In Figure 3 these rates are plotted versus 1/temperature. From these curves the volatilisation rates and recession rates, respectively, of iridium have been calculated. It can be seen that the linear weight loss strongly increases with increaseing pressure, at a constant temperature of 2000☌ and with increasing temperature, at a pressure of 1 pascal. The weight change of the iridium specimens as a function of oxidation time is shown in Figure 2. In the present study the metal losses of polycrystalline iridium in an oxygen atmosphere have been determined gravimetrically at high temperatures of 1500 to 2100☌ and low pressures of 0.1 to 100 pascals of oxygen. Ina recently published paper by Wimber et al, systematic investigations extended over a wider pressure and temperature range, 10 −2 to 1.3 × 10 5 pascals of oxygen and 1675 to 2260☌, have now been reported (7). The high-temperature oxidation of iridium metal has only rarely been studied ( 2 to 7). Platinum and rhodium have the lowest metal losses, followed by iridium and ruthenium, whileosmium has the highest losses. In that temperature range the oxidation rates in air of the various platinum metals show differences up to about six orders of magnitude ( 1, 2). However, volatile oxides are formed and increase the metal losses when compared with the evaporation of the pure metal in high vacuum. At low temperatures thin protective oxide films are formed but at temperatures above about 1100☌ the oxides decompose and the metal surfaces remain bare. They also show good chemical stability even in oxidising atmospheres. In both fields iridium metal and iridium alloys have considerable advantages due to their high melting points and good physical and mechanical properties.
At high temperatures the platinum metals and alloys are increasingly used as thermocouple pairings and as a crucible material for liquid glasses and oxides, that is for the growth of single crystals.
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