Chromium (Cr) is a brilliant, hard, refractory metal that melts at 1,857 °C (3,375 °F) and boils at 2,672 °C (4,842 °F). In the pure state it is resistant to ordinary corrosion, resulting in its application as an electroplated protective coating for other metals. It dissolves in nonoxidizing mineral acids but not in aqua regia or nitric acid, which passivate the metal.
||Body-centered cubic (BCC)
||a = 2.91 Å
|Coefficient of thermal expansion
||4.9 x 10-6 / K at 25°C
||94 W/m K at 20°C
Because chromium and chromium-rich alloys are brittle at room temperature, they have limited application. By far the largest consumption is as an alloying addition to iron. In amounts varying from 10 to 26 percent, chromium imparts corrosion resistance to steel; it is also used to improve hardenability, wear-resistance, and high-temperature strength.
As the mineral chromite, chromium is employed extensively as a refractory material. Other chromium chemicals are used as pigments and tanning agents.
Chromium is a relatively abundant element in Earth’s crust; the free metal is never found in nature. Although chromium occurs in many minerals, the only ore exploited commercially is chromite. This spinel mineral is ideally composed of ferrous oxide and chromic oxide with the chemical composition FeO · Cr2O3, but it is often found in nature with magnesia (MgO) substituting for FeO and alumina (A12O3) or ferric oxide (Fe2O3) substituting for Cr2O3. Other minerals such as silica (SiO2) are also present.
By the early 21st century, South Africa, India, Kazakhstan, and Turkey had become the world’s leading producers of chromite. The bulk of chromite reserves are found in stratiform deposits (thin, even layers covering a broad area), but podiform deposits (scattered pod-shaped formations of varying size) are also important.
Over 80% of the world's ferrochrome is utilised in the production of stainless steel. In 2006 28 Mt of stainless steel were produced.  Stainless steel depends on chromium for its appearance and its resistance to corrosion. The average chrome content in stainless steel is approximately 18%. It is also used when it is desired to add chromium to carbon steel. FeCr from Southern Africa, known as "charge chrome" and produced from a Cr containing ore with a low carbon content, is most commonly used in stainless steel production. Alternatively, high carbon FeCr produced from high grade ore found in Kazakhstan (among other places) is more commonly used in specialist applications such as engineering steels where a high Cr to Fe ratio and minimum levels of other elements such as sulfur, phosphorus and titanium are important and production of finished metals takes place in small electric arc furnaces compared to large scale blast furnaces.
Chromium surfaces are produced on other metals by electroplating and chromizing. There are two types of electroplating: decorative and “hard.” Decorative plate varies between 0.000254 and 0.000508 millimetre (0.00001 and 0.00002 inch) in thickness and is usually deposited over nickel. “Hard” plating is used because of its wear resistance and low coefficient of friction. For these types of plating, solutions of chromic acid (CrO3) are used.
In one method of chromizing, chromium is condensed on the surface from the vapour and is diffused into the metal by heating. In another method a chromium layer is fused on the surface and diffused in. Electron-beam deposition of chromium onto the surface followed by diffusion into the metal has also been used. Salt-bath chromizing using chromium chloride (CrCl2) has been tried.
The greatest consumption of ferrochromium is in the manufacture of stainless steel. Formerly, much of the alloy had to be of the low-carbon type, but since the advent of the argon-oxygen decarburization process, which allows steelmakers to burn off impurities such as silicon and carbon without also removing too much chromium, the demand has shifted to charge ferrochromium. Refined ferrochromium is now used principally as a trimming agent.
Stainless steels have a high resistance to oxidation and atmospheric corrosion, mainly because of the presence of chromium, which, at levels varying between 10 and 26 percent, forms a protective oxide film on the surface of the steel. The low-carbon ferritic stainless steels cannot be hardened by heat treatment; ferritic varieties containing 17 to 18 percent chromium are used in automobile trim and in equipment for handling nitric acid. High-carbon martensitic stainless steels are used when hardness and abrasion resistance are desired; steels of this type with 13 percent chromium are made into cutlery. Nickel and manganese can be added to high-chromium (16 to 26 percent) stainless steels to form the austenitic types, of which the 18-percent-chromium–8-percent-nickel variety is probably the best known. In addition to their resistance to oxidation and corrosion, austenitic steels maintain their strength at high temperatures better than do the plain chromium steels. Sometimes molybdenum, tungsten, niobium, or titanium are added to improve strength and corrosion resistance or to stabilize the carbides present. Steels of this type containing up to 26 percent chromium have excellent oxidation resistance at high temperatures; they are used in furnace parts, burner nozzles, and kiln linings.
Up to 2 percent chromium is added to low-alloy steel to improve hardenability, wear resistance, and high-temperature strength. Such steels, containing chromium in combination with other elements, such as molybdenum, nickel, manganese, and vanadium, are used for springs, roller and ball bearings, dies, rails, and high-strength structures. Steels containing 6–10 percent chromium have increased corrosion and oxidation resistance and are used in the form of tubes in the oil industry.
