SULFIDE PRECIPITATION ABOVE ANODIC ALUMINA FORMED IN THIOMOLYBDATE ELECTROLYTE

The composition, structure and morphology of the molybdenum sulphide, which forms at the anodic film surface during anodizing of aluminium i n thiomolybdate electrolyte, have been determined from Rutherford back scattering spectroscopy, electron microscopy and X-ray diffraction. Th e sulphide consists of a fragile layer of amorphous of microcrystallin e MoS3 that is attached loosely to the surface of the anodic film. The layer is formed near or at the film surface by the reaction between t hiomolybdate ions, from the electrolyte, and H+ ions, produced by film growth. The presence of the sulphide layer on the film surface does n ot affect significantly the growth of a typical barrier-type alumina f ilm, which, as generally occurs, is contaminated in part of its thickn ess by a low concentration of sulphur-containing species derived from electrolyte anions. (C) 1997 Elsevier Science S.A.