Abstract
Copper was codeposited with Sn in a weakly alkaline bath containing sodium tripolyphosphate as a complexing agent. The effect of plating variables on the composition of electrodeposited Cu-Sn alloys was then examined. The Cu content in the deposit was higher than that in the bath under all operating conditions, and increased with decreasing current density or increasing bath temperature. The results obtained indicate that the Cu-Sn alloy plating from tripolyphosphate baths is of the regular codposition, in which Cu behaves as a noble metal and Sn as a less noble one.
The surface appearance of the alloy deposits varied from copper-colored to golden-yellow, then silvery white with increasing Sn content. Bright and silvery white speculum deposits were obtained by using peptone as an additive. X-ray diffraction measurements have revealed that speculum deposits consist mainly of an η phase, and a phase, which appears and is stable only at elevated temperatures and low current densities. The hardness of the electrodeposited Cu-Sn alloys was high in the range of speculum composition. Speculum coatings had excellent resistance to some kinds of chemical attack and their corrosion resistance was slightly superior to that of bright Ni coating. In conclusion, the electrodeposited speculum seems to be applicable as a substitute for Ni plating.
