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Polyester Powder Coating After Zinc Plating [California] 

November 13, 2006

We have some Steel structures that are built using hot rolled high strength Steel. For corrosion prevention we plan to Zinc plate them then add a layer of polyester powder coat. Though there is an ASTM standard for the Zinc plating, there is no such thing for the powder coating. Does anybody know of an industry standard or procedure for that kind of application?

Thanks,

Salman Gharaibeh
ISE Corporation - Poway, CA, USA

You mention steel structures, which seems to imply substantial pieces of steel, but then state that you intend to zinc plate.
Electroplating isn't commonly used for large structures in hot rolled steel.
Hot dip galvanizing would be more common for HR structural Steel, and this can be followed by powdercoating.
The standards applicable vary from country to country. One such is ISO 13438 (prev BS6497).

Zinc plating as a corrosion protection is limited by the thickness of the zinc layer. Its application is usually limited by the size of the item.
Hot Dip galv is typically 10-15 times thicker that plating, and last correspondingly longer. Neither are easy to powdercoat, the pretreatment between zinc coating and powdercoating being the critical point. Either a chromate or a zinc phosphate pretreatment system of 8-12 stages is required. But if you do get it done this way you can expect a competent coater to offer a guarantee of 15-25 years, depending on the installation location.

Geoff Crowley
- Glasgow, Scotland