Chromium

CHROMIUM-a; m.[from Greek. chrōma - color, paint]

1. Chemical element (Cr), a steel-gray hard metal (used in the manufacture of hard alloys and for coating metal products).

2. Soft thin skin tanned with the salts of this metal. Chrome boots.

3. A genus of yellow paint obtained from chromates.

◁ Chrome (see).

(lat. Chromium), a chemical element of group VI of the periodic system. Named from the Greek chrōma - color, paint (due to the bright color of the compounds). Bluish silver metal; density 7.19 g / cm 3, t pl 1890°C. It does not oxidize in air. The main minerals are chrome spinels. Chromium is an essential component of stainless, acid-resistant, heat-resistant steels and a large number of other alloys (nichrome, chrome, stellite). Used for chrome plating. Chromium compounds - oxidizing agents, inorganic pigments, tanning agents.

CHROME (Latin chromium, from Greek chromium - color, color, chromium compounds are characterized by a wide color palette), Cr (read "chromium"), a chemical element with atomic number 24, atomic mass 51.9961. It is located in group VIB in the 4th period of the Periodic Table of the Elements.
Natural chromium consists of a mixture of four stable nuclides: 50 Cr (content in the mixture 4.35%), 52 Cr (83.79%), 53 Cr (9.50%) and 54 Cr (2.36%). Configuration of the two outer electron layers 3s 2 R 6 d 5 4s 1 . The oxidation states are from 0 to +6, the most characteristic are +3 (the most stable) and +6 (valencies III and VI).
The radius of the neutral atom is 0.127 nm, the radius of the ions (coordination number 6): Cr 2+ 0.073 nm, Cr 3+ 0.0615 nm, Cr 4+ 0.055 nm, Cr 5+ 0.049 nm and Cr 6+ 0.044 nm. Sequential ionization energies 6.766, 16.49, 30.96, 49.1, 69.3 and 90.6 eV. Electron affinity 1.6 eV. Electronegativity according to Pauling (cm. PAULING Linus) 1,66.
Discovery history
In 1766, a mineral was discovered in the vicinity of Yekaterinburg, which was called "Siberian red lead", PbCrO 4 . The modern name is crocoite. In 1797 the French chemist L. N. Vauquelin (cm. VAUCLAIN Louis Nicola) isolated a new refractory metal from it (most likely, Vauquelin obtained chromium carbide).
Being in nature
The content in the earth's crust is 0.035% by weight. In sea water, the content of chromium is 2·10 -5 mg/l. Chromium is almost never found in free form. Included in more than 40 various minerals(chromite FeCr 2 O 4 , volkonskoite, uvarovite, vokelenite, etc.). Some meteorites contain chromium sulfide compounds.
Receipt
Chromite is an industrial raw material in the production of chromium and alloys based on it. The reduction smelting of chromite with coke (reducing agent), iron ore and other components produces ferrochrome with a chromium content of up to 80% (by weight).
To obtain pure chromium metal, chromite with soda and limestone is fired in furnaces:
2Cr 2 O 3 + 2Na 2 CO 3 + 3O 2 \u003d 4Na 2 CrO 4 + 4CO 2
The resulting sodium chromate Na 2 CrO 4 is leached with water, the solution is filtered, evaporated and treated with acid. In this case, Na 2 CrO 4 chromate passes into Na 2 Cr 2 O 7 dichromate:
2Na 2 CrO 4 + H 2 SO 4 = Na 2 Cr 2 O 7 + Na 2 SO 4 + H 2 O
The resulting dichromate is reduced with sulfur:
Na 2 Cr 2 O 7 + 3S = Na 2 S + Cr 2 O 3 + 2SO 2,
The resulting pure chromium (III) oxide Cr 2 O 3 is subjected to aluminothermy:
Cr 2 O 3 + 2Al \u003d Al 2 O 3 + 2Cr.
Silicon is also used
2Cr 2 O 3 + 3Si = 3SiO 2 + 4Cr
To obtain high purity chromium, technical chromium is electrochemically purified from impurities.
Physical and chemical properties
In its free form, it is a bluish-white metal with a cubic body-centered lattice, a= 0.28845 nm. At a temperature of 39°C, it changes from a paramagnetic state to an antiferromagnetic state (the Neel point). Melting point 1890°C, boiling point 2680°C. Density 7.19 kg / dm 3.
Air resistant. At 300°C, it burns to form green chromium oxide (III) Cr 2 O 3 , which has amphoteric properties. By fusing Cr 2 O 3 with alkalis, chromites are obtained:
Cr 2 O 3 + 2NaOH \u003d 2NaCrO 2 + H 2 O
Uncalcined chromium (III) oxide is easily soluble in alkaline solutions and acids:
