Pickling (metal) 3088803 222380236 2008-06-29T01:24:47Z Mercurybds 7231775 {{Unreferenced|date=July 2007}} {{otheruses|Pickling (disambiguation)}} '''Pickling''' is a treatment of [[metal]]lic surfaces in order to remove impurities, stains, [[rust]] or [[Fouling|scale]] with a solution called [[pickle liquor]], containing [[strong acid|strong mineral acid]]s, before subsequent processing, such as [[extrusion]], [[rolling]], [[paint]]ing, [[galvanization|galvanizing]] or plating with [[tin]] or [[chromium]]. The two [[acid]]s commonly used are [[hydrochloric acid]] and [[sulfuric acid]]. Pickling liquor may be a combination of acids and may also contain [[nitric acid|nitric]] or [[hydrofluoric acid|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{{Fact|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. ==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 [http://www.delstar.com/pickling.htm] 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 [http://www.powder-coater.com/chemical_cleaning.htm]. 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. ==Waste products== Wastewaters from pickling include acidic rinse waters, metallics salts and waste acid. Spent pickle liquor is considered a [[hazardous waste]] by [[United States Environmental Protection Agency|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 [[Iron(II) sulfate|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 [[fertilizer]]s. Through the late 1980s, spent pickle liquor was traditionally land disposed by steel manufacturers after [[Lime (mineral)|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. == Jewelry == Similarly, pickling is used in [[jewelry]] making, after [[Flux (metallurgy)|fluxing]] or [[soldering]], in order to remove any scale and contaminants from the jewelry metal, which is often sterling [[silver]], [[copper]], or [[gold]]. ==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 was process developed in 2003 known as [[SCS: Smooth Clean Surface sheet steel|'''SCS''' for Smooth Clean Surface]]<ref name="a">[http://www.metalcenternews.com/2005/november/mcn05111cs_scs.htm ''SCS Polishes TMW's Prospects'' by Tim Triplett, '''Metal Center News''', November 2005.]</ref><ref name="b">[http://www.scsprocess.com/index.html '''SCS Process web site.''']</ref>, 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. ==Citations== <references/> ==External links== *[http://www.novelindia.com/Stainless_Steel_Pickling_Passivation_Chemical.htm `Fumeless' Pickling and Passivation Gel Chemical for Stainless Steel ] by Novel Surface Treatments *[http://www.delstar.com/pickling.htm] Delstar Metal Finishing Inc. *[http://www.powder-coater.com/chemical_cleaning.htm] Powder coating Manual [[Category:metalworking]] [[de:Beizen]] [[it:Decapaggio]] [[nl:Beitsen]] [[pt:Decapagem]]