Raman microscopy of chromate interactions with corroding aluminum alloy 2024-T3

Abstract

Raman microspectroscopy was used to observe actively corroding aluminum alloy 2024-T3 directly in a solution containing NaCl and dilute K 2Cr 2O 7. Raman spectra acquired in and near corrosion pits allowed identification of two products of Cr VI interactions with the corroding alloy. If the alloy potential was fixed negative of the pitting potential, an Al III–Cr VI mixed oxide formed in formerly active or metastable pits. This mixed oxide is similar to a covalent compound which can be made synthetically by adding NaOH to solutions containing Al 3+ and CrO 4 2−. If the alloy potential was held positive of the pitting potential, the corrosion product was primarily a Cr III–Cr VI mixed oxide, formed by partial reduction of Cr VI to Cr III by exposed Al metal or by H 2. At the pitting potential, a mixture of Al III and Cr III mixed oxides was observed, with the Al III species located primarily within pits. A model for the formation mechanism of the mixed oxides is proposed, and the consequences of the findings to corrosion inhibition by Cr VI are considered. In particular, the concentration of Cr VI in and near pits driven by mixed oxide formation may serve to direct the inhibitor from the solution to sites of corrosion.