Chromium
From Encyclopædia
The chemical element chromium is a lustrous
metal of the transition series (see
TRANSITION ELEMENTS). Its chemical symbol is Cr,
atomic number 24, and
atomic weight 51.996. Chromium was discovered in 1798 by N. L. Vauquelin. Its name is derived from the Greek word for
color, since most chromium compounds are brightly colored. Chromium does not occur free in nature; in bound form it makes up 0.1-0.3 parts per million of the Earth's crust. The only important chromium ore is CHROMITE. The red
color of rubies and the green
color of emeralds, serpentine, and chrome
mica are caused by chromium. Chromium is an essential trace element in human nutrition. It is involved in the
metabolism of the
sugar glucose.PreparationChromium
metal is prepared by reducing the ore in a blast furnace with
carbon (coke) or
silicon to form an alloy of chromium and
iron called ferrochrome, which is used as the starting material for the many
iron-containing alloys that use chromium. Chromium to be used in
iron-free alloys is obtained by reduction or electrolysis of chromium compounds. Chromium is difficult to work in the pure
metal form. It is brittle at low temperatures, and its high melting point (1,900 deg C/3,452 deg F) makes it difficult to cast.UsesThe most important use of chromium is in chrome plating, which creates a hard, wear-resistant, attractive surface. Chrome plating can be performed by immersion or by electrolysis. The latter method allows very thin layers to be deposited but uses a good deal of current; the cathode current efficiency is only 10-15 percent.Chromium is alloyed with
iron to improve its resistance to corrosion, its hardness, and its workability. Other
metals, such as vanadium,
manganese,
tungsten, and molybdenum, are added to these alloys in order to obtain special properties. Genuine stainless steel always contains
nickel and chromium. Super corrosion-resistant types of steel, such as those used for furnaces, heat exchangers, and burner heads, contain about 30 percent chromium.Important nonferrous (
iron-free) chromium alloys include stellite, which contains cobalt and
tungsten and is used in cutting, lathing, and milling tools; and
nickel-chromium (nichrome), which is used in resistance
wire in electrical heaters,
irons, and toasters.CompoundsThe most important valences of chromium are 3 and 6, although chromium with valences of 1, 2, 4, and 5 has also been shown to exist in a
number of compounds. Chromium compounds often have a green
color, but yellow, blue, red, and
violet compounds are also known. The most important one is chromic oxide, which is used as a pigment (chromic oxide green). Chrome alum forms beautiful
violet crystals and is used in the tanning of leather and in textile dyeing. A
number of other chromium salts are also used in the textile industry as mordants. Chromium compounds with a valence of 6 are called chromates; most have a yellow
color and all are toxic. When absorbed into the body they severely irritate the gastrointestinal tract, leading to circulatory shock and renal damage. Chromate yellow, one of the most important yellow pigments, is highly toxic because it contains both chromium and
lead. Chromates are used as anticorrodents in water-cooling systems. Unfortunately, because they are toxic, their runoff has severe effects on river flora. It is economically and ecologically wise to remove chromates before waste water is released.Stephen FleishmanBibliography: Burrows, Desmond, ed., Chromium:
metabolism and Toxicity (1983); Fairhurst, S., and Minty, C., Toxicity of Chromium and Inorganic Chromium Compounds (1989); Langard, S., Biological and Environmental Aspects of Chromium (1983); Rollinson, C. L., The Chemistry of Chromium, Molybdenum and
tungsten (1975).
