Titanium And Chromium

Titanium is a chemical element with the symbol Ti and atomic number 22. It has a low density and is a strong, lustrous, corrosion-resistant (including sea water, aqua regia and chlorine) transition metal with a silver color. Sony VAIO VPCF135Z1E/B battery

Titanium was discovered in Cornwall, Great Britain, by William Gregor in 1791 and named by Martin Heinrich Klaproth for the Titans of Greek mythology. The element occurs within a number of mineral deposits, principally rutile and ilmenite, which are widely distributed in the Earth's crust and lithosphere, and it is found in almost all living things, rocks, water bodies, and soilsSony VAIO VPCF137HG/BI battery.[2] The metal is extracted from its principal mineral ores via the Kroll process[3] or the Hunter process. Its most common compound, titanium dioxide, is a popular photocatalyst and is used in the manufacture of white pigments.[4] Other compounds include titanium tetrachloride (TiCl4), a component of smoke screens and catalysts; and titanium trichloride (TiCl3), which is used as a catalyst in the production of polypropyleneSony VAIO VPCF136FG/BI battery.[2]

Titanium can be alloyed with iron, aluminium, vanadium, molybdenum, among other elements, to produce strong lightweight alloys for aerospace (jet engines, missiles, and spacecraft), military, industrial process (chemicals and petro-chemicals, desalination plants, pulp, and paper), automotive, agri-food, medical prostheses, orthopedic implants, dental and endodontic instruments and filesSony VAIO VPCF135FG/B battery, dental implants, sporting goods, jewelry, mobile phones, and other applications.[2]

The two most useful properties of the metal form are corrosion resistance and the highest strength-to-weight ratio of any metal.[5] In its unalloyed condition, titanium is as strong as some steels, but 45% lighter.[6] There are two allotropic forms[7] and five naturally occurring isotopes of this element, 46Ti through 50Ti, with 48Ti being the most abundant (73.8%).Sony VAIO VPCF127HG/BI battery Titanium's properties are chemically and physically similar to zirconium, because both of them have the same number of valence electrons and are in the same group in the periodic table.

Characteristics

[edit]Physical properties

A metallic element, titanium is recognized for its high strength-to-weight ratio.[7] It is a strong metal with low density that is quite ductile (especially in an oxygen-free environment),[2] lustrous, and metallic-white in color.[9] The relatively high melting point (more than 1,650 °C or 3,000 °F) makes it useful as a refractory metal. It is paramagnetic and has fairly low electrical and thermal conductivity. Sony VAIO VPCF11Z1E battery

Commercial (99.2% pure) grades of titanium have ultimate tensile strength of about 63,000 psi (434 MPa), equal to that of common, low-grade steel alloys, but are 45% lighter.[6] Titanium is 60% more dense than aluminium, but more than twice as strong[6] as the most commonly used 6061-T6 aluminium alloy. Certain titanium alloys (e.g., Beta C) achieve tensile strengths of over 200,000 psi (1,400 MPa). Sony VAIO VPCF11S1E battery However, titanium loses strength when heated above 430 °C (806 °F).[11]

It is fairly hard (although not as hard as some grades of heat-treated steel), non-magnetic and a poor conductor of heat and electricity. Machining requires precautions, as the material will soften and gall if sharp tools and proper cooling methods are not used. Like those made from steel, titanium structures have a fatigue limit which guarantees longevity in some applicationsSony VAIO VPCF11M1E/H battery.[9] Titanium alloys specific stiffnesses are also usually not as good as other materials such as aluminium alloys and carbon fiber, so it is used less for structures which require high rigidity.

The metal is a dimorphic allotrope whose hexagonal alpha form changes into a body-centered cubic (lattice) β form at 882 °C (1,620 °F).[11] The specific heat of the alpha form increases dramatically as it is heated to this transition temperature but then falls and remains fairly constant for the β form regardless of temperatureSony VAIO VPCF11M1E battery.[11] Similar to zirconium and hafnium, an additional omega phase exists, which is thermodynamically stable at high pressures, but is metastable at ambient pressures. This phase is usually hexagonal (ideal) or trigonal (distorted) and can be viewed as being due to a soft longitudinal acoustic phonon of the β phase causing collapse of (111) planes of atoms.[12]

[edit]Chemical properties

The Pourbaix diagram for titanium in pure water, perchloric acid or sodium hydroxideSony VAIO VPCF11JFX/B battery

The most noted chemical property of titanium is its excellent resistance to corrosion; it is almost as resistant as platinum, capable of withstanding attack by dilute sulfuric acid and hydrochloric acid as well as chlorine gas, chloride solutions, and most organic acids.[3] However, it is soluble in concentrated acids.[14] The Pourbaix diagram in the image shows that titanium is actually thermodynamically a very reactive metalSony VAIO VPCF119FJ battery.

However, it is slow to react with water and air, because it forms a passive and protective oxide coating that protects it from further reaction.[2] When it first forms, this protective layer is only 1–2 nm thick but continues to slowly grow; reaching a thickness of 25 nm in four years.[15]

Titanium readily reacts with oxygen at 1,200 °C (2,190 °F) in air, and at 610 °C (1,130 °F) in pure oxygen, forming titanium dioxideSony VAIO VPCF119FC/BI battery.[7] As a result, the metal cannot be melted in open air since it burns before the melting point is reached. Melting is only possible in an inert atmosphere or in a vacuum. At 550 °C (1,022 °F), it combines with chlorine.[3] It also reacts with the other halogens and absorbs hydrogen.[4]

Titanium is one of the few elements that burns in pure nitrogen gas, reacting at 800 °C (1,470 °F) to form titanium nitride, which causes embrittlementSony VAIO VPCF119FC battery.[16]

Experiments have shown that natural titanium becomes radioactive after it is bombarded with deuterons, emitting mainly positrons and hard gamma rays.[3]

[edit]Compounds

See also categories: Titanium compounds and Titanium minerals

TiN coated drill bit

The +4 oxidation state dominates titanium chemistry,[17] but compounds in the +3 oxidation state are also common.[18] Because of this high oxidation state, many titanium compounds have a high degree of covalent bondingSony VAIO VPCF117HG/BI battery.

