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Chromium Passivity in Sulfuric Acid Solution

Passivity of chromium in 0.5 M sulfuric acid has been explored using cyclic voltammetry, chronoamperometry, and electrochemical impedance spectroscopy. The results of potentiostatic measurements have shown that the passive current density is potential-independent, while the thickness of the passive layer increases linearly on the applied potential, which is consistent with the postulation of the Point Defect Model. The Mott-Schottky analysis revealed that the passive film is an n-type semiconductor (related to the outer Cr(OH)3 layer). No evidence for p-type behavior was obtained, indicating that the cation vacancies do not have a significant population density in the film compared with the two donors (cation interstitial and oxygen vacancies). The transpassive state comprise a thick, porous oxide film on the surface, with characteristics consistent with the oxidative ejection of Cr(VI) species from the barrier layer.

Chromium; Passive film; Semiconducting properties; Point defect model

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