Thermal degradation of a palladium coating on hydrogen-tight niobium membranes

The thermal stability of palladium-covered niobium, a composite used in hydrogen-tight membranes, is studied. Palladium is applied by the method of plasma sputtering. The thermal degradation of the coating is judged from a decrease in its ability to prevent niobium oxidation. It is found that the degree of oxidation of the samples heated in air at 693 K drops when the thickness of the coating increases in the range 2.5-1000.0 nm. The reason is heating-induced destructive modifications of the coating morphology, which are observed under a scanning electron microscope. The thicker the coating, the higher the temperature of onset of destruction. At temperatures above 873 K, the initially smooth continuous coating becomes highly porous and this porous layer does not change upon subsequent heating (up to 1273 K). Critical factors and possible mechanisms of thermal degradation of palladium coatings on hydrogen-tight composite membranes are discussed.