Oxygen corrodes iron to form rust, a reddish form of iron oxide. A molecule of rust is larger than a molecule of iron. It puffs up, cracks and flakes. Citric acid, an inexpensive, widely available acid, is one of several acids used to remove rust. Gentle and safe to handle, it combines with rust to form a soluble iron citrate.
Removing Rust with Acid
Numerous commercial compounds containing phosphoric acid are used to remove rust. Hydrochloric acid and oxalic acid, found in many household cleaning and bleaching agents, are also effective. These acids are highly toxic. They produce noxious fumes and can burn your skin.
Citric acid, found naturally in many fruits, including lemons, will do the job just as well. It is environmentally friendly. You can flush it down a toilet or let it drain down a sink.
Obtaining Citric Acid
Brewing and canning supply stores sell citric acid as powdered crystals. It is sold as sour salt in kosher food stores and as a sprouting aid in health food stores.
Cleaning Rust from Metal
Clean the rusty item. Citric acid will remove rust, but won't strip paint or cut grease.
Wear rubber gloves to protect your hands and long sleeves to protect your arms.
Put the citric acid powder into hot water to form a solution that is 10 per cent citric acid. Do not add the water to the crystals. Use a plastic container, not metal.
Lower the rusty item into the solution. It will begin to bubble. If there are no bubbles, the acid is not working.
Leave the item in the acid bath until the bubbling stops. You can swirl the solution to remove outer layers of rust and expose more rust to the acid. A gunk will form. You can remove the item occasionally and scour the gunk with steel wool.
Removing rust can take from a few minutes to several hours. When the bubbles stop, the solution is spent. If you leave an item in the solution too long, the acid will make microscopic pits in the metal, making it look dull and rough.
When you finish removing the rust, scrub the item with a scouring pad under running water. Wash it, then neutralise any remaining acid with a solution of several tablespoons of baking soda mixed with a quart of water.
Cleaning Rust Stains
To remove rust stains from fabrics, mix 1 tsp of citric acid to 1 qt. of hot water. Put the rust-stained fabric in a sink. Drip the citric acid solution onto the rust stains. When the rust is gone, wash and rinse the fabric.
Lemon juice is a classic home citric acid solution to remove rust stains from delicate fabrics. Squeeze the lemon juice on the stain and let it soak.