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I'm currently removing rust from steel with citric acid dissolved in water. I generally use a wire brush as the final treatment to remove any surface rust that the acid loosened up. The pieces under treatment end up shiny before being painted over for renewed protection.

I haven't found any authoritative answer on this, but online search results seem to recommend soaking the parts in a solution of baking soda as the final treatment after the citric acid. The idea being to neutralize the acid.

What is the purpose of the neutralization step in this scenario? Does it effectively reduce how prone the acid-treated steel is to rusting again?

And assuming the acid actuates on the surface of the steel, would the wire brush treatment be a substitute for the baking soda, if it effectively removes some of the acid-treated metal from the outer layer?

Background: Electrical Engineering. But my Chemistry is indeed very rusty (no pun intended).

This question is about why and if it is necessary to neutralize the acid after treatment, whereas Acetic(vinegar) and citric acid as the rust remover is about how to mix acids and which other acids to use. In my view, the questions are completely different, despite of the fact that they both mention the usage of citric acid.

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  • $\begingroup$ The standard chemical treatment for rust ( when a good mechanical treatment can not be done ) is phosphoric acid with manganese. These are marketed as "conversion coatings" ; Navel jelly is a common brand. $\endgroup$
    – blacksmith37
    Commented Jul 4, 2018 at 15:35
  • $\begingroup$ @blacksmith37 Thanks for the reply. My question is on the acid citric treatment, but it's also good to know about the alternatives. So what does 'standard treatment' mean in this context? Does it mean it's better than acid citric, and if so, in which regard? $\endgroup$
    – John M
    Commented Jul 4, 2018 at 16:16
  • $\begingroup$ Is the steel stainless steel? Phosphoric acid is the usual anti-rust treatment for regular steel, but citric acid is recommended as a safe non-passivating cleaner for flash rust on stainless steel. (The wire brush must then also be stainless steel.) $\endgroup$
    – James Gaidis
    Commented Jul 5, 2018 at 14:02
  • $\begingroup$ @JamesGaidis: it's regular steel. Citric acid has worked well for me so far in removing rust (not just flash or surface rust) from regular steel. I'm aware of other methods (I have in fact tried phosphoric acid too). The question is rather on the neutralization step when citric acid is used, when that is the method of choice. $\endgroup$
    – John M
    Commented Jul 5, 2018 at 17:44
  • $\begingroup$ I believe the phosphoric acid cleans and leaves some iron phosphate which is a good substrate for paint ( over simplification , I am sure) . The citric just removes oxides ; we used it to clean steel fracture faces for examination. $\endgroup$
    – blacksmith37
    Commented Jul 5, 2018 at 18:22

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After removing a part from an acid solution bath, it should have no trace of the acid left on it, otherwise, it will rust quickly and aggressively. Depending on the strength of the acid, it can happen right before your eyes.

Flushing the part with water often suffices, especially for weak acids. To be on the safe side, however, it is best to soak the part for a minute or two in a weak base solution, such as baking soda and water.

This works due to basic chemistry. Back in chemistry class, we learned acids and bases neutralize each other, resulting in a chemical that's less reactive than either the acid or the base. It's the same principle when you pour vinegar on baking soda and it fizzes violently. The vinegar is an acid (acetic acid) while the baking soda (sodium bicarbonate) is a base. The final result of the reaction is water, carbon dioxide, and sodium acetate.

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