Abstract
A novel approach is proposed for constructing the adsorption isotherm in corrosion studies, based on a well-known interaction between citric acid and aluminium in alkaline electrolytes. Our approach utilizes the instantaneous impedance measurements via Dynamic Electrochemical Impedance Spectroscopy in galvanostatic mode (g-DEIS). Unlike other common tools, g-DEIS delivers exact information about the concentration required for full surface coverage with adsorbed inhibitor. A significantly larger data set enables observation of multi-stage mechanism of adsorption and complex interaction between citric acid and the metal surface. The obtained impedance results displayed a high convergence with other electrochemical, XPS and SEM studies carried out for verification purposes.It was shown that citric acid exhibits critical concentration with respect to the inhibition efficiency. Chemistry of the adsorbed layer was altered and secondary increase in the corrosion rate was observed in the excess of citric acid, a feature typical for surfactants. Furthermore, authors compared different aluminium alloys with respect to their corrosion resistance mediated by citric acid. The conclusion was reached that regardless of corrosion resistance, the kinetics and mechanism of passive layer formation in Al alloys depends only slightly on alloying additives. AA7020 alloy was an exception because of a specific interaction between zinc and citric acid.
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