Star sapphires and rubies get their asterism from the titanium dioxide impurities present in them.[15] Titanates are compounds made with titanium dioxide. Barium titanate has piezoelectric properties, thus making it possible to use it as a transducer in the interconversion of sound and electricity.[7] Esters of titanium are formed by the reaction of alcohols and titanium tetrachloride and are used to waterproof fabrics. Sony VAIO VPCF116FG/BI battery

Titanium nitride (TiN), having a hardness equivalent to sapphire and carborundum (9.0 on the Mohs Scale),[19] is often used to coat cutting tools, such as drill bits.[20] It also finds use as a gold-colored decorative finish, and as a barrier metal in semiconductor fabrication.[21]

Titanium tetrachloride (titanium(IV) chloride, TiCl4, sometimes called "tickle"[22]) is a colorless liquid which is used as an intermediate in the manufacture of titanium dioxide for paintSony VAIO VPCF115FG/B battery.[23] It is widely used in organic chemistry as a Lewis acid, for example in the Mukaiyama aldol condensation.[24] Titanium also forms a lower chloride, titanium(III) chloride (TiCl3), which is used as a reducing agent.[25]

Titanocene dichloride is an important catalyst for carbon-carbon bond formation. Titanium isopropoxide is used for Sharpless epoxidation. Other compounds include titanium bromide (used in metallurgy, superalloys, and high-temperature electrical wiring and coatings) and titanium carbide (found in high-temperature cutting tools and coatings) Sony VAIO VPCF21ZHJ battery.[4]

[edit]Occurrence

Titanium is always bonded to other elements in nature. It is the ninth-most abundant element in the Earth's crust (0.63% by mass)[27] and the seventh-most abundant metal. It is present in most igneous rocks and in sediments derived from them (as well as in living things and natural bodies of water).[2][3] Of the 801 types of igneous rocks analyzed by the United States Geological Survey, 784 contained titaniumSony VAIO VPCF21Z1E/BI battery.[27] Its proportion in soils is approximately 0.5 to 1.5%.[27]

It is widely distributed and occurs primarily in the minerals anatase, brookite, ilmenite, perovskite, rutile, titanite (sphene), as well in many iron ores.[15] Of these minerals, only rutile and ilmenite have any economic importance, yet even they are difficult to find in high concentrations. Significant titanium-bearing ilmenite deposits exist in western AustraliaSony VAIO VPCF21AHJ battery, Canada, China, India, Mozambique, New Zealand, Norway, and Ukraine.[15] Large quantities of rutile are also mined in North America and South Africa and help contribute to the annual production of 90,000 tonnes of the metal and 4.3 million tonnes of titanium dioxide.[15] Total reserves of titanium are estimated to exceed 600 million tonnesSony VAIO VPCF21AGJ battery.[15]

Titanium is contained in meteorites and has been detected in the sun and in M-type stars;[3] the coolest type of star with a surface temperature of 3,200 °C (5,790 °F).[28] Rocks brought back from the moon during the Apollo 17 mission are composed of 12.1% TiO2.[3] It is also found in coal ash, plants, and even the human bodySony VAIO VPCF21AFJ battery.

[edit]Isotopes

Main article: Isotopes of titanium

Naturally occurring titanium is composed of 5 stable isotopes: 46Ti, 47Ti, 48Ti, 49Ti, and 50Ti, with 48Ti being the most abundant (73.8% natural abundance). Eleven radioisotopes have been characterized, with the most stable being 44Ti with a half-life of 63 years, 45Ti with a half-life of 184.8 minutes, 51Ti with a half-life of 5.76 minutes, and 52Ti with a half-life of 1.7 minutesSony VAIO VPCF219FJ/BI battery. All of the remaining radioactive isotopes have half-lives that are less than 33 seconds and the majority of these have half-lives that are less than half a second.[8]

The isotopes of titanium range in atomic weight from 39.99 u (40Ti) to 57.966 u (58Ti). The primary decay mode before the most abundant stable isotope, 48Ti, is electron capture and the primary mode after is beta emission. The primary decay products before 48Ti are element 21 (scandium) isotopes and the primary products after are element 23 (vanadium) isotopesSony VAIO VPCF217HG/BI battery.[8]

[edit]History

Martin Heinrich Klaproth named titanium for the Titans of Greek mythology.

Titanium was discovered included in a mineral in Cornwall, United Kingdom, in 1791 by amateur geologist and pastor William Gregor, then vicar of Creed parish.[29] He recognized the presence of a new element in ilmenite[4] when he found black sand by a stream in the nearby parish of Manaccan and noticed the sand was attracted by a magnetSony VAIO VPCF217HG battery.[29] Analysis of the sand determined the presence of two metal oxides; iron oxide (explaining the attraction to the magnet) and 45.25% of a white metallic oxide he could not identify.[27] Gregor, realizing that the unidentified oxide contained a metal that did not match the properties of any known element, reported his findings to the Royal Geological Society of Cornwall and in the German science journal Crell's AnnalenSony VAIO VPCF14ZHJ battery.[29]

Around the same time, Franz-Joseph Müller von Reichenstein produced a similar substance, but could not identify it.[4] The oxide was independently rediscovered in 1795 by German chemist Martin Heinrich Klaproth in rutile from Hungary.[29] Klaproth found that it contained a new element and named it for the Titans of Greek mythology.[28] After hearing about Gregor's earlier discovery, he obtained a sample of manaccanite and confirmed it contained titaniumSony VAIO VPCF14AHJ battery.

The processes required to extract titanium from its various ores are laborious and costly; it is not possible to reduce in the normal manner, by heating in the presence of carbon, because that produces titanium carbide.[29] Pure metallic titanium (99.9%) was first prepared in 1910 by Matthew A. Hunter at Rensselaer Polytechnic Institute by heating TiCl4 with sodium at 700–800 °C in the Hunter processSony VAIO VPCF14AGJ battery.[3] Titanium metal was not used outside the laboratory until 1932 when William Justin Kroll proved that it could be produced by reducing titanium tetrachloride (TiCl4) with calcium.[30] Eight years later he refined this process by using magnesium and even sodium in what became known as the Kroll process.[30] Although research continues into more efficient and cheaper processes (e.g., FFC Cambridge), the Kroll process is still used for commercial productionSony VAIO VPCF14AFJ battery.

Titanium sponge, made by the Kroll process

Titanium of very high purity was made in small quantities when Anton Eduard van Arkel and Jan Hendrik de Boer discovered the iodide, or crystal bar, process in 1925, by reacting with iodine and decomposing the formed vapors over a hot filament to pure metal.[31]

In the 1950s and 1960s the Soviet Union pioneered the use of titanium in military and submarine applications (Alfa Class and Mike Class)[32] as part of programs related to the Cold WarSony VAIO VPCF149FJ/BI battery.[33] Starting in the early 1950s, titanium began to be used extensively for military aviation purposes, particularly in high-performance jets, starting with aircraft such as the F100 Super Sabre and Lockheed A-12.

In the USA, the Department of Defense realized the strategic importance of the metal[34] and supported early efforts of commercialization.[35] Throughout the period of the Cold War, titanium was considered a Strategic Material by the U.S. governmentSony VAIO VPCF148FJ/B battery, and a large stockpile of titanium sponge was maintained by the Defense National Stockpile Center, which was finally depleted in 2005.[36] Today, the world's largest producer, Russian-based VSMPO-Avisma, is estimated to account for about 29% of the world market share.[37]

In 2006, the U.S. Defense Agency awarded $5.7 million to a two-company consortium to develop a new process for making titanium metal powder. Under heat and pressure, the powder can be used to create strong, lightweight items ranging from armor plating to components for the aerospace, transport, and chemical processing industries. Sony VAIO VPCF13ZHJ battery

Production and fabrication

Titanium (mineral concentrate)

The processing of titanium metal occurs in 4 major steps:[39] reduction of titanium ore into "sponge", a porous form; melting of sponge, or sponge plus a master alloy to form an ingot; primary fabrication, where an ingot is converted into general mill products such as billet, bar, plate, sheet, strip, and tube; and secondary fabrication of finished shapes from mill productsSony VAIO VPCF13Z0E/B battery.

