Blisters on Zinc-Plated Steel

Q. We are experiencing blistering on steel parts, which are put through the following sequence:

1. Clean

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2. Zinc plate

3. Trivalent chromium conversion coating

4. Bake for 12 hours.

Do you have any suggestions for how to eliminate the blistering?— M.U.

A. You do not state the type of zinc plating bath you are using, but I’m assuming it is an acid-zinc-type plating bath to minimize the formation of hydrogen during the plating process. There are a number causes for the formation of blisters. The most common, of course, is inadequate cleaning of the steel surface prior to the plating step. But, assuming you have a good cleaning sequence in place, where else should you investigate?

Do you see signs of blisters after the plating step but prior to the chemical conversion step? If no, the blisters are probably due to your baking step. If during the plating step excessive amounts of hydrogen are liberated, some of this hydrogen is trapped under the zinc plate. Baking of the parts is designed to reduce the amount of hydrogen present at or near the surface of the part. Baking the parts for a period of time does drive off the hydrogen, but the hydrogen escaping through the plate may cause blisters to form. Of course, if you are using a cyanide or non-cyanide-based alkaline bath you will generate more hydrogen during the plating step.