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 aluminium 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.
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.
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.
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 1960s, 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]
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]
Nitric acid is an inorganic compound with the formula HNO3. It is a highly corrosive mineral acid. The compound is colorless, but samples tend to acquire a yellow cast over time due to decomposition into oxides of nitrogen. Most commercially available nitric acid has a concentration of 68% in water. When the solution contains more than 86% HNO3, it is referred to as fuming nitric acid. Depending on the amount of nitrogen dioxide present, fuming nitric acid is further characterized as red fuming nitric acid at concentrations above 86%, or white fuming nitric acid at concentrations above 95%.
Rust is an iron oxide, a usually reddish-brown oxide formed by the reaction of iron and oxygen in the catalytic presence of water or air moisture. Rust consists of hydrous iron(III) oxides (Fe2O3·nH2O) and iron(III) oxide-hydroxide (FeO(OH), Fe(OH)3), and is typically associated with the corrosion of refined iron.
Stainless steel, also known as inox, corrosion-resistant steel (CRES), and rustless steel, is an iron-based alloy containing a minimum level of chromium that is resistant to rusting and corrosion. Stainless steel's resistance to corrosion results from the 10.5%, or more, chromium content which forms a passive film that can protect the material and self-heal in the presence of oxygen. It can also be alloyed with other elements such as molybdenum, carbon, nickel and nitrogen to develop a range of different properties depending on its specific use.
Corrosion is a natural process that converts a refined metal into a more chemically stable oxide. It is the gradual deterioration of materials by chemical or electrochemical reaction with their environment. Corrosion engineering is the field dedicated to controlling and preventing corrosion.
In physical chemistry and engineering, passivation is coating a material so that it becomes "passive", that is, less readily affected or corroded by the environment. Passivation involves creation of an outer layer of shield material that is applied as a microcoating, created by chemical reaction with the base material, or allowed to build by spontaneous oxidation in the air. As a technique, passivation is the use of a light coat of a protective material, such as metal oxide, to create a shield against corrosion. Passivation of silicon is used during fabrication of microelectronic devices. Undesired passivation of electrodes, called "fouling", increases the circuit resistance so it interferes with some electrochemical applications such as electrocoagulation for wastewater treatment, amperometric chemical sensing, and electrochemical synthesis.
Hot-dip galvanization is a form of galvanization. It is the process of coating iron and steel with zinc, which alloys with the surface of the base metal when immersing the metal in a bath of molten zinc at a temperature of around 450 °C (842 °F). When exposed to the atmosphere, the pure zinc (Zn) reacts with oxygen (O2) to form zinc oxide (ZnO), which further reacts with carbon dioxide (CO2) to form zinc carbonate (ZnCO3), a usually dull grey, fairly strong material that protects the steel underneath from further corrosion in many circumstances. Galvanized steel is widely used in applications where corrosion resistance is needed without the cost of stainless steel, and is considered superior in terms of cost and life-cycle. It can be identified by the crystallization patterning on the surface (often called a "spangle").
Patina is a thin layer that variously forms on the surface of copper, brass, bronze, and similar metals and metal alloys, or certain stones and wooden furniture, or any similar acquired change of a surface through age and exposure.
In metallurgy, a flux is a chemical reducing agent, flowing agent, or purifying agent. Fluxes may have more than one function at a time. They are used in both extractive metallurgy and metal joining.
Chrome plating is a technique of electroplating a thin layer of chromium onto a metal object. A chrome plated part is called chrome, or is said to have been chromed. The chromium layer can be decorative, provide corrosion resistance, facilitate cleaning, and increase surface hardness. Sometimes a less expensive substitute for chrome, such as nickel, may be used for aesthetic purposes.
Industrial processes are procedures involving chemical, physical, electrical, or mechanical steps to aid in the manufacturing of an item or items, usually carried out on a very large scale. Industrial processes are the key components of heavy industry.
Anodizing is an electrolytic passivation process used to increase the thickness of the natural oxide layer on the surface of metal parts.
Pitting corrosion, or pitting, is a form of extremely localized corrosion that leads to the random creation of small holes in metal. The driving power for pitting corrosion is the depassivation of a small area, which becomes anodic while an unknown but potentially vast area becomes cathodic, leading to very localized galvanic corrosion. The corrosion penetrates the mass of the metal, with a limited diffusion of ions.
Bluing, sometimes spelled as blueing, is a passivation process in which steel is partially protected against rust using a black oxide coating. It is named after the blue-black appearance of the resulting protective finish. Bluing involves an electrochemical conversion coating resulting from an oxidizing chemical reaction with iron on the surface selectively forming magnetite, the black oxide of iron. In comparison, rust, the red oxide of iron, undergoes an extremely large volume change upon hydration; as a result, the oxide easily flakes off, causing the typical reddish rusting away of iron. Black oxide provides minimal protection against corrosion, unless also treated with a water-displacing oil to reduce wetting and galvanic action. In colloquial use, thin coatings of black oxide are often termed "gun bluing", while heavier coatings are termed "black oxide". Both refer to the same chemical process for providing true gun bluing.
Hydrochloric acid regeneration or HCl regeneration is a chemical process for the reclamation of bound and unbound HCl from metal chloride solutions such as hydrochloric acid.
In metallurgy, refining consists of purifying an impure metal. It is to be distinguished from other processes such as smelting and calcining in that those two involve a chemical change to the raw material, whereas in refining the final material is chemically identical to the raw material. Refining thus increases the purity of the raw material via processing. There are many processes including pyrometallurgical and hydrometallurgical techniques.
Nitinol biocompatibility is an important factor in biomedical applications. Nitinol (NiTi), which is formed by alloying nickel and titanium, is a shape-memory alloy with superelastic properties more similar to that of bone, when compared to stainless steel, another commonly used biomaterial. Biomedical applications that utilize nitinol include stents, heart valve tools, bone anchors, staples, septal defect devices and implants. It is a commonly used biomaterial especially in the development of stent technology.
Smooth clean surface (SCS) is a process applied to hot rolled sheet metal and coils to remove nearly all mill scale and clean the steel surface.
Hydrochloric acid, also known as muriatic acid or spirits of salt, is an aqueous solution of hydrogen chloride (HCl). It is a colorless solution with a distinctive pungent smell. It is classified as a strong acid. It is a component of the gastric acid in the digestive systems of most animal species, including humans. Hydrochloric acid is an important laboratory reagent and industrial chemical.
Eco pickled surface (EPS) is a process applied to hot rolled sheet steel to remove all surface oxides and clean the steel surface. Steel which has undergone the EPS process acquires a high degree of resistance to subsequent development of surface oxide (rust), so long as it does not come into direct contact with moisture. EPS was developed by The Material Works, Ltd., which has filed several patent applications covering the process. It is primarily intended to be a replacement of the acid pickling process wherein steel strip is immersed in solutions of hydrochloric and sulfuric acids to chemically remove oxides.
Cobalt extraction refers to the techniques used to extract cobalt from its ores and other compound ores. Several methods exist for the separation of cobalt from copper and nickel. They depend on the concentration of cobalt and the exact composition of the ore used.
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