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Is it posssible to OVER pretreat for powder coating?

August 6, 2009

My company is attempting to extend the corrosion resistance and improve paint adhesion on the '380' die cast components of an outdoor product we sell all over the U.S. As background information, the '380' appears (from weight and appearance) to contain a very high zinc content (which we know is wrong; we think the supplier is adding it to aid in the casting process). We've just finished a 500 hour cyclic salt spray / high-humidity test on some scribed castings and some of the test results puzzle me.

Part #1 was sandblasted, then received our paint line's iron phosphate wash (the first stage of our 3-stage pretreatment), then powder coated.

Part #2 was tumble deburred, alumniprep 33 washed, rinsed, Alodine 1201 treated, rinsed, then powder coated.

Part #3 was sandblasted, then alumniprep 33 washed, rinsed, Alodine 1201 treated, rinsed, then powder coated (skipped the pretreatment wash).

I was expecting Part #2 or 3 to out perform #1, but #1 greatly surpassed the other two parts (which had about the same amount of corrosion).

Can anyone explain why? Is it possible to OVER pretreat castings? Could the high zinc content be part of the answer?

Thank you.

David Maijala
Director of Quality in metal manufacturing - Huntington Beach, California, USA

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You can over pretreat and doing so can effect salt spray performance. You should be getting 700hrs or better with the iron phosphate and 3 stage system. The phosphate obviously is a multimetal type with fluorides added for aluminum(the iron is for steel substrates).
Much kudos on using the 3 stager, the first system I managed was 3 stages and it's performance was as good or better than the 5 and sometimes 7 stages that I've used since.
Many years and a significant amount of testing has proven to me over and over that a simple multimetal phosphate, rinse, DI system is all you need for good powder coating.

Sheldon Taylor
supply chain electronics
Wake Forest, North Carolina
August 10, 2009

Thanks for your answer!

David Maijala
- HUNTINGTON BEACH
August 11, 2009