Pickling (metal)

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 choiceFact|date=February 2007 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.

Process

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 is used to remove this scale. Acid cleaning is also used for removing inorganic contaminants not removable by other primary cleaning solutions. Acid cleaning has limitations in that it is difficult to handle because of its corrosiveness, and it is not applicable to all steels. Hydrogen embrittlement becomes a problem for some alloys and high-carbon steels. The hydrogen from the acid reacts with the surface and makes it brittle and causes cracks. Because of its high reactance to treatable steels, acid concentrations and solution temperatures must be kept under control to assure desired pickling rates.

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.

Waste products

Waste waters from pickling include acidic rinse waters, metallic 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 acidic ferrous sulfate or ferrous chloride to be disposed of. Since the 1960s total hydrochloric acid regeneration processes have reached widespread acceptance.Fact|date=February 2007 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 on-site by steel manufacturers.

Alternatives to acid pickling

Sheet steel that undergoes acid pickling will oxidize (rust) when exposed to atmospheric conditions of moderately high humidity. For this reason, a thin film of oil or similar waterproof coating is applied to create a barrier to moisture in the air. This oil film must later be removed for many fabrication, plating or painting processes.

A process was developed in 2003 known as smooth clean surface (SCS), [ [http://www.metalcenternews.com/2005/november/mcn05111cs_scs.htm "SCS Polishes TMW's Prospects" by Tim Triplett, Metal Center News, November 2005.] ] representing an alternative to acid pickling. In the SCS process, surface oxidation is removed using an engineered abrasive and the process leaves the surface resistant to subsequent oxidation without the need for oil film or other protective coating.

Pickle liquor

Pickle liquor is an acid solution used to descale or clean steel in various steelmaking processes. Typically, the acids employed in the pickling of steel are hydrochloric, nitric, sulfuric and hydrofluoric acids or combinations thereof. When the solution becomes spent, pickle liquor is considered a hazardous waste by EPA.

Through the late 1980s, spent pickle liquor was traditionally land disposed by steel manufacturers after lime neutralization. The lime neutralization process raised the pH of the spent acid and made heavy metals in the sludge less likely to leach into the environment. Today however, the majority of spent pickle liquor is recycled or regenerated on-site by steel manufacturers.

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