Chromium

The strengthening effect of forming stable metal carbides at grain boundaries, and the strong increase in corrosion resistance made chromium an important alloying material for steel. High-speed tool steels contain between 3 and 5% chromium. Stainless steel, the primary corrosion-resistant metal alloy, is formed when chromium is introduced to iron in concentrations above 11%.[58] For stainless steel's formation, ferrochromium is added to the molten iron. Also, nickel-based alloys have increased strength due to the formation of discrete, stable, metal, carbide particles at the grain boundaries. For example, Inconel 718 contains 18.6% chromium. Because of the excellent high-temperature properties of these nickel superalloys, they are used in jet engines and gas turbines in lieu of common structural materials.[59] ASTM B163 relies on Chromium for condenser and heat-exchanger tubes, while castings with high strength at elevated temperatures that contain Chromium are standardised with ASTM A567.[60] AISI type 332 is used where high temperature would normally cause carburization, oxidation or corrosion.[61] Incoloy 800 "is capable of remaining stable and maintaining its austenitic structure even after long time exposures to high temperatures".[62] Nichrome is used as resistance wire for heating elements in things like toasters and space heaters. These uses make chromium a strategic material. Consequently, during World War II, U.S. road engineers were instructed to avoid chromium in yellow road paint, as it "may become a critical material during the emergency."[63] The United Stated likewise considered chromium "essential for the German war industry" and made intense diplomatic efforts to keep it out of the hands of Nazi Germany.[64]

The high hardness and corrosion resistance of unalloyed chromium makes it a reliable metal for surface coating; it is still the most popular metal for sheet coating, with its above-average durability, compared to other coating metals.[65] A layer of chromium is deposited on pretreated metallic surfaces by electroplating techniques. There are two deposition methods: thin, and thick. Thin deposition involves a layer of chromium below 1 µm thickness deposited by chrome plating, and is used for decorative surfaces. Thicker chromium layers are deposited if wear-resistant surfaces are needed. Both methods use acidic chromate or dichromate solutions. To prevent the energy-consuming change in oxidation state, the use of chromium(III) sulfate is under development; for most applications of chromium, the previously established process is used.[54]

In the chromate conversion coating process, the strong oxidative properties of chromates are used to deposit a protective oxide layer on metals like aluminium, zinc, and cadmium. This passivation and the self-healing properties of the chromate stored in the chromate conversion coating, which is able to migrate to local defects, are the benefits of this coating method.[66] Because of environmental and health regulations on chromates, alternative coating methods are under development.[67]

Chromic acid anodizing (or Type I anodizing) of aluminium is another electrochemical process that does not lead to the deposition of chromium, but uses chromic acid as an electrolyte in the solution. During anodization, an oxide layer is formed on the aluminium. The use of chromic acid, instead of the normally used sulfuric acid, leads to a slight difference of these oxide layers.[68]
The high toxicity of Cr(VI) compounds, used in the established chromium electroplating process, and the strengthening of safety and environmental regulations demand a search for substitutes for chromium, or at least a change to less toxic chromium(III) compounds.[54]

Pigment[edit]

The mineral crocoite (which is also lead chromate PbCrO4) was used as a yellow pigment shortly after its discovery. After a synthesis method became available starting from the more abundant chromite, chrome yellow was, together with cadmium yellow, one of the most used yellow pigments. The pigment does not photodegrade, but it tends to darken due to the formation of chromium(III) oxide. It has a strong color, and was used for school buses in the United States and for the Postal Service (for example, the Deutsche Post) in Europe. The use of chrome yellow has since declined due to environmental and safety concerns and was replaced by organic pigments or other alternatives that are free from lead and chromium. Other pigments that are based around chromium are, for example, the deep shade of red pigment chrome red, which is simply lead chromate with lead(II) hydroxide (PbCrO4·Pb(OH)2). A very important chromate pigment, which was used widely in metal primer formulations, was zinc chromate, now replaced by zinc phosphate. A wash primer was formulated to replace the dangerous practice of pre-treating aluminium aircraft bodies with a phosphoric acid solution. This used zinc tetroxychromate dispersed in a solution of polyvinyl butyral. An 8% solution of phosphoric acid in solvent was added just before application. It was found that an easily oxidized alcohol was an essential ingredient. A thin layer of about 10–15 µm was applied, which turned from yellow to dark green when it was cured. There is still a question as to the correct mechanism. Chrome green is a mixture of Prussian blue and chrome yellow, while the chrome oxide green is chromium(III) oxide.[69]