Because the metal reacts with oxygen at high temperatures it cannot be produced by reduction of its dioxide.[9] Titanium metal is therefore produced commercially by the Kroll process, a complex and expensive batch process. (The relatively high market value of titanium is mainly due to its processing, which sacrifices another expensive metal, magnesiumSony VAIO VPCF13M8E/B battery.[40]) In the Kroll process, the oxide is first converted to chloride through carbochlorination, whereby chlorine gas is passed over red-hot rutile or ilmenite in the presence of carbon to make TiCl4. This is condensed and purified by fractional distillation and then reduced with 800 °C molten magnesium in an argon atmosphere.[7]

A more recently developed method, the FFC Cambridge process,[41] may eventually replace the Kroll processSony VAIO VPCF13AHJ battery. This method uses titanium dioxide powder (which is a refined form of rutile) as feedstock to make the end product which is either a powder or sponge. If mixed oxide powders are used, the product is an alloy manufactured at a much lower cost than the conventional multi-step melting process. The FFC Cambridge process may render titanium a less rare and expensive material for the aerospace industry and the luxury goods marketSony VAIO VPCF13AGJ battery, and could be seen in many products currently manufactured using aluminium and specialist grades of steel.

Common titanium alloys are made by reduction. For example, cuprotitanium (rutile with copper added is reduced), ferrocarbon titanium (ilmenite reduced with coke in an electric furnace), and manganotitanium (rutile with manganese or manganese oxides) are reduced.[42]

2 FeTiO3 + 7 Cl2 + 6 C → 2 TiCl4 + 2 FeCl3 + 6 CO (900 °C)

TiCl4 + 2 Mg → 2 MgCl2 + Ti (1100 °C) Sony VAIO VPCF13AFJ battery

About 50 grades of titanium and titanium alloys are designated and currently used, although only a couple of dozen are readily available commercially.[43] The ASTM International recognizes 31 Grades of titanium metal and alloys, of which Grades 1 through 4 are commercially pure (unalloyed). These four are distinguished by their varying degrees of tensile strengthSony VAIO VPCF138FJ/BI battery, as a function of oxygen content, with Grade 1 being the most ductile (lowest tensile strength with an oxygen content of 0.18%), and Grade 4 the least (highest tensile strength with an oxygen content of 0.40%).[15] The remaining grades are alloys, each designed for specific purposes, be it ductility, strength, hardness, electrical resistivity, creep resistance, resistance to corrosion from specific media, or a combination thereofSony VAIO VPCF138FJ/B battery.[44]

The grades covered by ASTM and other alloys are also produced to meet Aerospace and Military specifications (SAE-AMS, MIL-T), ISO standards, and country-specific specifications, as well as proprietary end-user specifications for aerospace, military, medical, and industrial applications. Sony VAIO VPCF138FC/BI battery

In terms of fabrication, all welding of titanium must be done in an inert atmosphere of argon or helium in order to shield it from contamination with atmospheric gases such as oxygen, nitrogen, or hydrogen.[11] Contamination will cause a variety of conditions, such as embrittlement, which will reduce the integrity of the assembly welds and lead to joint failureSony VAIO VPCF135FG battery. Commercially pure flat product (sheet, plate) can be formed readily, but processing must take into account the fact that the metal has a "memory" and tends to spring back. This is especially true of certain high-strength alloys.[46][47] Titanium cannot be soldered without first pre-plating it in a metal that is solderable.[48] The metal can be machined using the same equipment and via the same processes as stainless steelSony VAIO VPCF12AHJ battery.[11]

[edit]Applications

A titanium cylinder, "Grade 2" quality

Titanium is used in steel as an alloying element (ferro-titanium) to reduce grain size and as a deoxidizer, and in stainless steel to reduce carbon content.[2] Titanium is often alloyed with aluminium (to refine grain size), vanadium, copper (to harden), iron, manganese, molybdenum, and with other metals.[49] Applications for titanium mill products (sheet, plate, bar, wire, forgings, castings) can be found in industrial, aerospace, recreational, and emerging marketsSony VAIO VPCF12AGJ battery. Powdered titanium is used in pyrotechnics as a source of bright-burning particles.

Pigments, additives and coatings

Titanium dioxide is the most commonly used compound of titanium

About 95% of titanium ore extracted from the Earth is destined for refinement into titanium dioxide (TiO2), an intensely white permanent pigment used in paints, paper, toothpaste, and plastics.[50] It is also used in cement, in gemstones, as an optical opacifier in paper,[51] and a strengthening agent in graphite composite fishing rods and golf clubsSony VAIO VPCF12AFJ battery.

TiO2 powder is chemically inert, resists fading in sunlight, and is very opaque: this allows it to impart a pure and brilliant white color to the brown or gray chemicals that form the majority of household plastics.[4] In nature, this compound is found in the minerals anatase, brookite, and rutile.[2] Paint made with titanium dioxide does well in severe temperaturesSony VAIO VPCF129FJ/BI battery, and stands up to marine environments.[4] Pure titanium dioxide has a very high index of refraction and an optical dispersion higher than diamond.[3] In addition to being a very important pigment, titanium dioxide is also used in sunscreens due to its ability to protect skin by itself.[9] Recently, titanium oxide has been put to use in air purifiers (as a filter coating), or in film used to coat windows on buildings so that when titanium oxide becomes exposed to UV lightSony VAIO VPCF128FJ/B battery (either solar or artificial) and moisture in the air, reactive redox species like hydroxyl radicals are produced so that they can purify the air or keep window surfaces clean.[52]

[edit]Aerospace and marine

Due to their high tensile strength to density ratio,[7] high corrosion resistance,[3] fatigue resistance, high crack resistance,[53] and ability to withstand moderately high temperatures without creeping, titanium alloys are used in aircraft, armor plating, naval ships, spacecraftSony VAIO VPCF127FJ/W battery, and missiles.[3][4] For these applications titanium alloyed with aluminium, vanadium, and other elements is used for a variety of components including critical structural parts, fire walls, landing gear, exhaust ducts (helicopters), and hydraulic systems. In fact, about two thirds of all titanium metal produced is used in aircraft engines and frames.[54] The SR-71 "Blackbird" was one of the first aircraft to make extensive use of titanium within its structureSony VAIO VPCF11ZHJ battery, paving the way for its use in modern military and commercial aircraft. An estimated 59 metric tons (130,000 pounds) are used in the Boeing 777, 45 in the Boeing 747, 18 in the Boeing 737, 32 in the Airbus A340, 18 in the Airbus A330, and 12 in the Airbus A320. The Airbus A380 may use 146 metric tons, including about 26 tons in the engines.[55] In engine applications, titanium is used for rotors, compressor blades, hydraulic system components, and nacellesSony VAIO VPCF11AHJ battery. The titanium 6AL-4V alloy accounts for almost 50% of all alloys used in aircraft applications.[56]

