AN RBS STUDY OF THE SULFIDATION BEHAVIOR OF NIOBIUM AND NB-AL ALLOYS

The growth mechanisms of the sulphide scales on niobium and niobium-al uminium alloys at a temperature of 1073 K and sulphur pressure of 1 kP a have been studied by means of Rutherford backscattering (RES) with t he aid of a marker technique. A gold marker film of about 1 nm thickne ss 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. Afte r sulphidation the gold marker distribution in the scale has been dete rmined by RES. It has been found that sulphidation of niobium occurs b y the outward diffusion of niobium cations since the gold marker has b een 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 allo y aluminium content is too low to form a separate phase of aluminium s ulphide. 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.