Chromium oxides are also used as a green pigment in the field of glassmaking and also as a glaze for ceramics.[70] Green chromium oxide is extremely lightfast and as such is used in cladding coatings. It is also the main ingredient in infrared reflecting paints, used by the armed forces to paint vehicles and to give them the same infrared reflectance as green leaves.[71]

Other uses[edit]

Chromium(III) ions present in corundum crystals (aluminium oxide) cause them to be colored red; when corundum appears as such, it is known as a ruby. If the corundum is lacking in chromium(III) ions, it is known as a sapphire.[note 3] A red-colored artificial ruby may also be achieved by doping chromium(III) into artificial corundum crystals, thus making chromium a requirement for making synthetic rubies.[note 4][72] Such a synthetic ruby crystal was the basis for the first laser, produced in 1960, which relied on stimulated emission of light from the chromium atoms in such a crystal. Ruby has a laser transition at 694.3 nanometers, in a deep red color.[73]

Because of their toxicity, chromium(VI) salts are used for the preservation of wood. For example, chromated copper arsenate (CCA) is used in timber treatment to protect wood from decay fungi, wood-attacking insects, including termites, and marine borers.[74] The formulations contain chromium based on the oxide CrO3 between 35.3% and 65.5%. In the United States, 65,300 metric tons of CCA solution were used in 1996.[74]

Chromium(III) salts, especially chrome alum and chromium(III) sulfate, are used in the tanning of leather. The chromium(III) stabilizes the leather by cross linking the collagen fibers.[75] Chromium tanned leather can contain between 4 and 5% of chromium, which is tightly bound to the proteins.[37] Although the form of chromium used for tanning is not the toxic hexavalent variety, there remains interest in management of chromium in the tanning industry. Recovery and reuse, direct/indirect recycling,[76] and "chrome-less" or "chrome-free" tanning are practiced to better manage chromium usage.[77]

The high heat resistivity and high melting point makes chromite and chromium(III) oxide a material for high temperature refractory applications, like blast furnaces, cement kilns, molds for the firing of bricks and as foundry sands for the casting of metals. In these applications, the refractory materials are made from mixtures of chromite and magnesite. The use is declining because of the environmental regulations due to the possibility of the formation of chromium(VI).[55] [78]

Several chromium compounds are used as catalysts for processing hydrocarbons. For example, the Phillips catalyst, prepared from chromium oxides, is used for the production of about half the world's polyethylene.[79] Fe-Cr mixed oxides are employed as high-temperature catalysts for the water gas shift reaction.[80][81] Copper chromite is a useful hydrogenation catalyst.[82]

Uses of compounds[edit]

The biologically beneficial effects of chromium(III) are debated.[91][92] Chromium is accepted by the U.S. National Institutes of Health as a trace element for its roles in the action of insulin, a hormone that mediates the metabolism and storage of carbohydrate, fat, and protein.[7] The mechanism of its actions in the body, however, have not been defined, leaving in question the essentiality of chromium.[93][94]

In contrast, hexavalent chromium (Cr(VI) or Cr6+) is highly toxic and mutagenic.[95] Ingestion of chromium(VI) in water has been linked to stomach tumors, and it may also cause allergic contact dermatitis (ACD).[96]

