Chromium (Cr) is an element with atomic number 24 and atomic mass 51.996 of a secondary subgroup of the sixth group of the fourth period of the periodic system of chemical elements of D. I. Mendeleev. Chrome is a hard metal with a bluish-white color. Has high chemical resistance. At room temperature Cr is resistant to water and air. This element is one of the most important metals used in industrial alloying of steels. Chromium compounds have bright colors of various colors, which is why it got its name. After all, translated from Greek, “chrome” means “paint”.
There are 24 known isotopes of chromium from 42Cr to 66Cr. Stable natural isotopes are 50Cr (4.31%), 52Cr (87.76%), 53Cr (9.55%) and 54Cr (2.38%). Of the six artificial radioactive isotopes, the most important is 51Cr, with a half-life of 27.8 days. It is used as an isotope indicator.
Unlike the metals of antiquity (gold, silver, copper, iron, tin and lead), chromium has its own “discoverer”. In 1766, a mineral was found in the vicinity of Yekaterinburg, which was called “Siberian red lead” - PbCrO4. In 1797, L. N. Vauquelin discovered element No. 24 in the mineral crocoite, a natural lead chromate. Around the same time (1798), independently of Vauquelin, chromium was discovered by German scientists M. G. Klaproth and Lowitz in a sample of heavy black mineral (it was chromite FeCr2O4), found in the Urals. Later in 1799, F. Tassert discovered a new metal in the same mineral found in southeastern France. It is believed that it was Tassert who first managed to obtain relatively pure metal chromium.
Metal chromium is used for chrome plating, and also as one of the most important components of alloy steels (in particular stainless steels). In addition, chromium has found application in a number of other alloys (acid-resistant and heat-resistant steels). After all, the introduction of this metal into steel increases its resistance to corrosion both in aqueous environments at normal temperatures and in gases at elevated temperatures. Chromium steels are characterized by increased hardness. Chromium is used in thermochrome plating, a process in which the protective effect of Cr is due to the formation of a thin but durable oxide film on the surface of the steel, which prevents the interaction of the metal with the environment.
Chromium compounds are also widely used; chromites are successfully used in the refractory industry: open-hearth furnaces and other metallurgical equipment are lined with magnesite-chromite bricks.
Chromium is one of the biogenic elements that are constantly included in the tissues of plants and animals. Plants contain chromium in their leaves, where it is present in the form of a low-molecular complex not associated with subcellular structures. Until now, scientists have not been able to prove the necessity of this element for plants. However, in animals, Cr is involved in the metabolism of lipids, proteins (part of the enzyme trypsin), and carbohydrates (a structural component of the glucose-resistant factor). It is known that only trivalent chromium is involved in biochemical processes. Like most other important nutrients, chromium enters the animal or human body through food. A decrease in this microelement in the body leads to slower growth, sharp increase blood cholesterol levels and decreased sensitivity of peripheral tissues to insulin.
At the same time at pure form chromium is very toxic - Cr metal dust irritates lung tissue, chromium (III) compounds cause dermatitis. Chromium (VI) compounds lead to various human diseases, including cancer.
Chromium is an important biogenic element, which is certainly included in the tissues of plants, animals and humans. The average content of this element in plants is 0.0005%, and almost all of it accumulates in the roots (92-95%), the rest is contained in the leaves. Higher plants do not tolerate concentrations of this metal above 3∙10-4 mol/l. In animals, the chromium content ranges from ten thousandths to ten millionths of a percent. But in plankton, the coefficient of chromium accumulation is amazing - 10,000-26,000. In the adult human body, the Cr content ranges from 6 to 12 mg. Moreover, the physiological need for chromium for humans has not been established quite precisely. It largely depends on the diet - when eating food high in sugar, the body's need for chromium increases. It is generally accepted that a person needs approximately 20–300 mcg of this element per day. Like other biogenic elements, chromium can accumulate in body tissues, especially in hair. It is in them that the chromium content indicates the degree of provision of the body with this metal. Unfortunately, with age, the “reserves” of chromium in tissues are depleted, with the exception of the lungs.
Chromium is involved in the metabolism of lipids, proteins (present in the enzyme trypsin), carbohydrates (is a structural component of the glucose-resistant factor). This factor ensures the interaction of cellular receptors with insulin, thereby reducing the body's need for it. Glucose tolerance factor (GTF) enhances the action of insulin in all metabolic processes involving it. In addition, chromium takes part in the regulation of cholesterol metabolism and is an activator of certain enzymes.
The main source of chromium in animals and humans is food. Scientists have found that the concentration of chromium in plant foods is significantly lower than in animal foods. The richest sources of chromium are brewer's yeast, meat, liver, legumes and whole unprocessed grains. A decrease in the content of this metal in food and blood leads to a decrease in growth rate, an increase in cholesterol in the blood, and a decrease in the sensitivity of peripheral tissues to insulin (diabetes-like state). In addition, the risk of developing atherosclerosis and disorders of higher nervous activity increases.
However, even at concentrations of a fraction of a milligram per cubic meter In the atmosphere, all chromium compounds have a toxic effect on the body. Poisoning with chromium and its compounds is common during their production, in mechanical engineering, metallurgy, in textile industry. The degree of toxicity of chromium depends on the chemical structure of its compounds - dichromates are more toxic than chromates, Cr+6 compounds are more toxic than Cr+2 and Cr+3 compounds. Signs of poisoning include a feeling of dryness and pain in the nasal cavity, a sore throat, difficulty breathing, coughing and similar symptoms. If there is a slight excess of chromium vapors or dust, the signs of poisoning disappear soon after work in the workshop stops. With prolonged constant contact with chromium compounds, signs of chronic poisoning appear - weakness, constant headaches, weight loss, dyspepsia. Disturbances in the functioning of the gastrointestinal tract, pancreas, and liver begin. Bronchitis, bronchial asthma, and pneumosclerosis develop. Skin diseases appear - dermatitis, eczema. In addition, chromium compounds are dangerous carcinogens that can accumulate in body tissues, causing cancer.
Prevention of poisoning includes periodic medical examinations of personnel working with chromium and its compounds; installation of ventilation, dust suppression and dust collection equipment; use of personal protective equipment (respirators, gloves) by workers.
The root "chrome" in its concept of "color", "paint" is part of many words used in a wide variety of fields: science, technology and even music. So many names of photographic films contain this root: “orthochrome”, “panchrome”, “isopanchrome” and others. The word chromosome is made up of two Greek words: chromo and soma. Literally this can be translated as “painted body” or “body that is painted.” Structural element chromosomes formed in the interphase of the cell nucleus as a result of chromosome doubling are called “chromatids”. “Chromatin” is a substance of chromasomes located in the nuclei of plant and animal cells, which is intensely stained with nuclear dyes. “Chromatophores” are pigment cells in animals and humans. In music, the concept of “chromatic scale” is used. “Khromka” is one of the types of Russian accordion. In optics, there are the concepts of “chromatic aberration” and “chromatic polarization”. “Chromatography” is a physical and chemical method for separating and analyzing mixtures. “Chromoscope” is a device for obtaining a color image by optically combining two or three color-separated photographic images, illuminated through specially selected differently colored filters.
The most toxic is chromium (VI) oxide CrO3; it belongs to hazard class I. Lethal dose for humans (orally) 0.6 g. Ethyl alcohol ignites on contact with freshly prepared CrO3!
The most common grade of stainless steel contains 18% Cr, 8% Ni, about 0.1% C. It has excellent resistance to corrosion and oxidation, and retains strength at high temperatures. It is from this steel that the sheets used in the construction of the sculptural group of V.I. were made. Mukhina "Worker and Collective Farm Woman".
Ferrochrome, used in the metallurgical industry in the production of chromium steels, was of very poor quality at the end of the 19th century. This is due to the low chromium content in it - only 7-8%. Then it was called “Tasmanian cast iron” due to the fact that the original iron-chrome ore was imported from Tasmania.
