Electronoptical study of anodic film growth on rapidly solidified Al-Si based alloys

Summary

Electronoptical examination of rapidly solidified Al-Si based alloys, containing 8.5-10 wt%Si, shows that solidification proceeds via pre-dendritic nuclei from which a cellular texture emanates; in regions of relatively low cooling rate, secondary dendritic arms are evident. Typically, cooling rates are estimated to be in the range of 10⁴-10⁵ Ks⁻¹, which give rise to an alloy structure of fine featured cells comprised of an aluminium-rich phase surrounded by a silicon-rich phase.

Anodizing of the alloys in near neutral electrolytes proceeds relatively inefficiently until anodic alumina formation over the aluminium-rich phase bridges over the oxidizing silicon-rich phase; oxidation of the latter phase, whereby silica film material develops, is also associated with oxygen evolution and electroluminescence. In acid electrolytes, that is phosphoric acid, the presence of the silicon-rich phase persists throughout the anodizing run, since porous alumina development proceeds only by OH⁻IO^2- ingress and no film bridging mechanism exists. Consequently, anodic alumina film formation proceeds at a relatively low current efficiency, and chemical dissolution of the outer regions of the alumina film generates a friable coating. Eventually a three-dimensional network of the oxidized silicon-rich phase is observed with associated regions of disrupted alumina.

Heat treatment of the as-received alloys develops discrete silicon particles with an improvement in the efficiency of porous anodic alumina formation; however, porosity within the bulk alloy is highlighted by anodizing.