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).

  • $\begingroup$ 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 Jul 4 '18 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 Jul 4 '18 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 Jul 5 '18 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 Jul 5 '18 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 Jul 5 '18 at 18:22

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