Copper removal during formation of corrosion resistant alkaline oxide coatings on Al-Cu-Mg alloys

The performance of corrosion resistant inorganic oxide coatings formed on Al-Cu-Mg alloys is often degraded by Cu enrichment that occurs during oxide formation. This is particularly true of coating processes conducted in basic solutions. A modification to an alkaline oxide coating process has been made that simultaneously eliminates Cu enrichment and forms a corrosion resistant coating. In this paper, the modified process is described and the resulting coating morphology, structure and composition are reported. Results from electrochemical and exposure corrosion tests show that useful gains in corrosion resistance are achieved. Cu removal during the modified process is rationalized using an argument based on the increase in Cu solubility that occurs in solutions with a pH greater than the solubility minimum for Cu (9.8), and the effect of Cu complexing by carbonate.