Characterization of chromate conversion coating on AA7075-T6 aluminum alloy

Chromate conversion coatings (CCCs) on AA7075-T6 were characterized using scanning electron microscopy, focused ion beam sectioning and scanning transmission electron microscopy with nanoelectron dispersive spectroscopy line profiling. The thickness and composition of the CCC was different at different locations on the heterogeneous microstructure of AA7075-T6. The coating formed on the matrix phase was much thicker than that formed on the coarse Al-Cu-Mg, Al-Fe-Cu and Mg-Si intermetallic particles. Nano-electron dispersive spectroscopy line profiling indicated that the coating on the Al-Fe-Cu particles was similar to the CCC formed on the phase matrix, primarily a chromium oxide. However, the coatings on the Al-Cu-Mg and Mg-Si particles were mixed Al/Mg/Cr oxide and Mg oxide, respectively. The growth of CCC followed a linear-logarithmic kinetic rate law. The observations of this study support the sol-gel model of CCC formation. Copyright (C) 2004 John Wiley Sons, Ltd.