It was previously mentioned that chrome alum is used in tanning leather. Thanks to this, the concept of “chrome” boots appeared. Leather tanned with chromium compounds acquires shine, gloss and strength.
Many laboratories use a “chromic mixture” - a mixture of a saturated solution of potassium dichromate with concentrated sulfuric acid. It is used in degreasing the surfaces of glass and steel laboratory glassware. It oxidizes fat and removes its remains. Just handle this mixture with caution, because it is a mixture of a strong acid and a strong oxidizing agent!
Nowadays, wood is still used as a building material, because it is inexpensive and easy to process. But she has a lot negative properties- susceptibility to fires and fungal diseases that destroy it. To avoid all these troubles, wood is impregnated with special compounds containing chromates and dichromates, plus zinc chloride, copper sulfate, sodium arsenate and some other substances. Thanks to such compositions, wood increases its resistance to fungi and bacteria, as well as to open fire.
Chrome has occupied a special niche in printing. In 1839, it was discovered that paper impregnated with sodium bichromate suddenly turned brown when exposed to bright light. Then it turned out that bichromate coatings on paper, after exposure, do not dissolve in water, but, when wetted, acquire a bluish tint. Printers took advantage of this property. The desired pattern was photographed on a plate with a colloidal coating containing dichromate. The illuminated areas did not dissolve during washing, and the unexposed areas dissolved, and a pattern remained on the plate from which it was possible to print.
The history of the discovery of element No. 24 began in 1761, when an unusual red mineral was found in the Berezovsky mine (the eastern foot of the Ural Mountains) near Yekaterinburg, which, when ground into dust, gave a yellow color. The find belonged to St. Petersburg University professor Johann Gottlob Lehmann. Five years later, the scientist delivered the samples to the city of St. Petersburg, where he conducted a series of experiments on them. In particular, he treated the unusual crystals with hydrochloric acid, resulting in a white precipitate in which lead was found. Based on the results obtained, Lehman named the mineral Siberian red lead. This is the story of the discovery of crocoite (from the Greek “krokos” - saffron) - a natural lead chromate PbCrO4.
Interested in this find, Peter Simon Pallas, a German naturalist and traveler, organized and led an expedition of the St. Petersburg Academy of Sciences to the heart of Russia. In 1770, the expedition reached the Urals and visited the Berezovsky mine, where samples of the mineral being studied were taken. This is how the traveler himself describes it: “This amazing red lead mineral is not found in any other deposit. When ground into powder it turns yellow and can be used in artistic miniatures.” German enterprise overcame all the difficulties of mining and delivering crocoite to Europe. Despite the fact that these operations took at least two years, soon the carriages of noble gentlemen of Paris and London were traveling painted with finely ground crocoite. The collections of the mineralogical museums of many universities of the old world have been enriched with the best examples of this mineral from the Russian depths. However, European scientists could not figure out the composition of the mysterious mineral.
This lasted for thirty years, until a sample of Siberian red lead fell into the hands of Nicolas Louis Vauquelin, professor of chemistry at the Paris Mineralogical School, in 1796. After analyzing the crocoite, the scientist found nothing in it except oxides of iron, lead and aluminum. Subsequently, Vauquelin treated crocoite with a solution of potash (K2CO3) and, following the precipitation of a white precipitate of lead carbonate, isolated a yellow solution of an unknown salt. After conducting a series of experiments on processing the mineral with salts of various metals, the professor, with the help of of hydrochloric acid isolated a solution of “red lead acid” - chromium oxide and water (chromic acid exists only in dilute solutions). By evaporating this solution, he obtained ruby-red crystals (chromic anhydride). Further heating of the crystals in a graphite crucible in the presence of coal gave a lot of fused gray needle-shaped crystals - a new, hitherto unknown metal. The next series of experiments showed the high refractoriness of the resulting element and its resistance to acids. The Paris Academy of Sciences immediately witnessed the discovery; the scientist, at the insistence of his friends, gave the name to the new element - chromium (from the Greek “color”, “color”) due to the variety of shades of the compounds it forms. In his further works, Vauquelin confidently stated that the emerald color of some precious stones, as well as natural beryllium and aluminum silicates, is explained by the admixture of chromium compounds in them. An example is the emerald, which is colored in green color beryl, in which aluminum is partially replaced by chromium.
It is clear that Vauquelin did not obtain pure metal, most likely its carbides, which is confirmed by the needle-shaped shape of light gray crystals. Pure chromium metal was later obtained by F. Tassert, probably in 1800.
Also, independently of Vauquelin, chromium was discovered by Klaproth and Lowitz in 1798.
In the bowels of the earth, chromium is a fairly common element, despite the fact that it is not found in free form. Its clarke (average content in the earth's crust) is 8.3.10-3% or 83 g/t. However, its distribution among breeds is uneven. This element is mainly characteristic of the Earth’s mantle; the fact is that ultramafic rocks (peridotites), which are presumably close in composition to the mantle of our planet, are the richest in chromium: 2 10-1% or 2 kg/t. In such rocks, Cr forms massive and disseminated ores, and the formation of the largest deposits of this element is associated with them. The chromium content is also high in basic rocks (basalts, etc.) 2 10-2% or 200 g/t. Much less Cr is found in acidic rocks: 2.5 10-3%, sedimentary rocks (sandstones) - 3.5 10-3%, shales also contain chromium - 9 10-3%.
It can be concluded that chromium is a typical lithophile element and is almost entirely contained in deep minerals in the Earth’s interior.
There are three main chromium minerals: magnochromite (Mn, Fe)Cr2O4, chromopicotite (Mg, Fe)(Cr, Al)2O4 and aluminochromite (Fe, Mg)(Cr, Al)2O4. These minerals have a single name - chrome spinel and the general formula (Mg, Fe)O (Cr, Al, Fe)2O3. By appearance they are indistinguishable and are inaccurately called "chromites". Their composition is variable. The content of the most important components varies (weight %): Cr2O3 from 10.5 to 62.0; Al2O3 from 4 to 34.0; Fe2O3 from 1.0 to 18.0; FeO from 7.0 to 24.0; MgO from 10.5 to 33.0; SiO2 from 0.4 to 27.0; TiO2 impurities up to 2; V2O5 up to 0.2; ZnO up to 5; MnO up to 1. Some chromium ores contain 0.1-0.2 g/t of platinum group elements and up to 0.2 g/t of gold.
In addition to various chromites, chromium is part of a number of other minerals - chrome vesuvian, chrome chlorite, chrome tourmaline, chrome mica (fuchsite), chrome garnet (uvarovite), etc., which often accompany ores, but are not of industrial importance. Chromium is a relatively weak aquatic migrant. Under exogenous conditions, chromium, like iron, migrates in the form of suspensions and can precipitate in clays. The most mobile form is chromates.
Of practical importance, perhaps, is only chromite FeCr2O4, which belongs to spinels - isomorphic minerals of the cubic system with the general formula MO Me2O3, where M is a divalent metal ion, and Me is a trivalent metal ion. In addition to spinels, chromium is found in many much less common minerals, for example, melanochroite 3PbO 2Cr2O3, vokelenite 2(Pb,Cu)CrO4(Pb,Cu)3(PO4)2, tarapacaite K2CrO4, ditzeite CaIO3 CaCrO4 and others.
Chromites are usually found in the form of granular masses of black color, less often - in the form of octahedral crystals, have a metallic luster, and occur in the form of continuous masses.
At the end of the 20th century, chromium reserves (identified) in almost fifty countries of the world with deposits of this metal amounted to 1674 million tons. The leading position is occupied by the Republic of South Africa - 1050 million tons, where the main contribution is made by the Bushveld complex (about 1000 million tons ). The second place in chrome resources belongs to Kazakhstan, where very high quality ore is mined in the Aktobe region (Kempirsay massif). Other countries also have reserves of this element. Turkey (in Guleman), Philippines on the island of Luzon, Finland (Kemi), India (Sukinda), etc.
