Lemon juice retards or interferes with the rusting of iron. Actually, lemon juice is a good method for removing rust stains from clothing. Rust is composed of iron oxides, mostly Fe2O3. This contains the iron (III) ion. The citric acid in lemon juice (part of the tart taste) chelates with the iron (III) in the rust to form a complex. UV radiation from sunlight then converts the iron (III) citrate to iron (II) citrate which is water soluble. The rust stain can then be removed from the fabric by washing with water.