Ask the Expert Question-and-Answer Archive

by Ron Joseph

March, 2006

Zinc Phosphate Coating Weight

Q. I have been approached to help layout some specifications, regarding a sizable order of 12-gauge x 6-inch heavy duty T-hinges for exterior durability. We are targeting a salt spray test (ASTM B117) of 500-750 hours, and a QUV test (ASTM G53) of 1,000-1,500 hours.

The zinc phosphate conversion coating looks too light at 70 mg/sq ft, what are your recommendations. Ron, your help is greatly appreciated.

1. Molten zinc bath, substrate centrifuged to zinc thickness of 15-microns, (.015mm), (.6 mils).

2. Fine grain zinc phosphate conversion coating of 70 mg/sq ft. with a dry clear organic seal

3. TGIC Black Polyester powder coat paint to thickness of 75 microns, (.075mm), (3 mils).

A. The coating system you present should give you the desired 500-750 hours salt spray results per ASTM B117; however, I agree that the zinc phosphate coating weight appears to be low. Usual thicknesses are in the range of 150 - 300 mg/ sq ft, but you can check this by referring to TT-C-490 Type I, which is the Federal standard for zinc phosphate. In fact, you can use the Federal standard in your specification.

Best wishes,

Ron Joseph