Our country has its own developed chromium deposits in the Urals (Donskoye, Saranovskoye, Khalilovskoye, Alapaevskoye and many others). Moreover, in early XIX centuries, it was the Ural deposits that were the main sources of chromium ores. It was only in 1827 that the American Isaac Tison discovered a large deposit of chrome ore on the border of Maryland and Pennsylvania, seizing the mining monopoly for many years. In 1848, deposits of high-quality chromite were found in Turkey, near Bursa, and soon (after the depletion of the Pennsylvania deposit) it was this country that took over the role of monopolist. This continued until 1906, when rich deposits of chromite were discovered in South Africa and India.
Total consumption of pure chromium metal today is approximately 15 million tons. The production of electrolytic chromium - the purest - accounts for 5 million tons, which is a third of total consumption.
Chromium is widely used to alloy steels and alloys, giving them corrosion and heat resistance. More than 40% of the resulting pure metal is consumed in the production of such “superalloys”. The most well-known resistance alloys are nichrome with a Cr content of 15-20%, heat-resistant alloys - 13-60% Cr, stainless alloys - 18% Cr and ball bearing steels 1% Cr. The addition of chromium to ordinary steels improves them physical properties and makes the metal more susceptible to heat treatment.
Metallic chromium is used for chrome plating - applying a thin layer of chromium to the surface of steel alloys in order to increase the corrosion resistance of these alloys. The chrome coating perfectly resists the effects of humid atmospheric air, salty sea air, water, nitric and most organic acids. Such coatings have two purposes: protective and decorative. The thickness of the protective coatings is about 0.1 mm; they are applied directly to the product and give it increased wear resistance. Decorative coatings have an aesthetic value; they are applied to a layer of another metal (copper or nickel), which actually performs a protective function. The thickness of such a coating is only 0.0002–0.0005 mm.
Chromium compounds are also actively used in various fields.
The main chromium ore - chromite FeCr2O4 is used in the production of refractories. Magnesite-chromite bricks are chemically passive and heat-resistant; they can withstand sudden, repeated temperature changes, which is why they are used in the structures of the arches of open-hearth furnaces and the working space of other metallurgical devices and structures.
The hardness of chromium (III) oxide crystals - Cr2O3 is comparable to the hardness of corundum, which ensures its use in the compositions of grinding and lapping pastes used in mechanical engineering, jewelry, optical and watch industries. It is also used as a catalyst for the hydrogenation and dehydrogenation of certain organic compounds. Cr2O3 is used in painting as a green pigment and for coloring glass.
Potassium chromate - K2CrO4 is used in leather tanning, as a mordant in the textile industry, in the production of dyes, and in wax bleaching.
Potassium dichromate (chrompic) - K2Cr2O7 is also used for tanning leather, as a mordant for dyeing fabrics, and is a corrosion inhibitor for metals and alloys. Used in the manufacture of matches and for laboratory purposes.
Chromium (II) chloride CrCl2 is a very strong reducing agent, easily oxidized even by atmospheric oxygen, which is used in gas analysis for the quantitative absorption of O2. In addition, it is used to a limited extent in the production of chromium by electrolysis of molten salts and chromatometry.
Chromium-potassium alum K2SO4.Cr2(SO4)3 24H2O is used mainly in the textile industry - for tanning leather.
Anhydrous chromium chloride CrCl3 is used for applying chromium coatings to the surface of steels by chemical vapor deposition, is integral part some catalysts. CrCl3 hydrates are a mordant for dyeing fabrics.
Various dyes are made from lead chromate PbCrO4.
A solution of sodium dichromate is used to clean and etch the surface of steel wire before galvanizing, and also to brighten brass. Chromic acid is obtained from sodium dichromate, which is used as an electrolyte in chrome plating of metal parts.
In nature, chromium is found mainly in the form of chromium iron ore FeO∙Cr2O3; when it is reduced with coal, an alloy of chromium with iron is obtained - ferrochrome, which is directly used in the metallurgical industry in the production of chromium steels. The chromium content in this composition reaches 80% (by weight).
The reduction of chromium (III) oxide with coal is intended to obtain high-carbon chromium necessary for the production of special alloys. The process is carried out in an electric arc furnace.
To obtain pure chromium, chromium(III) oxide is first prepared and then reduced by an aluminothermic method. In this case, a mixture of powdered or in the form of aluminum shavings (Al) and a charge of chromium oxide (Cr2O3) are first heated to a temperature of 500-600 ° C. Then, reduction is initiated with a mixture of barium peroxide with aluminum powder, or by igniting part of the charge, followed by adding the remaining part . In this process, it is important that the resulting thermal energy is sufficient to melt the chromium and separate it from the slag.
Cr2O3 + 2Al = 2Cr + 2Al2O3
The chromium obtained in this way contains a certain amount of impurities: iron 0.25-0.40%, sulfur 0.02%, carbon 0.015-0.02%. The content of pure substance is 99.1–99.4%. This chromium is fragile and easily ground into powder.
The reality of this method was proven and demonstrated back in 1859 by Friedrich Wöhler. On an industrial scale, aluminothermic reduction of chromium became possible only after it became accessible method obtaining cheap aluminum. Goldschmidt was the first to develop safe way regulation of the highly exothermic (hence explosive) reduction process.
When it is necessary to obtain high-purity chromium, industry uses electrolytic methods. Electrolysis is carried out using a mixture of chromic anhydride, chromoammonium alum or chromium sulfate with dilute sulfuric acid. Chromium deposited on aluminum or stainless steel cathodes during the electrolysis process contains dissolved gases as impurities. Purity of 99.90–99.995% can be achieved using high-temperature (1500-1700° C) purification in a hydrogen flow and vacuum degassing. Advanced electrolytic chromium refining techniques remove sulfur, nitrogen, oxygen and hydrogen from the raw product.
In addition, it is possible to obtain Cr metal by electrolysis of CrCl3 or CrF3 melts in a mixture with potassium, calcium, and sodium fluorides at a temperature of 900 ° C in an argon environment.
The possibility of an electrolytic method for obtaining pure chromium was proved by Bunsen in 1854 by subjecting an aqueous solution of chromium chloride to electrolysis.
The industry also uses a silicothermic method for producing pure chromium. In this case, chromium is reduced from oxide by silicon:
2Cr2O3 + 3Si + 3CaO = 4Cr + 3CaSiO3
Chromium is silicothermally smelted in arc furnaces. The addition of quicklime allows you to convert refractory silicon dioxide into low-melting calcium silicate slag. The purity of silicothermic chromium is approximately the same as aluminothermic chromium, however, naturally, the silicon content in it is slightly higher and the aluminum content is slightly lower.
Cr can also be obtained by the reduction of Cr2O3 with hydrogen at 1500° C, the reduction of anhydrous CrCl3 with hydrogen, alkali or alkaline earth metals, magnesium and zinc.
To obtain chromium, they also tried to use other reducing agents - carbon, hydrogen, magnesium. However, these methods are not widely used.
The Van Arkel-Kuchman-De Boer process uses the decomposition of chromium (III) iodide on a wire heated to 1100° C with the deposition of pure metal on it.
Chrome is a hard, very heavy, refractory, malleable metal of a steel-gray color. Pure chromium is quite plastic, crystallizes in a body-centered lattice, a = 2.885 Å (at a temperature of 20 ° C). At a temperature of about 1830° C, there is a high probability of transformation into a modification with a face-centered lattice, a = 3.69 Å. Atomic radius 1.27 Å; ionic radii of Cr2+ 0.83 Å, Cr3+ 0.64 Å, Cr6+ 0.52 Å.
The melting point of chromium directly depends on its purity. Therefore, the determination of this indicator for pure chromium is very difficult task- after all, even a small content of nitrogen or oxygen impurities can significantly change the melting point. Many researchers have been studying this issue for decades and received results that are far from each other: from 1513 to 1920 ° C. Previously, it was generally accepted that this metal melts at a temperature of 1890 ° C, but modern research indicates a temperature of 1907 ° C, chromium boils at temperatures above 2500° C - the data also varies: from 2199° C to 2671° C. The density of chromium is less than that of iron; it is 7.19 g/cm3 (at a temperature of 200° C).
