Dissolution Behavior of Al2CuMg (S Phase) in Chloride and Chromate Conversion Coating Solutions

Abstract

The dissolution rate and Cu surface enrichment of sputter-deposited thin films exposed to 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 NaCl solutions, dissolves rapidly by dealloying, forming a passivating Cu-rich surface layer. Upon layer breakdown a dissolution rate of about was observed. In chromate conversion coating baths, 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. samples that were first chromate conversion coated and then exposed to NaCl solution exhibited low dissolution rates, , suggesting that the film formed in the conversion bath was somewhat protective. EDS showed that Cu enrichment occurred on conversion-coated during exposure to NaCl, indicating that conversion coatings on cannot completely suppress Cu surface enrichment, Cu redistribution, and deposition corrosion phenomena.