Citric Acid Rust

Citric acid reacts rapidly with iron, but is not itself the rusting of iron. The water in the lemon juice helps iron rust, but in the presence of air. The same happens when you open bottle of Coca-Cola, including citric acid, phosphoric acid, carbonic acid and other things in the fall for the taste.

Citric acid has an advantage over some other acids, because it is less corrosive, not as dangerous to the environment, while remaining efficient, cheap (but not as cheap as the old battery acid). The chemistry is relatively simple, rust (iron oxide) is Solubilised by the acid (ammonium ion is also acidic) and citrate ion coordinates to the iron-forming soluble iron citrate.