Iridium oxide formation

Alloys with iridium Iridium alloys with platinum in all proportions, and alloys containing up to about 40% iridium are workable, although considerably harder than pure platinum. The creep resistance of iridium-platinum alloys is better than that of rhodium-platinum alloys at temperatures below 500°C. Their stability at high temperatures, however, is substantially lower, owing to the higher rate of formation of a volatile iridium oxide. 

Alloys with ruthenium Additions of ruthenium have a most marked effect upon the hardness of platinum, but the limit of workability is reached at about 15% ruthenium, owing to the fact that ruthenium belongs to a crystallographic system different from that of platinum. Apart from a somewhat greater tendency to oxide formation at temperatures above 800°C, the resistance to corrosion of ruthenium-platinum alloys is comparable to that of iridium-platinum alloys of similar composition. 

While the positive chemical shift for Ru, indicating thick oxide formation, is virtually absent for x < 0.8 in RuxIr, x,a positive chemical shift is observed for Ir4/ for x > 0.2. Obviously the oxidation and corrosion of the ruthenium component is inhibited at the expense of increased oxidation of the iridium sites. Similarly, no preferential corrosion was observed for a Ru0 8Ir0.2 mixed oxide during 02 evolution at 1.5 Vsce for 8 hours [83],... 

In the Ir-AljOa system migration does not play a role in crystallite growth when heating in an oxygen environment above 713 K. Sintering was inhibited by BaO, CaO, and SrO up to 923 K oxidative stabilization of iridium crystallites is consistent with the formation of an immobile surface iridate by reaction between a mobile molecular iridium oxide species and the well dispersed Group IIA oxide. 

While in the previous two examples we have discussed the initial stages of oxidation, we now shall focus our attention on the optical properties of thicker oxide layers. Anodically formed iridium oxide films have attracted particular attention because of their pronounced electrochromic effect. When an Ir electrode is scanned anodically in 0.5 M H2SO4, oxidation starts at +0.6 V versus SCE. On the cathodic scan, however, the oxide layer is not reduced to the metallic state but to a low-conductivity hydroxide film facilitating further oxide formation with each anodic potential cycle. Continuous cycling of the iridium electrode between -0.25 and +1.3 V (SCE), at a frequency of 1 cps, therefore has been used as a standard treatment for the formation of thick anodic iridium oxide films. ... 

High Temperature Properties. There are marked differences in the abihty of PGMs to resist high temperature oxidation. Many technological appHcations, particularly in the form of platinum-based alloys, arise from the resistance of platinum, rhodium, and iridium to oxidation at high temperatures. Osmium and mthenium are not used in oxidation-resistant appHcations owing to the formation of volatile oxides. High temperature oxidation behavior is summarized in Table 4. 

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