Chromium
Industrial production of chromium proceeds from chromite ore (mostly FeCr2O4) to produce ferrochromium, an iron-chromium alloy, by means of aluminothermic or silicothermic reactions.Pure chromium metal is produced by a different process: roasting and leaching of chromite to separate it from iron, followed by reduction with carbon and then aluminium.In contrast, iron forms a more porous oxide which is weak and flakes easily and exposes fresh metal to the air, causing continued rusting.Modern archaeologists discovered that bronze-tipped crossbow bolts at the tomb of Qin Shi Huang showed no sign of corrosion after more than 2,000 years, because they had been coated in chromium.[50][51] In multiple Warring States period tombs, sharp jians and other weapons were also found to be coated with 10 to 15 micrometers of chromium oxide, which left them in pristine condition to this day.On 26 July 1761, Johann Gottlob Lehmann found an orange-red mineral in the Beryozovskoye mines in the Ural Mountains which he named Siberian red lead.[56] In 1770, Peter Simon Pallas visited the same site as Lehmann and found a red lead mineral that was discovered to possess useful properties as a pigment in paints.In 1827, a larger chromite deposit was discovered near Baltimore, United States, which quickly met the demand for tanning salts much more adequately than the crocoite that had been used previously.[62] This made the United States the largest producer of chromium products until the year 1848, when larger deposits of chromite were uncovered near the city of Bursa, Turkey.For the production of ferrochromium, the chromite ore (FeCr2O4) is reduced in large scale in electric arc furnace or in smaller smelters with either aluminium or silicon in an aluminothermic reaction.[67] 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.Also, nickel-based alloys have increased strength due to the formation of discrete, stable, metal, carbide particles at the grain boundaries.[73] The United States likewise considered chromium "essential for the German war industry" and made intense diplomatic efforts to keep it out of the hands of Nazi Germany.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.A wash primer was formulated to replace the dangerous practice of pre-treating aluminium aircraft bodies with a phosphoric acid solution.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.[101][102] Although chromium is regarded as a trace element and dietary mineral, its suspected roles in the action of insulin – a hormone that mediates the metabolism and storage of carbohydrate, fat, and protein – have not been adequately established.[13][14] 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 (chromodulin), which might play a role in the insulin signaling pathway.[13][15] The U.S. National Academy of Medicine (NAM) updated the Estimated Average Requirements (EARs) and the Recommended Dietary Allowances (RDAs) for chromium in 2001.Collectively, the EARs, RDAs, AIs, and ULs are the parameters for the nutrition recommendation system known as Dietary Reference Intake (DRI).[15][113][114] 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).As of 27 May 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.[122] Although the mechanism of action in biological roles for chromium is unclear, in the United States chromium-containing products are sold as non-prescription dietary supplements in amounts ranging from 50 to 1,000 μg.[13][123] The notion of chromium as a potential regulator of glucose metabolism began in the 1950s when scientists performed a series of experiments controlling the diet of rats.[124] The experimenters subjected the rats to a chromium deficient diet, and witnessed an inability to respond effectively to increased levels of blood glucose.[124] In 2005, the U.S. Food and Drug Administration had approved a qualified health claim for chromium picolinate with a requirement for specific label wording: In other parts of the petition, the FDA rejected claims for chromium picolinate and cardiovascular disease, retinopathy or kidney disease caused by abnormally high blood sugar levels.Looking at the results from four meta-analyses, one reported a statistically significant decrease in fasting plasma glucose levels and a non-significant trend in lower hemoglobin A1C.One, limited to chromium picolinate, a common supplement ingredient, reported a statistically significant −1.1 kg (2.4 lb) weight loss in trials longer than 12 weeks.[145] In the US, the Occupational Safety and Health Administration (OSHA) has designated an air permissible exposure limit (PEL) in the workplace as a time-weighted average (TWA) of 1 mg/m3.The National Institute for Occupational Safety and Health (NIOSH) has set a recommended exposure limit (REL) of 0.5 mg/m3, time-weighted average.
Chromium (disambiguation)Standard atomic weightperiodic tableHydrogenHeliumLithiumBerylliumCarbonNitrogenOxygenFluorineSodiumMagnesiumAluminiumSiliconPhosphorusSulfurChlorinePotassiumCalciumScandiumTitaniumVanadiumManganeseCobaltNickelCopperGalliumGermaniumArsenicSeleniumBromineKryptonRubidiumStrontiumYttriumZirconiumNiobiumMolybdenumTechnetiumRutheniumRhodiumPalladiumSilverCadmiumIndiumAntimonyTelluriumIodineCaesiumBariumLanthanumCeriumPraseodymiumNeodymiumPromethiumSamariumEuropiumGadoliniumTerbiumDysprosiumHolmiumErbiumThuliumYtterbiumLutetiumHafniumTantalumTungstenRheniumOsmiumIridiumPlatinumMercury (element)ThalliumBismuthPoloniumAstatineFranciumRadiumActiniumThoriumProtactiniumUraniumNeptuniumPlutoniumAmericiumCuriumBerkeliumCaliforniumEinsteiniumFermiumMendeleviumNobeliumLawrenciumRutherfordiumDubniumSeaborgiumBohriumHassiumMeitneriumDarmstadtiumRoentgeniumCoperniciumNihoniumFleroviumMoscoviumLivermoriumTennessineOganessonAtomic numbergroup 6Periodperiod 4Electron configurationMelting pointBoiling pointDensityHeat of fusionkJ/molHeat of vaporizationMolar heat capacityVapor pressureOxidation statesElectronegativityIonization energies