Zinc Alloy Electrodeposition for Corrosion Protection

t is known that the protective value of zinc is partially a result of its ability to form a protective film when plated on a base metal, such as steel. The zinc is sacrificial and corrodes I preferentially to protectthe steel. Although pure zinc coating provides some degree of protection for the substrate, in a severe corrosion environment, zinc is consumed rapidly. For this reason, it is usual practice to increase the thickness of the zinc coating. This is not commercially cost effective and the thick coating causes problems during forming and welding of the steel. Consequently, in the last 20 years, most industries, especiallythe automotive and building industries, began to look for an alternative process. The result was the development of zinc alloy systems, such as zinc-nickel,' zinc-c0balt,2~~~~ zinciron,',@ zinc-manganese,g,1° and the most current process that includes a combination of zinc-cobalt-chromium alloy" and zinc-iron-manganese alloy12 deposits. These alloy systems offer superior corrosion resistance and mechanical properties, weldability, paintability and thinnercoating. Such coatings are always high in zinc content, have less noble metal to maintainanodic protection ofthe steel substrate, and remain less active than plated zinc alone.',*,'3 The zinc in the alloy will still corrode sacrificially to protect steel, but at a much slower rate. The selection of a particular alloy process dependson the app1ication;this may includeease of controlling plating bath composition, the degree of corrosion protection it can provide and, last but not least, the operating cost. For this reason, great attention must be paid to selection of a suitable zinc alloy system. This paper points out some of the advantages and merits of each process and draws attention to their applicability for individual use. Many zinc electroplating processes are being developed for corrosion protection, and they contain various constituents (in addition to the main ions in solution) to control and increasethe life of the electrolyte. This paper reports on the investigation of four types of commercially used zinc alloy processes, namely zinc-nickel, zinc-cobalt,zinc-iron andzinc-manganese. Tables 1 and2offersome examples of zincalloysystems.Theseform the basis of this study and are a guide to the constituents, operating conditions and some of their industrial applications.