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Pickling is a metal surface treatment used to remove impurities, such as stains, inorganic contaminants, and rust or scale from ferrous metals, copper, precious metals and aluminum alloys.[1] A solution called pickle liquor, which usually contains acid, is used to remove the surface impurities. It is commonly used to descale or clean steel in various steelmaking processes.
Process
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Metal surfaces can contain impurities that may affect usage of the product or further processing like plating with metal or painting. Various chemical solutions are usually used to clean these impurities. Strong acids, such as hydrochloric acid and sulfuric acid are common, but different applications use various other acids. Also alkaline solutions can be used for cleaning metal surfaces. Solutions usually also contain additives such as wetting agents and corrosion inhibitors. Pickling is sometimes called acid cleaning if descaling is not needed.[2][3]
Many hot working processes and other processes that occur at high temperatures leave a discoloring oxide layer or scale on the surface. In order to remove the scale the workpiece is dipped into a vat of pickle liquor. Prior to cold rolling operation, hot rolled steel is normally passed through a pickling line so as to eradicate the scale from the surface.
The primary acid used in steelmaking is hydrochloric acid, although sulfuric acid was previously more common. Hydrochloric acid is more expensive than sulfuric acid, but it pickles much faster while minimizing base metal loss. The speed is a requirement for integration in automatic steel mills that run production at speeds as high as 800 ft/min (&#;243 metres/min).[4]
Carbon steels, with an alloy content less than or equal to 6%, are often pickled in hydrochloric or sulfuric acid. Steels with an alloy content greater than 6% must be pickled in two steps and other acids are used, such as phosphoric, nitric and hydrofluoric acid. Rust- and acid-resistant chromium-nickel steels are pickled traditionally in a bath of hydrofluoric and nitric acid.[5] Most copper alloys are pickled in dilute sulfuric acid, but brass is pickled in concentrated sulfuric and nitric acid mixed with sodium chloride and soot.[1]
In jewelry making, pickling is used to remove the copper oxide layer that results from heating copper and sterling silver during soldering and annealing. A diluted sulfuric acid pickling bath is traditionally used,[6] but may be replaced with citric acid.
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.
Disadvantages
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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, causing cracks. Because of its high reactivity with treatable steels, acid concentrations and solution temperatures must be kept under control to ensure desired pickling rates.
Waste products
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Pickling sludge is the waste product from pickling, and includes acidic rinse waters, iron chlorides, and metallic salts and waste acid.[7] Spent pickle liquor is considered a hazardous waste by the EPA.[8] Pickle sludge from steel processes is usually neutralized with lime and disposed of in a landfill since the EPA no longer deems it a hazardous waste after neutralization.[8] The lime neutralization process raises the pH of the spent acid. The waste material is subject to a waste determination to ensure no characteristic or listed waste is present.[9] Since the s, hydrochloric pickling sludge is often treated in a hydrochloric acid regeneration system, which recovers some of the hydrochloric acid and ferric oxide. The rest must still be neutralized and disposed of in land fills[10] or managed as a hazardous waste based on the waste profile analysis.[9] The by-products of nitric acid pickling are marketable to other industries, such as fertilizer processors.[11]
Alternatives
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Smooth clean surface (SCS) and eco pickled surface (EPS) are more recent alternatives. 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.[citation needed] EPS is a more direct replacement for acid pickling. Acid pickling relies on chemical reactions while EPS uses mechanical means.[which?] The EPS process is considered "environmentally friendly" compared with acid pickling[by whom?] and it imparts to carbon steel a high degree of rust resistance,[how?] eliminating the need to apply the oil coating that serves as a barrier to oxidation for acid-pickled carbon steel.[citation needed]
Alternative methods include also mechanical cleaning such as abrasive blasting, grinding, wire brushing, hydrocleaning and Laser cleaning. These methods generally do not provide as clean a surface as pickling does.[12][13]
See also
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References
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Rolling steel is a nonsubtractive metal processing technique that utilizes a series of rollers to form steel. Rolling steel allows for thick slabs of steel to be rolled into narrower sheets. Along with being rolled to specific thicknesses, the act of rolling itself work hardens the material. There are two primary rolling techniques &#; hot rolling and cold rolling. Both techniques provide different benefits useful for different applications.
Hot rolling is a process that applies heat to the steel before it is passed through a series of rollers, making it easier to form it into the desired shape. Hot rolled steel becomes cold rolled steel as its components undergo additional processing after the hot rolling process is complete and that steel has cooled at room temperature. The steel becomes cold rolled when it is once again put through a series of rollers without the application of heat. This allows for greater precision regarding the specifications of the end product.
Zeeco Metals warehouses and slits both hot rolled pickled and oiled material and cold rolled material as well as several other types of steel coils. All steel carried by Zeeco Metals, consists of the following grades:
After hot rolling steel, it is common for steel to carry surface contaminants such as mill scale. Pickling and oiling are processes that can remove contaminants and prevent future corrosion during storage.
The pickling process is a procedure where hot rolled steel is run through a highly acidic cleaner, called pickling liquor, to remove impurities from the material. After pickling, the steel is at risk of rapid corrosion from atmospheric exposure. To remedy this problem, the pickled steel is immediately oiled to prevent oxidation. After being oiled, the steel can be stored for long periods with minimal risk of oxidation, which would cause rusting to occur. It is typically recommended that the oil remains in place until the metal is ready for fabrication.
There are many benefits to using hot rolled pickled and oiled steel, including:
Hot rolled pickled and oiled steel is best utilized in environments that are likely to rust the steel and where the application requires specific dimensional parameters. The oil that is applied to the steel promotes resistance to corrosion and allows the steel to be used for parts in automotive, agriculture, construction, and tubing products.
Cold rolled steel is hot rolled steel that undergoes additional processing. After hot rolling, the steel is allowed to cool and is then re-rolled at room temperature. Since the steel is no longer hot, greater amounts of pressure are required to compress the steel into its desired shape. This process requires more work, but results in tighter dimensional tolerances and an improved surface finish.
There are many benefits to cold rolling steel, including:
Although more expensive, cold rolled steel is preferred when precision and optimal surface quality is needed. Popular applications include aerospace parts, metal furniture, at-home appliances, and mechanical components.
For over four decades, Zeeco Metals has been an ISO certified and trusted supplier of flat-rolled coil steel products. We pride ourselves on offering quick turnaround times that can be measured in days and weeks for our slitting services. We hold ourselves to the highest business standards&#; from our production capabilities, impeccable customer service, and our competitive pricing. Contact us to learn more about our current inventory of hot rolled pickled and oiled and cold rolled steel coils.
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