Question:
We anneal a platinum-iridium alloy at around 2550ºF (1400ºC) using a direct gas-fired box furnace, followed by water quenching into an external tank. We want to wind up with less surface oxidation and are hoping to “improve" the atmosphere. What atmosphere conditions are needed? What changes or improvements can you suggest that might help and what is the "correct/best" atmosphere for keeping this material at least clean if not bright?

Answer:
Most people are not familiar with Iridium (Element #77). Iridium, discovered in 1804, comes from the Latin word iris, meaning rainbow. It was so named because of its colorful salts. Iridium is known as a "platinum metal" because it is found in natural deposits of platinum (#78) and along with osmium (#76), its twin element. Iridium's most common application is as an alloying agent for hardening platinum. Iridium is the most corrosion resistant of all platinum-group metals.

Interestingly, annealing of platinum-iridium alloys is most often done in air. Your oxidation problems would be minimized if you used a furnace that was not direct gas-fired as the products of combustion (water vapor, carbon dioxide) are contributing to the problem. Furnaces with reducing atmospheres should not be used for platinum alloys.

Annealing should be done in an oxidizing atmosphere in order to avoid a reduction of the other metals. Be aware that iridium forms a volatile oxide above 1650ºF (900ºC), and the loss by evaporation increases with temperature (and iridium content).

While most of the platinum alloys will be fully annealed at or slightly above 1830ºF (1000ºC), other alloys require higher temperatures. The usual anneal time will depend on part thickness and is normally in the 15-20 minute range. Finally, unnecessary long heating at high temperature is to be avoided because this can lead to a coarsening of its crystal structure resulting in embrittlement and cracking.