Cr 2 O 3 + 6HCl = 2CrCl 3 + 3H 2 O
Thermal decomposition of chromium carbonyl Cr(OH) 6 produces red basic chromium(II) oxide CrO. Brown or yellow Cr(OH) 2 hydroxide with weakly basic properties precipitates when alkalis are added to solutions of chromium(II) salts.
With careful decomposition of chromium oxide (VI) CrO 3 under hydrothermal conditions, chromium dioxide (IV) CrO 2 is obtained, which is a ferromagnet and has metallic conductivity.
When concentrated sulfuric acid reacts with solutions of dichromates, red or violet-red crystals of chromium oxide (VI) CrO 3 are formed. Typically acid oxide, when interacting with water, it forms strong unstable chromic acids: chromic H 2 CrO 4 , dichromic H 2 Cr 2 O 7 and others.
known halides corresponding to varying degrees chromium oxidation. Chromium dihalides CrF 2 , CrCl 2 , CrBr 2 and CrI 2 and trihalides CrF 3 , CrCl 3 , CrBr 3 and CrI 3 have been synthesized. However, unlike similar compounds of aluminum and iron, CrCl 3 trichloride and CrBr 3 chromium tribromide are non-volatile.
Among chromium tetrahalides, CrF 4 is stable, chromium tetrachloride CrCl 4 exists only in vapor. Chromium hexafluoride CrF 6 is known.
Chromium oxyhalides CrO 2 F 2 and CrO 2 Cl 2 have been obtained and characterized.
Synthesized compounds of chromium with boron (borides Cr 2 B, CrB, Cr 3 B 4, CrB 2, CrB 4 and Cr 5 B 3), with carbon (carbides Cr 23 C 6, Cr 7 C 3 and Cr 3 C 2), with silicon (silicides Cr 3 Si, Cr 5 Si 3 and CrSi) and nitrogen (nitrides CrN and Cr 2 N).
Chromium(III) compounds are the most stable in solutions. In this oxidation state, chromium corresponds to both the cationic form and anionic forms, for example, the anion 3- existing in an alkaline medium.
When chromium(III) compounds are oxidized in an alkaline medium, chromium(VI) compounds are formed:
2Na 3 + 3H 2 O 2 \u003d 2Na 2 CrO 4 + 2NaOH + 8H 2 O
Cr (VI) corresponds to a number of acids that exist only in aqueous solutions: chromic H 2 CrO 4, dichromic H 2 Cr 2 O 7, trichromic H 3 Cr 3 O 10 and others that form salts - chromates, dichromates, trichromats, etc. .
Depending on the acidity of the medium, the anions of these acids are easily converted into each other. For example, when a yellow solution of potassium chromate is acidified with K 2 CrO 4, orange potassium dichromate K 2 Cr 2 O 7 is formed:
2K 2 CrO 4 + 2HCl \u003d K 2 Cr 2 O 7 + 2KCl + H 2 O
But if an alkali solution is added to an orange solution of K 2 Cr 2 O 7, how does the color turn yellow again, because potassium chromate K 2 CrO 4 is formed again:
K 2 Cr 2 O 7 + 2KOH \u003d 2K 2 CrO 4 + H 2 O
When a barium salt solution is added to a yellow solution containing chromate ions, a yellow precipitate of barium chromate BaCrO 4 precipitates:
Ba 2+ + CrO 4 2- = BaCrO 4
Chromium(III) compounds are strong oxidizing agents, for example:
K 2 Cr 2 O 7 + 14 HCl \u003d 2CrCl 3 + 2KCl + 3Cl 2 + 7H 2 O
Application
The use of chromium is based on its heat resistance, hardness and corrosion resistance. They are used to obtain alloys: stainless steel, nichrome, etc. A large amount of chromium is used for decorative corrosion-resistant coatings. Chromium compounds are refractory materials. Chromium oxide (III) - a pigment of green paint, is also part of abrasive materials (GOI paste). The color change during the reduction of chromium(VI) compounds is used to conduct an express analysis for the alcohol content in the exhaled air.
The Cr 3+ cation is part of the potassium chromium KCr(SO 4) 2 ·12H 2 O alum used in leather dressing.
Physiological action
Chromium is one of the biogenic elements that is constantly included in the tissues of plants and animals. In animals, chromium is involved in the metabolism of lipids, proteins (part of the trypsin enzyme), and carbohydrates. A decrease in the content of chromium in food and blood leads to a decrease in the growth rate, an increase in blood cholesterol.
Chromium metal is practically non-toxic, but 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 is 0.0015 mg/m 3 .

encyclopedic Dictionary. 2009 .

