Francis, Ashburton, Victoria, Australia-email: J.R. Source: NACE International member Robert A. An iridium-enriched meteor is believed to have hit Mexico’s Yucatán Peninsula, and dust from the impact caused darkening of the skies that wiped out most large animals. The rocks in the boundary layer between the Cretaceous and Tertiary geological periods, laid down ~65 million years ago, contain a higher proportion of this element than other rocks. Second, iridium is probably best known because of its association with the likely cause of extinction of the dinosaurs. It is expected that a new definition of the kilogram based on fundamental physical constants will be adopted within the next year or two. The iridium is added to increase the hardness to resist wear during cleaning. One is that the last remaining base unit still defined by an artifact rather than a fundamental constant of nature is the kilogram, which is a cylinder of platinum with 10% iridium kept under nested bell jars at the International Bureau of Weights and Measures near Paris, France. There are two interesting facts regarding iridium not related to corrosion properties. ![]() Those involved in nondestructive testing may come across iridium-192 as an isotope in radiography. It is one of the mixed metals in mixed metal oxide-coated titanium anodes for impressed current cathodic protection. Despite its corrosion resistance, you are most likely to come across iridium indirectly. As an alloying element, it increases the corrosion resistance of titanium and palladium, and the strength, hardness, and corrosion resistance of platinum. In a pure form, it is used for electrodes in high-performance spark plugs and crucibles for the preparation of single crystals of certain electronic and optical glasses. It is produced as a by-product of nickel smelting and has few applications as a pure metal. World production of iridium is small, only a few metric tons (tonnes) per year, and it is one of the least abundant and most expensive metals. It was named after Iris, the goddess of the rainbow, because its salts were so colorful. Iridium was discovered by Yorkshire chemist Smithson Tennant (1761-1813) in 1803, by separating and analyzing the residue left after dissolving impure platinum in aqua regia. It is only a fraction less dense than the densest element, its neighbor osmium in the periodic table, and approximately double the density of lead. Iridium is a hard, brittle member of the so-called platinum group of metals in the periodic table. The only substances that will dissolve the metal are molten sodium cyanide (NaCN) and potassium cyanide (KCN). It is resistant to alkalis and acids, including boiling aqua regia (a mixture of nitric acid and hydrochloric acid ), as shown in Table 1. According to ASM International’s Metals Handbook 1 and numerous other sources, that superlative honor goes to one of the rarest elements, iridium. ![]() We know that corrosion is a reaction between a material and its environment, so why not have a look at the most corrosion resistant metal in the periodic table. It is designed to show that our chosen field is as interesting, historic, and artistic as any other. This series of short articles-in honor of NACE International’s 75th Anniversary-attempts to paint a more positive and optimistic picture of the corrosion profession by looking at examples of successful resistance to the ravages of corrosion from around the world (and beyond) and from the distant past as well as present. But we all know our work involves much more than this. Rust-your nontechnical friends and family will probably think this is all you deal with in your work in the corrosion industry.
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