Effect of pre-treatment on corrosion resistance of chromate conversion coated and anodized AA2219 alloy

The study pertains to the effect of prior surface copper removal treatment on the corrosion resistance of chromate conversion coated(CCC) and anodized AA2219 Aluminium-copper alloy. Pre-treatment of test samples was done using AC Polarization in nitric acid to remove the copper rich precipitates from the surface. Pitting resistance of the base metal with and without copper rich phase was evaluated by potentiodynamic polarization test and transmission electron microscopy. Significant improvement in pitting resistance of base metal(T6) was observed after the copper removal. Two layers of organic coatings have been given on chromate conversion coated and normal anodized base metal with and without the copper removal. Zinc rich self etching and strontium chromate based primers are used as under coats. Epoxy based alkyd and polyurethane based finish coats have been used as topcoats. Electrochemical impedance spectroscopy has been used to evaluate the corrosion resistance of the coated surfaces. It has been found that CCC is more effective after the copper removal treatment. Lower corrosion resistance of CCC on base metal without copper removal may be attributed to the heterogeneous microstructure and partial covering of CCC on the intermetallic compounds. The copper removal treatment is not effective in improving corrosion resistance of anodized surfaces which has been attributed to possible consumption of base metal and nature of the growth of the anodic oxide layer.