See what "chrome" is in other dictionaries:

chromium- chrome and... Russian spelling dictionary

chromium- chrome/… Morphemic spelling dictionary

- (from Greek chroma color, paint). A grayish metal mined from chromium ore. Dictionary of foreign words included in the Russian language. Chudinov A.N., 1910. CHROME grayish metal; in pure x. not used; connections with... Dictionary of foreign words of the Russian language

CHROMIUM- see CHROME (Cr). Chromium compounds are found in sewage many industrial enterprises producing chromium salts, acetylene, tannins, aniline, linoleum, paper, paints, pesticides, plastics, etc. Trivalent ones are found in water ... ... Fish Diseases: A Handbook

CHROME, ah, husband. 1. Chemical element, solid light gray shiny metal. 2. Kind of yellow paint (special). | adj. chrome, oh, oh (to 1 value) and chrome, oh, oh. Chrome steel. Chrome ore. II. CHROME, ah, husband. Soft thin leather. | adj… Dictionary Ozhegov

chromium- a, m. chrome m. Novolat. chromium lat. chroma gr. dye. 1. The chemical element is a hard silvery metal used in the manufacture of hard alloys and for coating metal products. BAS 1. The metal discovered by Vauquelin, ... ... Historical dictionary gallicisms of the Russian language

CHROMIUM- CHROME, Chromium (from Greek chroma paint), I symbol. SG, chem. element with at. weighing 52.01 (isotopes 50, 52, 53, 54); ordinal number 24, for! occupies a place in an even subgroup VI of group j of the periodic table. Compounds X. often i occur in nature ... Big Medical Encyclopedia

- (lat. Chromium) Cr, a chemical element of Group VI of the Periodic Table of Mendeleev, atomic number 24, atomic mass 51.9961. Name from Greek. chroma color, paint (due to the bright color of the Compound). Bluish silver metal; density 7.19 ... ... Big Encyclopedic Dictionary

CHROME 1, a, m. Ozhegov's Explanatory Dictionary. S.I. Ozhegov, N.Yu. Shvedova. 1949 1992 ... Explanatory dictionary of Ozhegov

CHROME 2, a, m. A grade of soft thin leather. Explanatory dictionary of Ozhegov. S.I. Ozhegov, N.Yu. Shvedova. 1949 1992 ... Explanatory dictionary of Ozhegov

Chromium

Item #24. One of the hardest metals. It has high chemical resistance. One of the most important metals used in the production of alloy steels. Most chromium compounds have a bright color, and a variety of colors. For this feature, the element was named chromium, which means “paint” in Greek.

How was it found

A mineral containing chromium was discovered near Yekaterinburg in 1766 by I.G. Lehmann and named "Siberian red lead". Now this mineral is called crocoite. Its composition is also known - РbCrО4. And at one time, "Siberian red lead" caused a lot of controversy 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 disputes) was called chromium.

Vauquelin treated crocoite with K2 CO3 potash: lead chromate was converted to potassium chromate. Then using of hydrochloric acid potassium chromate was converted to chromium oxide and water (chromic acid exists only in dilute solutions). By heating the green powder of chromium oxide in a graphite crucible with coal, Vauquelin obtained a new refractory metal.

The Paris Academy of Sciences in all its form witnessed the discovery. But, most likely, Vauquelin singled out not elemental chromium, but its carbides. This is evidenced by the needle-like 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 differ in a special color. However, friends managed to persuade the chemist, referring to the fact that from brightly colored chromium compounds one can 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; as Vauquelin found out, they were colored by impurities of chromium compounds.) And this name was established for the new element.

Incidentally, the syllable "chrome", precisely in the sense of "colored", is included in many scientific, technical and even musical terms. Widely known photographic films are "isopanchrome", "panchrome" and "orthochrome". The word "chromosome" in Greek means "the body that is colored." There is a "chromatic" scale (in music) and there is a harmonic "hromka".

Where is he located

There is quite a lot of chromium in the earth's crust - 0.02%. The main mineral from which industry obtains chromium is chromium spinel of variable composition with the general formula (Mg, Fe) O (Cr, Al, Fe) 2 O3. Chrome ore is called chromites or chromium iron ore (because it almost always contains iron). There are deposits of chromium ores in many places. Our country has huge reserves of chromites. One of the largest deposits is located in Kazakhstan, in the Aktyubinsk region; it was discovered in 1936. Significant reserves of chrome ores are also in the Urals.

Chromites are mostly used for the smelting of ferrochromium. It is one of the most important ferroalloys and absolutely essential for the mass production of alloy steels.

Ferroalloys are alloys of iron with other elements used in the main rite for alloying and deoxidizing steel. Ferrochrome contains at least 60% Cr.

Tsarist Russia almost did not produce ferroalloys. Several blast furnaces of southern plants smelted low-percentage (for alloying metal) ferrosilicon and ferromanganese. Moreover, in 1910, a tiny factory was built on the Satka River, which flows in the Southern Urals, which smelted scanty amounts of ferromanganese and ferrochromium.

