IMPROVEMENT IN ELECTROPLATING OF TIN-ZINC ALLOY COATING

Tin-zinc electrodeposit offers outstanding corrosion protection for steel by combining the barrier protection of tin with the galvanic protection of zinc, without the bulky corrosion product associated with wholly zinc deposit. It has high corrosion resistance, especially against sulfur dioxide and salt water for aircraft parts, automobile components, and equipment used for offshore oilfield applications. Tin-zinc coating was developed as a direct replacement for environmentally unfriendly cadmium coating.  In this publication, the technical improvements achieved in the electrodeposition of tin-zinc alloy coating from a newly developed alkaline non-cyanide bath and the structure-corrosion property of the deposit will be discussed.  The 25 ± 5% zinc (balance tin) alloy coating offers excellent corrosion protection with no obvious white corrosion product. Corrosion test revealed that the deposit offers protection which is better than cadmium, zinc and zinc-nickel alloy coatings for equal thicknesses. The tin-zinc deposit obtained from the stannate/zincate bath is fine grained, semi-bright, and shows considerable improvements over the tin-zinc deposit from the previously used cyanide plating process.  Electron microscopy and X-ray methods reveal microstructural information and structure-corrosion properties of tin-zinc alloy coating and in the context of its potentials for industrial applications.