Growth Mechanism of Porous Anodic Films on Aluminium

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Abstract

Fundamental research on the growth of porous anodic alumina (PAA) films has been undertaken for many years because of the complexity of the processes involved and the wide range of commercial applications. In this study, a tungsten tracer approach has been used to determine the influences of current density and electrolyte temperature on the incorporation of the tracer and its distribution and consequently, the growth mechanisms of PAA films.The efficiencies of growth of PAA films, formed during anodizing at 5 mA cm-2 in the three major forming acids at 25 °C, are ~60%, due to loss of outwardly migrating Al3+ ions at the film/electrolyte interface. Thus, only the inwardly migrating O2- ions contribute to formation of the anodic oxide at the film/metal interface. The pores are developed due to flow of alumina from beneath the pore base regions toward the cell walls, which is indicated by distortion of the incorporated Al-W alloy layers and retention of the tungsten species within the anodic films.PAA films formed at a low range of current densities (<2 mA cm-2) develop by a field-assisted dissolution mode, with significant losses of aluminium and tungsten species to the electrolyte, and low expansion factors of less than 1.2. Conversely, films formed at current densities ≥2 mA cm-2 grow by a flow mechanism: flow of film material transports the alumina oxide, including the incorporated tungsten tracers, from the barrier layer regions to the cell walls, resulting in relatively thicker films at higher current densities and retention of the tungsten within the films. The tungsten remains mainly within the inner cell region of the films, with a tungsten-free region present next to the pore wall. The efficiency of film growth increases from ~0.29 to ~0.73 with increase of current density from 0.5 to 30 mA cm-2, and from ~0.26 to ~0.88 with increasing current density between 0.5 and 50 mA cm-2 for anodizing in sulphuric and oxalic acids respectively.Comparatively, for PAA films formed at 15 mA cm-2 in oxalic acid, reduction of electrolyte temperature from 20 to 1 °C gives rise to a slight increase of the anodizing efficiency from ~0.67 to ~0.74; the film expansion factor also increases from ~1.32 to ~1.43. The previous arises from reduced field-assisted ejection of Al3+ ions at the decreased electrolyte temperature.Anodizing of the aluminium substrates in phosphoric acid or neutral phosphate solution generates barrier anodic alumina films and the barrier layers of porous films respectively, which comprise phosphorus-containing outer regions and a phosphorus-free inner regions. The phosphorus-containing outer region accounts for ~0.67 of the barrier films and the ~0.80 of the barrier layer of the porous films. Further, the distributions of phosphorus species are not significantly affected by the incorporation of the tungsten tracer nanolayer into the films; the influence of the phosphorus species on the outward migration of the tungsten species is also negligible.This tungsten tracer study suggests a significant influence of the flow of alumina oxide, under the high electric field, on the formation of PAA films at current densities ≥2 mA cm-2.

Keyword(s)

Porous anodic alumina; anodizing; mechanism; tungsten tracer

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