Zinc plating - recent developments and practical considerations

Abstract

Zinc offers very effective sacrificial protection as well as brightness to steel. About 40% of the total zinc produced is used for coating, mostly by hot dip galvanizing. But there is a definite trend towards zinc plating, which has certain advantages like much better control of coating thickness, no ash or dross etc. resulting in considerable saving in zinc. Zinc is electroplated basically from cyanide, alkaline non-cyanaide and acid baths, and each bath has some advantages and disadvantages. However, the emphasis has been shifted towards non-cyanide baths, especially acid chloride baths which give mirror bright coatings. There is approximately 30% savings in running non-cyanide baths compared to cyanide baths, although the cleaning has to be as good as in nickel plating. Recent trends in zinc plating, with special reference to energy, water and metal conservation, high speed and continuous plating for wires, strips and importance of anodes are mentioned in the paper. Electro-deposited alloys of zinc, especially iron-cobalt-zinc developed by the author, and low chromate passivator developed at NML to provide better resistance to corrosion are also highlighted. The importance and placement of zinc anodes and a high speed fluoborate bath suitable for bright zinc plating of steel wires, developed at NML, has been underscored. It is estimated that shares about 4% of the world metal finishing market. Zinc plating is sure to see a major growth in the 21st century.