I wanted to remove the rust on numerous steel parts with phosphoric acid. After removing all the grease in trichloroethylene, I put my parts into a 50% solution of phosphoric acid ad 40°C for 5 minutes.

When all the rust was removed, I rinsed the parts with clean water to remove all the acid. At that time, I decided to not neutralize the solution in NaOH.

In about 5 minutes all the cleaned parts exposed to air rusted. The parts into the water became greenish (I guess it is FePO4 residue).

I would like to understand why I get this ultra fast oxidation and how I can prevent it to occur.

  • $\begingroup$ What steel was it? Also why you used such harsh method? $\endgroup$ – Mithoron Oct 16 '15 at 11:55
  • $\begingroup$ Some regular steel I guess not an stainless one obviously. I have 3-4 hundred parts to treat. Doing all by hand is a tough work. Also I think I like harsh methods :) $\endgroup$ – nowox Oct 16 '15 at 14:17

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