Application of EIS to In Situ Characterization of Hydrothermal Sealing of Anodized Aluminum Alloys: Comparison between Hexavalent Chromium-Based Sealing, Hot Water Sealing and Cerium-Based Sealing

Abstract

Chromic acid anodizing has been used for almost a century to enhance corrosion protection of aerospace alloys. For some applications,<br/>hydrothermal sealing in hexavalent chromium-containing solution is required to enhance further the corrosion resistance but, due to<br/>environmental concerns, the use of hexavalent chromium must be discontinued. Good progress has been made to replace chromates<br/>during anodizing but comparatively less effort has focused on the sealing process. In this work, for the first time, electrochemical<br/>impedance spectroscopy (EIS) has been used to characterize in-situ the sealing processes occurring during hot water sealing,<br/>sodium chromate sealing and cerium sealing. The results suggest that the processes occurring during sodium chromate sealing are<br/>significantly different compared to hot water and cerium sealing. In particular, during chromate sealing, the porous skeleton is<br/>significantly attacked, suggesting that the anticorrosion performance is likely to arise from the residuals of chromate rather than from<br/>the improvement of the barrier properties. In contrast, during hot water sealing, little attack occurs on the porous skeleton, and the<br/>improved corrosion protection is due to the enhanced barrier effect. During cerium sealing, precipitation of cerium products occurs,<br/>providing an inhibitor reservoir, and little, if any, attack occurs on the pre-existing oxide.