Chromium, Molybdenum, and Tungsten
Authors: LangÅrd Sverre , Lison Dominique 
Published: January 2012

The physical and chemical characteristics of chromium and some of its compounds are summarized.

The termchromiumis derived from the Greek word for color, because most chromium compounds are brightly pigmented. The element chromium was discovered in Europe in 1798 by N. L. Vauquelin, but it had already been used in swords by the Hittites about 1300 BC.Chromium occurs in nature in bound‐form chromite ore, which is the only chromium ore of any importance, and it makes up 0.1–0.3 ppm of the earth's crust. The red color of rubies and green color of emeralds, serpentine, and chrome mica are produced by chromium.

Chromium 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 employ 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 makes it difficult to cast.

The U.S. National Occupational Exposure Survey estimated that a total of about 200,000 workers, including about 30,000 women, were potentially exposed to hexavalent chromium compounds. The typical airborne concentrations in various industrial operations are given; however, the combustion of coal and oil is the largest single source of air pollution.

The use of chromium in stainless steel (SS) (+18%) is a major use of the element.

Chromium in the trivalent form is an essential trace element to humans. It is involved in the metabolism of glucose. Chromium deficiency may result in impaired glucose tolerance, peripheral neuropathy, and elevated serum insulin, cholesterol, and triglycerides, similar to those symptoms observed in diabetic patients.

Accumulated molybdenum in euxinic shales has been of great importance for understanding the stepwise oxygenation of the earth atmosphere. Mo is incorporated into metal cofactors by complex machineries, and is an essential trace element for microorganisms, plants, animals, and humans. As Mo in trace amounts are quite abundant in nature today, deficiency due to low intake from food and drinking water is hardly known in humans. Deficiency is reported after prolonged total parenteral nutrition with clinical signs characterized by tachycardia, headache, mental disturbances, and coma. XO deficiency is relatively benign, still patients with isolated deficiencies of SO or Mo cofactor exhibit mental retardation, neurological problems, and ocular lens dislocation. Symptoms of intoxication may be general, such as flulike and CNS symptoms, fever, coughing, nausea, vomiting, diarrhea, anemia, and neuropathy.

Tungsten has the highest melting point of all the metals (3400°C) and it retains its strength at high temperatures. Tungsten carbide is second to diamond in hardness. The prime uses of tungsten are in cemented carbides, mills products, and steel/alloys. Exposure to tungsten‐containing compounds may occur during production and uses of tungsten, its alloys, and compounds.

Toxicological information on tungsten and its compounds is limited in comparison to other metals. There is a need for more systematic studies of the behavior and effects of tungsten and its various compounds in different animal species.

The early determinations of toxicity of tungsten and its compounds showed a difference between soluble and insoluble forms. Soluble compounds (tungstate) were distinctly more toxic than the insoluble forms (oxide), resulting in two separate permissible limits for industrial exposure (1 and 5 mg/m3TWA, respectively). Most existing data concern the toxicity and health effects of cemented WC and its constituents, particularly in humans. The most significant exposure‐related health effects are an interstitial lung disease (hard metal disease) and an increased risk of lung cancer. Experimental studies have also reported a worrying carcinogenic activity of tungsten containing alloys.