Element 77 · Transition Metal
Iridium
Iridium is the hardest, most corrosion-resistant metal known — and now critical to green hydrogen production.
Element Facts
SYMBOL
Ir
ATOMIC NO.
77
ATOMIC WEIGHT
192.22
CATEGORY
Transition Metal
PERIOD
Period 6
GROUP
Group 9
Historical Uses
Iridium was discovered in 1803 by British chemist Smithson Tennant while studying the black residue left after dissolving crude platinum in aqua regia. Tennant identified two new elements in the residue: osmium and iridium. He named iridium after Iris, the Greek goddess of the rainbow, reflecting the striking variety of colours in its salts.
In 1875, the International Bureau of Weights and Measures chose a platinum-iridium alloy (90% platinum, 10% iridium) to fabricate the International Prototype Kilogram — the physical artefact that defined the kilogram until 2019. The same alloy was used for the International Prototype Metre bar. Iridium's extreme hardness and corrosion resistance made it ideal for this purpose.
Through the 20th century, iridium was used in specialised applications demanding extreme durability: fountain pen tips, electrical contacts, spark plug electrodes, and as a hardening agent in platinum jewellery. Its density (the highest of any element) and corrosion resistance at high temperatures made it irreplaceable in certain industrial applications.
Overview
Iridium is element 77, the second-densest element and one of the most corrosion-resistant materials known. Iridium is also notable for the iridium anomaly at the Cretaceous-Paleogene boundary — the geological evidence for the asteroid impact that ended the dinosaurs.
Current Uses
PEM (proton exchange membrane) electrolyser anodes for green hydrogen production — emerging strategic use. Spark plug electrodes in high-performance engines. Crucibles for crystal growth in semiconductor manufacture.
Not Commercially Viable for Scrap
Why QuickStop Metals doesn’t buy Iridium:
Iridium recovery is among the most specialist precious metal recycling activities. UK scrap yards do not handle iridium — it goes to PGM refiners.
Price Context
Iridium has been one of the most extreme price movers in the precious metals — peaking near $6,000/oz in 2021.