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An Introduction to Chromium

Vauquelin, who later was the first to produce it on its own, discovered chromium in 1797. Chromium (Cr) occurs in nature only in combination. In its pure state chromium is a steel-silvery grey with bluish tinge lustre. It is an extremely hard metal that takes a high polish. The principal ore is chromite (FeO.Cr2O3), from which it is obtained by reduction with aluminium and electrolysis. Chromium metal lacks ductility and is susceptible to nitrogen embrittlement, and is not used as a structural metal. It is resistant to oxidation and does not react with nitric acid. However, it dissolves in hydrochloric acid and slowly in sulphuric acid. It is subject to intergranular corrosion at temperatures above 816°C (1500°F).

When plated on highly polished metal chromium gives a smooth surface that has no capillary attraction to water or oil and chromium-plated bearing surfaces can be run without oil.

Commercially available grades of chromium steels are:

  • Chromium metal – pure grade containing greater than 99% chromium.
  • High-carbon chromium – contains a minimum of 86% chromium, 8 to 11% carbon with a maximum of 0.5% of both iron and silicon.
  • Isochrome – 99.99% pure (made by the reduction of iodide).
  • Alphatised (chromised) steel – is steel coated with chromium by a diffusion process.
  • Securacoat GPX 9160 (of Securamax International) – is a plasma-sprayed chromium oxide coating with high resistance to oxidation, corrosion and wear. It is applied to stainless steels and titanium ball valves used in the separation of gold from sulphide ore slurries by autoclave processing in the mining industry.

Hexavalent chromium compounds are toxic.

Applications

Chromium is used in:

  • Stainless steels
  • Heat-resistant alloys
  • High-strength alloy steels
  • Electrical-resistance alloys
  • Wear-resistance
  • Corrosion resistance
  • Decorative coatings and electroplating

Chromium compounds are used for:

  • Pigments (e.g. lead chromate for yellow)
  • Chemicals
  • The refractory industry uses chromite in the form of bricks and shapes, due to its high melting point, moderate thermal expansion and high temperature stability.
  • Chrome compounds are added to glass and give it an emerald green colour.
  • Some chromium compounds can be used as catalysts.
  • Dichromate’s (e.g. K2Cr2O7) are used as an oxidising agent in quantitative analysis and in the tanning of leather.

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