Potentials of Iron, 18-8, and Titanium in Passivating Solutions

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Abstract
Potentials of iron in chromates show a linear relation for , suggesting that they follow the Langmuir adsorption isotherm. Maximum potential change corresponding to an adsorbed monolayer of chromate ions occurs at 0.0025 molar , which approximates the minimum concentration for passivity reported by Robertson. The evidence agrees with a primary mechanism of passivity based on adsorption rather than on oxide film formation. For example, potentials of iron exposed to several organic inhibitors, where the mechanism is undoubtedly one of adsorption, also follow the adsorption isotherm as shown by Hackerman and coworkers. Passivity of 18‐8 stainless steel and titanium in sulfuric acid containing cupric or ferric salts appears similarly to be accompanied by adsorption of Cu++ or Fe+++. The irreversible nature of the potentials is in accord with the view that the adsorbate, in part, is chemisorbed.

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