Chromium

Appearance / physical state / colour

Description of key information

Chromium metal is a grey, hard, solid, inert, inorganic metal.

Key value for chemical safety assessment

Physical state at 20°C and 1013 hPa:

solid

Additional information

Chromium is found in many minerals, but its only commercial source is chromite. In its pure form, chromite comprises 68 % of Cr2O3 and 32 % of FeO. Chromium is found in all phases of the environment, including air, water, soil and living organisms. Chromium is the 21st most abundant element in the Earth’s crust at an average concentration of about 125 ppm. Chromium metal is grey, hard, solid, inorganic metal. It is practically insoluble in water.

Under normal environmental conditions, the chromium on the surface of the metal and chromium-containing alloys such as stainless steel is rapidly converted to the trivalent oxide providing the corrosion resistance. It is well known in metallurgy that on contact with air or other oxygen-containing media, metallic chromium and other metals like iron and nickel rapidly oxidize resulting in a layer of oxides on the surface. This so-called passivation layer is so thin that it does not affect the natural brightness of the metal/alloy, but it very effectively separates the metal/alloy from the surrounding medium. It is continuous, non-porous and insoluble under normal conditions. If it is broken (e. g. by scratching), it is immediately self-healing under the influence of oxygen from air or water. Any effects of other elements are only to influence the effectiveness of chromium in forming or maintaining the film (e. g. nickel promotes re-passivation, especially in reducing environments; and molybdenum stabilizes the passive film in the presence of chlorides). Increasing the chromium content, from the minimum of 10.5 % necessary for ”stainless steel”, to 17 to 20 %, greatly increases the stability of the passive film.

The oxidation states of chromium range from –2 to +6. The important valences are 0, +3 and +6. Elemental chromium(0) does not occur naturally on the Earth but is produced either by metallurgical or electrolytic processes. The trivalent state is the thermodynamically most stable among chromium species. In nature, almost all the chromium exists in trivalent compounds, other forms being mainly of anthropogenic origin. The divalent state is a strong reductant and this form of chromium rapidly decomposes in air or water to form the relatively stable trivalent state. The tetravalent form is only normally encountered as the black magnetic oxide CrO2 used in magnetic recording tapes. The pentavalent state, as the ion (CrO4)3-, is encountered in compounds such as LaCrO4and Ca3(CrO4)2. It is unstable in aqueous solution, disproportionating into chromium(VI) and (III), and is a transient intermediate in the intracellular reduction of the chromate ion, (CrO4)2-. The hexavalent form is the second most stable chromium species. It is a strong oxidant, especially in acidic media. The ease of reduction of hexavalent forms to trivalent increases with increasing acidity. Chromium(VI) occurs only rarely in nature in the mineral crocoite (lead chromate), from which the element was first isolated.

Chromium metal is made mainly by thermal reduction of chromium(III) oxide using aluminium or silicon as reducing agent.