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Pickling:

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Acid Cleaning for Scale Removal

Pickling is a finishing process in which heavy oxide crust or scale that develops on iron or strip steel, steel wire, and some other forms of steel surface during hot forming or heat treating in air, is removed chemically by immersion in a bath of suitable mixtures of acids called pickle liquor.

Most used to treat Carbon steel products, with an alloy content less than or equal to 6% is generally a solution of either hydrochloric (HCl) or sulfuric acid (H₂SO₄).

Hydrochloric acid is more expensive than sulfuric acid, but its action is much faster while minimizing base metal loss. It avoids the formation of smut and decreases the danger of overpickling. It also permits to economize on heating expenses because the process runs at lower temperature than that based on sulfuric acid.

Scale must be thoroughly removed before applying further finishing layers like plating or paint, that otherwise would not make contact with the metal, but most important would not protect the metal surfaces.

Results of this cleaning process depend on the type, strength and temperature of the acid mix employed, but it is generally agreed that it is an economic process adaptable to continuous cleaning of large quantities of materials.

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Sheet steel processed by acid Pickling will oxidize (rust) rapidly when exposed to moist atmospheric conditions. To prevent this, a thin film of oil or similar waterproof coating is applied to avoid contact of pickled steel with moisture in the air. The oil film must be removed for any further fabrication, plating or painting processes.

Inhibitors are natural or synthetic organic compounds that are added to acid solutions to protect plain carbon steels being cleaned, by slowing or stopping the chemical action of the acids on the base metal.

Inhibitors reduce the loss of iron, minimize hydrogen embrittlement, protect the surface from pitting attack caused by excessive pickling or from poor surface condition quality, reduce fumes resulting from excessive reaction of acids on metal, and help economize on acid consumption, without affecting appreciably the rate of scale or rust removal.

For Pickling alloy steels, uninhibited acid solutions are preferred because a more energetic chemical action is required to remove the oxides. If the surfaces covered with scale and rust are generally and uniformly clean from additional contaminants and if the Pickling time is not critical, no other pretreatment is required.

If the rusted or scaled surfaces submitted to Pickling are contaminated by soils that do not react with acids, like grease, oils, soaps, lubricants and other coatings, then they must be precleaned by an additional process, usually alkaline cleaning, especially advantageous when acid Pickling time is limited to a few seconds in a continuous working line.

Limitations of acid cleaning consist in the difficulty to handle the products because of their corrosiveness, and in the problem of hydrogen embrittlement that may significantly affect some alloy and high-carbon steels. The hydrogen liberated from the acid reacts with the surface, makes it brittle and may cause cracks. To assure desired pickling rates without endangering these steels, acid concentrations and solution temperatures must be kept under stricter control.

Pickling sludge is the waste product from this process, that includes acidic rinse waters, metallic salts and waste spent acid. EPA considers all these as hazardous waste to be treated before being disposed of. Sludge from steel processes should be neutralized with lime before being discharged in a land fill.

Those engaging in these processes should adopt responsible attitudes, be familiar with environmental protection requirements and make sure to meet them always in their small or big industrial activities.

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