Introduction

ZINC ALLOY PLATING has found significant use since about 1980 in Japan and Europe, and more recently in the United States. The driving force behind the development of this technology was the quest for higher-performance coatings, especially in the automotive industry. Another driver was the urgent need to find an adequate replacement for cadmium plating; cadmium is highly toxic and has been banned in many industrial countries.

The use of zinc alloys provides several advantages. Electrochemically, alloys have different corrosion potentials from their alloying elements. Alloys of zinc, for example, can be designed to maintain anodic protection to steel, but remain less electrochemically active than pure zinc. Thus, a zinc alloy coating can still be sacrificial to steel components, but corrodes much more slowly than zinc when exposed to a corrosive environment.

Several zinc alloy processes are currently in commercial use. The choice of a particular process depends on the end-product requirements and conditions of use. Available alloys are zinc-iron, zinc-cobalt, zinc-nickel, and tin-zinc. As in unalloyed zinc plating, chromate conversion coating post-treatments are used to improve the overall corrosion resistance of the alloy, and especially to retard the bulky "white rusting" characteristic of unalloyed zinc. Specialty chromating processes designed to work with these alloys are used for this purpose. See the article "Zinc Plating" in this Volume for more information.

0 0

Post a comment