Platinum and the Refractory Oxides

Platinum reacts exothermically with aluminium, zirconium and thorium, but is very reluctant to alloy with magnesium. It dissolves 2.2 per cent by weight of aluminium at 1500°C and 10 per cent by weight of zirconium at 1650°C. These wide solid solutions are in equilibrium with very stable inter-metallic compounds, thus accounting for the severe reactions which can occur when platinum is heated in contact with alumina and zirconia. Zirconium reacts peritectically with platinum. The effects of these refractory decomposition processes on the long-term stability of platinum thermocouples will be discussed in the fourth and concluding article in this series.