Iridium has the reputation of being the most corrosion resistant of all metals. It was discovered in 1803 by Smithson Tennant in 1803 and named for the Latin iris, or "rainbow" because it forms a large number of very colorful compounds. The pure metal is very difficult to machine into useful shapes because of its hardness and its principal use is as a hardening agent for platinum. It will withstand attack by hot aqua regia (a mixture of concentrated hydrochloric and nitric acids---which dissolves even gold) and can be used routinely at temperatures up to 2000oC.
Stephen R. Marsden (ChemTopics)