The corrosion inhibition mechanism of chromium(VI): Probing the interaction of chromium(VI) with aluminum alloy surfaces

Aluminum alloys used in the manufacture of aircraft are highly susceptible to localized corrosion due to the presence of copper-rich intermetallic particles. These particles act as sites for pit initiation and are catalytic toward oxygen reduction. Chromium(VI) has been found to be an efficient inhibitor of corrosion on these alloys, but the method by which it inhibits is not well understood. Raman spectroscopy is a powerful tool to study the interactions of Cr(VI) with corroding alloy surfaces due to the ability to monitor species in situ as well as image surfaces for specific chemical species.; In order to characterize the surface interactions, the speciation of Cr(VI) in solution and adsorbed to oxide surfaces was probed. Under conditions where most corrosion occurs, Cr(VI) exists predominantly as chromate (CrO ₄−2) and, at lower pH, bichromate (HCrO₄ −). On adsorbed aluminum oxide surfaces, similar behavior was observed with Cr(VI) predominantly in a monomeric state. On copper oxide surfaces, mainly monomer species were adsorbed under service conditions, but polymerization of Cr(VI) to dichromate and higher polymeric Cr(VI) was observed.; Interaction of Cr(VI) with alloy surfaces was probed by initiating corrosion, introducing Cr(VI) solution, and observing the interaction. In situ observation of an active pit following addition of Cr(VI) resulted in the formation of an Al/Cr(VI) mixed oxide near to previously active sites on the aluminum alloy surface. At applied potential where the anodic film on aluminum is subject to breakdown, reduction of Cr(VI) is observed and deposits of a Cr(III)/Cr(VI) mixed oxide were observed. The anodic film acts a barrier and promotes Cr(VI) adsorption. The Al/Cr(VI) appears to deposit at previously active sites of oxygen reduction and provides a concentrated supply of inhibitor.; By separating the oxidation and reduction reactions onto different electrodes, the mechanism by which Cr(VI) inhibits corrosion can be determined. Cr(VI) acts as an inhibitor of both aluminum dissolution and oxygen reduction with the inhibition being highly dependent on the pH. A mechanism for inhibition by Cr(VI) was proposed which contains two specific steps—the adsorption of Cr(VI) to the electrode surface and subsequent reduction to Cr(III) forming an inhibiting layer.