Pickling is a metal surface treatment used to remove impurities, such as stains, inorganic contaminants, rust or crust from ferrous metals, copper, precious metals, and aluminum alloys. A solution called liquor, usually acidic, is used to remove surface impurities. This is usually used for cleaning or cleaning steel in various steel-making processes.
Video Pickling (metal)
Process
Metal surfaces may contain impurities that may affect product use or further processing such as coating 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 also use a variety of other acids. Alkali solution can also be used to clean metal surfaces. Solutions usually also contain additives such as wetting agents and corrosion inhibitors. Preservation is sometimes called acid cleaning if the crust is not needed.
Many hot work processes and other processes that occur at high temperatures leave a layer of oxide that changes color or scale on the surface. To remove the workpiece scale dipped in a pickle liquor vat. Before cold rolling operations, hot rolled steel is usually passed through a pickling line so as to eradicate scale from the surface.
The main acid used in steelmaking is hydrochloric acid, although sulfuric acid was previously more common. Hydrochloric acid is more expensive than sulfuric acid, but the pickle is much faster while minimizing the loss of base metal. Speed ​​is a requirement for integration in automatic steel plants that run production at high speed; speeds as high as 800 feet/min (243 meters/min) have been reported.
Carbon steels, with alloying content of less than or equal to 6%, are often acidified in hydrochloric or sulfuric acid. Steels with an alloy content of more than 6% should be acidified in two steps and other acids are used, such as phosphoric acid, nitrate and hydrofluorate. Stainless and acid-stainless nickel-chromium-nickel is acidified in aqueous solutions of hydrochloric and nitrate acids. Most copper alloys are acidified in dilute sulfuric acid, but the brass is acidified in concentrated sulfuric acid and nitrate mixed with sodium chloride and soot.
In jewelry making, preservation is used to remove the copper oxide layers produced from heating of copper and silver during soldering and softening. Aqueous sulfuric preservative baths are traditionally used, but can be replaced with citric acid.
Sheet steel that undergoes acid preservation will oxidize (rust) when exposed to atmospheric conditions with high moisture. For this reason, thin films of oil or similar waterproof coatings are applied to create moisture barriers in the air. This oil film should then be discarded for many fabrication, coating or painting processes.
Maps Pickling (metal)
Disadvantages
Acid cleansing has limitations because it is difficult to handle due to its corrosive nature, and does not apply to all steel. Embrittlement of hydrogen becomes a problem for some alloys and high carbon steels. Hydrogen from the acid reacts with the surface and makes it brittle and causes cracks. Due to the high reactance of the processible steel, the acid concentration and the temperature of the solution must be kept under control to ensure the desired level of preservation.
The waste product
Pickling Mud is a waste product of preservation, and includes acid rinsing water, iron chloride, and metal salts and waste acids. Pickled liquor consumed is considered a hazardous waste by the EPA. The pickled sludge from the steel process is usually neutralized with lime and dumped at landfills because the EPA no longer considers it a hazardous waste after neutralization. The process of lime neutralization increases the pH of the spent acid. The waste material is subject to the determination of waste to ensure no characteristics or waste are listed. Since the 1960s, hydrochloric preservation mud is often treated in hydrochloric acid regeneration systems, which recover some hydrochloric acid and iron oxide. The remainder should still be neutralized and disposed of in land that is filled or managed as hazardous waste based on the analysis of the waste profile. The byproducts of nitric acid preservation can be marketed to other industries, such as fertilizer processors.
Alternative
Smooth net surface (SCS) and eco pickled surface (EPS) are a newer alternative. In the SCS process, surface oxidation is removed by using engineered abrasion and the process leaves the surface resistant to subsequent oxidation without the need for oil film or other protective films. EPS is a more direct replacement for acid preservation. Pickled acids depend on chemical reactions while EPS uses mechanical means. The EPS process is considered "environmentally friendly" compared to acid preservation and imparts to high carbon stainless steels, eliminating the need to apply an oil layer that serves as a barrier for oxidation for pickled carbon steels.
Alternative methods include mechanical cleaning such as abrasive blasting, milling, wire polishing and hydrocleaning. These methods generally do not provide a clean surface such as acidification.
References
Source of the article : Wikipedia
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