Letter 7082

Powdercoating of galvanized steel 

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Is powdercoating of galvanized steel acceptable with phosphate cleaning, or is other treatment nessasary?

Mike Lowe
- shenandoah,VA. USA

You can powder coat galvanized steel just fine, just make sure you don't get passivated galvinized. There is a little more involved with treating galvinized, but with the right process it can be successfully done.

Just remember, Iron Phosphate will not form in the areas where there is no iron. What you get will be a clean part, a little etch (micro) and possibly some small amount of phosphates of the coating metals.

An adhesion promoting final rinse can also be a huge benefit if you have durability standards to meet.

Jeff Watson - Pearland, TX

Mike

Phosphating is a good method of preparing galvanized surfaces for powder paint. It is important though to use the right phosphate for the application. For example, for outdoor use, zinc phosphate with a good seal is best, while certain types of iron phosphate may be acceptable in less demanding applications. For indoor applications, an iron phosphate without a seal is probably fine.

Also, be sure to specify no "Chem Treat" from your steel supplier. Chem Treat (chemical treatment) is a chromium-based passivation which protects the unpainted galvanize from corrosion, but seriously reduces paint adhesion.

Good luck.

Gary Nelson Chemetall Oakite Berkeley Heights, NJ, USA

Hi

Cleaner-coaters may indeed be used for pretreat providing the surface has not been chromate passivated. If you require higher performance (outside exposure) you will need to use special final rinse additives (simplest being Cr containing - but effluent!) or subsequent dry-in-place process.

R

Roger Bridger
- Croydon UK

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