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Pickling is a treatment of metallic surfaces in order to remove impurities, stains, rust or scale with a solution called pickle liquor, containing strong mineral acids, before subsequent processing, such as extrusion, rolling, painting, galvanizing or plating with tin or chromium. The two acids commonly used are hydrochloric acid and sulphuric acid. Pickling liquor may be a combination of acids and may also contain nitric or hydrofluoric acids.

Carbon steel is pickled usually by either sulphuric acid or hydrochloric acid. At one time, sulphuric acid was the pickling agent of choice^{[citation needed]} for picklers running integrated steel works. Hydrochloric acid is chosen in more modern lines when bright surfaces, low energy consumption, reduced overpickling and the total recovery of the pickling agent from the waste pickle liquor are desired.

Wastewaters from pickling include acidic rinse waters, metallics salts and waste acid. Spent pickle liquor is considered a hazardous waste by EPA. Some plants, particularly those using hydrochloric acid, operate acid recovery plants where the mineral acid is boiled away from the iron salts, but there still remains a large volume of highly acid ferrous sulphate or ferrous chloride to be disposed of. Since the 1960s total hydrochloric acid regeneration processes have reached widespread acceptance.^{[citation needed]} The by-product of nitric acid pickling is marketable to a couple of secondary industries including fertilizers.

Through the late 1980s, spent pickle liquor was traditionally land disposed by steel manufacturers after lime neutralization. The lime neutralization process raises the pH of the spent acid and makes heavy metals in the sludge less likely to leach into the environment. Today however, the some of spent pickle liquor can be recycled or regenerated onsite by steel manufacturers.

Similarly, pickling is used in jewelry making, after fluxing or soldering, in order to remove any scale and contaminants from the jewelry metal, which is often sterling silver, copper, or gold.

• `Fumeless' Pickling and Passivation Gel Chemical for Stainless Steel by Novel Surface Treatments