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

Sn-Co electrodeposits alloy of approximate composition 80% Sn-20% Co (wt%) can be obtained from a gluconate bath as single phase CoSn2, which is simil ar in appearance to decorative chromium. The potentiodynamic and cyclic vol tammogram techniques were used to study the corrosion behaviour of CoSn2 in sodium berate solutions (Na2B4O7) at pH = 9.6. The effect of different fac tors such as concentration of berate ions, pH, potential scan rate, success ive cyclic voltammetry, and progressive addition of halide ions (Cl- and Br -) on the electrochemical behaviour of CoSn2 alloys are discussed. The obse rved corrosion resistance of electrodeposited CoSn2 alloy is due to the for mation 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 SnO2 and the third and fourth related to the formation of cobalt o xides. SEM examination confirms that pitting corrosion takes place in prese nce of borax and is increased by adding halide ions. (C) 1999 Elsevier Scie nce B.V. All rights reserved.