Chromium (Chemical symbol: Cr)
Chromium was discovered in 1761 by Johann Gottlob Lehmann, in the form of the red crystalline mineral crocoite or lead II chromate (Fig. 1). Lehmann found the orange-red mineral, which he named Siberian red lead, in the Ural Mountains. It was initially used as a pigment due to its bright color, and its use as a pigment became widespread. Nearly all chromium is commercially extracted from chromite ore, which is iron-chromium oxide (FeCr2O4). At first, crocoite from Russia was the main source, but a large chromite deposit was discovered in 1827 near Baltimore, Md. This made the U.S. the largest producer of chromium products until 1848, when large deposits of chromite were found near Bursa, Turkey. Chromium is the 22nd most abundant element in Earth's crust.
Chromium's use as a pigment is widespread. It includes chrome yellow, which was one of the most used yellow pigments in the mid-20th century. It was used on school buses in the U.S. and by Deutsche Post postal service in Europe, among others. The use of chrome yellow has declined due to environmental and toxicity concerns and was replaced by organic pigments or other alternatives. Chromium oxides are also used to provide a green color in glassmaking and as a glaze in ceramics. Green chromium oxide is also the principle ingredient in infrared reflecting paints and used by the armed forces for infrared camouflage, which provides vehicles a similar infrared profile to green leaves. It is resistant to ultraviolet attack from sunlight.
Perhaps the most important use of chromium is as an alloying element for steel – stainless steel and other heat-resistant alloys. Stainless steel, the main corrosion-resistant metal alloy, is formed when chromium is added to iron in concentrations above 11%. In this process, ferrochromium is added to molten iron. For example, Inconel 718 contains 18.6% chromium, and high-speed tool steel contains up to 5% chromium. The 300-series stainless steels are the most commonly used due to workability (Fig. 2) and low cost. Chrome is combined with nickel to form superalloys used in jet engines and gas turbines. Chromium-molybdenum alloy steel or chrome-moly (CrMo) is another chrome alloy used for high-temperature and high-pressure services. Chrome-moly is used in the power-generation industry and the petrochemical industry because of its heat resistance, tensile strength and corrosion resistance at high temperature.
Chromium is also vital to the plating industry, important for its luster and wear-resistance. It is valued as a protective and decorative coating on car parts, plumbing fixtures, furniture parts and many other items. It is usually applied by electroplating (Fig. 3). Chromium was used for electroplating starting in 1848 and became widespread with the development of an improved process in 1924. Chromium has also been used as a wood preservative, in the leather tanning process and as a catalyst in the manufacture of polyethylene. The toxic effects of chromium to humans have more recently led to its substitution with environmentally friendly alternatives whenever possible.
Here are a few important facts about chromium.
- Melting point: 2180K (1907°C, 3465°F)
- Boiling point: 2944K (2671°C, 4840°F)
- Density at 20°C: 7.19 g/cm3
- Heat of fusion: 21.0 kJ/mole
- Heat of vaporization: 347 kJ/mole
- Molar heat capacity: 23.35 J/(mole·K)