IMPEDANCE-SPECTROSCOPY RESPONSE AND BREAKDOWN POTENTIAL OF ALUMINUM-COPPER ALLOYS

The effects of increased Cu additions and heat treatment on the oxide-layer resistance, oxide-layer capacitance, open-circuit potential, and breakdown potential of Al-Cu alloys were evaluated. The addition of Cu to Al lowered the oxide-layer resistance and raised the oxide-layer capacitance in both the solution-treated (550-degrees-C; water-quenched) and artificially aged (30 min at 250-degrees-C) conditions. However, the artificially aged alloys had a lower oxide-layer resistance and a higher oxide-layer capacitance compared to the solution-treated alloys. Open-circuit potentials increased with Cu content. Breakdown potentials became more positive with increasing Cu content beyond 0.5%. Artificial aging at 250-degrees-C for 30 min did not significantly alter the open-circuit potential or the breakdown potential. An increase in aging time between 0.5 and 2 h of an Al-Cu (2%) alloy did lower its oxide-layer resistance, and a peak in the oxide-layer capacitance was observed after an aging time of 2 h. Further aging led to a decrease in the oxide-layer capacitance and a decrease in the total "dc" resistance. In addition, the open-circuit and breakdown potentials became more negative with increased aging time. During anodic dc polarization, Al and the solution-treated alloys exhibited general pitting corrosion, while short-term aged alloys were prone to pronounced grain-boundary attack. The electrochemical response and the type of corrosion attack observed are discussed in terms of the microstructural changes which occur during age hardening.