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°C; water‐quenched) and artificially aged (30 min at 250°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°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.