Dissolution Behavior of Al[sub 2]CuMg (S Phase) in Chloride and Chromate Conversion Coating Solutions

The dissolution rate and Cu surface enrichment of sputter-deposited Al 2 Cu Mg thin films exposed to 0.5 M NaCl solutions and chromate conversion coating solutions has been characterized. Dissolution rates were measured using an electrochemical quartz crystal microbalance, and Cu surface enrichment was characterized by energy-dispersive spectroscopy (EDS). Surface morphology and Volta potential were characterized by scanning probe microscopy. In 0.5 M NaCl solutions, Al 2 Cu Mg dissolves rapidly by dealloying, forming a passivating Cu-rich surface layer. Upon layer breakdown a dissolution rate of about 0.08 μ g ∕ cm 2 s was observed. In chromate conversion coating baths, Al 2 Cu Mg demonstrated a net mass loss rate, which included mass loss from dissolution and mass gain from film formation. Cu surface enrichment was not detected after conversion coating. By comparison, pure Al thin films exhibited a net mass gain under similar conditions. Al 2 Cu Mg samples that were first chromate conversion coated and then exposed to 0.5 M NaCl solution exhibited low dissolution rates, 0.001 μ g ∕ cm 2 s , suggesting that the film formed in the conversion bath was somewhat protective. EDS showed that Cu enrichment occurred on conversion-coated Al 2 Cu Mg during exposure to 0.5 M NaCl, indicating that conversion coatings on Al 2 Cu Mg cannot completely suppress Cu surface enrichment, Cu redistribution, and deposition corrosion phenomena.