Zinc coating of steel components has been thus far considered the most economical and viable industrial finishing process for steel, where sacrificial type corrosion resistance is required. For most applications, zinc finishes afford from 24 hours to “white” rust and up to 240 hours to “red rust” in accelerated neutral salt spray testing, depending on zinc thickness, type of chromate and availability of organic top coat.
Zinc-Iron The process produces zinc alloys containing l5-25% iron. The alloy has good weldability, workability and can be adapted to commercially electroplated strip steel. The alloy composition and process can be varied to enhance weldability or adhesion of electropainting processes. Black chromating is the most suitable for this type of alloy.
Zinc-Cobalt Commercially available processes are similar to low ammonia or ammonia free acid chloride zinc baths. Some newer baths operate on the alkaline side. The deposit contains from 1-3% cobalt. The acid type bath has a higher cathode efficiency, and reduced hydrogen embrittlement, but its plating thickness distribution varies substantially between low and high current density areas. Table X – Typical bath composition. Chromate conversion coatings in iridescent, black and yellow are available.
Zinc-Nickel There are two types of zinc-nickel processes currently available commercially. Acid and alkaline non cyanide types. Alloys deposited contain from 5 to 15% nickel. Corrosion resistance improves with nickel content up to 15-18% but the deposit becomes more noble and loses its sacrificial protection property. Chromate film formation was found to be at optimum in the 5-10% nickel content range. Above this range the deposit tends to be passive and chromating becomes very difficult.