The young Soviet country in the first years of development had to import ferroalloys from abroad. Such dependence on the 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 firstborn of the USSR ferroalloy industry, was put into operation. In 1933, two more plants were launched - in Zaporozhye and Zestaponi. This made it possible to stop the import of ferroalloys. In just a few years, the production of many types of special steels was organized in the Soviet Union - ball-bearing, heat-resistant, stainless, automotive, high-speed ... All these steels include chromium.

At the 17th Party Congress, Sergo Ordzhonikidze, People's Commissar for Heavy Industry, said: “... if we didn’t have high-quality steels, we wouldn’t have an autotractor industry. The cost of high-quality steels we are currently using 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 be in bondage to the capitalists ... "

The plant on the basis of the Aktobe field was built later, during the years of the Great Patriotic War. He gave the first melting of ferrochromium on January 20, 1943. The workers of the city of Aktobe took part in the construction of the plant. The building was declared popular. The ferrochrome of the new plant was used to manufacture metal for tanks and cannons, for the needs of the front.

Years have passed. Now Aktobe Ferroalloy Plant - largest enterprise, producing ferrochromium of all grades. Highly qualified national cadres of metallurgists have grown up at the plant. From year to year, the plant and chromite mines are increasing their capacity, providing our ferrous metallurgy with high-quality ferrochromium.

Our country has a unique deposit of naturally alloyed iron ore rich in chromium and nickel. It is located in the Orenburg steppes. On the basis of this deposit, the Orsk-Khalilovsky metallurgical plant was built and operates. In the blast furnaces of the plant, naturally alloyed cast iron is smelted, which has a high heat resistance. Partly it is used in the form of casting, but most of it is sent for processing into nickel steel; chromium burns out when steel is smelted from cast iron.

Cuba, Yugoslavia, many countries of Asia and Africa have large reserves of chromites.

How to get it

Chromite is used mainly in three industries: metallurgy, chemistry and refractory production, and metallurgy consumes about two thirds of all chromite.

Steel alloyed with chromium has increased strength, resistance to corrosion in aggressive and oxidizing environments.

Obtaining pure chromium is an expensive and time-consuming process. Therefore, for alloying steel, mainly ferrochromium is used, which is obtained in electric arc furnaces directly from chromite. The reducing agent is coke. The content of chromium oxide in chromite should not be lower than 48%, and the ratio of Cr:Fe should not be less than 3:1.

Ferrochrome obtained 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, ferrochromium is needed, which contains little carbon (the reasons for this are discussed below, in the chapter “Chromium in Alloys”). Therefore, a part of high-carbon ferrochrome is subjected to special treatment in order to reduce the carbon content in it to tenths and hundredths of a percent.

Elemental, metallic chromium is also obtained from chromite. The production of commercially 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 aluminum oxides, the reaction is accompanied by a significant release of heat.

But first you need to get pure chromium oxide Cr2 O3. To do this, finely ground chromite is mixed with soda and limestone or iron oxide is added to this mixture. The whole mass is fired, and sodium chromate is formed:

2Cr2 O3 + 4Na2 CO3 + 3O2 → 4Na2 CrO4 + 4CO2 .

Then sodium chromate is leached from the calcined mass with water; the lye is filtered, evaporated and treated with acid. The result is sodium bichromate Na2 Cr2 O7. By reducing it with sulfur or carbon when heated, green chromium oxide is obtained.

Chromium metal can be obtained by mixing pure chromium oxide with aluminum powder, heating this mixture in a crucible to 500...600°C and setting it on fire with barium peroxide. Aluminum takes away oxygen from chromium oxide. This reaction Cr2 O3 + 2Al → Al2 O3 + 2Cr is the basis of the industrial (aluminothermic) method for obtaining chromium, although, of course, the factory technology is much more complicated. Chromium, obtained aluminothermally, contains tenths of a percent of aluminum and iron, and hundredths of a percent of silicon, carbon and sulfur.

The silicothermic method for obtaining commercially pure chromium is also used. In this case, chromium oxide is reduced by silicon according to the reaction

2Cr2 O3 + 3Si → 3SiO2 + 4Cr.

This reaction takes place in arc furnaces. To bind silica, limestone is added to the mixture. The purity of silicothermal chromium is approximately the same as that of aluminothermic chromium, although, of course, the content of silicon in it is somewhat higher, and aluminum is somewhat lower. To obtain chromium, they tried to use other reducing agents - carbon, hydrogen, magnesium. However, these methods are not widely used.

High purity chromium (about 99.8%) is produced electrolytically.

Commercially pure and electrolytic chromium is used mainly for the production of complex chromium alloys.

Constants and properties of chromium

The atomic mass of chromium is 51.996. In the periodic table, he occupies a place in the sixth group. Its closest neighbors and analogues are molybdenum and tungsten. It is characteristic that the neighbors of chromium, as well as 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 have obtained values ​​from 1513 to 1920°C. Such a large "scatter" is primarily due to the amount and composition of impurities contained in chromium. It is now believed that chromium melts at about 1875°C. Boiling point 2199°C. The density of chromium is less than that of iron; it is equal to 7.19.

