Sciencemadness Discussion Board

Phosphoric acid and rust

DrMario - 21-12-2015 at 05:29

I am trying to figure out why does phosphoric acid, allegedly, work for rust... removal? Accounts differ: some say it transforms rust into iron (III) phosphate, others say it simply dissolves it (producing, I guess, soluble phosphates of iron). My own experience has been that all rust is removed. I only tried pure phosphoric acid (75%), not one of the commercial products containing H3PO4 + SECRET_STUFF.


Anyone willing to clue me in what is supposed to happen?

ave369 - 21-12-2015 at 05:40

I've used both the acid itself and sprays based on it. It works, it derusts my stand and clamps. But how exactly...

DrMario - 21-12-2015 at 06:20

Quote: Originally posted by ave369  
I've used both the acid itself and sprays based on it. It works, it derusts my stand and clamps. But how exactly...


It derusts, i.e. produces a soluble compound which is then removed, right? This is my experience also, but if I would be to believe accounts from other people, it's supposed to transform the rust into an insoluble and protective compound (consistent with the behavior of iron(III) phosphate).

aga - 21-12-2015 at 09:09

< 10 seconds googling ...
https://en.wikipedia.org/wiki/Phosphoric_acid

"Phosphoric acid may be used to remove rust by direct application to rusted iron, steel tools, or other surfaces. The phosphoric acid changes the reddish-brown iron(III) oxide, Fe2O3 (rust) to ferric phosphate, FePO4. An empirical formula for this reaction is:

2 H3PO4 + Fe2O3 → 2 FePO4 + 3 H2O "

I have heard that strong (8% ?) vinegar is also used for de-rusting iron and steel, probably making iron acetate from the iron oxide.

DrMario - 21-12-2015 at 10:57

Quote: Originally posted by aga  
< 10 seconds googling ...
https://en.wikipedia.org/wiki/Phosphoric_acid

"Phosphoric acid may be used to remove rust by direct application to rusted iron, steel tools, or other surfaces. The phosphoric acid changes the reddish-brown iron(III) oxide, Fe2O3 (rust) to ferric phosphate, FePO4. An empirical formula for this reaction is:

2 H3PO4 + Fe2O3 → 2 FePO4 + 3 H2O "


Well, that isn't necessarily accurate: ferric phosphate (Fe(III) phosphate) is NOT water soluble. And yet, both I and the person above experienced first hand that treating rust with phosphoric acid produced something which was dissolved in water.

Don't assume everybody else is too dumb to search the internet - maybe we are smart enough, but the answers we found are apparently inconsistent with empirical evidence.

Etaoin Shrdlu - 21-12-2015 at 11:22

Quote: Originally posted by DrMario  
Well, that isn't necessarily accurate: ferric phosphate (Fe(III) phosphate) is NOT water soluble. And yet, both I and the person above experienced first hand that treating rust with phosphoric acid produced something which was dissolved in water.

Dissolved in water, or dissolved in your strong acid solution?

deltaH - 21-12-2015 at 11:40

Perhaps as it's in excess acid, Fe(H2PO4)3 forms, not FePO4, which is highly insoluble?

aga - 21-12-2015 at 11:41

Quote: Originally posted by DrMario  
Don't assume everybody else is too dumb to search the internet - maybe we are smart enough

Then it would be better to say something like :-

"hey ! wiki says This about phosphoric acid and rust, and i can't see how that works"

instead of "how does this work ?" with no evidence of having done even a few seconds research on your own.

Edit:

Even that level of adding a supporting reference would make it qualify for Chemistry in General.

As it stands, it's unreferenced speculation, which is what Beginnings is for.

[Edited on 21-12-2015 by aga]