Film Breakdown on Nickel under Transpassive Dissolution Conditions in Sodium Nitrate Solutions

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Abstract
Film breakdown phenomena leading to transpassive high rate dissolution of nickel in nitrate electrolytes were investigated under controlled flow conditions. Current efficiencies for metal dissolution were determined as a function of current density at different pH, temperature, and nitrate ion concentration. Initiation of film breakdown was investigated by performing experiments at very small dissolution times using polycrystalline and single crystal anodes and observing the resulting surface textures by means of optical and scanning electron microscopes. Results indicate that during high rate nickel dissolution, current efficiency is mostly governed by anode potential and nitrate ion concentration while bulk electrolyte pH and temperature have relatively little effect. Transpassive dissolution is initiated by local breakdown of the passive film. Dissolution takes place at the activated sites from a few well‐defined crystal planes. With increasing time, these sites grow until they merge together resulting in an etched surface microtexture. Similarities between observed film breakdown phenomena leading to high rate dissolution at potentials above those for oxygen evolution and the pitting behavior of passive metals under corrosion conditions are discussed.
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