By chemical properties chromium is close to molybdenum and tungsten. Its highest oxide CrO3 is acidic, it is chromic anhydride H2 CrO4. The mineral crocoite, from which we began our acquaintance with element No. 24, is a salt of this acid. In addition to chromic acid, dichromic acid H2 Cr2 O7 is known, and its salts, bichromates, are widely used in chemistry. The most common chromium oxide Cr2 O3 is amphoterene. In general, under different conditions, chromium can exhibit valencies from 2 to 6. Only compounds of tri- and hexavalent chromium are widely used.

Chromium has all the properties of a metal - a good conductor of heat and electricity, has a characteristic metallic luster. main feature chromium - its resistance to acids and oxygen.

For those who constantly deal with chromium, another of its features has become a byword: at a temperature of about 37 ° C, some of the physical properties of this metal change abruptly, abruptly. At this temperature, there is a pronounced maximum of internal friction and a minimum of the modulus of elasticity. The electrical resistance, the coefficient of linear expansion, and the thermoelectromotive force change almost as sharply.

Scientists have yet to explain this anomaly.

Four natural isotopes of chromium are known. Their mass numbers are 50, 52, 53 and 54. The share of the most common isotope 52 Cr is about 84%

Chromium in alloys

It would probably be unnatural if the story of 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 the iron and steel industry. The addition of chromium to ordinary steels (up to 5% Cr) improves their physical properties and makes the metal more susceptible to heat treatment. Chromium is alloyed with spring, spring, tool, die and ball bearing steels. In them (except for ball-bearing steels), chromium is present together with manganese, molybdenum, nickel, vanadium. And ball bearing steels contain only chromium (about 1.5%) and carbon (about 1%). The latter forms with chromium carbides of exceptional hardness: Cr3 C. Cr7 C3 and Cr23 C6. They give ball bearing steel high wear resistance.

If the chromium content of the steel is increased to 10% or more, the steel becomes more resistant to oxidation and corrosion, but here a factor that can be called carbon limitation comes into play. The ability of carbon to bind large quantities chromium leads to depletion of steel with this element. Therefore, metallurgists face 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 their strength at high temperatures. From sheets of such steel, a sculptural group by V.I. Mukhina "Worker and Collective Farm Girl", 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 to get a scratch, a microcrack, 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.

More valuable alloys of chromium with Nickel (they are often introduced as alloying additives and other elements). The most common alloys of this group - nichrome contain up to 20% chromium (the rest is nickel) and are used for the manufacture of heating elements. Nichromes have a large electrical resistance for metals; when current is passed, they heat up very much.

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. Blades are made from these alloys, for example. gas turbines.

Heat resistance is also possessed by chromium-cobalt alloys containing 25 ... 30% chromium. The industry also uses chromium as a material for anti-corrosion and decorative coatings.

The main chromium ore, chromite, is also used in the production of refractories. Magnesite-chromite bricks are chemically passive and heat-resistant, they can withstand repeated sharp temperature changes. Therefore, they are used in the construction of the arches of open-hearth furnaces. The resistance of magnesite-chromite arches is 2...3 times greater than that of Dinas ones.

Dinas is an acid refractory brick containing at least 93% silica. Dinas fire resistance is 1680...1730°C. In the 14th volume of the Great Soviet Encyclopedia (2nd edition), published in 1952, dynas is called an indispensable material for the arches of open-hearth furnaces. This statement should be considered obsolete, although dinas is still widely used as a refractory.

Chemists obtain mainly potassium and sodium bichromates from chromite K2 Cr2 O7 and Na2 Cr2 O7.

Phromates and chrome alums KCr(SO4); used for tanning leather. Hence the name "chrome" boots. Leather. tanned with chromium compounds, it has a beautiful sheen, is durable and easy to use.

From lead chromate PbCrO4. manufacture various dyes. Clean and etch the surface with a solution of sodium bichromate steel wire before galvanizing, and also lighten brass. Chromite and other chromium compounds are widely used as dyes for ceramic glazes and glass.

Finally, chromic acid is obtained from sodium bichromate, which is used as an electrolyte in chromium plating of metal parts.

Chromium will retain its importance as an alloying addition to steel and as a material for metal coatings in the future; 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 brittleness and the exceptional complexity of machining do not yet allow these alloys to be widely used, although they can compete with any materials in terms of heat resistance and wear resistance. In recent years, a new direction in the production of chromium-containing alloys has been outlined - alloying them with nitrogen. This gas, which is usually harmful in metallurgy, forms strong compounds with chromium - nitrides. Nitriding of chromium steels increases their wear resistance and reduces the content of deficient nickel in "stainless steels". Perhaps this method will also overcome the "machinability" of chromium-based alloys? Or will other, yet unknown methods come to the rescue here? One way or another, one must think that in the future these alloys will take their rightful place among necessary equipment materials.

