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I actually tested submerging a bunch of old rusty knives blades in a cup of plain cola for a few days not expecting much. To my amazement the myth held true, coke is a really good rust remover.

So what is the chemistry behind it?

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Rust consists of insoluble iron oxides and hydroxides, $\ce {Fe2O3·nH2O}$ and $\ce{FeO(OH)·Fe(OH)3}$. Acidifying these compounds protonates the oxide and hydroxide anions to for water:

$$\ce{O^{2-} + 2H^+ \rightarrow H_2O}$$ $$\ce{OH^{-} + H^+ \rightarrow H_2O}$$

This leaves the soluble iron $\ce{Fe(III)}$, which can be washed away.

Many colas contain phosphoric acid, and are quite acidic (Coke has a pH of ~2.5 according to this science fair project), thus they can dissolve the oxides.

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    $\begingroup$ It's not that Fe(III) gets washed away necessarily but that Fe(III) phosphates gets formed which works afterwards as a protection layer like the oxides for e.g. aluminium. $\endgroup$ Jul 10 '15 at 8:46
  • $\begingroup$ Ah, yes. I didn't think about the phosphate ion. Good point. $\endgroup$
    – buckminst
    Jul 14 '15 at 19:08
  • $\begingroup$ It's not just a point but what happens :) $\endgroup$ Jul 15 '15 at 1:08

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