Abstract
The growth mechanisms of the sulphide scales on niobium and niobium-aluminium alloys at a temperature of 1073 K and sulphur pressure of 1 kPa have been studied by means of Rutherford backscattering (RBS) with the aid of a marker technique. A gold marker film of about 1 nm thickness was deposited on the surface of niobium as well as on Nb-0.6 at% Al, Nb-15.2 at% Al and Nb-63.4 at% Al alloys by vacuum evaporation. After sulphidation the gold marker distribution in the scale has been determined by RBS. It has been found that sulphidation of niobium occurs by the outward diffusion of niobium cations since the gold marker has been located at the metal/scale interface. The scale structure for the Nb-0.6Al alloy is almost the same as that for niobium because the alloy aluminium content is too low to form a separate phase of aluminium sulphide. Other niobium-aluminium alloys form a double layer scale with an outer aluminium sulphide layer and an inner niobium sulphide layer, both of which are formed by the outward diffusion of cations.
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