Rust or iron oxide, is the reddish brown crust that forms on materials containing iron and is the result of the corrosion of iron. The corrosion itself is an electrochemical reaction, oxidation, that occurs when iron comes in contact with water and oxygen, in other words, humid air. Since corrosion causes metal to disintegrate if left untreated, it is best to treat the problem in its initial stages. Once the metal gets completely corroded, it cannot be restored or repaired. Though there are various methods to remove rust and corrosion, the best way is to use certain chemicals, namely acids.
Strong Acids Used in Rust and Corrosion Removal
Since Iron oxide is a base, its soluble salts include chlorides, sulphates and nitrates. Hence, the best known acids used in its removal are hydrochloric, sulphuric and nitric acid. Hydrochloric acid, better known as muriatic acid, works as a solvent for rust by converting it to iron chloride. It is readily available in hardware stores, though in a diluted form because it is dangerous when concentrated. Sulphuric acid, or battery acid, reacts with rust to form non-toxic iron sulphate and is also used in its diluted form. Nitric acid is also a strong solvent but again, it involves dangerous risks and moreover, due to its use in explosives, it is not readily available.
All of these acids require proper precautions before handling, such as masks, goggles and thick rubber gloves.
Lesser Agents Used in Removing Rust and Corrosion
Phosphoric and oxalic acids are two other acids that effectively remove rust and corrosion from metallic objects, but both have some drawbacks. Phosphoric acid removes rust by converting it with ferric phosphate, but leaves behind a coating of iron phosphate, which is insoluble in water. Oxalic acid also removes rust, but leaves a residue of iron oxalate which is also largely insoluble.
Organic Acids Used in Rust Removal
Though there are some well-known household items used in removing rust and corrosion, the most effective, is the use of acetic acid, which is an organic acid and the only one of its kind used in rust removal. Acetic acid is found in its diluted form in vinegar and in concentrated form as glacial acetic acid, or naval jelly. Since acetic acid in vinegar is a weaker acid, adding sodium chloride, or chloride ions, will increase the rate of rust and corrosion removal.
Other Household Chemicals Used to Remove Rust/Corrosion
Weaker acids used to remove rust stains are lemon juice (citric acid), onion and raw potato juice (chemical components) with salt (sodium chloride). However, these are very weak solvents and can only remove mild rust stains.