Evidence for Cu ion formation by dissolution and dealloying the Al₂CuMg intermetallic compound in rotating ring-disk collection experiments

Rotating ring-disk collection experiments and stripping voltammetry have been used to detect Cu ions generated by dissolution of the Al₂CuMg intermetallic compound in aqueous chloride solutions. Cu ions are generated under open-circuit conditions (open-circuit potential -0.930 V_SCE), or by slight anodic or cathodic polarization (±0.050 V from the open-circuit potential). In all cases, Cu ion generation occurs even though the electrode potential is hundreds of millivolts negative of the equilibrium potential, E_Cu. Results show that Cu ion generation is strongly dependent on the degree of solution aeration, indicating a role for Cu oxidants in the liberation process. Copper chloride-complex formation and local surface curvature effects can shift the Cu equilibrium potential in the active direction, but neither phenomenon appears to play a primary role in oxidation of Cu from the intermetallic compound. Rather, the results of this study support the notion that Cu ion generation involves (i) dealloying of the intermetallic compound, (ii) nonfaradaic liberation of mechanically and electrically isolated metallic Cu clusters by physical coarsening of the dealloyed particle, and (iii) oxidation of the electrically isolated Cu clusters. This Cu ion generation mechanism is believed to be a significant factor contributing to the poor corrosion resistance and poor conversion coating characteristics of Al₂CuMg-bearing alloys.