Chrome has all the basic characteristics of metals - it conducts heat well, its resistance to electric current is very low, like most metals, chrome has a characteristic shine. In addition, this item has one very interesting feature: the fact is that at a temperature of 37° C its behavior cannot be explained - a sharp change in many physical properties occurs, this change has an abrupt nature. Chrome, like a sick person at a temperature of 37° C, begins to act up: the internal friction of chromium reaches a maximum, the elastic modulus drops to minimum values. The value of electrical conductivity jumps, the thermoelectromotive force and the coefficient of linear expansion constantly change. Scientists cannot yet explain this phenomenon.
The specific heat capacity of chromium is 0.461 kJ/(kg.K) or 0.11 cal/(g °C) (at a temperature of 25 °C); thermal conductivity coefficient 67 W/(m K) or 0.16 cal/(cm sec °C) (at a temperature of 20 °C). Thermal coefficient of linear expansion 8.24 10-6 (at 20 °C). Chromium at a temperature of 20 ° C has a specific electrical resistivity of 0.414 μΩ m, and its thermal coefficient of electrical resistance in the range of 20-600 ° C is 3.01 10-3.
It is known that chromium is very sensitive to impurities - the smallest fractions of other elements (oxygen, nitrogen, carbon) can make chromium very brittle. It is extremely difficult to obtain chromium without these impurities. For this reason, this metal is not used for structural purposes. But in metallurgy it is actively used as an alloying material, since its addition to the alloy makes the steel hard and wear-resistant, because chromium is the hardest of all metals - it cuts glass like diamond! The Brinell hardness of high-purity chromium is 7-9 Mn/m2 (70-90 kgf/cm2). Spring, spring, tool, stamp and ball bearing steels are alloyed with chromium. In them (except for ball bearing steels) chromium is present along with manganese, molybdenum, nickel, and vanadium. The addition of chromium to conventional steels (up to 5% Cr) improves their physical properties and makes the metal more susceptible to heat treatment.
Chromium is antiferromagnetic, specific magnetic susceptibility 3.6 10-6. Electrical resistivity 12.710-8 Ohm. The temperature coefficient of linear expansion of chromium is 6.210-6. The heat of vaporization of this metal is 344.4 kJ/mol.
Chrome is resistant to corrosion in air and water.
Chemically, chromium is quite inert, this is explained by the presence of a durable thin oxide film on its surface. Cr does not oxidize in air, even in the presence of moisture. When heated, oxidation occurs exclusively on the metal surface. At 1200°C the film is destroyed and oxidation occurs much faster. At 2000° C, chromium burns to form green chromium (III) oxide Cr2O3, which has amphoteric properties. By fusing Cr2O3 with alkalis, chromites are obtained:
Cr2O3 + 2NaOH = 2NaCrO2 + H2O
Uncalcined chromium(III) oxide easily dissolves in alkaline solutions and acids:
Cr2O3 + 6HCl = 2CrCl3 + 3H2O
In compounds, chromium mainly exhibits oxidation states Cr+2, Cr+3, Cr+6. The most stable are Cr+3 and Cr+6. There are also some compounds where chromium has oxidation states Cr+1, Cr+4, Cr+5. Chromium compounds are very diverse in color: white, blue, green, red, purple, black and many others.
Chromium easily reacts with dilute solutions of hydrochloric and sulfuric acids to form chromium chloride and sulfate and release hydrogen:
Cr + 2HCl = CrCl2 + H2
Aqua regia and nitric acid passivate chromium. Moreover, chromium passivated by nitric acid does not dissolve in dilute sulfuric and hydrochloric acids even after prolonged boiling in their solutions, but at some point dissolution does occur, accompanied by violent foaming from the liberated hydrogen. This process is explained by the fact that chromium goes from a passive state to an active one, in which the metal is not protected protective film. Moreover, if nitric acid is added again during the dissolution process, the reaction will stop, since chromium is again passivated.
Under normal conditions, chromium reacts with fluorine to form CrF3. At temperatures above 600° C, interaction with water vapor occurs, the result of this interaction is chromium (III) oxide Cr2O3:
4Cr + 3O2 = 2Cr2O3
Cr2O3 is green microcrystals with a density of 5220 kg/m3 and a high melting point (2437° C). Chromium(III) oxide exhibits amphoteric properties, but is very inert and difficult to dissolve in aqueous acids and alkalis. Chromium(III) oxide is quite toxic. When it comes into contact with the skin, it can cause eczema and other skin diseases. Therefore, when working with chromium (III) oxide, it is imperative to use personal protective equipment.
In addition to the oxide, other compounds with oxygen are known: CrO, CrO3, obtained indirectly. The greatest danger is from inhaled oxide aerosol, which causes severe diseases of the upper respiratory tract and lungs.
Chromium forms a large number of salts with oxygen-containing components.
It is characteristic that chromium's neighbors, like chromium itself, are widely used for alloying steels.
The melting point of chromium depends on its purity. Many researchers tried to determine it and obtained values from 1513 to 1920 ° C. Such a large “scatter” is explained primarily by the amount and composition of impurities contained in chromium. It is now believed that it melts at a temperature of about 1875° C. The boiling point is 2199° C. The density of chromium is less than that of iron; it is equal to 7.19.
Its chemical properties are similar to molybdenum and tungsten. Its highest oxide CrO3 is acidic, it is chromic acid anhydride H2CrO4. The mineral with which we began our acquaintance with element No. 24 is a salt of this acid. In addition to chromic acid, dichromic acid H2Cr2O7 is known; its salts, dichromates, are widely used in chemistry.
The most common chromium oxide, Cr2O3, is amphoteric. In general, in different conditions can exhibit valencies from 2 to 6. Only tri- and hexavalent chromium compounds are widely used.
Chrome has all the properties of a metal - it conducts heat and electricity well, and has a characteristic metallic luster. The main feature of chromium is its resistance to acids and oxygen.
For those who constantly deal with chromium, another of its features has become the talk of the town: at a temperature of about 37° C, some of the physical properties of this metal change sharply and abruptly. At this temperature there is a clearly expressed maximum of internal friction and a minimum of elasticity modulus. Electrical resistance, coefficient of linear expansion, and thermoelectromotive force change almost as sharply.
Scientists cannot yet explain this anomaly.
There are four known natural isotopes of chromium. Their mass numbers are 50, 52, 53 and 54. The share of the most common isotope 52Cr is about 84%.
It would probably be unnatural if the story about the use of chromium and its compounds began not with steel, but with something else. Chromium is one of the most important alloying elements used in ferrous metallurgy. The addition of chromium to conventional steels (up to 5% Cr) improves their physical properties and makes the metal more susceptible to heat treatment. Spring, spring, tool, stamp and ball bearing steels are alloyed with chromium. In them (except for ball bearing steels) chromium is present along with manganese, molybdenum, nickel, and vanadium. And ball bearing steels contain only chromium (about 1.5%) and (about 1%). The latter forms with chromium carbides of exceptional hardness: Cr3C, Cr7C3 and Cr23C6. They give ball bearing steel high wear resistance.
If the chromium content of steel is increased to 10% or more, the steel becomes more resistant to oxidation and corrosion, but this is where a factor that can be called carbon limitation comes into play. The ability of carbon to bind large amounts of chromium leads to the depletion of steel in this element. Therefore, metallurgists are faced with a dilemma: do you want to get corrosion resistance- reduce the carbon content and lose on wear resistance and hardness.
The most common grade of stainless steel contains 18% chromium and 8% nickel. The carbon content in it is very low - up to 0.1%. Stainless steels resist corrosion and oxidation well and retain strength at high temperatures. The sculptural group “Worker and Collective Farm Woman” by V.I. Mukhina was made from sheets of such steel, which was installed in Moscow at the Northern entrance to the Exhibition of National Economic Achievements. Stainless steels are widely used in the chemical and petroleum industries.
