# Why do vinegar and other acids remove rust?

I have started a project on restoring a very rusty knife. I had used vinegar to remove the rust. A question dawned on me: why does vinegar remove rust? So being the thoughtful person I am, I googled it. Unfortunately, Dr. Google could only provide me with what it is doing (which I already know).

But what I really want to know is: what chemical reaction causes the rust to disappear?

• Googling without result usually means you are asking the wrong question. There are millions of chemical reactions, nobody remembers them all. Instead, we remember that certain classes of compounds tend to react with each other in such-and-such way. You identified one of the classes correctly, it is acids. Now what is rust? – Ivan Neretin Apr 5 '17 at 8:04
• I don't know whether this applies for vinegar (I don't believe so). But some acids remove rust by making it soluble in water, and hence it comes off, whereas others may aggravate it and make it worse. Sometimes it etches into the rust and flakes it off. Hope this helps others – Anonymus Jan 14 '18 at 20:58

The acid can dissolve rust (e.g. $\ce{FeOOH}$). The chemical reaction is:

$\ce{3CH_3COOH + FeOOH -> Fe(CH_3COO)_3 + 2H_2O}$

EDIT: ($\ce{Fe(OAc)_3}$is NOT soluble in water) The acid forms a water-insoluble salt with the iron oxide, which then probably just crumbles from the rust layer and precipitates in the container.

Mind that rust also contains for example $\ce{FeO(OH)_3}$ and $\ce{Fe_2O_3·nH_2O}$. The mechanism is the same.

Also, the acid oxidizes the upper layer of the iron under the rust to form a rust-resisting oxide-layer.

Vinegar contains $\ce{CH3COOH}$ (acetic acid) which reacts with rust $\ce{FeOOH}$:

$\ce{3CH3COOH + FeOOH -> Fe(CH3COO)3 + 2H2O}$

and $\ce{Fe(CH3COO)3}$ (iron(III) acetate) is water soluble.