Temperature-induced discolouration of platinum alloy

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Category: Metallurgy and Materials

Subject: Temperature-induced discolouration of platinum alloy

Question

What can cause discolouration of the surface of a platinum-iridium alloy at high temperature?

Answer

Although the platinum group metals are assumed to be completely inert, they all actually develop an oxide layer in air at high temperature. Usually the oxide layer is so thin (a few angstroms) that it is effectively transparent, and the metal surface remains bright. However, rhodium and iridium particularly can build up a thicker oxide which is grey/black in colour, if held within the temperature range 900-1100ºC, and with iridium we have seen a slight colouration at temperatures as low as 600ºC. A similar characteristic can be seen with rhodium-platinum and iridium-platinum alloys.

The good news is that the oxide layer is harmless. It doesn't affect the properties or performance of components made from platinum group metal alloys in any way.

Answer posted 29 November 2007

Submitted by: Ms Sara Coles

Affiliation: Platinum Metals Review

Answered by: Robert McGrath

Affiliation: Johnson Matthey Noble Metals