Electrodeposited amorphous alloy coatings present a promising alternative to single metal coatings. Alloy coatings with an amorphous or nanocrystalline structure combine properties, like high hardness and increased wear and corrosion properties, not obtained by coatings of the single alloy components. Apart from the unique properties increasingly strict environmental and safety legislation is an important driver to replace existing single metal plating technology.

Amorphous white-bronze, CuSn(Zn), electroplating was investigated as alternative to Ni plating as undercoat for noble metals in jewellery and PVD coatings on tools. A methanesulphonic acid based bath for CuSn alloy deposition was developed. The relation between alloy composition and the bath composition and the process conditions was established and supported by basic electrochemical and chemical speciation data. The coating composition was optimized to the desired coating properties and the resulting amorphous/crystalline structure can roughly be explained from the Cu-Sn binary phase diagram.

The optimal CuSn composition was determined as 50 wt Cu / 50 wt% Sn giving bright white coatings with attractive appearence. The hardness of the amorphous bronze coatings, 500 HV, is higher than that of Ni, but also that of single Cu or Sn coatings. The corrosion and tarnishing resistance (< 200h NSS) of the white bronze coating outperforms Ni and even white bronze coatings from traditional cyanide based baths (Table 1). Upscaling at an industrial facility shows promising results for application.

Table 1: Comparison of the corrosion and tarnishing resistance of the acid CuSn to standard nickel and cyanide based CuSn. (5 μm coatings on brass as undercoat to 1 μm gold finish)

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