Materials Performance

MAR 2018

Materials Performance is the world's most widely circulated magazine dedicated to corrosion prevention and control. MP provides information about the latest corrosion control technologies and practical applications for every industry and environment.

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20 MARCH 2018 W W W.MATERIALSPERFORMANCE.COM MATERIAL MATTERS Iridium—The Ultimate in Corrosion Resistance R ust—your nontechnical friends and family will probably think this is all you deal with in your work in the corro- sion industry. But we all know our work involves much more than this. This series of short articles—in honor of NACE Inter- national's 75th Anniversary—attempts to paint a more positive and optimistic picture of the corrosion profession by looking at examples of successful resis- tance to the ravages of corrosion from around the world (and beyond) and from the distant past as well as present. It is designed to show that our chosen f ield is as interesting, historic, and artistic as any other. Let's start at the beginning. We know that corrosion is a reaction between a material and its environment, so why not have a look at the most corrosion resis- tant metal in the periodic table. Accord- ing to ASM International's Metals Hand- book 1 and numerous other sources, that superlative honor goes to one of the rarest elements, iridium. It is resistant to alkalis and acids, including boiling aqua regia (a mixture of nitric acid [HNO 3 ] and hydrochloric acid [HCl]), as shown in Table 1. The only substances that will dissolve the metal are molten sodium cyanide (NaCN) and potassium cyanide (KCN). Iridium is a hard, brittle member of the so-called platinum group of metals in the periodic table. It is only a fraction less dense than the densest element, its neigh- bor osmium in the periodic table, and approximately double the density of lead. Iridium was discovered by Yorkshire chemist Smithson Tennant (1761-1813) in 1803, by separating and analyzing the res- idue left after dissolving impure platinum in aqua regia. It was named after Iris, the goddess of the rainbow, because its salts were so colorful. 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 is produced as a by-product of nickel smelting and has few applications as a pure metal. 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. As an alloying ele- ment, it increases the corrosion resis- tance of titanium and palladium, and the strength, hard- ness, and corrosion resistance of plati- num. Despite its corrosion resis- tance, you are most likely to come across iridium indi- rectly. It is one of the mixed metals in mixed metal oxide- coated titanium an- odes for impressed current cathodic protection. Those involved in nondestruc- tive testing may come across iridium- 192 as an isotope in radiography. There are two interesting facts regarding iridium not related to corro- sion properties. One is that the last remaining base unit still def ined by an artifact rather than a fundamental con- stant of nature is the kilogram, which is a cylinder of platinum with 10% iridium kept under nested bell jars at the Interna- tional Bureau of Weights and Measures near Paris, France. The iridium is added to increase the hardness to resist wear during cleaning. It is expected that a new def inition of the kilogram based on fun- damental physical constants will be adopted within the next year or two. Second, iridium is probably best known because of its association with the likely cause of extinction of the dino- saurs. The rocks in the boundary layer between the Cretaceous and Tertiary geo- logical periods, laid down ~65 million years ago, contain a higher proportion of this element than other rocks. An irid- ium-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. Our next tale on successes in resisting corrosion is about metals arriving from outer space, which will be covered in the April 2018 issue of MP. Source: NACE International member Robert A. Francis, Ashburton, Victoria, Australia—email: Robfrancis766@gmail. com. Reference 1 J.R. Davis, ed., Metals Handbook Desk Edition, 2nd Edition (Materials Park, OH: ASM Inter- national, 1998). TABLE 1. ELEMENTS' RESISTANCE TO CORROSIVE MEDIA Corrosive Gold Platinum Iridium Concentrated HCl at 100 °C ✔ ¢ ✔ Concentrated HNO 3 at 100 °C ✘ ✔ ✔ Aqua regia at 100 °C ✘ ✘ ✔ Moist chlorine at room temperature ✘ ¢ ✔ ✔: Resistant ¢: Some attack ✘: Severe attack

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