Acid Zinc and Hardened Steel

Question:

We are plating hardened steel parts using an acid zinc chloride plating bath. The parts look great out of the plating bath but after baking the zinc deposit peels and in some cases blisters. We have tried more vigorous cleaning without any success. What can we do to solve this problem? M.L.

ANSWER:

A classic example of the old cliché, “beauty is only skin deep” or putting it another way, what you see is not necessarily what you get! Processing of hardening steels is somewhat more complicated than processing of other types of steels. The reason for this is conceptually simple—hardened steels tend to collect hydrogen at the surface of the part. The hydrogen is formed during the various steps of the finishing process, cleaning, acid dipping and plating. The hydrogen can cause embrittlement of the part if the part is not properly baked after the plating process. The hydrogen must have an easy route to escape from the surface of the part during the baking step.

Here’s where the problem arises. If the grain structure of the plated deposit is very fine the hydrogen cannot easily escape from the surface of the part. The result is peeling and/or blistering of the deposit. Finer grain size is usually achieved by putting additives, usually organic, in the plating bath. We routinely add these organic materials to many types of plating baths to obtain bright deposits. Acid zinc chloride is no exception. The organic additives not only give a brighter deposit and in the case of this bath help give better covering power in low-current-density areas.

There are a couple of things you might try to solve your peeling/blistering problem. Try running your plating bath with a minimum amount of organic additives. Your chemical vendor may have some suggestions for you. The other thing you might try is to change your bake cycle. Sometimes baking at a lower temperature will reduce or eliminate the peeling/blistering problem.