Corrosion of Sn-Co alloy in alkaline media and the effect of Cl exp - and Br exp - ions

Sn-Co electrodeposits alloy of approximate composition 80% Sn-20% Co (wt%) can be obtained from a gluconate bath as single phase CoSn sub 2 , which is similar in appearance to decorative chromium. The potentiodynamic and cyclic voltammogram techniques were used to study the corrosion behaviour of CoSn sub 2 in sodium borate solutions (Na sub 2 B sub 4 O sub 7 ) at pH=9.6. The effect of different factors such as concentration of borate ions, pH, potential scan rate, successive cyclic voltammetry, and progressive addition of halide ions (Cl exp - and Br exp - ) on the electrochemical behaviour of CoSn sub 2 alloys are discussed. The observed corrosion resistance of electrodeposited CoSn sub 2 alloy is due to the formation of a thin passive film, which is examined by X-ray spectroscopy and believed to be mainly tin and cobalt oxides. The voltammograms involve four anodic peaks, the first and second of which correspond to the formation of SnO and SnO sub 2 and the third and fourth related to the formation of cobalt oxides. SEM examination confirms that pitting corrosion takes place in presence of borax and is increased by adding halide ions.