Due to its high corrosion resistance to sea water, titanium is used to make propeller shafts and rigging and in the heat exchangers of desalination plants;[3] in heater-chillers for salt water aquariums, fishing line and leader, and for divers' knives. Titanium is used to manufacture the housings and other components of ocean-deployed surveillance and monitoring devices for scientific and military useSony VAIO VPCF11AGJ battery. The former Soviet Union developed techniques for making submarines largely out of titanium.[57]

[edit]Industrial

High-purity (99.999%) titanium with visible crystal structure

Welded titanium pipe and process equipment (heat exchangers, tanks, process vessels, valves) are used in the chemical and petrochemical industries primarily for corrosion resistance. Specific alloys are used in downhole and nickel hydrometallurgy applications due to their high strength (e. g.: titanium Beta C alloy), corrosion resistance, or combination of bothSony VAIO VPCF11AFJ battery. The pulp and paper industry uses titanium in process equipment exposed to corrosive media such as sodium hypochlorite or wet chlorine gas (in the bleachery).[58] Other applications include: ultrasonic welding, wave soldering,[59] and sputtering targets.[60]

Titanium tetrachloride (TiCl4), a colorless liquid, is important as an intermediate in the process of making TiO2 and is also used to produce the Ziegler-Natta catalyst, and is used to iridize glass and because it fumes strongly in moist air it is also used to make smoke screensSony VAIO VPCF118FJ battery.[9]

[edit]Consumer and architectural

Titanium metal is used in automotive applications, particularly in automobile or motorcycle racing, where weight reduction is critical while maintaining high strength and rigidity.[61] The metal is generally too expensive to make it marketable to the general consumer market, other than high-end products, particularly for the racing/performance market. Late model Corvettes have been available with titanium exhaustsSony VAIO VPCF117FJ battery.[62]

Titanium is used in many sporting goods: tennis rackets, golf clubs, lacrosse stick shafts; cricket, hockey, lacrosse, and football helmet grills; and bicycle frames and components. Although not a mainstream material for bicycle production, titanium bikes have been used by race teams and adventure cyclists.[63] Titanium alloys are also used in spectacle framesSony VAIO VGN-CS33H battery.[64] This results in a rather expensive, but highly durable and long lasting frame which is light in weight and causes no skin allergies. Many backpackers use titanium equipment, including cookware, eating utensils, lanterns, and tent stakes.[64] Though slightly more expensive than traditional steel or aluminium alternatives, these titanium products can be significantly lighter without compromising strength. Titanium is also favored for use by farriers, since it is lighter and more durable than steel when formed into horseshoesSony VAIO VGN-CS33H/Z battery.[64]

Titanium has occasionally been used in architectural applications: the 40 m (131 foot) memorial to Yuri Gagarin, the first man to travel in space, in Moscow, is made of titanium for the metal's attractive color and association with rocketry.[65] The Guggenheim Museum Bilbao and the Cerritos Millennium Library were the first buildings in Europe and North AmericaSony VAIO VGN-CS33H/B battery, respectively, to be sheathed in titanium panels.[54] Other construction uses of titanium sheathing include the Frederic C. Hamilton Building in Denver, Colorado[66] and the 107 m (350 foot) Monument to the Conquerors of Space in Moscow.[67]

Because of its superior strength and light weight when compared to other metals traditionally used in firearms (steel, stainless steel, and aluminium), and advances in metalworking techniques, the use of titanium has become more widespread in the manufacture of firearmsSony VAIO VGN-CS31Z/Q battery. Primary uses include pistol frames and revolver cylinders. For these same reasons, it is also used in the body of laptop computers (for example, in Apple's PowerBook line).[68]

Some upmarket categories of tools made to be lightweight and corrosion-resistant, such as shovels and flashlights, are made of titanium or titanium alloys as well.

[edit]Jewelry

Because of its durability, titanium has become more popular for designer jewelry (particularly, titanium rings).[64] Its inertness makes it a good choice for those with allergies or those who will be wearing the jewelry in environments such as swimming poolsSony VAIO VGN-CS31S/W battery. Titanium is also alloyed with gold to produce an alloy that can be marketed as 24-carat gold, as the 1% of alloyed Ti is insufficient to require a lesser mark. The resulting alloy is roughly the hardness of 14-carat gold and thus is more durable than a pure 24-carat gold item would be.[69]

Titanium's durability, light weight, dent- and corrosion resistance makes it useful in the production of watch cases.[64] Some artists work with titanium to produce artworks such as sculptures, decorative objects and furnitureSony VAIO VGN-CS31S/V battery.[70]

The inertness and ability to be attractively colored makes titanium a popular metal for use in body piercing.[71] Titanium may be anodized to produce various colors, which varies the thickness of the surface oxide layer and causes interference fringes.[72]

[edit]Medical

Because it is biocompatible (non-toxic and is not rejected by the body), titanium is used in a gamut of medical applications including surgical implements and implants, such as hip balls and sockets (joint replacement) that can stay in place for up to 20 yearsSony VAIO VGN-CS31S/T battery.[29] The titanium is often alloyed with about 4% aluminium[73] or 6% Al and 4% vanadium.

Titanium has the inherent property to osseointegrate, enabling use in dental implants that can remain in place for over 30 years. This property is also useful for orthopedic implant applications.[29] These benefit from titanium's lower modulus of elasticity (Young's modulus) to more closely match that of the bone that such devices are intended to repairSony VAIO VGN-CS31S/R battery. As a result, skeletal loads are more evenly shared between bone and implant, leading to a lower incidence of bone degradation due to stress shielding and periprosthetic bone fractures which occur at the boundaries of orthopedic implants. However, titanium alloys' stiffness is still more than twice that of bone so adjacent bone bears a greatly reduced load and may deteriorateSony VAIO VGN-CS31S/P battery.[74]

Since titanium is non-ferromagnetic, patients with titanium implants can be safely examined with magnetic resonance imaging (convenient for long-term implants). Preparing titanium for implantation in the body involves subjecting it to a high-temperature plasma arc which removes the surface atoms, exposing fresh titanium that is instantly oxidizedSony VAIO VGN-CS28 battery.[29]

Titanium is also used for the surgical instruments used in image-guided surgery, as well as wheelchairs, crutches, and any other products where high strength and low weight are desirable.

Precautions

Nettle contains up to 80 parts per million of titanium.

