Iridium has the second highest density of all elements, the second highest melting point among the platinum group metals and the highest elastic modulus of all face-centered cubic metals. Iridium is very hard and strong (HV 240), and is therefore ductile only at high temperatures of approximately 1100–1500°C. Iridium displays high mechanical stability even at temperatures around 2000°C. Because it is very brittle, working the metal, for example by turning it on a lathe or milling it, is all but impossible. Iridium is considered the most resistant material; it is resistant to acids, including aqua regia, and it has the highest stability of all metals against oxide melts. It is only affected by salt baths and to a lesser degree by alkali hydroxide baths. It does not tarnish at room temperature, but forms oxides at temperatures under 800°C, which become volatile at approximately 1000°C. In compounds iridium occurs with valences of –1, 0, +1, +2, +3, +4, +6, with oxidation states +1 and +4 being the most frequent.Iridium is used as a component in alloys with the other platinum group metals, in particular with platinum itself. Iridium makes alloys stronger and imparts greater corrosion resistance. Heraeus manufactures components and semifinished products made from platinum-iridium alloys for the chemical industry, aerospace technology and spark plug electrodes.Due to the metal's stability at a range of temperatures and its outstanding corrosion resistance, components for high-temperature applications are made of iridium. Heraeus in particular makes crucibles for growing refractory oxidic crystals.
