THE COMPOSITION AND MORPHOLOGY OF ANODIC FILMS ON AL-MO ALLOYS

The growth of barrier-type anodic films on amorphous Al-Mo alloys, con taining 16 to 40 atom percent Mo, in berate electolyte was investigate d by analytical transmission electron microscopy, Rutherford backscatt ering spectroscopy, secondary ion mass spectrometry, and extended x-ra y absorption spectroscopy. During growth at 1 and 5 mA cm(-2), two-lay ered amorphous oxide films formed at high efficiency by outward migrat ion of cations and inward migration of oxygen ions; the outer layer is composed of units of alumina contaminated by boron species derived fr om the electrolyte and the inner layer contains units of both Al2O3 an d MoO3 distributed uniformly at the resolution of the analyses. At 0.1 mA cm(-2) the films form at reduced faradaic efficiency, which result s in a decreased thickness of the outer alumina layer. The two-layered films develop as a consequence of faster migration of Al3+ ions than Mo6+ ions within the inner layers of the films. New film material form s at the alloy/film and film/electrolyte interfaces and at the interfa ce between the inner and outer layers. At the alloy/film interface, a thin layer of alloy is highly enriched in molybdenum as a consequence of anodic oxidation. Due to the mechanism of film growth, a high conce ntration of boron accumulates in a sublayer of film material just abov e the inner/outer layer interface.