High chromium steels (containing 25-30% Cr) are particularly resistant to oxidation when high temperature. They are used for the manufacture of parts for heating furnaces.
Now a few words about chromium-based alloys. These are containing more than 50% chromium. They have very high heat resistance. However, they have a very big drawback that negates all the advantages: they are very sensitive to surface defects: all it takes is a scratch or microcrack to appear, and the product will quickly collapse under load. For most alloys, such shortcomings are eliminated by thermomechanical treatment, but chromium-based alloys cannot be treated in this way. In addition, they are too brittle at room temperature, which also limits their application.
Alloys of chromium and nickel are more valuable (they often contain alloying additives and other elements). The most common alloys of this group - nichromes contain up to 20% chromium (the rest) and are used for the manufacture of heating elements. Nichromes have high electrical resistance for metals; when current is passed through, they become very hot.
The addition of molybdenum and cobalt to chromium-nickel alloys makes it possible to obtain materials with high heat resistance and the ability to withstand heavy loads at 650-900 ° C. For example, gas turbine blades are made from these alloys. Cobalt-chromium alloys containing 25-30% chromium also have heat resistance. Industry also uses chromium as a material for anti-corrosion and decorative coatings.
Element No. 24. One of the most hard metals. Has high chemical resistance. One of the most important metals used in the production of alloy steels. Most chromium compounds are brightly colored, and the most different colors. For this feature, the element was named chromium, which means “paint” in Greek.
A mineral containing chromium was discovered near Yekaterinburg in 1766 by I.G. Lehmann called it “Siberian red lead”. Now this mineral is called crocoite. Its composition is also known - PbCrO 4. And at one time, “Siberian red lead” caused a lot of disagreement among scientists. For thirty years they argued about its composition, until, finally, in 1797, the French chemist Louis Nicolas Vauquelin isolated a metal from it, which (also, by the way, after some controversy) was called chromium.
Vauquelin treated crocoite with potash K 2 CO 3: lead chromate turned into potassium chromate. Potassium chromate was then converted into chromium oxide and water using hydrochloric acid (chromic acid exists only in dilute solutions). By heating green chromium oxide powder in a graphite crucible with coal, Vauquelin obtained a new refractory metal.
The Paris Academy of Sciences witnessed the discovery in its entirety. But, most likely, Vauquelin isolated not elemental chromium, but its carbides. This is evidenced by the needle-shaped shape of the light gray crystals obtained by Vauquelin.
The name “chrome” was suggested by Vauquelin’s friends, but he did not like it - the metal did not have a special color. However, friends managed to persuade the chemist, citing the fact that brightly colored chromium compounds can be used to obtain good paints. (By the way, it was in the works of Vauquelin that the emerald color of some natural beryllium and aluminum silicates was first explained; they, as Vauquelin found out, were colored by impurities of chromium compounds.) And so this name was adopted for the new element.
By the way, the syllable “chrome”, precisely in the sense of “colored”, is included in many scientific, technical and even musical terms. Isopanchrome, panchrome and orthochrome photographic films are widely known. The word "chromosome" translated from Greek means "body that is colored." There is a “chromatic” scale (in music) and there is a “chromatic” harmonic.
There is quite a lot of chromium in the earth's crust - 0.02%. The main mineral from which the industry obtains chromium is chrome spinel of variable composition with the general formula (Mg, Fe) O · (Cr, Al, Fe) 2 O 3. Chrome ore is called chromite or chromium iron ore (because it almost always contains iron). There are deposits of chrome ores in many places. Our country has huge reserves of chromites. One of the largest deposits is located in Kazakhstan, in the Aktobe region; it was discovered in 1936. There are significant reserves of chrome ores in the Urals.
Chromites are mostly used for smelting ferrochrome. It is one of the most important ferroalloys*, absolutely necessary for the mass production of alloy steels.
* Ferroalloys are alloys of iron with other elements used mainly for alloying and deoxidizing steel. Ferrochrome contains at least 60% Cr.
Tsarist Russia produced almost no ferroalloys. Several blast furnaces at southern factories smelted low-percentage (alloying metal) ferrosilicon and ferromanganese. Moreover, on the Satka River, which flows in the Southern Urals, in 1910 a tiny factory was built that smelted tiny amounts of ferromanganese and ferrochrome.
Young Soviet country In the first years of development, ferroalloys had to be imported from abroad. Such dependence on capitalist countries was unacceptable. Already in 1927...1928. The construction of Soviet ferroalloy plants began. At the end of 1930, the first large ferroalloy furnace was built in Chelyabinsk, and in 1931 the Chelyabinsk plant, the first-born of the ferroalloy industry of the USSR, came into operation. In 1933, two more factories were launched - in Zaporozhye and Zestafoni. This made it possible to stop the import of ferroalloys. In just a few years, the Soviet Union organized the production of many types of special steels - ball bearing, heat-resistant, stainless, automotive, high-speed... All these steels contain chromium.
At the 17th Party Congress, People's Commissar of Heavy Industry Sergo Ordzhonikidze said: “...if we did not have high-quality steels, we would not have an automobile and tractor industry. The cost of the high-quality steel we currently use is estimated at over 400 million rubles. If it were necessary to import, it would be 400 million rubles. every year, damn it, you would end up in bondage to the capitalists...”
The plant on the basis of the Aktobe field was built later, during the Great Patriotic War. He produced the first ferrochrome smelting on January 20, 1943. The workers of the city of Aktyubinsk took part in the construction of the plant. The construction was declared public. The ferrochrome of the new plant was used to produce metal for tanks and guns, for the needs of the front.
Years have passed. Now the Aktobe Ferroalloy Plant is the largest enterprise producing ferrochrome of all grades. The plant has produced highly qualified national metallurgical personnel. From year to year, the plant and chromite mines are increasing their capacity, providing our ferrous metallurgy with high-quality ferrochrome.
Our country has a unique deposit of naturally alloyed iron ores rich in chromium and nickel. It is located in the Orenburg steppes. The Orsko-Khalilovsky Metallurgical Plant was built and operates on the basis of this deposit. Naturally alloyed cast iron, which has high heat resistance, is smelted in the plant’s blast furnaces. Part of it is used in the form of casting, but most of it is sent for processing into nickel steel; chromium burns out when smelting steel from cast iron.
Cuba, Yugoslavia, and many countries in Asia and Africa have large reserves of chromites.
Chromite is used primarily in three industries: metallurgy, chemistry, and refractories, with metallurgy consuming approximately two-thirds of all chromite.
Steel alloyed with chromium has increased strength and resistance to corrosion in aggressive and oxidizing environments.
Obtaining pure chromium is expensive and labor-intensive process. Therefore, for alloying steel, ferrochrome is mainly used, which is obtained in electric arc furnaces directly from chromite. The reducing agent is coke. The chromium oxide content in chromite must be at least 48%, and the Cr:Fe ratio must be at least 3:1.
Ferrochrome produced in an electric furnace usually contains up to 80% chromium and 4...7% carbon (the rest is iron).
But for alloying many high-quality steels, ferrochrome containing little carbon is needed (the reasons for this are discussed below, in the chapter “Chrome in Alloys”). Therefore, part of the high-carbon ferrochrome is subjected to special treatment to reduce the carbon content in it to tenths and hundredths of a percent.
Elementary metallic chromium is also obtained from chromite. The production of technically pure chromium (97...99%) is based on the aluminothermy method, discovered back in 1865 by the famous Russian chemist N.N. Beketov. The essence of the method is the reduction of oxides with aluminum; the reaction is accompanied by a significant release of heat.
But first you need to obtain pure chromium oxide Cr 2 O 3. To do this, finely ground chromite is mixed with soda and limestone or iron oxide is added to this mixture. The entire mass is burned, and sodium chromate is formed:
2Cr 2 O 3 + 4Na 2 CO 3 + 3O 2 → 4Na 2 CrO 4 + 4CO 2.
