Good Vs. Bad Pretreatment

Q. I am the engineer for two electrostatic epoxy powder coating lines, one is an immersion pretreatment and the other is spray. The product being painted is steel sheet metal with heat treat scale and surface oils to be removed.

A few years ago we made an economic decision to go from an “engineered” chemical for pretreatment to generic phosphoric acid. We manage it to a pH 1.5 and titration of 7% by volume. Currently we are experiencing equipment damage due to pickling, and I am about to go back to the “engineered chemical” strategy.

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My problem is to identify a metric for “good” and “bad” pretreatment prior to powder coating. We do have a 25-year old reference for a different chemical, but it was by film weight. I really don’t trust that. I already have a chemical company ready to help, but I would like your input on this before I go further. N.C.

A. Pretreatment requirements are fairly simple for powder coating. You only need a clean surface for proper adhesion. After that requirement, the additional benefit of pretreatment is strictly for corrosion performance. So here is what I recommend:

1. Ensure that your parts pass the “water-break-free” and “white” towel tests to ensure there are no surface contaminants on the part. Now you know the part is clean.
2. Then you can have the chemical supplier recommend and test to verify the iron phosphate coating on the surface for improved corrosion performance. The amount of iron phosphate is directly related to the amount of corrosion resistance you are looking for. This measurement value is called coating weight and is measured in mg/sq ft.

I hope that this helps you.