How to Remove Rust From Metal Using Acid
Phosphoric acid is generally the acid of preference for rust removal. It reacts with the rust much faster than it etches (erodes) the underlying iron. Furthermore, it leaves a fine, blue-black coating of iron phosphate behind, which prevents rusting. It will leave a hard, bright metal finish that is both attractive and ready to prime and paint. Does this Spark an idea?
Things You'll Need
- Phosphoric acid
- Steel wool or wire wheel
- Non-steel brush
- Baking soda
- Paint and Primer
Instructions
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1
Purchase phosphoric acid. Anything around 85% concentration is fine. There's no need to dilute it further.
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2
Clean the rusty surface with steel wool or a wire wheel, then with water and soap.
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Dunk the piece into a container of the acid and let it soak a few minutes. If the item is too large to be submerged, pour the acid into a spray bottle and spray the rusty part or wet paper towels with the acid and lay them on the rusted portions.
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4
Take off any towels or pull item out of solution.
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Brush off the rust with a non-steel brush.
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Rinse with water and re-do if rust remains.
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Neutralize the remaining acid on the iron piece with a solution of baking soda and water.
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Dry the item with a towel and heat it---for example, with a hair dryer--to drive out the remaining moisture.
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Prime and paint the iron. The metal needs to be painted as soon as possible since the rust protection from the thin layer of iron phosphate left behind is short-lived.
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Tips & Warnings
Mix and use the acid in a well-ventilated area. Use rubber gloves and eye protection.
Because phosphoric acid removes rust, some people get the idea that a stronger acid, such as muriatic (hydrochloric) acid, would work even better. Phosphoric acid and muriatic acid are two different chemicals that affect iron in drastically different ways.
Do not use muriatic acid on cast iron, which is porous and thus difficult to neutralize deeply enough to stop its etching the iron for an extended period. Pitting results.
Some people use Coca Cola, which contains phosphoric acid. Unfortunately, it also contains carbonic acid. It is better to stick with the phosphoric acid.
