I do a fair bit of home powdercoating. One step of the surface pretreatment involves dipping the part in a solution of iron phosphate.
When I have a fresh batch of solution, it’s free of precipitate. However, after dunking many steel parts into it, a yellowish-white precipitate forms.
What is that precipitate?
The powder you see is Iron phosphate !
The result of using iron phosphate solution on the steel is a layer of insoluble iron phosphate.
Iron atoms are multi-valent , it may be the valency change that changes it from soluble to insoluble.
But the solution is also called “Detergent” which I assume means a bunch of chemicals also present, with the aim of keeping the iron phosphate from precipitating… trade secret recipes.
I am not sure, but if I had to guess, I would say I don’t think your “iron phosphate” solution is that, since neither ferrous nor ferric phosphate is soluble in water. What I would guess it is is, among other things, a solution of phosphoric acid. Phosphoric acid is a well-known cleaner for rusted steel (“naval jelly” contains it). So I would guess the acid cleans the surface, and then oxidizes the iron to a few atoms’ depth, producing Fe+3 which combines with the PO4-3 in solution to give you a surface layer of iron (III) phosphate. Apparently that surface layer adheres tightly to the underlying Fe in the way that Fe2O3 so famously and sadly does not.
As for what the precipitate could be: almost anything, since most phosphate salts are insoluble, due to that massive charge on the anion. The few exceptions are big soft cations like K+, Na+ or NH4+. So any cation that’s lying around in your solution would probably combine with the PO4-3 and form an insoluble salt. Also any iron (III) phosphate that formed but didn’t adhere to the iron.
If it’s not part of the solution, I reckon it must be part of the problem.
My guess would be that the “solution of iron phosphate” was never a solution to begin with, but a suspension, or maybe a colloid.
It arrives in a 1-gallon jug, and I dilute it with 30 gallons of tap water. If it’s a suspension/colloid, it doesn’t appear to be settled in the original jug despite sitting on my storage shelf for months before being used.
I shouldn’t have called it an “iron phosphate solution” in my OP. The product web page calls it “iron phosphate primer.” So Carl Pham may be onto something.