Chromium is added to cobalt alloys in amounts up to 25 percent to obtain corrosion resistance and hardness. Cobalt-chromium-tungsten alloys are used for cutting tools and hard facings.
Nickel-chromium superalloys with up to 60 percent chromium and sometimes a little iron are used for high-temperature applications. Chromium is also added to aluminum alloys in quantities up to 0.5 percent to improve their strength and corrosion resistance.
The use of chromite as a refractory is next in importance to the metallurgical applications of chromium. A typical analysis of a chromite suitable for refractory purposes is 38 to 48 percent Cr2O3, 12 to 24 percent Al2O3, 14 to 24 percent Fe2O3, 14 to 18 percent MgO, and less than 10 percent SiO2. The usefulness of chromite as a refractory is based on its high melting point of 2,180 °C (3,960 °F), moderate thermal expansion, the stability of its crystalline form at elevated temperatures, and its neutral chemical behaviour.
Bricks of 100 percent chromium ore have been largely replaced by bricks composed of mixtures of chromite and added magnesia for greater refractoriness, volume stability, and resistance to spalling. One of the refractories used in the fused-cast condition is composed of 80 percent alumina and 20 percent chromite. This product is highly resistant to the corrosive action of a variety of fluxes, slags, and glasses.
Pigments account for about one-third of the primary production of chromium chemicals. Chrome oxide green, which is nearly pure Cr2O3, is the most stable green pigment known. It is used for colouring roofing granules, cements, and plasters. It is also employed as a fine powder for polishing. Chromium yellow varies greatly in the shades available and is essentially lead chromate, or crocoite. This pigment makes an excellent paint for both wood and metal. Zinc yellow, a basic zinc chromate, is used as a corrosion-inhibiting primer on aircraft parts fabricated from aluminum or magnesium. Molybdate orange is a combination of lead chromate with molybdenum salts. Chrome green is a mixture of lead chromate with iron blue. This pigment has excellent covering and hiding power and is widely used in paints.
About 25 percent of the chromium chemicals produced go into chrome tanning of leather. This process uses chrome reagents in the form of basic chromic sulfates that, in turn, are produced from sodium dichromate. This reagent is produced by heating the ore with soda ash and then leaching out soluble chromate, which is then converted to the dichromate by treatment with sulfuric acid.
More than one-fourth of the production of primary chromium chemicals is employed in metal-surface treatments and corrosion control. Such applications include chromium plating, chromizing, anodizing of aluminum, and treatment of zinc and magnesium. Chromium chemicals are used in dips for iron, steel, brass, and tin and also as inhibitors for brines and for recirculating water systems.
Ferrochrome production is essentially a carbothermic reduction operation taking place at high temperatures. Cr Ore (an oxide of chromium and iron) is reduced by coal and coke to form the iron-chromium alloy. The heat for this reaction can come from several forms, but typically from the electric arc formed between the tips of the electrodes in the bottom of the furnace and the furnace hearth. This arc creates temperatures of about 2,800 °C (5,070 °F). In the process of smelting, huge amounts of electricity are consumed, making production in countries with high power cost very expensive.
Tapping of the material from the furnace takes place intermittently. When enough smelted ferrochrome has accumulated in the hearth of the furnace, the tap hole is drilled open and a stream of molten metal and slag rushes down a trough into a chill or ladle. The ferrochrome solidifies in large castings, which are crushed for sale or further processed.
Ferrochrome is often classified by the amount of carbon and chrome it contains. The vast majority of FeCr produced is charge chrome from Southern Africa. With high carbon being the second largest segment followed by the smaller sectors of low carbon and intermediate carbon material.
Most ores smelted with coke in an electric furnace produce metals that are saturated with carbon. For ferrochromium, the saturation point is approximately 9 percent, but actual carbon content varies with the condition of the ore and the composition of the slag. For example, with a lumpy, refractory ore and a slag containing approximately equal amounts of magnesia, alumina, and silica, a ferrochromium is produced that contains 4 to 6 percent carbon and less than 1.5 percent silicon. This is a result of high temperatures generated by a viscous slag, of a slowly reacting bulky ore, and, possibly, of refining of the molten metal by undissolved ore in the slag. When the rate of the reducing reaction is increased by using fine ore, or when the slag is made less viscous by adding lime to the melt, the carbon level of the ferrochromium approaches saturation.
Adding more silica to the charge produces what is called charge ferrochromium, a carbon-saturated alloy with an increased silicon content. Some South African ores produce charge ferrochromium containing 52–54 percent chromium, 6–7 percent carbon, and 2–4 percent silicon; ores from Zimbabwe with a higher chromium-iron ratio yield a product that is 63–67 percent chromium, 5–7 percent carbon, and 3–6 percent silicon.
During the smelting of high-carbon or charge ferrochromium, slag and metal are tapped from the furnace into a ladle and separated by decanting or skimming. The recovery of chromium from the ore varies: in a good operation 90 percent is recovered in the molten metal; of the 10 percent remaining in the slag, some is in metallic form and can be recovered by mineral processing techniques. The smelting of charge ferrochromium consumes 4,000 kilowatt-hours of electric power per ton of alloy produced, compared with 4,600 kilowatt-hours per ton for high-carbon ferrochromium.
