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Phosphoric acid + steel = zinc phosphate?

I've used several rust converter [on eBay or Amazon] , metal etching and metal prep products on sheet steel automobile bodies over the years in which the active ingredient was always phosphoric acid. They all claim to leave a "zinc phosphate coating". Where does the zinc come from? Does it magically appear from the chemical reaction? -Or must zinc be an ingredient in the solution being used? -Or are there some traces of zinc in the sheet steel that react with the phosphoric acid to produce zinc phosphate? Thanks, JM

John Millard
auto restoration hobbyist - Somerville, New Jersey
2004

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Simple phosphoric acid can produce an iron phosphate coating which offers a measure of corrosion prevention, John. That is the typical "rust converter [on eBay or Amazon] " / "naval jelly" / "rust reformer" stuff you can buy in a hardware store. But zinc phosphate is a more capable corrosion preventer than phosphoric acid, and the zinc is an ingredient in the solution. There is a lot more to a high quality zinc phosphatizing solution than phosphoric acid and zinc though; the suppliers are not going to give away to the world the full benefit of all their research expenditures by describing exactly what is in it and how they make it, but there are good books covering the generalities, and searching expired patents should be very revealing regarding some specifics.

Something which might be a little confusing to you is that zinc phosphating is also done on zinc galvanized surfaces. In that case though not as additional corrosion prevention but as a procedure which helps paint adhere to the galvanized surface. Good luck.

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
2004