Pickling (metal)

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For other meanings of pickling see pickling (disambiguation).

Pickling is a treatment of metallic surfaces in order to remove impurities, stains, or scales with a solution called pickle liquor before subsequent processing, such as extrusion, rolling, and galvanizing.

Final treatment of iron and steel products before onward sale into manufacturing includes pickling in strong mineral acid to remove rust or iron oxide and prepare the surface for tin or chromium plating or for other surface treatments such as galvanization or painting. The two acids commonly used are hydrochloric acid and sulphuric acid. Wastewaters from pickling include acidic rinse waters together with waste acid. Some plants operate acid recovery plants, (particularly those using hydrochloric acid), 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. However since the 1960s total hydrochloric acid regeneration processes have reached widespread acceptance.

[edit] Pickling of carbon steel

Carbon Steel Grades are pickled by either sulphuric acid or hydrochloric acid.

Sulphuric Acid is the pickling agent of choice for picklers running integrated steel works and obtaining it from their coke oven batteries. The by-product of sulphuric acid pickling, ferrous sulphate, is marketable to a couple of secondary industries including fertilizers.

Hydrochloric Acid is chosen, when bright surfaces, low energy consumption, reduced overpickling and the total recovery of the pickling agent from the waste pickle liquor are desired.

The spent acid from hydrochloric acid pickling of carbon steel is commonly recovered in hydrochloric acid regeneration plants.

[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.