Metal finishing Q&As since 1989

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Salt Spray Tests of Galvanizing plus Powder Coating are tanking!

Recently our salt spray tests have gone from 1000 hrs down to 72 hours for our salt spray tests. We are using a galvanized coated A500 steel covered by 3 mils of powder coat. After 72 hours,the powder coat will be decimated leaving the galvanization exposed.I would appreciate any information on why the powder coat would be failing prematurely when in the past we were passing our spray test. Thank You.

Nicholas P Howard
Engineer - Abingdon, Virginia
2007

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Although the powder coating could be defective, I suspect it is peeling. You should specify a zinc phosphate pretreatment before the powder coating for proper adhesion, Nicholas.

Another possibility is that your applicator previously used a capable chromate conversion coating as a pretreatment, and has converted to an unsatisfactory replacement in an attempt to eliminate chromates from his plant.

Galvanizing plus paint or powder coating is called a duplex system, so you might want to study that. Good luck.

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