Tungsten and molybdenum possess high melting points and good mechanical strengths at elevated temperatures, but even at moderate temperatures both oxidise rapidly. Previous studies have shown that tungsten-chromium-palladium alloys have remarkable oxidation resistance when heated in air, and different mechanisms have been proposed to explain the advantageous action of the palladium.
A recent investigation of the oxidation mechanism and of the characteristics of this alloy system, and of some quaternary alloys produced by substituting large amounts of molybdenum for some of the tungsten, has now been reported (D.-B. Lee and G. Simkovich, J. Less-Common Met., 1990, 163, (1), 51-62).
Between 1000 and I250°C, the oxidation resistance of the alloys increases with temperature, the molybdenum-containing alloys being the more resistant. The palladium enhances the formation of a protective chromic oxide scale, acts as a reservoir for chromium, facilitates the outward movement of chromium and prevents oxygen diffusing inwards.