Electrodeposition and corrosion resistance of nanocrystalline white bronze (CuSn) coatings

For jewellery applications electroplated white bronze (CuSn) was investigated as undercoating for noble metal finishes as alternative to nickel. A strongly acidic plating bath was developed with an organic additive to suppress hydrogen evolution and obtain bright coatings. An electrochemical study of the bath shows that the additive strongly inhibits the hydrogen evolution reaction around the equilibrium potential for Sn deposition. Whereas at more anodic potentials Sn UPD is observed, at more cathodic potentials mass transfer controlled CuSn deposition takes place with 95% current efficieny. Under these conditions bright nanocrystalline CuSn alloy coatings with 0 - 50 % Sn are formed. The corrosion resistance of gold finished jewellery items with a 5 um 50% Sn bronze undercoating was compared to that with a 5 um nickel undercoating. Various accelerated corrosion tests show that both the first onset of corrosion and the corrosion propagation are equal or slower for bronze compared to nickel. The bronze coating prevents tarnishing in a thioacetemide test for more than 144 hours whereas with a nickel undercoating tarnishing is seen after 8 hours. EIS analysis indicates slower coating degradation with a CuSn undercoating when abraded in artificial sweat actual jewellery items coated in an industrial pilot bath gave similar promising results.