Iron Phosphating on Zinc Electroplating?

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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!

SUGGESTED PHOSPHATING BOOKS

"Optimization of Phosphating Process" by Vinod Prasad Sharma (2022)
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"Phosphating of Metals" by Werner Rausch (1991)
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"Phosphating & Metal Pretreatment" by D.B. Freeman (1986)
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A. I cannot 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.

Thanks, 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!

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:

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,