Chromium (Cr) is a lustrous, silver transition metal. After boron and carbon (diamond modification) it is the third hardest element (Mohs hardness 8.5). It is known for its high resistance to corrosion and tarnishing (passivation through the formation of a chromium oxide layer). Its mass fraction in the earth’s crust is 0.019%.

The name chromium is derived from the Greek word “chroma” (colour), as an expression of the colourful compounds chromium forms. A clear illustration of the colour inducing behaviour of chromium is apparent in ruby, where incorporations of trace amounts of chromium are responsible for its vibrant red colour.

In stainless steel, chromium forms a passive layer that protects the underlying material from corrosion. In superalloys, chromium provides both strength and corrosion resistance.

Chromium sputtering targets are used for glass coating, decorative coating, flat display coating and hardware tool coating. Chromium thin films improve the wear resistance and lifetime of engine components. In DLC coatings chromium functions as a bonding layer.