Three or six?

Because chromium resists air oxidation and acids well, 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 in the development of the electrolytic chromium plating process.

It is known that conventional electroplating is applied using electrolytes in which the ion of the applied element has a positive charge. With chromium, this did not work out: the coatings turned out to be porous and easily peeled off.

For almost three quarters of a century, scientists have been working on the problem of chromium plating, and only in the 20s of our century did they find that the electrolyte of a chrome bath should contain not trivalent chromium, but chromic acid, i.e. hexavalent chromium. In industrial chromium 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 passes first into trivalent, and then is reduced to metal. However, most experts agree that chromium at the cathode is restored immediately from the hexavalent state. Some scientists believe that atomic hydrogen is involved in this process, some that hexavalent chromium simply gains six electrons.

Decorative and solid

Chrome coatings are of two types: decorative and hard. More often you have to deal with decorative ones: on watches, door handles and other items. Here, a layer of chromium is deposited on top of another metal, most commonly nickel or copper. Steel is protected from corrosion by this sublayer, and a thin (0.0002 ... 0.0005 mm) layer of chromium gives the product a formal look.

Solid surfaces are constructed 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, as well as reduce the coefficient of friction.

Chrome plating without electrolyte

There is another way of applying chromium coatings - diffusion. This process takes place not in galvanic baths, but in furnaces.

The steel part is placed in chromium powder and heated in a reducing atmosphere. Within 4 hours at a temperature of 1300°C, a chromium-enriched layer 0.08 mm thick forms on the surface of the part. The hardness and corrosion resistance of this layer is much greater than the hardness of steel in the mass of the part. But this seemingly simple method had to be repeatedly improved. Chromium carbides formed on the surface of the steel, which prevented the diffusion of chromium into the steel. In addition, chromium powder sinters at a temperature of about a thousand degrees. To prevent this from happening, neutral refractory powder is mixed into it. Attempts to replace chromium powder with a mixture of chromium oxide and charcoal did not give positive results.

More vital was the proposal to use its volatile halide salts, for example, CrCl2, as a carrier of chromium. Hot gas washes the chrome-plated product, and the following reaction occurs:

CrCl2 + Fe ↔ FeCl2 + Cr.

The use of volatile halide salts made it possible to lower 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 ferrochromium. 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 more strongly bonded to the base material than the galvanically applied one.

It all started with washing dishes...

In any analytical laboratory there is a large bottle with a dark liquid. This is a "chromium mixture" - a mixture of a saturated solution of potassium bichromate with concentrated sulfuric acid. Why is she needed?

On the fingers of a person there is always fatty contamination, which easily transfers to glass. It is these deposits that the chromium mixture is designed to wash off. It oxidizes fat and removes its residues. But this substance must be handled with care. A few drops of a chromium mixture that fell 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.

Chrome and wood

Even in our age of glass, aluminium, concrete and plastics, wood cannot but be recognized as excellent. building material. Its main advantage is ease of processing, and its main disadvantages are 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 bichromates plus zinc chloride, copper sulfate, sodium arsenate and some other substances. Impregnation greatly increases the resistance of wood to the action of fungi, insects, flames.

Looking at a drawing

Illustrations in printed publications are made with cliches - metal plates on which this pattern (or rather, its mirror image) is engraved chemically or manually. Before the invention of photography, clichés were only engraved by hand; it is laborious work that requires great skill.

But back in 1839 there was a discovery that seemed to have nothing to do with printing. It has been found that paper impregnated with sodium or potassium dichromate, after being illuminated with a bright light, suddenly turns brown. Then it turned out that bichromate coatings on paper, after exposure, do not dissolve in water, but, when wetted, acquire a bluish tint. This property was used by printers. The desired pattern was photographed on a plate with a colloidal coating containing bichromate. The illuminated areas did not dissolve during washing, but the non-exposed ones dissolved, and a pattern remained on the plate from which it was possible to print.

Now other photosensitive materials are used in printing, the use of bichromate gels is declining. But do not forget that chromium helped the "pioneers" of the photomechanical method in printing.

Chromium(lat. Cromium), Cr, a chemical element of Group VI of the Mendeleev periodic system, atomic number 24, atomic mass 51.996; steel-blue metal.

Natural stable isotopes: 50 Cr (4.31%), 52 Cr (87.76%), 53 Cr (9.55%) and 54 Cr (2.38%). Of the artificial radioactive isotopes, the most important is 51 Cr (half-life T ½ = 27.8 days), which is used as an isotope tracer.

History reference. Chromium was discovered in 1797 by LN Vauquelin in the mineral crocoite - natural lead chromate РbCrО 4 . Chrome got its name from the Greek word chroma - color, paint (because of the variety of colors of its compounds). Independently of Vauquelin, chromium was discovered in crocoite in 1798 by the German scientist M. G. Klaproth.

