Electro-Deposition of Tin-Zinc Alloy as a Protective Coating of Ferrous Metals

Abstract

Alloys of tin and zinc are now deposited electrolytically for their high intrinsic resistance to corrosion. The plating of this alloy is intended primarily for the protection of ferrous metals, the protection being considered to be better than with zinc, cadmium or tin alone. According to information and data available on corrosion resistance under salt spray conditions and in cyclic humidity tests, the best protection is obtained from a deposit between the range 75-80 per cent Sn and 25-20 per cent Zn and a coating of 0.0003 in. thickness. Tin-zinc alloy plating has been commercially practised in England and U.S.A. from a bath in which the use of potassium cyanide is essential. Since the cyanide bath is hazardous to operate, investigations are being made of the possibility of replacing the conventional cyanide plating bath by a non-cyanide bath consisting of a mixture of sodium stannate,sodium zincate and excess of caustic soda in solution.Preliminary observations have been completed on the cathode potential current density curves,separately of tin and zinc in the sodium stannate and sodium zincate solutions respectively. The results show the possibility of the co-deposition of the two metals from the proposed electrolyte.Further investigations are in progress to ascertain the influences of various factors, e.g. C.D., pH,concentrations, temperature, etc., on the character and composition of the deposits and to arrive at the best operative conditions for the commercial plating from the bath.