Sodium chromate is then leached from the calcined mass with water; the liquor is filtered, evaporated and treated with acid. The result is sodium bichromate Na 2 Cr 2 O 7 . By reducing it with sulfur or carbon when heated, green chromium oxide is obtained.
Metallic chromium can be obtained by mixing pure chromium oxide with aluminum powder, heating this mixture in a crucible to 500...600°C and igniting it with barium peroxide. Aluminum takes oxygen away from chromium oxide. This reaction Cr 2 O 3 + 2Al → Al 2 O 3 + 2Сr is the basis of the industrial (aluminothermic) method for producing chromium, although, of course, the factory technology is much more complicated. Chromium obtained aluminothermically contains tenths of a percent of aluminum and iron, and hundredths of a percent of silicon, carbon and sulfur.
A silicothermic method is also used to obtain technically pure chromium. In this case, chromium is reduced from oxide by silicon according to the reaction 2Сr 2 О 3 + 3Si → 3SiO 2 + 4Сr.
This reaction occurs in arc furnaces. To bind silica, limestone is added to the charge. The purity of silicothermic chromium is approximately the same as aluminothermic chromium, although, of course, the silicon content in it is slightly higher and the aluminum content is slightly lower. To obtain chromium, they also tried to use other reducing agents - carbon, hydrogen, magnesium. However, these methods are not widely used.
High purity chromium (approximately 99.8%) is obtained electrolytically.
Technically pure and electrolytic chromium is used mainly for the production of complex chromium alloys.
The atomic mass of chromium is 51.996. In the periodic table it occupies a place in the sixth group. Its closest neighbors and analogues are molybdenum and tungsten. It is characteristic that chromium's neighbors, like chromium itself, are widely used for alloying steels.
The melting point of chromium depends on its purity. Many researchers have tried to determine it and obtained values from 1513 to 1920°C. Such a large “scatter” is explained primarily by the amount and composition of impurities contained in chromium. It is now believed that chromium melts at a temperature of about 1875°C. Boiling point 2199°C. The density of chromium is less than that of iron; it is equal to 7.19.
In terms of chemical properties, chromium is close to molybdenum and tungsten. Its highest oxide CrO 3 is acidic, it is chromic acid anhydride H 2 CrO 4. The mineral crocoite, with which we began our acquaintance with element No. 24, is a salt of this acid. In addition to chromic acid, dichromic acid H 2 Cr 2 O 7 is known; its salts, dichromates, are widely used in chemistry. The most common chromium oxide, Cr 2 O 3, is amphoteric. In general, under different conditions, chromium can exhibit valencies from 2 to 6. Only compounds of tri- and hexavalent chromium are widely used.
Chrome has all the properties of a metal - it conducts heat and electricity well, and has a characteristic metallic luster. The main feature of chromium is its resistance to acids and oxygen.
For those who constantly deal with chromium, another of its features has become the talk of the town: at a temperature of about 37°C, some of the physical properties of this metal change sharply and abruptly. At this temperature there is a clearly expressed maximum of internal friction and a minimum of elasticity modulus. Electrical resistance, coefficient of linear expansion, and thermoelectromotive force change almost as sharply.
Scientists cannot yet explain this anomaly.
There are four known natural isotopes of chromium. Their mass numbers are 50, 52, 53 and 54. The share of the most common isotope 52 Cr is about 84%
It would probably be unnatural if the story about the use of chromium and its compounds began not with steel, but with something else. Chromium is one of the most important alloying elements used in ferrous metallurgy. The addition of chromium to conventional steels (up to 5% Cr) improves their physical properties and makes the metal more susceptible to heat treatment. Spring, spring, tool, stamp and ball bearing steels are alloyed with chromium. In them (except for ball bearing steels) chromium is present along with manganese, molybdenum, nickel, and vanadium. And ball bearing steels contain only chromium (about 1.5%) and carbon (about 1%). The latter forms carbides of exceptional hardness with chromium: Cr 3 C. Cr 7 C 3 and Cr 23 C 6. They give ball bearing steel high wear resistance.
If the chromium content of steel is increased to 10% or more, the steel becomes more resistant to oxidation and corrosion, but this is where a factor that can be called carbon limitation comes into play. The ability of carbon to bind large amounts of chromium leads to the depletion of steel in this element. Therefore, metallurgists are faced with a dilemma: if you want to get corrosion resistance, reduce the carbon content and lose on wear resistance and hardness.
The most common grade of stainless steel contains 18% chromium and 8% nickel. The carbon content in it is very low - up to 0.1%. Stainless steels resist corrosion and oxidation well and retain strength at high temperatures. The sculptural group of V.I. was made from sheets of such steel. Mukhina “Worker and Collective Farm Woman”, which is installed in Moscow at the Northern entrance to the Exhibition of Achievements of the National Economy. Stainless steels are widely used in the chemical and petroleum industries.
High-chromium steels (containing 25...30% Cr) are particularly resistant to oxidation at high temperatures. They are used for the manufacture of parts for heating furnaces.
Now a few words about chromium-based alloys. These are alloys containing more than 50% chromium. They have very high heat resistance. However, they have a very big drawback that negates all the advantages: these alloys are very sensitive to surface defects: it is enough for a scratch or microcrack to appear, and the product will quickly collapse under load. For most alloys, such deficiencies are eliminated by thermomechanical treatment, but chromium-based alloys cannot be treated in this way. In addition, they are too brittle at room temperature, which also limits their application.
Alloys of chromium and nickel are more valuable (they often contain alloying additives and other elements). The most common alloys of this group - nichromes contain up to 20% chromium (the rest is nickel) and are used for the manufacture of heating elements. Nichromes have high electrical resistance for metals; when current is passed through, they become very hot.
The addition of molybdenum and cobalt to chromium-nickel alloys makes it possible to obtain materials with high heat resistance and the ability to withstand heavy loads at 650...900°C. For example, gas turbine blades are made from these alloys.
Cobalt-chromium alloys containing 25...30% chromium also have heat resistance. Industry also uses chromium as a material for anti-corrosion and decorative coatings.
The main chrome ore, chromite, is also used in the production of refractories. Magnesite-chromite bricks are chemically passive and heat-resistant, they can withstand repeated sudden temperature changes. Therefore, they are used in the designs of open-hearth furnace roofs. The durability of magnesite-chromite vaults is 2...3 times greater than that of dinas vaults*.
* Dinas is an acidic refractory brick containing at least 93% silica. Fire resistance of dinas is 1680...1730°C. In the 14th volume of the Bolshoi, published in 1952, Soviet Encyclopedia(2nd edition) dinas is called an indispensable material for the arches of open-hearth furnaces. This statement should be considered outdated, although dinas is still widely used as a refractory.
Chemists mainly obtain potassium and sodium bichromates K 2 Cr 2 O 7 and Na 2 Cr 2 O 7 from chromite.
Bpchromates and chrome alum KCr(SO 4); used for tanning leather. This is where the name “chrome” boots comes from. Leather. tanned with chrome compounds, has a beautiful shine, is durable and easy to use.
From lead chromate PbCrO 4. produce various dyes. A solution of sodium dichromate is used to clean and etch the surface of steel wire before galvanizing, and also to brighten brass. Chromite and other chromium compounds are widely used as colorants for ceramic glazes and glass.
Finally, chromic acid is obtained from sodium dichromate, which is used as an electrolyte in the chrome plating of metal parts.
Chromium will continue to remain important in the future as an alloying additive to steel and as a material for metal coatings; Chromium compounds used in the chemical and refractory industries will not lose their value.
