Zinc Alloy Deposits

We are considering zinc-alloy-plated deposits, to improve corrosion resistance in our fasteners. Is this the right approach?

Zinc alloy processes will improve the salt spray resistance of a fastener over the same thickness of a pure zinc deposit, because by alloying the zinc with a more noble metal, the activity of the zinc is reduced, while still maintaining "cathodic" protection of the steel. Whether substitution of pure zinc plating with alloys such as zinc-cobalt, zinc-nickel, zinc-tin or zinc-iron is the right answer must be determined on an individual basis. All of these alloy processes are more difficult to control than pure-zinc plating and are, therefore, more expensive (1.5 to 2 times the cost).

Remember that the corrosion resistance of zinc is almost directly related to the thickness, while the cost of zinc plating is not. You can, therefore, also get improved corrosion resistance by simply applying more zinc, without significantly affecting the cost, though dimensional tolerance then becomes a factor.

Is there a difference in corrosion resistance of zinc deposits obtained from the various types of plating processes, such as cyanide, alkaline non-cyanide and chlorine?

The generally accepted answer to this question is that zinc affords protection by galvanic action and that the amount of galvanic protection is directly related to the amount of corrodable metal present (i.e., thickness). In theory, the answer is "No," there is no difference. If your part is difficult to rinse, however, you may find reduced corrosion resistance caused by trapped chlorides from the chloride process. Such is not the case with the other two processes you mention.

Should I specify whether my parts are to be plated in a certain type of zinc bath?

It depends on your needs. (See above questions, for example.) In one recent case, a company, manufacturing a part needing low electrical-contact resistance, discovered that parts plated in a chloride process failed to complete a low-amperage circuit, while the exact same parts, plated in an alkaline, non-cyanide process were successful. If coverage is important (deep recesses, drilled holes), the chloride process is also at a disadvantage.

Based on the above deficiencies, why would I want to specify chloride zinc?

Chloride zinc is accepted as the deposit with the most pleasing appearance. It is extremely bright, and when a blue-bright dip is used after plating, it closely matches chromium-over-nickel, at far less cost. Chloride zinc also has "leveling" ability. That is, fine scratches can be hidden by the plating, thereby improving the appearance of the part. Each process has its advantages over the others.