Titanium is non-toxic even in large doses and does not play any natural role inside the human body.[28] An estimated quantity of 0.8 milligrams of titanium is ingested by humans each day, but most passes through without being absorbedSony VAIO VGN-CS28/Q battery.[28] It does, however, have a tendency to bio-accumulate in tissues that contain silica. An unknown mechanism in plants may use titanium to stimulate the production of carbohydrates and encourage growth. This may explain why most plants contain about 1 part per million (ppm) of titanium, food plants have about 2 ppm, and horsetail and nettle contain up to 80 ppmSony VAIO VGN-CS27 battery.[28]

As a powder or in the form of metal shavings, titanium metal poses a significant fire hazard and, when heated in air, an explosion hazard.[75] Water and carbon dioxide–based methods to extinguish fires are ineffective on burning titanium; Class D dry powder fire fighting agents must be used insteadSony VAIO VGN-CS27/W battery.[4]

When used in the production or handling of chlorine, care must be taken to use titanium only in locations where it will not be exposed to dry chlorine gas which can result in a titanium/chlorine fire.[76] A fire hazard exists even when titanium is used in wet chlorine due to possible unexpected drying brought about by extreme weather conditionsSony VAIO VGN-CS27/R battery.

Titanium can catch fire when a fresh, non-oxidized surface comes in contact with liquid oxygen.[77] Such surfaces can appear when the oxidized surface is struck with a hard object, or when a mechanical strain causes the emergence of a crack. This poses the possible limitation for its use in liquid oxygen systems, such as those found in the aerospace industrySony VAIO VGN-CS27/P battery.

Chromium ( /ˈkroʊmiəm/ kroh-mee-əm) is a chemical element which has the symbol Cr and atomic number 24. It is the first element in Group 6. It is a steely-gray, lustrous, hard metal that takes a high polish and has a high melting point. It is also odorless, tasteless, and malleable. The name of the element is derived from the Greek word "chrōma" (χρώμα) Sony VAIO VGN-CS27/C battery, meaning colour,[2] because many of its compounds are intensely coloured. Chromium oxide was used by the Chinese in the Qin dynasty over 2,000 years ago to coat weapons such as bronze crossbow bolts and steel swords found at the Terracotta Army. It later came to the attention of the West when it was discovered by Louis Nicolas Vauquelin in the mineral crocoite (lead(II) chromate) in 1797Sony VAIO VGN-CS26T/W battery. Crocoite was used as a pigment, and after the discovery that the mineral chromite also contains chromium, this mineral was used to produce pigments as well.

Chromium was regarded with great interest because of its high corrosion resistance and hardness. A major development was the discovery that steel could be made highly resistant to corrosion and discoloration by adding chromium to form stainless steel. This application, along with chrome plating Sony VAIO VGN-CS26T/V battery (electroplating with chromium) are currently the highest-volume uses of the metal. Chromium and ferrochromium are produced from the single commercially viable ore, chromite, by silicothermic or aluminothermic reaction or by roasting and leaching processes.

Although trivalent chromium (Cr(III)) is required in trace amounts for sugar and lipid metabolism, few cases have been reported where its complete removal from the diet has caused chromium deficiency. In larger amounts and in different forms, chromium can be toxic and carcinogenicSony VAIO VGN-CS26T/T battery. The most prominent example of toxic chromium is hexavalent chromium (Cr(VI)). Abandoned chromium production sites often require environmental cleanup.

Characteristics

[edit]Physical

Chromium is remarkable for its magnetic properties: it is the only elemental solid which shows antiferromagnetic ordering at room temperature (and below). Above 38 °C, it transforms into a paramagnetic state.[1]

[edit]Passivation

Chromium metal left standing in air is passivated by oxygen, forming a thin protective oxide surface layer. This layer is a spinel structure only a few atoms thickSony VAIO VGN-CS26T/R battery. It is very dense, and prevents the diffusion of oxygen into the underlying material. This barrier is in contrast to iron or plain carbon steels, where the oxygen migrates into the underlying material and causes rusting.[3] The passivation can be enhanced by short contact with oxidizing acids like nitric acid. Passivated chromium is stable against acidsSony VAIO VGN-CS26T/Q battery. The opposite effect can be achieved by treatment with a strong reducing agent that destroys the protective oxide layer on the metal. Chromium metal treated in this way readily dissolves in weak acids.[4]

Chromium, unlike metals such as iron and nickel, does not suffer from hydrogen embrittlement. However, it does suffer from nitrogen embrittlement, reacting with nitrogen from air and forming brittle nitrides at the high temperatures necessary to work the metal partsSony VAIO VGN-CS26T/P battery.[5]

[edit]Occurrence

Crocoite (PbCrO4)

Chromite ore

Chromium is the 21st most abundant element in Earth's crust with an average concentration of 100 ppm.[6] Chromium compounds are found in the environment, due to erosion of chromium-containing rocks and can be distributed by volcanic eruptions. The concentrations range in soil is between 1 and 3000 mg/kg, in sea water 5 to 800 µg/literSony VAIO VGN-CS26T/C battery, and in rivers and lakes 26 µg/liter to 5.2 mg/liter.[7] Chromium is mined as chromite (FeCr2O4) ore.[8] About two-fifths of the chromite ores and concentrates in the world are produced in South Africa, while Kazakhstan, India, Russia, and Turkey are also substantial producers. Untapped chromite deposits are plentiful, but geographically concentrated in Kazakhstan and southern AfricaSony VAIO VGN-CS25H battery.[9]

Although rare, deposits of native chromium exist.[10][11] The Udachnaya Pipe in Russia produces samples of the native metal. This mine is a kimberlite pipe, rich in diamonds, and the reducing environment helped produce both elemental chromium and diamond.[12]

The relation between Cr(III) and Cr(VI) strongly depends on pH and oxidative properties of the location, but in most cases, the Cr(III) is the dominating speciesSony VAIO VGN-CS25H/W battery,[7] although in some areas the ground water can contain up to 39 µg/liter of total chromium of which 30 µg/liter is present as Cr(VI).[13]

Isotopes

Main article: Isotopes of chromium

Naturally occurring chromium is composed of three stable isotopes; 52Cr, 53Cr and 54Cr with 52Cr being the most abundant (83.789% natural abundance). 19 radioisotopes have been characterized with the most stable being 50Cr with a half-life of (more than) 1.8×1017 years, and 51Cr with a half-life of 27.7 daysSony VAIO VGN-CS25H/R battery. All of the remaining radioactive isotopes have half-lives that are less than 24 hours and the majority of these have half-lives that are less than 1 minute. This element also has 2 meta states.[14]

53Cr is the radiogenic decay product of 53Mn. Chromium isotopic contents are typically combined with manganese isotopic contents and have found application in isotope geology. Mn-Cr isotope ratios reinforce the evidence from 26Al and 107Pd for the early history of the solar systemSony VAIO VGN-CS25H/Q battery. Variations in 53Cr/52Cr and Mn/Cr ratios from several meteorites indicate an initial 53Mn/55Mn ratio that suggests Mn-Cr isotopic composition must result from in-situ decay of 53Mn in differentiated planetary bodies. Hence 53Cr provides additional evidence for nucleosynthetic processes immediately before coalescence of the solar systemSony VAIO VGN-CS25H/P battery.[15]

The isotopes of chromium range in atomic mass from 43 u (43Cr) to 67 u (67Cr). The primary decay mode before the most abundant stable isotope, 52Cr, is electron capture and the primary mode after is beta decay.[14] 53Cr has been posited as a proxy for atmospheric oxygen concentrationSony VAIO VGN-CS25H/C battery.[16]

