Salt spray tests were conducted to evaluate the corrosion resistance of Au plating with an underlying metal layer composed of a Cu-Sn alloy plating film with a given alloy composition; the Cu-Sn alloy was deposited from an environmentally friendly sulfosuccinic acid bath containing methionine and a surfactant as additives. The results demonstrated that the Cu-Sn (40-55%Sn) speculum alloy plating drastically improved the corrosion resistance in comparison to conventional bright-Ni plating. Furthermore, the results of studying the mechanism by which the corrosion resistance was improved on the basis of evaluations of the crystal structure and anodic polarization properties attributed the corrosion resistance behavior to the peculiar characteristics of the η phase of the intermetallic compound (Cu_6Sn_5). This enables prompt passivation of the Cu-Sn (40-55%Sn) alloy exposed through the pinholes in the surface Au plating layer.
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