Raman microscopy of chromate interactions with corroding aluminum alloy 2024-T3
By Seman, Michael; Marino, Joey; Yang, Wenli & Wolden, Colin A.
Published in Corrosion Science
2004
Abstract
Raman microspectroscopy was used to observe actively corroding aluminum alloy 2024-T3 directly in a solution containing NaCl and dilute K2Cr2O7. Raman spectra acquired in and near corrosion pits allowed identification of two products of CrVI interactions with the corroding alloy. If the alloy potential was fixed negative of the pitting potential, an AlIII–CrVI 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 Al3+ and CrO42-. If the alloy potential was held positive of the pitting potential, the corrosion product was primarily a CrIII–CrVI mixed oxide, formed by partial reduction of CrVI to CrIII by exposed Al metal or by H2. At the pitting potential, a mixture of AlIII and CrIII mixed oxides was observed, with the AlIII 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 CrVI are considered. In particular, the concentration of CrVI in and near pits driven by mixed oxide formation may serve to direct the inhibitor from the solution to sites of corrosion.