[edit]Compounds

Chromium is a member of the transition metals, in group 6. Chromium(0) has an electronic configuration of 4s13d5, owing to the lower energy of the high spin configuration. Chromium exhibits a wide range of possible oxidation states, where the +3 state is most stable energetically; the +3 and +6 states are most commonly observed in chromium compounds, whereas the +1, +4 and +5 states are rareSony VAIO VGN-CS23T/W battery.[17]

The following is the Pourbaix diagram for chromium in pure water, perchloric acid or sodium hydroxide:[7][18]

Chromium(III)

Chromium(III) chloride hexahydrate ([CrCl2(H2O)4]Cl·2H2O)

Anhydrous chromium(III) chloride (CrCl3)

A large number of chromium(III) compounds are known. Chromium(III) can be obtained by dissolving elemental chromium in acids like hydrochloric acid or sulfuric acid. The Cr3+ ion has a similar radius (63 pm) to the Al3+ ion (radius 50 pm), so they can replace each other in some compounds, such as in chrome alum and alum. When a trace amount of Cr3+ replaces Al3+ in corundum (aluminium oxide, Al2O3), the red-colored ruby is formedSony VAIO VGN-CS23T/Q battery.

Chromium(III) ions tend to form octahedral complexes. The colors of these complexes is determined by the ligands attached to the Cr centre. The commercially available chromium(III) chloride hydrate is the dark green complex [CrCl2(H2O)4]Cl. Closely related compounds have different colours: pale green [CrCl(H2O)5]Cl2 and the violet [Cr(H2O)6]Cl3Sony VAIO VGN-CS23H battery. If water-free green chromium(III) chloride is dissolved in water then the green solution turns violet after some time, due to the substitution of water by chloride in the inner coordination sphere. This kind of reaction is also observed with solutions of chrome alum and other water-soluble chromium(III) saltsSony VAIO VGN-CS23H/S battery.

Chromium(III) hydroxide (Cr(OH)3) is amphoteric, dissolving in acidic solutions to form [Cr(H2O)6]3+, and in basic solutions to form [Cr(OH)6]3−. It is dehydrated by heating to form the green chromium(III) oxide (Cr2O3), which is the stable oxide with a crystal structure identical to that of corundumSony VAIO VGN-CS23H/B battery.[4]

[edit]Chromium(VI)

Chromium(VI) oxide

Chromium(VI) compounds are powerful oxidants at low or neutral pH. Most important are chromate anion (CrO2−

4) and dichromate (Cr2O72-) anions, which exist in equilibrium:

2 [CrO4]2- + 2 H+  [Cr2O7]2- + 2 H2O

Chromium(VI) halides are known also and include hexafluoride CrF6 and chromyl chloride (CrO2Cl2).[4]

Sodium chromate is produced industrially by the oxidative roasting of chromite ore with calcium or sodium carbonateSony VAIO VGN-CS23G battery. The dominant species is therefore, by the law of mass action, determined by the pH of the solution. The change in equilibrium is visible by a change from yellow (chromate) to orange (dichromate), such as when an acid is added to a neutral solution of potassium chromate. At yet lower pH values, further condensation to more complex oxyanions of chromium is possibleSony VAIO VGN-CS23G/W battery.

Both the chromate and dichromate anions are strong oxidizing reagents at low pH:[4]

Sodium chromate (Na2CrO4)

Cr2O2−

7 + 14 H3O+ + 6 e− → 2 Cr3+ + 21 H2O (ε0 = 1.33 V)

They are, however, only moderately oxidizing at high pH:[4]

CrO2−

4 + 4 H2O + 3 e− → Cr(OH)3 + 5 OH− (ε0 = −0.13 V)

Chromium(VI) compounds in solution can be detected by adding an acidic hydrogen peroxide solution. The unstable dark blue chromium(VI) peroxide (CrO5) is formed, which can be stabilized as an ether adduct CrO5·OR2.[4]

Chromic acid has the hypothetical formula H2CrO4. It is a vaguely described chemical, despite many well-defined chromates and dichromates are known. The dark red chromium(VI) oxide CrO3, the acid anhydride of chromic acid, is sold industrially as "chromic acid".Sony VAIO VGN-CS23G/Q battery It can be produced by mixing sulfuric acid with dichromate, and is a strong oxidizing agent.

[edit]Chromium(V) and chromium(IV)

The oxidation state +5 is only realized in few compounds but are intermediates in many reactions involving oxidations by chromate. The only binary compound is the volatile chromium(V) fluoride (CrF5). This red solid has a melting point of 30 °C and a boiling point of 117 °C. It can be synthesized by treating chromium metal with fluorine at 400 °C and 200 bar pressureSony VAIO VGN-CS23G/P battery. The peroxochromate(V) is another example of the +5 oxidation state. Potassium peroxochromate (K3[Cr(O2)4]) is made by reacting potassium chromate with hydrogen peroxide at low temperatures. This red brown compound is stable at room temperature but decomposes spontaneously at 150–170 °C.[19]

Compounds of chromium(IV) (in the +4 oxidation state) are slightly more common than those of chromium(V) Sony VAIO VGN-CS21Z/Q battery. The tetrahalides, CrF4, CrCl4, and CrBr4, can be produced by treating the trihalides (CrX3) with the corresponding halogen at elevated temperatures. Such compounds are susceptible to disproportionation reactions and are not stable in water.

[edit]Chromium(II) and chromium(I)

Many chromium(II) compounds are known, including the water-stable chromium(II) chloride, CrCl2, which can be made by reduction of chromium(III) chloride with zinc. The resulting bright blue solution is only stable at neutral pH.[4] Many chromous carboxylates are also known, most famously, the red chromous acetate (Cr2(O2CCH3)4), which features a quadruple bondSony VAIO VGN-CS21S/W battery.

[edit]Chromium(I)

Most Cr(I) compounds are obtained by oxidation of electron-rich, octahedral Cr(0) complexes. Other Cr(I) complexes contain cyclopentadienyl ligands. As verified by X-ray diffraction, a Cr-Cr quintuple bond (length 183.51(4)  pm) has also been described.[20] Extremely bulky monodentate ligands stabilize this compound by shielding the quintuple bond from further reactionsSony VAIO VGN-CS21S/V battery.

Chromium compound determined experimentally to contain a Cr-Cr quintuple bond

[edit]Chromium(0) and chromium(I)

Main article: organochromium chemistry

Many chromium(0) compounds are known. Most are derivatives of chromium hexacarbonyl or bis(benzene)chromium.

[edit]History

Weapons found in burial pits dating from the late 3rd century BC Qin Dynasty of the Terracotta Army near Xi'an, China have been analyzed by archaeologists. Although buried more than 2,000 years ago, the ancient bronze tips of crossbow bolts and swords found at the site showed no sign of corrosion, because the bronze was coated with chromiumSony VAIO VGN-CS21S/T battery.[21]

Chromium later came to the attention of westerners in the 18th century. 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. Though misidentified as a lead compound with selenium and iron components, the mineral was Crocoite (lead chromate) with a formula of PbCrO4Sony VAIO VGN-CS21S/R battery.[22]

In 1770, Peter Simon Pallas visited the same site as Lehmann and found a red lead mineral that had useful properties as a pigment in paints. The use of Siberian red lead as a paint pigment developed rapidly. A bright yellow pigment made from crocoite also became fashionable.[22]

The red colour of rubies is from a small amount of chromium(III) Sony VAIO VGN-CS21S/P battery.

