Electrodeposition of zinc alloys

Electrolytic coatings offer a number of advantages, both in terms of their functional uses (e.g. anticorrosion) and their aesthetic properties. They are used in a wide variety of industries, including the chemical, construction, automotive and aerospace sectors...

In the field of anticorrosion, cadmium coatings and, to a lesser extent, zinc coatings have long been used to protect steels and light or copper alloys. Until the 1970s, cadmium plating was the most widespread solution in all industrial sectors, due to the exceptional qualities of these coatings (excellent corrosion resistance, low contact resistance, mechanical and tribological properties). Today, the use of cadmium-based components is highly regulated, given their recognized toxicity to the environment and to the health of those exposed. Although cadmium is banned from equipment and machinery in many fields, it is still used in certain specific applications in the aeronautics, space and nuclear sectors, which require high levels of safety.

Spurred on by the automotive sector's search for new coatings to provide ten-year warranties against corrosion, research and development efforts have focused on the development of zinc-based co-deposits that perform better than pure zinc coatings. In this context, a great deal of work has been devoted to alloy systems: zinc-nickel, zinc-nickel-iron, zinc-cobalt, zinc-iron, tin-zinc and zinc-manganese. In practice and since 1970, the main electrolyte formulations, available on a highly competitive market, concern zinc-nickel, zinc-cobalt and zinc-iron systems. They enable :

• to produce coatings with corrosion resistance comparable or even superior to that of their pure zinc counterparts;

• achieve good finishes (phosphating, cosmetics) for specific applications.

Moreover, even if the performance of these codeposits is better than, if not equal to, that of their unalloyed zinc counterparts, electroplating with bimetallic electrolytes is more delicate and requires a high level of process expertise, which is often difficult to reconcile with the cost requirements of customers.

This dossier, dedicated to Zn-X coatings, where X = Fe, Co, Ni, Mn, Sn, will deal with electroplating baths, coating characteristics, the hydrogen involved in their development and, finally, their service life.