If silica is added to the charge until its weight equals that of the ore, the smelting processes will yield what is known as ferrochrome silicon. Containing 38–42 percent silicon and less than 0.1 percent carbon, this alloy is used as a deoxidizer in the production of stainless steel and as an intermediate material in the production of low-carbon ferrochromium. Because of the greater energy required to reduce silica to silicon, the smelting of ferrochrome silicon consumes 7,600 kilowatt-hours per ton of product.
When chromite and lime are melted in an open electric-arc furnace and then contacted with ferrochrome silicon, a low-carbon (0.05 percent) ferrochromium can be obtained. In an alternate process, high-carbon ferrochromium is oxidized and then blended with additional high-carbon ferrochromium. The briquetted mixture is placed in a large vacuum furnace, which is heated by graphite resistors to 1,400 °C (2,550 °F) at a reduced pressure of 30 pascals. The carbon is removed from the alloy (going off as carbon monoxide) to a level as low as 0.01 percent.
Pure chromium is produced either by the thermal reduction of Cr2O3 with aluminum or by the electrolysis of trivalent chromium solutions.
The aluminothermic process begins with the roasting of fine ore, soda, and lime in air at 1,100 °C (2,000 °F). This creates a calcine containing sodium chromate, which is leached from the insoluble gangue and then reduced and precipitated as Cr2O3. The Cr2O3 is blended with finely divided aluminum powder, charged to a refractory-lined container, and ignited. The combustion quickly generates temperatures in excess of 2,000 °C (3,600 °F), giving a clean separation of chromium from the slag. The purity of the metal, varying from 97 to 99 percent chromium, depends on the starting materials.
In the electrolytic process, milled high-carbon ferrochromium is leached by a mixture of reduced anolyte (electrolytic solution recycled from the anode side of the smelting cell), a chrome alum mother liquor based on a solution of ammonium sulfate recycled from a later stage in the process, and sulfuric acid. The resultant slurry is cooled, and silica and other undissolved solids are filtered from the solution. The iron forms crude ferrous ammonium sulfate crystals, which also are filtered out. The mother liquor is then clarified in a filter press, and about 80 percent of the chromium is stripped by precipitation as ammonium chrome alum. The mother liquor is sent back to the leach circuit while the chrome alum crystals are dissolved in hot water and fed into the cathode portion of an electrolytic cell. The cell is divided by a diaphragm in order to prevent the chromic and sulfuric acids formed at the anode from mixing with the catholyte (cathode electrolyte). With the passage of electric current from a lead anode to a thin stainless-steel cathode sheet, chromium is plated onto the cathode and, every 72 hours, is stripped from the sheet, sealed in stainless steel cans, and heated to 420 °C (790 °F) to remove water and hydrogen. This electrolytic chromium contains 0.5 percent oxygen, which is too high for some applications; combining it with carbon and heating the briquettes to 1,400 °C (2,550 °F) at 13 pascals lowers the oxygen content to 0.02 percent, resulting in a metal more than 99.9 percent pure.
Future Chromium Supplies
World chromium reserves, mining capacity, and ferrochromium production capacity are largely concentrated in the Eastern Hemisphere. Because there is no viable substitute for chromiumin the production of stainless steel and because the United States has small chromium resources, there has been concern about domestic supply during every national military emergency since World War I. In recognition of the vulnerability of lengthy supply routes during military emergencies, chromium (in various forms, including chromite ore, chromiumferro alloys, and chromium metal) has been held in the National Defense Stockpile since before World War II. Since 1991, however, changes in national security considerations have resulted in reduced stockpile goals, and inventories are being sold. At the current rate, it is estimated that these stockpiles will be exhausted by 2015. In 2009, recycled chromium from stainless steel scrap accounted for 61 percent of U.S. chromium apparent consumption, making recycled material the only domestic commercial chromium supply source.
To help predict where future chromium supplies might be located, USGS scientists study how and where identified chromium resources are concentrated in the Earth's crust and use that knowledge to assess the likelihood that undiscovered chromium resources exist. Studies of the distribution of podiform chromite depositsin ultramafic rocks in California and Oregon have helped to refine techniques used to estimate undiscovered chromium resources. These kinds of USGS studies provide unbiased scientific information to decision makers responsible for the stewardship of Federal lands, as well as data required to better evaluate mineral resource availability in a global context.
Mineral resource assessments are dynamic. Because they provide a snapshot that reflects our best understanding of how and where resources are located, the assessments must be updated periodically as better data and concepts are developed. Current research by the USGS involves updating mineral deposit models and mineral environmental models for chromium and other important nonfuel commodities and improving the techniques used to assess for concealed mineral resource potential. The results of this research will provide new information to decrease uncertainty in future mineral resource assessments.
1- Wikipedia, https://en.wikipedia.org/wiki/Ferrochrome
2- Encyclopædia Britannica, https://www.britannica.com/
4- Minerals Yearbook, Ferro Alloys, https://minerals.usgs.gov/