Pickling (metal)

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For other uses, see Pickling (disambiguation).

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 sulfuric acid. Pickling liquor may be a combination of acids and may also contain nitric or hydrofluoric acids.

Carbon steel is pickled usually by either sulfuric acid or hydrochloric acid. At one time, sulfuric 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.

1 When Acid Pickling (Cleaning) is Used
2 Waste products
3 Jewelry
4 External links

[edit] When Acid Pickling (Cleaning) is Used

Laser cutting, welding, and hot working leave a discolored oxidized layer or scale on the surface of the worked steel. This must be removed in order to perform many of the surface finishing processes. Acid pickling [1] is used to remove this scale. Acid cleaning is also used for removing inorganic contaminant not removable by other primary cleaning solutions. Acid cleaning has its limitations in that it is difficult to handle because of it corrosiveness and in not applicable to all steels. Hydrogen embrittlement becomes a problem for some alloys and high-carbon steels [2]. The hydrogen from the acid reacts with the surface and makes it brittle and crack. Because of its high reactance to treatable steels, acid concentrations and solution temperatures must be kept under control to assure desired pickling rates.

[edit] Waste products

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 sulfate 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.

[edit] Jewelry

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.