In 1797, Louis Nicolas Vauquelin received samples of crocoite ore. He produced chromium trioxide (CrO3) by mixing crocoite with hydrochloric acid. In 1798, Vauquelin discovered that he could isolate metallic chromium by heating the oxide in a charcoal oven.[23] He was also able to detect traces of chromium in precious gemstones, such as ruby or emerald. Sony VAIO VGN-CS215J/R battery

During the 1800s, chromium was primarily used as a component of paints and in tanning salts. At first, crocoite from Russia was the main source, but in 1827, a larger chromite deposit was discovered near Baltimore, United States. This made the United States the largest producer of chromium products till 1848 when large deposits of chromite where found near Bursa, TurkeySony VAIO VGN-CS215J/Q battery.[8]

Chromium is also known for its luster when polished. It is used as a protective and decorative coating on car parts, plumbing fixtures, furniture parts and many other items, usually applied by electroplating. Chromium was used for electroplating as early as 1848, but this use only became widespread with the development of an improved process in 1924Sony VAIO VGN-CS19/W battery.[25]

Metal alloys now account for 85% of the use of chromium. The remainder is used in the chemical industry and refractory and foundry industries.

[edit]Production

Approximately 4.4 million metric tons of marketable chromite ore were produced in 2000, and converted into ~3.3 million tons of ferro-chrome with an approximate market value of 2.5 billion United States dollars.[26] The largest producers of chromium ore have been South Africa (44%) India (18%), Kazakhstan (16%) Zimbabwe (5%), Finland (4%) Iran (4%) and Brazil (2%) with several other countries producing the rest of less than 10% of the world productionSony VAIO VGN-CS19/R battery.[26]

The two main products of chromium ore refining are ferrochromium and metallic chromium. For those products the ore smelter process differs considerably. 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 reactionSony VAIO VGN-CS19/Q battery.[27]

Chromium ore output in 2002[26]

For the production of pure chromium, the iron has to be separated from the chromium in a two step roasting and leaching process. The chromite ore is heated with a mixture of calcium carbonate and sodium carbonate in the presence of air. The chromium is oxidized to the hexavalent form, while the iron forms the stable Fe2O3Sony VAIO VGN-CS19/P battery. The subsequent leaching at higher elevated temperatures dissolves the chromates and leaves the insoluble iron oxide. The chromate is converted by sulfuric acid into the dichromate.[27]

4 FeCr2O4 + 8 Na2CO3 + 7 O2 → 8 Na2CrO4 + 2 Fe2O3 + 8 CO2

2 Na2CrO4 + H2SO4 → Na2Cr2O7 + Na2SO4 + H2O

The dichromate is converted to the chromium(III) oxide by reduction with carbon and then reduced in an aluminothermic reaction to chromium.[27]

Na2Cr2O7 + 2 C → Cr2O3 + Na2CO3 + CO

Cr2O3 + 2 Al → Al2O3 + 2 Cr

Applications

Metallurgy

Decorative chrome plating on a motorcycle.

The strengthening effect of forming stable metal carbides at the grain boundaries and the strong increase in corrosion resistance made chromium an important alloying material for steel. The high speed tool steels contain between 3 and 5% chromium. Stainless steelSony VAIO VGN-CS17H/W battery, the main corrosion-proof metal alloy, is formed when chromium is added to iron in sufficient concentrations, usually above 11%. For its formation, ferrochromium is added to the molten iron. Also nickel-based alloys increase in strength due to the formation of discrete, stable metal carbide particles at the grain boundaries. For example, Inconel 718 contains 18.6% chromiumSony VAIO VGN-CS17H/Q battery. 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.[28]

The relative high hardness and corrosion resistance of unalloyed chromium makes it a good surface coating, being still the most "popular" metal coating with unparalleled combined durability. A thin layer of chromium is deposited on pretreated metallic surfaces by electroplating techniques. There are two deposition methods: Thin, below 1 µm thicknessSony VAIO VGN-CS16T/W battery, layers are deposited by chrome plating, and are used for decorative surfaces. If wear-resistant surfaces are needed then thicker chromium layers are deposited. Both methods normally use acidic chromate or dichromate solutions. To prevent the energy consuming change in oxidation state, the use of Chromium(III) sulfate is under development, but for most applications, the established process is usedSony VAIO VGN-CS16T/T battery.[25]

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 by the chromate stored in the chromate conversion coating, which is able to migrate to local defects, are the benefits of this coating methodSony VAIO VGN-CS16T/R battery.[29] Because of environmental and health regulations on chromates, alternative coating method are under development.[30]

Anodizing of aluminium is another electrochemical process, which does not lead to the deposition of chromium, but uses chromic acid as 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 layersSony VAIO VGN-CS16T/Q battery.[31] 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.[25]

[edit]Dye and pigment

School bus painted in chrome yellow[32]

The mineral crocoite (lead chromate PbCrO4) was used as a yellow pigment shortly after its discoverySony VAIO VGN-CS16T/P battery. 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 photo degrade 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 US and for Postal Service (for example Deutsche Post) in EuropeSony VAIO VGN-CS13T/W battery. The use of chrome yellow declined due to environmental and safety concerns and was replaced by organic pigments or alternatives free from lead and chromium. Other pigments based on chromium are, for example, the bright red pigment chrome red, which is a basic lead chromate (PbCrO4·Pb(OH)2). A very important chromate pigment, which was used widely in metal primer formulations was zinc chromateSony VAIO VGN-CS13H/W battery, now replaced by zinc phosphate. A wash primer was formulated to replace the dangerous practice of pretreating 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 applicationSony VAIO VGN-CS13H/R battery. It was found that an easily oxidized alcohol was an essential ingredient. A thin layer of about 10–15 microns 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.[33]

A red color is achieved by doping chromium(III) into the crystals of corundum, which are then called ruby. Therefore, chromium is used in producing synthetic rubiesSony VAIO VGN-CS13H/Q battery.[34]

Chromium oxides are also used as a green color in glassmaking and as a glaze in ceramics.[35] Green chromium oxide is extremely light-fast and as such is used in cladding coatings. It is also the main ingredient in IR reflecting paints, used by the armed forces, to paint vehicles, to give them the same IR reflectance as green leavesSony VAIO VGN-CS13H/P battery.[36]

[edit]Wood preservative

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.[37] 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 have been used in 1996Sony VAIO VGN-CS11Z/T battery.[37]

[edit]Tanning

Main article: Tanning

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.[38] Chromium tanned leather can contain between 4 and 5% of chromium, which is tightly bound to the proteins.[8] Although the form of chromium used for tanning is not the toxic hexavalent varietySony VAIO VGN-CS11Z/R battery, there remains interest in management of chromium in the tanning industry such as recovery and reuse, direct/indirect recycling,[39] use of less chromium or "chrome-less" tanning are practiced to better manage chromium in tanning.

