COPPER REMOVAL DURING FORMATION OF CORROSION-RESISTANT ALKALINE OXIDECOATINGS 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 dur ing oxide formation. This is particularly true of coating processes co nducted in basic solutions. A modification to an alkaline oxide coatin g process has been made that simultaneously eliminates Cu enrichment a nd forms a corrosion resistant coating. In this paper, the modified pr ocess is described and the resulting coating morphology, structure and composition are reported. Results from electrochemical and exposure c orrosion tests show that useful gains in corrosion resistance are achi eved. Cu removal during the modified process is rationalized using an argument based on the increase in Cu solubility that occurs in solutio ns with a pH greater than the solubility minimum for Cu (9.8), and the effect of Cu complexing by carbonate.