Corrosion Protection on Steel

Ask an Expert From: Products Finishing, from Products Finishing magazine

Posted on: 5/20/2013

What is the best zinc phosphate process for protecting steel?

Q. Currently we use a galvanized sheet and form it for our fence products using the ERW welding process (high frequency). We then do a multi-step zinc phosphate conversion coating before we polyester powder coat. Some of our smaller parts made of carbon steel can be phosphated and powder coated, but I was wondering if I zinc electroplated the parts, then zinc phosphate conversion coated the zinc and then powder coated, wouldn’t this be superior to just the zinc phosphate and powder coating? J.B.

A. I think that your basic assumption is correct and it will be better if you add the zinc plating step. However, zinc plating and galvanizing are not the same process. My concern is that any new coating step can cause issues with inner-coat reactions and adhesion. Zinc plating has caused some out-gassing and adhesion problems for coaters. The zinc phosphate should help limit the risk, but I would test a few parts to make certain you do not create a new problem.  

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