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Powder coating for aluminum die casting

We are using powder coat (polyester TGIC) on an aluminum die cast part used on construction equipment. I have been presented two option by our vendors for pre-treatment prior to coating. Vendor one proposes bead blast prior to coating. Vendor 2 proposes chromate prior to coating. We are looking for good corrosion protection and durability (from flying debris and abuse). Which is the better method of preparation? Or what are the questions I should be asking the vendors?

Michael O\'Hara
Engineer - Naples, Florida, USA
2007

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Chromating is better. Conversion coatings like chromate will limit undercutting corrosion in the event of a scratch for one thing. Secondly, organic coatings like powder "breathe"; moisture/humidity pass through them (not like a sieve, but to an extent) and it is better for the long term to have a conversion coating between the aluminum and the vapor.

That's opinion. Before committing to manufacture, however, salt spray testing of samples should be done, and that will offer evidence. I'd try both or I'd try the chromate first.

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
2007