Distribution of Chromium in nature. The average content of Chromium in the earth's crust (clarke) is 8.3·10 -3%. This element is probably more characteristic of the Earth's mantle, since the ultramafic rocks, which are believed to be closest in composition to the Earth's mantle, are enriched in Chromium (2·10 -4%). Chromium forms massive and disseminated ores in ultramafic rocks; the formation of the largest deposits of Chromium is associated with them. In basic rocks, the content of Chromium reaches only 2 10 -2%, in acidic rocks - 2.5 10 -3%, in sedimentary rocks (sandstones) - 3.5 10 -3%, shale - 9 10 -3 %. Chromium is a comparatively weak water migrant; Chromium content in sea water is 0.00005 mg/l.

In general, Chromium is a metal of the deep zones of the Earth; stony meteorites (analogues of the mantle) are also enriched in Chromium (2.7·10 -1%). Over 20 chromium minerals are known. Only chrome spinels (up to 54% Cr) are of industrial importance; in addition, chromium is contained in a number of other minerals that often accompany chromium ores, but are of no practical value in themselves (uvarovite, volkonskoite, kemerite, fuchsite).

Physical properties of Chromium. Chromium is a hard, heavy, refractory metal. Pure Chrome is plastic. Crystallizes in a body-centered lattice, a = 2.885Å (20 °C); at 1830°C, transformation into a modification with a face-centered lattice is possible, a = 3.69Å.

Atomic radius 1.27 Å; ionic radii Cr 2+ 0.83Å, Cr 3+ 0.64Å, Cr 6+ 0.52 Å. Density 7.19 g/cm 3 ; t pl 1890 °C; t kip 2480 °C. Specific heat capacity 0.461 kJ/(kg K) (25°C); thermal coefficient of linear expansion 8.24 10 -6 (at 20 °C); thermal conductivity coefficient 67 W/(m K) (20 °С); electrical resistivity 0.414 μm m (20 °C); the thermal coefficient of electrical resistance in the range of 20-600 °C is 3.01·10 -3 . Chromium is antiferromagnetic, specific magnetic susceptibility is 3.6·10 -6 . The hardness of high-purity Chromium according to Brinell is 7-9 MN / m 2 (70-90 kgf / cm 2).

Chemical properties of Chromium. External electronic configuration Chromium atom 3d 5 4s 1 . In compounds, it usually exhibits oxidation states +2, +3, +6, among which Cr 3+ is the most stable; individual compounds are known in which Chromium has oxidation states +1, +4, +5. Chromium is chemically inactive. At normal conditions resistant to oxygen and moisture, but combines with fluorine to form CrF 3 . Above 600 °C, it interacts with water vapor, giving Cr 2 O 3; nitrogen - Cr 2 N, CrN; carbon - Cr 23 C 6, Cr 7 C 3, Cr 3 C 2; gray - Cr 2 S 3. When fused with boron, it forms CrB boride; with silicon, it forms silicides Cr 3 Si, Cr 2 Si 3, CrSi 2. Chromium forms alloys with many metals. The interaction with oxygen proceeds at first quite actively, then it slows down sharply due to the formation of an oxide film on the metal surface. At 1200°C, the film breaks down and oxidation proceeds rapidly again. Chromium ignites in oxygen at 2000°C to form dark green chromium (III) oxide Cr 2 O 3 . In addition to the oxide (III), there are other compounds with oxygen, such as CrO, CrO 3 obtained indirectly. Chromium easily reacts with dilute solutions of hydrochloric and sulfuric acids to form chloride and chromium sulfate and release hydrogen; aqua regia and nitric acid passivate Chromium.

With an increase in the degree of oxidation, the acidic and oxidizing properties of Chromium increase. Cr 2+ derivatives are very strong reducing agents. The Cr 2+ ion is formed at the first stage of dissolution of Chromium in acids or during the reduction of Cr 3+ in an acidic solution with zinc. Nitrous hydrate Cr(OH) 2 during dehydration passes into Cr 2 O 3 . Cr 3+ compounds are stable in air. They can be both reducing and oxidizing agents. Cr 3+ can be reduced in an acidic solution with zinc to Cr 2+ or oxidized in an alkaline solution to CrO 4 2- with bromine and other oxidizing agents. Hydroxide Cr (OH) 3 (more precisely, Cr 2 O 3 nH 2 O) is an amphoteric compound that forms salts with the Cr 3+ cation or salts of chromic acid HCrO 2 - chromites (for example, KC-O 2, NaCrO 2). Cr 6+ compounds: CrO 3 chromic anhydride, chromic acids and their salts, among which the most important are chromates and dichromates - strong oxidizing agents. Chromium forms a large number of salts with oxygen-containing acids. Chromium complex compounds are known; complex compounds of Cr 3+ are especially numerous, in which Chromium has a coordination number of 6. There is a significant number of Chromium peroxide compounds

Get Chrome. Depending on the purpose of use, chromium is obtained in various degrees of purity. The raw material is usually chrome spinels, which are enriched and then fused with potash (or soda) in the presence of atmospheric oxygen. With regard to the main component of ores containing Cr 3 +, the reaction is as follows:

2FeCr 2 O 4 + 4K 2 CO 3 + 3.5O 2 \u003d 4K 2 CrO 4 + Fe 2 O 3 + 4CO 2.

