The effect of Cu content on chromate conversion coating formation of compositional analogs of the η phase Mg(Zn,xCu)₂

Thin film compositional analogs of the intermetallic compound η, Mg(Zn,xCu)₂ (0 ≤ x ≤ 35 atom %), found in high strength Al-Zn-Mg-Cu alloys were fabricated using a flash evaporation technique for characterization of electrochemical behavior and for chromate conversion coating formation experiments. In potentiodynamic polarization measurements in deaerated 0.1 M NaCl, the open-circuit and breakdown potentials increased and dissolution rates decreased with increasing Cu content of the analogs. When the analogs were chromate conversion coated from a ferricyanide-accelerated coating bath, Raman spectra showed that coating thicknesses decreased with increasing Cu content. Other coating formation experiments showed that ferricyanide additions to the coating bath acted as a coating formation accelerator for MgZn ₂. However, coating formation was so strongly inhibited for the Cu-bearing analogs, that the effect ferricyanide on coating formation was indiscernible.