The situation is much more complicated with chromium-based alloys. The great fragility and exceptional complexity of machining do not yet allow these alloys to be widely used, although in terms of heat resistance and wear resistance they can compete with any materials. In recent years, a new direction has emerged in the production of chromium-containing alloys - alloying them with nitrogen. This gas, usually harmful in metallurgy, forms strong compounds with chromium - nitrides. Nitriding of chromium steels increases their wear resistance and makes it possible to reduce the content of scarce nickel in “stainless steels”. Perhaps this method will also overcome the “unprocessability” of chromium-based alloys? Or will other, as yet unknown methods come to the rescue? One way or another, we must think that in the future these alloys will take their rightful place among the materials needed by technology.
Because chromium resists oxidation in air and acids, it is often applied to the surface of other materials to protect them from corrosion. The application method has long been known - this is electrolytic deposition. However, at first, unexpected difficulties arose during the development of the electrolytic chromium plating process.
It is known that conventional electroplating is applied using electrolytes in which the ion of the element being deposited has a positive charge. This did not work with chrome: the coatings turned out to be porous and peeled off easily.
For almost three quarters of a century, scientists worked on the problem of chrome plating and only in the 20s of our century they found that the electrolyte of a chrome bath should contain not trivalent chromium, but chromic acid, i.e. hexavalent chromium. During industrial chrome plating, salts of sulfuric and hydrofluoric acids are added to the bath; free acid radicals catalyze the process of galvanic deposition of chromium.
Scientists have not yet come to a consensus on the mechanism of deposition of hexavalent chromium on the cathode of a galvanic bath. There is an assumption that hexavalent chromium first transforms into trivalent chromium, and then is reduced to metal. However, most experts agree that chromium at the cathode is reduced immediately from the hexavalent state. Some scientists believe that atomic hydrogen is involved in this process, while others believe that hexavalent chromium simply gains six electrons.
There are two types of chrome coatings: decorative and hard. More often you have to deal with decorative ones: on the clock, door handles and other items. Here, a layer of chromium is applied to an underlayer of another metal, most often nickel or copper. The steel is protected from corrosion by this sublayer, and a thin (0.0002...0.0005 mm) layer of chrome gives the product a formal appearance.
Hard surfaces are built differently. Chromium is applied to steel in a much thicker layer (up to 0.1 mm), but without sublayers. Such coatings increase the hardness and wear resistance of steel, and also reduce the coefficient of friction.
There is another method of applying chrome coatings - diffusion. This process does not take place in galvanic baths, but in furnaces.
The steel piece is placed in chromium powder and heated in a reducing atmosphere. In 4 hours at a temperature of 1300°C, a chromium-enriched layer 0.08 mm thick is formed on the surface of the part. The hardness and corrosion resistance of this layer is much greater than the hardness of the steel in the mass of the part. But this seemingly simple method had to be improved several times. Chromium carbides formed on the surface of the steel, which prevented the diffusion of chromium into the steel. In addition, chromium powder sinteres at temperatures of about a thousand degrees. To prevent this from happening, neutral refractory powder is added to it. Attempts to replace chromium powder with a mixture of chromium oxide and coal did not produce positive results.
A more viable proposal was to use its volatile halide salts, for example CrCl 2 , as a chromium carrier. Hot gas washes the chrome-plated product, and the reaction occurs:
СrСl 2 + Fe ↔ FeСl 2 + Сr.
The use of volatile halide salts made it possible to reduce the chromium plating temperature.
Chromium chloride (or iodide) is usually obtained in the chromium plating plant itself, by passing vapors of the corresponding hydrohalic acid through powdered chromium or ferrochrome. The resulting gaseous chloride washes the chrome-plated product.
The process takes a long time – several hours. The layer applied in this way is much stronger connected to the base material than the one applied galvanically.
In any analytical laboratory there is a large bottle with a dark liquid. This is a “chromic mixture” - a mixture of a saturated solution of potassium dichromate with concentrated sulfuric acid. Why is it needed?
There is always grease on a person's fingers, which easily transfers to glass. It is these deposits that the chrome mixture is designed to wash away. It oxidizes fat and removes its remains. But this substance must be handled with care. A few drops of a chrome mixture falling on a suit can turn it into a kind of sieve: there are two substances in the mixture, and both are “robbers” - a strong acid and a strong oxidizing agent.
Even in our age of glass, aluminum, concrete and plastics, one cannot help but recognize wood as an excellent building material. Its main advantage is its ease of processing, and its main disadvantages are its fire hazard, susceptibility to destruction by fungi, bacteria, and insects. Wood can be made more resistant by impregnating it with special solutions, which necessarily include chromates and dichromates, plus zinc chloride, copper sulfate, sodium arsenate and some other substances. Impregnation greatly increases the resistance of wood to fungi, insects, and flame.
Illustrations in printed publications are made from cliches - metal plates on which this design (or rather, its mirror image) is engraved chemically or manually. Before the invention of photography, clichés were only engraved by hand; This is labor-intensive work that requires great skill.
But back in 1839, a discovery occurred that seemed to have nothing to do with printing. It was found that paper impregnated with sodium or potassium bichromate suddenly turns brown after being illuminated with bright light. Then it turned out that bichromate coatings on paper, after exposure, do not dissolve in water, but, when wetted, acquire a bluish tint. Printers took advantage of this property. The desired pattern was photographed on a plate with a colloidal coating containing dichromate. The illuminated areas did not dissolve during washing, but the unexposed areas dissolved, and a pattern remained on the plate from which it was possible to print.
Nowadays, other photosensitive materials are used in printing; the use of bichromate gels is being reduced. But we should not forget that chromium helped the “pioneers” of the photomechanical method in printing.
In 1766, professor of chemistry and head of the Chemical Laboratory of the St. Petersburg Academy of Sciences I.G. Lehman described a new mineral found in the Urals at the Berezovsky mine, which was called "Siberian red lead", PbCrO 4. The modern name is crocoite. In 1797, the French chemist L. N. Vauquelin isolated a new refractory metal from it.
The element received its name from the Greek. χρῶμα - color, paint - due to the variety of colors of its compounds.
The most common chromium mineral is chromium iron ore FeCr 2 O 4 (chromite), rich deposits of which are found in the Urals and Kazakhstan; the second most important mineral is crocoite PbCrO 4. The mass fraction of chromium in the earth's crust is 0.03%. Natural chromium consists of a mixture of five isotopes with mass numbers 50, 52, 53, 54 and 56; Other radioactive isotopes have also been artificially obtained.
The main quantities of chromium are obtained and used in the form of an alloy with iron, ferrochrome, by reducing chromite with coke: FeCr 2 O 4 + 4C = Fe + 2Cr + 4CO
Pure chromium is obtained by reducing its oxide with aluminum: Cr 2 O 3 + 2Al = 2Cr + Al 2 O 3
or electrolysis of aqueous solutions of chromium compounds.
Chrome is a grayish-white shiny metal, similar in appearance to steel, one of the hardest metals, r= 7.19 g/cm 3, Tmelt=2130K, Tboil=2945K. Chrome has all the properties characteristic of metals - it conducts heat and electricity well, and has the luster characteristic of most metals.
Chromium is stable in air due to passivation - the formation of a protective oxide film. For the same reason, it does not react with concentrated sulfur and nitric acids. At 2000°C it burns to form green chromium(III) oxide Cr 2 O 3 .
When heated, it reacts with many non-metals, often forming compounds of non-stoichiometric composition: carbides, borides, silicides, nitrides, etc.
Chromium forms numerous compounds in various oxidation states, mainly +2, +3, +6.
Oxidation state +2- basic oxide CrO (black), hydroxide Cr(OH) 2 (yellow). Chromium(II) salts (solutions blue color) are obtained by reducing chromium(III) salts with zinc in an acidic environment. Very strong reducing agents, they are slowly oxidized by water, releasing hydrogen.
Oxidation state +3- the most stable oxidation state of chromium, it corresponds to: amphoteric oxide Cr 2 O 3 and hydroxide Cr (OH) 3 (both gray-green), chromium (III) salts - gray-green or purple, chromites MCrO2, which are obtained by fusing chromium oxide with alkalis, tetra- and hexahydroxochromates(III) obtained by dissolving chromium(III) hydroxide in alkali solutions (green), numerous chromium complex compounds.