"Chromium deficiency", involving a lack of Cr(III) in the body, or perhaps some complex of it, such as glucose tolerance factor, is controversial.[7] Some studies suggest that the biologically active form of chromium (III) is transported in the body via an oligopeptide called low-molecular-weight chromium-binding substance (LMWCr), which might play a role in the insulin signaling pathway.[97]

The chromium content of common foods is generally low (1-13 micrograms per serving).[7][98] The chromium content of food varies widely, due to differences in soil mineral content, growing season, plant cultivar, and contamination during processing.[98] Chromium (and nickel) leach into food cooked in stainless steel, with the effect being largest when the cookware is new. Acidic foods that are cooked for many hours also exacerbate this effect.[99][100]

Dietary recommendations[edit]

There is disagreement on chromium's status as an essential nutrient. Governmental departments from Australia, New Zealand, India, Japan, and the United States consider chromium essential[101][102][103][104] while the European Food Safety Authority (EFSA), representing the European Union, does not.[105]

The U.S. National Academy of Medicine (NAM) updated the Estimated Average Requirements (EARs) and the Recommended Dietary Allowances (RDAs) for chromium in 2001. For chromium, there was insufficient information to set EARs and RDAs, so its needs are described as estimates for Adequate Intakes (AIs). The current AIs of chromium for women ages 14 through 50 is 25 μg/day, and the AIs for women ages 50 and above is 20 μg/day. The AIs for women who are pregnant are 30 μg/day, and for women who are lactating, the set AIs are 45 μg/day. The AIs for men ages 14 through 50 are 35 μg/day, and the AIs for men ages 50 and above are 30 μg/day. For children ages 1 through 13, the AIs increase with age from 0.2 μg/day up to 25 μg/day. As for safety, the NAM sets Tolerable Upper Intake Levels (ULs) for vitamins and minerals when the evidence is sufficient. In the case of chromium, there is not yet enough information, hence no UL has been established. Collectively, the EARs, RDAs, AIs, and ULs are the parameters for the nutrition recommendation system known as Dietary Reference Intake (DRI).[104] Australia and New Zealand consider chromium to be an essential nutrient, with an AI of 35 μg/day for men, 25 μg/day for women, 30 μg/day for women who are pregnant, and 45 μg/day for women who are lactating. A UL has not been set due to the lack of sufficient data.[101] India considers chromium to be an essential nutrient, with an adult recommended intake of 33 μg/day.[102] Japan also considers chromium to be an essential nutrient, with an AI of 10 μg/day for adults, including women who are pregnant or lactating. A UL has not been set.[103] The EFSA of the European Union however, does not consider chromium to be an essential nutrient; chromium is the only mineral for which the United States and the European Union disagree.[105][106]

For U.S. food and dietary supplement labeling purposes, the amount of the substance in a serving is expressed as a percent of the Daily Value (%DV). For chromium labeling purposes, 100% of the Daily Value was 120 μg. As of May 27, 2016, the percentage of daily value was revised to 35 μg to bring the chromium intake into a consensus with the official Recommended Dietary Allowance.[107][108] Compliance with the updated labeling regulations was required by 1 January 2020 for manufacturers with US$10 million or more in annual food sales, and by 1 January 2021 for manufacturers with lower volume food sales.[109][110] A table of the old and new adult daily values is provided at Reference Daily Intake.

Food sources[edit]

Food composition databases such as those maintained by the U.S. Department of Agriculture do not contain information on the chromium content of foods.[111] A wide variety of animal and vegetable foods contain chromium.[104] Content per serving is influenced by the chromium content of the soil in which the plants are grown, by foodstuffs fed to animals, and by processing methods, as chromium is leached into foods if processed or cooked in stainless steel equipment.[112] One diet analysis study conducted in Mexico reported an average daily chromium intake of 30 micrograms.[113] An estimated 31% of adults in the United States consume multi-vitamin/mineral dietary supplements,[114] which often contain 25 to 60 micrograms of chromium.

Supplementation[edit]