The resulting potassium chromate K 2 CrO 4 is leached with hot water and the action of H 2 SO 4 converts it into dichromate K 2 Cr 2 O 7 . Further, by the action of a concentrated solution of H 2 SO 4 on K 2 Cr 2 O 7, chromic anhydride C 2 O 3 is obtained or by heating K 2 Cr 2 O 7 with sulfur - Chromium oxide (III) C 2 O 3.

The purest Chromium is obtained under industrial conditions either by electrolysis of concentrated aqueous solutions of CrO 3 or Cr 2 O 3 containing H 2 SO 4 , or by electrolysis of Chromium sulfate Cr 2 (SO 4) 3 . In this case, chromium is precipitated on an aluminum or stainless steel cathode. Complete purification from impurities is achieved by treating Chromium with highly pure hydrogen at high temperature (1500-1700 °C).

It is also possible to obtain pure Chromium by electrolysis of CrF 3 or CrCl 3 melts mixed with sodium, potassium, calcium fluorides at a temperature of about 900 °C in an argon atmosphere.

Chromium is obtained in small quantities by reduction of Cr 2 O 3 with aluminum or silicon. In the aluminothermic method, a preheated mixture of Cr 2 O 3 and Al powder or shavings with the addition of an oxidizing agent is loaded into a crucible, where the reaction is initiated by igniting a mixture of Na 2 O 2 and Al until the crucible is filled with Chromium and slag. Chromium is smelted silicothermally in arc furnaces. The purity of the resulting Chromium is determined by the content of impurities in Cr 2 O 3 and in Al or Si used for recovery.

In industry, chromium alloys are produced on a large scale - ferrochrome and silicochrome.

Chromium application. The use of Chromium is based on its heat resistance, hardness and corrosion resistance. Most of all Chromium is used for smelting chromium steels. Alumino- and silicothermic chromium is used for smelting nichrome, nimonic, other nickel alloys, and stellite.

A significant amount of Chromium is used for decorative corrosion-resistant coatings. Chromium powder has been widely used in the production of metal-ceramic products and materials for welding electrodes. Chromium in the form of the Cr 3+ ion is an impurity in ruby, which is used as a gemstone and laser material. Chromium compounds are used to etch fabrics during dyeing. Some Chromium salts are used as component tanning solutions in the leather industry; PbCrO 4 , ZnCrO 4 , SrCrO 4 - as art paints. Chromite-magnesite refractory products are made from a mixture of chromite and magnesite.

Chromium compounds (especially Cr 6 + derivatives) are toxic.

Chromium in the body. Chromium is one of the biogenic elements that is constantly included in the tissues of plants and animals. The average content of Chromium in plants is 0.0005% (92-95% of Chromium accumulates in the roots), in animals - from ten thousandths to ten millionths of a percent. In planktonic organisms, the accumulation coefficient of Chromium is enormous - 10,000-26,000. Higher plants do not tolerate Chromium concentrations above 3-10 -4 mol/l. In leaves, it is present as a low molecular weight complex not associated with subcellular structures. In animals, chromium is involved in the metabolism of lipids, proteins (part of the trypsin enzyme), carbohydrates (a structural component of the glucose-resistant factor). The main source of Chromium in the body of animals and humans is food. A decrease in the content of Chromium in food and blood leads to a decrease in growth rate, an increase in blood cholesterol and a decrease in the sensitivity of peripheral tissues to insulin.

Chromium poisoning and its compounds occur during their production; in mechanical engineering (electroplated coatings); metallurgy (alloying additives, alloys, refractories); in the manufacture of leather, paints, etc. The toxicity of chromium compounds depends on their chemical structure: dichromates are more toxic than chromates, Cr (VI) compounds are more toxic than Cr (II), Cr (III) compounds. The initial forms of the disease are manifested by a feeling of dryness and pain in the nose, sore throat, difficulty breathing, coughing, etc.; they may disappear when contact with Chrome is discontinued. With prolonged contact with Chromium compounds, signs of chronic poisoning develop: headache, weakness, dyspepsia, weight loss, and others. Functions of a stomach, a liver and a pancreas are broken. Bronchitis, bronchial asthma, diffuse pneumosclerosis are possible. When exposed to Chromium, dermatitis and eczema may develop on the skin. According to some reports, Chromium compounds, mainly Cr(III), have a carcinogenic effect.