth by dissolution of the oxide. The second step is associated with the growth of pits under these patches. At higher positive potentials, mackinawite may be oxidized to and sulfur with simultaneous formation of pyrite. The resistance of passive films to breakdown is affected not only by its thickness but also by its composition (expressed as deviation from stoichiometry). Both are determined by the potential and time of growth. Emphasis was given to the kinetics and mechanism of mackinawite deposition on passive iron, and its relationship with the passivity breakdown process.

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