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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 $\ce{NaOH}$.

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

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

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  • $\begingroup$ 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
  • $\begingroup$ Rust removers, based on phosphoric acid, contain also tributylphosphate and perhaps some corrosion inhibitors. They are not supposed to get rinsed out. The surface gets rather dark violetish and can be painted. $\endgroup$ – Poutnik Nov 26 '19 at 2:22

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