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.