[edit]Refractory material

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 metalsSony VAIO VGN-CS11S/W battery. 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).[27]

[edit]Catalysts

Several chromium compounds are used as catalysts for processing hydrocarbons. For example the Phillips catalysts for the production of polyethylene are mixtures of chromium and silicon dioxide or mixtures of chromium and titanium and aluminium oxide.[40] Fe-Cr mixed oxides are employed as high-temperature catalysts for the water gas shift reaction. Copper chromite is a useful hydrogenation catalystSony VAIO VGN-CS11S/Q battery.[43]

[edit]Other use

Chromium(IV) oxide (CrO2) is a magnetic compound. Its ideal shape anisotropy, which imparts high coercivity and remnant magnetization, made it a compound superior to the γ-Fe2O3. Chromium(IV) oxide is used to manufacture magnetic tape used in high-performance audio tape and standard audio cassettes.[44] Chromates can prevent corrosion of steel under wet conditions, and therefore chromates are added to drilling muds. Sony VAIO VGN-CS11S/P battery

Chromium(III) oxide is a metal polish known as green rouge.

Chromic acid is a powerful oxidizing agent and is a useful compound for cleaning laboratory glassware of any trace of organic compounds. It is prepared in situ by dissolving potassium dichromate in concentrated sulfuric acid, which is then used to wash the apparatus. Sodium dichromate is sometimes used because of its higher solubility (50 g/L versus 200 g/L respectively) (Sony VAIO VGN-AW11M/H battery). Potassium dichromate is a chemical reagent, used in cleaning laboratory glassware and as a titrating agent. It is also used as a mordant (i.e., a fixing agent) for dyes in fabric.

[edit]Biological role

Chromium has no verified biological role and has been classified as not essential for mammals.[46] (Cr(III) or Cr3+) occurs in trace amounts and appears to be benign.[47] Chromium deficiency is controversial or is at least extremely rare. It has been attributed to only three people on parenteral nutrition(Sony VAIO VGN-AW11S/B battery), which is when a patient is fed a liquid diet through intravenous drips.[48] In contrast, hexavalent chromium (Cr(VI) or Cr6+) is very toxic and mutagenic when inhaled. Cr(VI) has not been established as a carcinogen when in solution, although it may cause allergic contact dermatitis (ACD).[49] Although no biological role for chromium has ever been demonstrated, dietary supplements for chromium include chromium(III) picolinate, chromium(III) polynicotinate, and related materials(Sony VAIO VGN-AW11Z/B battery). The benefit of those supplements is questioned by some studies.[50]

The use of chromium-containing dietary supplements is controversial owing to the absence of any verified biological role, the expense of these supplements, and the complex effects of their use.[51] The popular dietary supplement chromium picolinate complex generates chromosome damage in hamster cells (due to the picolinate ligand) (Sony VAIO VGN-AW170C battery).[52] In the United States the dietary guidelines for daily chromium uptake were lowered from 50–200 µg for an adult to 35 µg (adult male) and to 25 µg (adult female).[53]

[edit]Precautions

Main article: Chromium toxicity

Water insoluble chromium(III) compounds and chromium metal are not considered a health hazard, while the toxicity and carcinogenic properties of chromium(VI) have been known for a long time.[54] Because of the specific transport mechanisms, only limited amounts of chromium(III) enter the cells(Sony VAIO VGN-AW19/Q battery). Several in vitro studies indicated that high concentrations of chromium(III) in the cell can lead to DNA damage.[55] Acute oral toxicity ranges between 1.5 and 3.3 mg/kg.[56] The proposed beneficial effects of chromium(III) and the use as dietary supplements yielded some controversial results, but recent reviews suggest that moderate uptake of chromium(III) through dietary supplements poses no risk(Sony VAIO VGN-AW19 battery).[55]

[edit]Cr(VI)

The acute oral toxicity for chromium(VI) ranges between 50 and 150 µg/kg.[56] In the body, chromium(VI) is reduced by several mechanisms to chromium(III) already in the blood before it enters the cells. The chromium(III) is excreted from the body, whereas the chromate ion is transferred into the cell by a transport mechanism(Sony VAIO VGN-AW21M/H battery), by which also sulfate and phosphate ions enter the cell. The acute toxicity of chromium(VI) is due to its strong oxidational properties. After it reaches the blood stream, it damages the kidneys, the liver and blood cells through oxidation reactions. Hemolysis, renal and liver failure are the results of these damages. Aggressive dialysis can improve the situation(Sony VAIO VGN-AW21S/B battery).[57]

The carcinogenity of chromate dust is known for a long time, and in 1890 the first publication described the elevated cancer risk of workers in a chromate dye company.[58][59] Three mechanisms have been proposed to describe the genotoxicity of chromium(VI). The first mechanism includes highly reactive hydroxyl radicals and other reactive radicals which are by products of the reduction of chromium(VI) to chromium(III) (Sony VAIO VGN-AW21VY/Q battery). The second process includes the direct binding of chromium(V), produced by reduction in the cell, and chromium(IV) compounds to the DNA. The last mechanism attributed the genotoxicity to the binding to the DNA of the end product of the chromium(III) reduction.[60]

Chromium salts (chromates) are also the cause of allergic reactions in some people. Chromates are often used to manufacture, amongst other things, leather products(Sony VAIO VGN-AW21XY/Q battery), paints, cement, mortar and anti-corrosives. Contact with products containing chromates can lead to allergic contact dermatitis and irritant dermatitis, resulting in ulceration of the skin, sometimes referred to as "chrome ulcers". This condition is often found in workers that have been exposed to strong chromate solutions in electroplating, tanning and chrome-producing manufacturers. (Sony VAIO VGN-AW21Z/B battery)

[edit]Environmental issues

As chromium compounds were used in dyes and paints and the tanning of leather, these compounds are often found in soil and groundwater at abandoned industrial sites, now needing environmental cleanup and remediation per the treatment of brownfield land. Primer paint containing hexavalent chromium is still widely used for aerospace and automobile refinishing applications. (Sony VAIO VGN-AW31M/H battery)

In 2010, the Environmental Working Group studied the drinking water in 35 American cities. The study was the first nationwide analysis measuring the presence of the chemical in U.S. water systems. The study found measurable hexavalent chromium in the tap water of 31 of the cities sampled, with Norman, Oklahoma, at the top of list(Sony VAIO VGN-AW31S/B battery); 25 cities had levels that exceeded California's proposed limit.[64] Note: Concentrations of Cr VI in US municipal drinking water supplies reported by EWG are within likely, natural background levels for the areas tested and not necessarily indicative of industrial pollution (CalEPA Fact Sheet), as asserted by EWG. This factor was not taken into consideration in their report(Sony VAIO VGN-AW41MF battery).