Zinc phosphate vs zinc plating as base for powder coat

A. Zinc electroplating which will be left as a final finish (rather than being electrocoated) requires chromate conversion coating, or the components would quickly get white rust. Because of this, most zinc electroplating lines include chromate conversion coating right into the process cycle of the plating line.

If the components therefore have a chromate conversion coating on them, the zinc phosphating will not take properly, and I suppose the chromate could be partially removed within the phosphatizing cycle and cause contamination of the phosphating line.

But zinc plating per se (without a chromating post-treatment on it) will probably phosphatize fine and benefit from it. And unless the zinc plated parts were processed poorly, I wouldn't expect them to harm the phosphatization chemistry.
Luck & Regards,


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

A. ASTM B633 under Table 2 Type IV describes 'with phosphate conversion coating.' I don't see much of this in the real world.

Tom Rochester
- Jackson, Michigan USA
August 30, 2023

A. Hi again,
I have never heard of this being done and don't know if it is possible. I personally know of nothing that will remove the chromate without also severely damaging if not completely removing the zinc plating. To my limited knowledge stripping both the chromate and the zinc, and then replating but not chromating, is the only workable approach.
Luck & Regards,


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

A. My former employer was a coil coating line we routinely zinc phosphate HDG coil, however we had to test for passivation prior to application. Millions of pounds no issue,

Ron Zeeman
- Brampton ON Canada
February 3, 2024

A. I hope I could help you with your problem, this is the way we go about doing our powder coat ... first off they check for rust, if so they get dipped in a hot acid bath to dissolve any rust, then they get rinsed really well. Next they go to a 15% caustic soak cleaner which is run at 180 °F, then rinsed (really well) again. Next we go to a zinc phosphate immersion coating for approximately 2 minutes, then rinsed again, then they are rinsed in a hot water chromate rinse and air dried, then sent down to powder coat. This also puts a good rust inhibitor on the substrate.

Regards,

Brian C. Gaylets
- Scranton, Pennsylvania, U.S.A.

A. I believe that the zinc phosphate will provide much better product for several reasons. The zinc phosphate covers the entire part, inside and out. The zinc plating will coat outside surfaces only (for example, the outside of tubing will plate, the inside is bare.) The zinc phosphate is an excellent paint base, the zinc plate is less able to promote adhesion because it is a smoother surface.

The zinc phosphate is a stand alone product. It is used to coat military weapons. Combined with the powder coating it should be much superior.

Iron phosphate is a standard coating used in the paint and powder coating industry as a pretreatment. Zinc phosphate is more corrosion resistant than iron phosphate.

Daryl Spindler, CEF
decorative nickel-chrome plating - Greenbrier, Tennessee

A. Hi Prashant. Sorry, no. Zinc plating must metallurgically bond with the steel.

Regards,

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