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Iron Phosphating on Zinc Electroplating?




Q. I have a prospective client who is trying to put an iron phosphate coating over zinc electroplated parts. I've explained to him that he needs to use zinc phosphate with a grain-refining agent, but he insists that he is not able to convert to zinc phosphate. Why? I don't know at this time. I was thinking of researching an activating agent for the zinc-plate surface, but still don't think the iron would bond with the Zn. I'm on the road all the time, so can't research. Anyone have experience with a process similar to this?

randy fowler
Randall Fowler - Fowler Industrial Plating, LLC
Cleveland, Tennessee, USA
2006


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Ed. note: C'mon folks, this is Randy Fowler of letter 12044 asking. As the star figure of that letter -- for answering by experiment the question of whether stripping chromium in HCl generates hexavalent chromium -- we owe him one!


"Phosphating of Metals"
by Werner Rausch

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"Phosphating & Metal Pretreatment"
by D.B. Freeman

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A. I can not do anything but guess, but I think that you would get a very poor cross between iron phosphate and zinc phosphate. I do know that it is very difficult to visually control zinc phosphate on zinc plate. It comes out looking like an etch rather than a zincate that you get on iron. Two different zinc phosphate solutions and two different work sites with several attempts at each site.
My bet is that he wants the slightly cheaper iron because of speed and cost. Till it is tried, I will go with the opinion that it is a waste of time and money unless you are looking for an etched surface for "tooth" for the paint. I seriously doubt any gain in corrosion resistance and possibly even less as it will remove some of the zinc plating thickness.

James Watts
- Navarre, Florida
2006



thumbs up signThanks, James. I'm still having trouble understanding exactly what the customer wants, even though I've been told. Hopefully, Ted's little "side-bar" note will draw in some more info. I thank you for your input. Strange situation, indeed. I have some ideas, but still need more. Once again, thanks, Ted!

randy fowler
Randall Fowler - Fowler Industrial Plating, LLC
Cleveland, Tennessee, USA
2006



A. Hi Randy. I think you were right from the get-go. To my knowledge an iron-phosphate treatment is performed by immersing ferrous parts into a solution of phosphoric acid (with perhaps various other chemicals to assist in the cleaning and the film formation). The phosphoric acid reacts with the iron in the substrate and forms iron phosphates.

But I don't think there is any substantial amount of iron in the treatment chemistry itself, so where/how is iron phosphate supposed to form if you immerse zinc into phosphoric acid? Here's what Brad Gruss says in the Iron Phosphating chapter of the Metal Finishing Guidebook:

42740

My guess is that putting zinc coatings into iron phosphate chemicals is a bastardized attempt to do "something" while not spending the money for proper zinc phosphate chemistry and treatment stages. And as we all know from reading history (or at least historical fiction), bastards aren't entitled to the Phosphate family name :-)

Regards,

Ted Mooney, finishing.com
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
February 2014




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