Oxidation state +6- the second characteristic oxidation state of chromium, it corresponds to the acidic chromium(VI) oxide CrO 3 (red crystals, dissolves in water, forming chromic acids), chromic H 2 CrO 4, dichromic H 2 Cr 2 O 7 and polychromic acids, the corresponding salts : yellow chromates and orange dichromates. Chromium(VI) compounds are strong oxidizing agents, especially in an acidic environment, reduced to chromium(III) compounds
In an aqueous solution, chromates turn into dichromates when the acidity of the medium changes:
2CrO 4 2- + 2H + Cr 2 O 7 2- + H 2 O, which is accompanied by a color change.
Chromium, in the form of ferrochrome, is used in the production of alloyed steels (in particular, stainless steel) and other alloys. Chromium alloys: chromium-30 and chromium-90, indispensable for the production of nozzles for powerful plasma torches and in the aerospace industry, an alloy with nickel (nichrome) - for the production of heating elements. Large amounts of chromium are used as wear-resistant and beautiful galvanic coatings(chrome plating).
Chromium is one of the biogenic elements and is constantly included in the tissues of plants and animals. In animals, chromium is involved in the metabolism of lipids, proteins (part of the enzyme trypsin), and carbohydrates. A decrease in chromium content in food and blood leads to a decrease in growth rate and an increase in cholesterol in the blood.
In its pure form, chromium is quite toxic; chromium metal dust irritates lung tissue. Chromium(III) compounds cause dermatitis. Chromium(VI) compounds lead to various human diseases, including cancer. MPC of chromium(VI) in atmospheric air 0.0015 mg/m 3
Kononova A.S., Nakov D.D., Tyumen State University, 501(2) group, 2013
Sources:
Chromium (element) // Wikipedia. URL: http://ru.wikipedia.org/wiki/Chrome (access date: 01/06/2014).
Popular library of chemical elements: Chromium. // URL:
And fats.
Scientists say cholesterol levels are affected by chromium. Element It is considered biogenic, that is, it is necessary for the body, not only the human one, but also all mammals.
With a lack of chromium, their growth slows down and cholesterol “jumps.” The norm is 6 milligrams of chromium from the total weight of a person.
Ions of the substance are found in all tissues of the body. You should get 9 micrograms per day.
You can take them from seafood, pearl barley, beets, liver and duck meat. While you are purchasing products, we will tell you about other purposes and properties of chromium.
Properties of chromium
Chrome – chemical element related to metals. The color of the substance is silver-blue.
The element has the 24th atomic number, or, as they also say, atomic number.
The number indicates the number of protons in the nucleus. As for the electrons rotating near it, they have special property- fail.
This means that one or two particles can move from one sublevel to another.
As a result, the 24th element is able to half fill the 3rd sublevel. A stable electronic configuration is obtained.
Electron failure is a rare phenomenon. Apart from chromium, the only ones that come to mind are, perhaps, , , and .
Like the 24th substance, they are chemically inactive. It is not then that the atom reaches a stable state in order to react with everyone.
Under normal conditions chromium is an element of the periodic table, which can only be “stirred up”.
The latter is the antipode of the 24th substance and is maximally active. The reaction produces fluoride chromium.
Element, properties which are discussed, does not oxidize, is not afraid of moisture and refractory materials.
The latter characteristic “delays” reactions that are possible during heating. Thus, interaction with water vapor starts only at 600 degrees Celsius.
The result is chromium oxide. The reaction with also starts, giving the nitride of the 24th element.
At 600 degrees, several compounds with and the formation of sulfide are also possible.
If the temperature is increased to 2000, the chromium will ignite upon contact with oxygen. The result of combustion will be a dark green oxide.
This precipitate easily reacts with solutions and acids. The result of the interaction is chromium chloride and sulfide. All compounds of the 24th substance are, as a rule, brightly colored.
In its pure form, basic chromium element characteristics– toxicity. Metal dust irritates lung tissue.
Dermatitis, that is, allergic diseases, may appear. Accordingly, it is better not to exceed the norm of chromium for the body.
There is also a standard for the content of element 24 in the air. There should be 0.0015 milligrams per cubic meter of atmosphere. Exceeding the standard is considered pollution.
Chromium metal has a high density - more than 7 grams per cubic centimeter. This means the substance is quite heavy.
The metal is also quite high. It depends on the electrolyte temperature and current density. Fungi and mold seem to respect this.
If you impregnate wood with a chrome composition, microorganisms will not begin to destroy it. Builders use this.
They are also happy with the fact that treated wood burns worse, because chromium is a refractory metal. We will tell you further how and where else it can be applied.
Application of chromium
Chromium is an alloying element during smelting. Remember that under normal conditions the 24th metal does not oxidize or rust?
The basis of steels is . It cannot boast of such properties. That's why chromium is added, which increases corrosion resistance.
In addition, the addition of the 24th substance reduces the critical cooling rate point.
Siliconothermic chromium is used for smelting. This is a duet of the 24th element with nickel.
The additives used are silicon, . Nickel is responsible for its ductility, and chromium is responsible for its oxidation resistance and hardness.
Combine chrome and s. The result is super-hard stellite. Additives to it are molybdenum and.
The composition is expensive, but is necessary for surfacing machine parts in order to increase their wear resistance. Stellite is also sprayed onto working machines.
As a rule, decorative corrosion-resistant coatings use chromium compounds.
The bright range of their colors comes in handy. In metal-ceramics, color is not needed, therefore, powdered chrome is used. It is added, for example, for strength to the bottom layer of crowns for.
Chromium formula– component. This is a mineral from the group, but it does not have the usual color.
Uvarovite is a stone, and it is chromium that makes it so. It's no secret that they are used.
The green variety of the stone is no exception, and is valued higher than the red one because it is rare. Also, it will boil down a little to the standard ones.
This is also a plus, because mineral inserts are more difficult to scratch. The stone is cut facetted, that is, by forming angles, which increases the play of light.
Chromium mining
It is not profitable to extract chromium from minerals. Most with the 24th element are used entirely.
In addition, the chromium content in, as a rule, is low. The substance is extracted, basically, from ores.
Associated with one of them opening chrome. He was found in Siberia. In the 18th century, crocoite was found there. This is a red lead ore.
Its base is , the second element is chrome. A German chemist named Lehmann managed to discover it.
At the time of the discovery of crocoite, he was visiting St. Petersburg, where he conducted experiments. Now, the 24th element is obtained by electrolysis of concentrated aqueous solutions of chromium oxide.
Electrolysis of sulfate is also possible. These are 2 ways to get the purest chromium. Molecule oxide or sulfate is destroyed in a crucible, where the original compounds are set on fire.
The 24th element is separated, the rest goes to slag. All that remains is to smelt the chromium in an arc. This is how the purest metal is extracted.
There are other ways to get chromium element, for example, the reduction of its oxide with silicon.
But this method gives metal with big amount impurities and, moreover, is more expensive than electrolysis.
Chrome price
In 2016, the cost of chromium is still decreasing. January started at $7,450 per ton.
By mid-summer they are asking for only 7,100 conventional units per 1,000 kilograms of metal. Data provided by Infogeo.ru.
That is, Russian prices were considered. The global cost of chromium reached almost $9,000 per ton.
The lowest summer mark differs from the Russian one by only 25 dollars upward.
If we are not considering the industrial sector, for example, metallurgy, but benefits of chromium for the body, you can study the offers of pharmacies.
So, “Picolinate” of the 24th substance costs about 200 rubles. For “Cartnitin Chrome Forte” they ask for 320 rubles. This is the price tag for a package of 30 tablets.
Turamine Chrome can also compensate for the deficiency of the 24th element. Its cost is 136 rubles.
Chromium, by the way, is part of tests for detecting drugs, in particular marijuana. One test costs 40-45 rubles.
