THE GROWTH OF A POROUS OXIDE FILM OF A UNIQUE MORPHOLOGY BY ANODIC-OXIDATION OF AN AL-0.5 WT-PERCENT NI-ALLOY

Porous anodic oxide film growth on an Al-0.5 wt% Ni alloy, containing finely dispersed Al3Ni intermetallic particles, has been investigated by transmission electron microscopy of ultramicrotomed sections. It wa s found that film growth is influenced greatly by the localised proces ses proceeding at the intermetallic Al3Ni particles, with consequent d evelopment of a unique film morphology, which has not been observed pr eviously. At the initial stage of anodizing, the majority of the appli ed current is consumed by dissolution of the Al3Ni particles at the al loy surface, accompanied by oxygen gas evolution, which leads to the s uppression of the forming voltage, at 10 to 15V, for a sufficiently lo ng period until the particles are removed from the alloy surface. Duri ng this period, a thin porous film of fine dimensions, corresponding t o the voltage of 10 to 15 V, is formed over the matrix surface general ly. After the particles have been removed from the surface, the applie d current is used for the growth of the porous oxide over the matrix s urface as well as over the walls of cavities formed by dissolution of the particles. As the forming voltage increases, some of the pores gro w further, at the expense of other pores in the immediate surrounding regions, with their dimensions controlled precisely by the increasing forming voltage, resulting in the formation of large pores of a ''tear -drop'' shape. Implications of these findings for forming porous anodi c oxide films of controlled morphologies on aluminium and its alloys a re discussed. (C) 1998 Elsevier Science Ltd. All rights reserved.