Heat-treated Parts and E-coat

I would like to start this clinic with a question that was asked of a friend of mine and share with you his answer. The friend is Dennis Cook, president of DC Coaters Inc. His company is a custom E-coater. I have been working on a project with DC Coaters and they keep me in the e-mail loop. That is where I picked up on this question. I thought I should share it and the answer that Dennis wrote with you. So, with his permission, here it is.

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Q. I noticed the parts we sent you last week looked different than the other parts we have been receiving from you. As it happens, the parts in question have been heat treated. Does the fact that they were heat treated make a difference? If that is the case, please explain why they look different? S.D.

A. Heat treated parts paint a little differently than plain steel parts, so I will try to explain the differences.

First of all, we typically see heat treated parts arriving with a black heat treat scale on them. In some cases, we see burnt-on quenching oil or even salt residue. Our cleaner tanks are designed to remove stamping oils from 95% of the products we process and do the job very well. These high-pH (alkaline) tanks do not de-scale the heat treated parts. They also will not totally remove burnt-on oils. To properly clean heat treat scale and/or burnt-on oils it is common to pickle and oil, wheelabrate or vibratory finish. Pickle tanks are acid (very low pH) and will de-scale but will not clean the oil off as well as high-pH, which is why we don’t have pickle stages.

The zinc phosphate we use is a primer under the E-coat that gives the best adhesion and salt spray resistance to our final E-coat finish. In our business, we say we don’t paint parts, we paint phosphate, meaning it is as important as the paint. This phosphate is designed to actually grow zinc on to the steel parts. When we process heat-treated parts that have scale, the phosphate sees the scale and not the steel and does not “grow” as well. This issue from time to time could cause poor adhesion and poor appearance.

It all has to do with the heat treat and not our phosphate. As our quote letter states, we strongly suggest removing scale and burnt oil resides from parts before electrocoating. Of course, this is an added operation. D.C.