Summary for zinc phosphate vs zinc plating Discussion
Zinc phosphate vs zinc plating is discussed as a pretreatment question for electrocoating and powder coating. In the main answer, Ted Mooney says zinc-plated parts can usually be zinc phosphated if they have not already been chromate-converted; if a chromate film is present, the phosphate may not take properly and could contaminate the bath. He adds that plain zinc plating, without chromating, should phosphatize fine and may benefit from the treatment. Ron Zeeman later notes that his former employer routinely zinc-phosphated HDG coil after testing for passivation, with millions of pounds processed successfully. In the related powder-coat discussion, Brian C. Gaylets describes a cleaning, zinc phosphate, chromate rinse, and powder coat sequence, while Ted Mooney argues zinc phosphate is generally the better paint base because it covers all surfaces, is rougher for adhesion, and is more corrosion resistant than iron phosphate.
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Q. Is it feasible for those zinc plated part still going for zinc phosphating prior to electro-coating (ED coating) as top finishing?
Zinc plated part will release a lots of contaminants into zinc phosphating bath and cause unstable. Is it true?
Thank You
PUA KAI CHEK
Employed Manager – Kuala Lumpur Malaysia
August 13, 2023
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
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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
Q. Hello Sir … if the workpiece have been passivated with chromate aft zinc phosphating, how to remove away the chromate layer before proceed for zinc phosphating process since chromate layer will cause problem on zinc phosphating such as imperfection phosphated and contamination?
PUA KAI CHEK [returning]
– Kuala Lumpur Malaysia
September 3, 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
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finishing.com – Pine Beach, New Jersey
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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
⇩ Similar, related, Q&As — oldest first ⇩
Q. Our company supplies fittings to the fence industry and we now have the parts zinc plated then powder coated after. The powder coater doesn’t like to paint over the zinc plating and wants us to use a zinc phosphate primer
⇦ on
or
Amazon [affil link] with a powder coat finish. Some of the product use includes getting banged around and I am wondering if one process is better than another. We would use hot-dipped galvanized if possible – but no powder coater wants to paint over it. Any help?
Cal Kooistra
– Grand Rapids, Michigan
2005
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
Q. Can we do Zinc Plating on part after phosphating?
Prashant Kumat
Vishwakarma Industries – DELHI, India
June 26, 2017
A. Hi Prashant. Sorry, no. Zinc plating must metallurgically bond with the steel.
Regards,
Ted Mooney, P.E. RET
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finishing.com – Pine Beach, New Jersey
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