Formation of nano-scale composite anodic films on aluminium-holmium alloys

This study demonstrates the formation of barrier-type anodic films in a phosphate buffer (pH 8.4) on thermally evaporated aluminium-holmium alloys covering a 2.0–12.0 at.% Ho compositional spread. Scanning x-ray diffraction and scanning electron microscopy reveal that structure and morphology of the parent alloys library drastically change above 6.5 at.% Ho. Anodic films were formed when the as-evaporated alloys were stepwise anodized potentiodynamically to a maximum of 10 V in phosphate buffer regardless of film composition and morphology. Simultaneous in-situ electrochemical impedance spectroscopy discloses a good co-relation between film thickness and formation factor with the composition and morphology of parent alloys, suggesting a strong influence of holmium concentration on the structure, morphology of parent alloys as well as on the dielectric properties of resulting anodic films. Inductively coupled plasma optical emission spectroscopy measurements allowed a quantitative determination of the dissolved species during anodization.