Porous anodic films

O Sullivan, J. P. and G. C. Wood. 1970. The morphology and mechanism of formation of porous anodic films on aluminum. Proc. Roy. Soc. London A317 511-43. 

Takahashi, H., M. Nagayama, H. Akahori and A. Kitahara. 1973. Electron-microscopy of porous anodic films on aluminum by ultrathin section technique. Part 1. The structural change of the film during the current recovery. J. Electron Microscopy 22(2) 149-57. 

Figure 27 Schematic illustration of the film formation morphology across the anodization voltage-time response for A1 in sulfuric acid. In this environment, porous anodic films form. (From J. De Laet, H. Terryn, J. Vereecken. Electrochim Acta 41, 7/8, 1151 (1996).)...

Figure 29 (a) Explicit equivalent circuit model for an unsealed porous anodized film... 

Figure 2. The dependences of porous anodising process parameters (a) and porous anodic film parameters (b) on the forming voltage.

Figure 3, SEM images of porous anodic films formed by anodbing of A1 films in the meniscal region at 70 V a) cross section, b) top view, c) A - A plaiar section, d) B - B planar seaion.

Patermarakis, G. 2006. Aluminium anodizing in low acidity sulphate baths Growth mechanism and nanostructure of porous anodic films. Journal of Solid State Electrochemistry 10, 211-222. 

An equivalent circuit can be derived for the surface-bound membrane formed in this work similar in a manner to the approach taken for porous anodic films and porous electrodes (41-46). An equivalent circuit network, proposed in Figure 8a, corresponds to the model in Figure 7. This network has three RC subnetworks that represent the oxide layer, the surface-bound membrane layer, and the double layer. Cox and Rox are the capacitance and resistance of oxide. and Rdl are the double-layer capacitance and the polarization resistance, known as the charge transfer resistance at the membrane-water interface. For the subnetwork of the surface-bound membrane layer, one branch represents a tightly packed alkylsilane and lipid bilayer in series, and the other branch represents the pores and defects through the bilayer. Calk, Clip and Ralk, Rhp are the capacitances and resistances of... 

Taking into account the stoichiometric ratio of metal and oxygen atoms in A12O3 and WO3 one can calculate that the formed oxide is oxygen-deficient and has excess of W atoms. The oxygen deficiency can be explained by an appearance of oxygen vacancies. Moreover, a part of W atoms can be in unbound states like it takes place for Si atoms in anodized Al-Si alloys [4,5]. The higher concentration of excess W is in the bottom part of the porous anodic film. 

To achieve the maximum protective qualities and corrosion resistance required for finished articles, the anodic oxide must be sealed after it is formed and/or colored. This produces a hydrated oxide layer with improved protective properties. Figure 5.44 illustrates how the initially porous anodic film evolves during the sealing process. 

Figure 5.44 Schematic description of the evolution of a porous anodic film on aluminum as a function of the sealing time at 85°C.

However, if the current charge is not controlled, then a high current density can lead to a thicker and more porous anodic film. It has also been found that (Chai et al, 2008) the corrosion resistance of an anodic film can be related to the applied cnrrent density. An anodized coating formed at a higher cnrrent density should be thicker and have higher corrosion resistance. 

Garcia-Vergara S.J., Skeldon R, Thompson G.E. and Habazaki H. (2006), A flow model of porous anodic film growth on aluminium , ii/ec/rac/izw. Acta, 52, 681-87. 

Thompson G.E., FumeauxR.C. and Wood G.C. (1978), Electron microscopy of ion beam thinned porous anodic films formed on aluminium , Corros. Sci., 18,481-98. 

Given the ability of incorporating nanoparticles into porous anodic films and conducting polymer matrices, with the latter forming within the pores, the incorporation of both TiOi nanoparticles and polyaniline in single step anodising is considered here. The corrosion performance of the coated aluminium alloy has been examined by salt spray testing and by electrochemical impedance spectroscopy. 

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