Zinc Alloy Plating
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 endproduct
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.
Zinc-Iron Plating
Zinc-iron plating produces alloys containing 15 to 25% Fe as-plated. Advantages of this alloy are good weldability and
ductility. It is electroplated on steel coil and strip for auto bodies. Strip for the manufacture of automotive components is
also plated in baths that produce 1% Fe in the alloy deposit; a special feature of this alloy is its suitability for deep black
chromating.
The corrosion resistance of zinc-iron is generally lower than that of the other zinc alloys, especially after exposure to high
temperatures such as those encountered by under-the-hood automotive components. A typical zinc-iron solution
composition used in strip line plating is:

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