Aluminium anodising

Anodic passivation also allows titanium to be employed as a Jig for aluminium anodising baths ", because the protective anodic film formed on titanium allows passage of electronic current to the metal contact while virtually suppressing flow of ionic current through the anodically-formed surface film. This aspect is discussed in more detail in relation to special applications. 

Paulet J.F., Fuchs R., Gillich V., Le phenomene d iridescence sur les surfaces en aluminium anodise et colore, Alusuisse Lonza, note FWEO, 1994. 

Patrie J., Les methodes de controle applicables a I aluminium anodise en architecture. Revue de VAluminium, 1972, p. 869-877. 

Sheet aluminium can be given a colour by a similar process. The aluminium is first made the anode in a bath of chromic acid (p. 377) when, instead of oxygen being evolved, the aluminium becomes coated with a very adherent film of aluminium oxide which is very adsorbent. If a dye is added to the bath the oxide film is coloured, this colour being incorporated in a film which also makes the remaining aluminium resistant to corrosion. This process is called anodising aluminium. 

Steel is particularly bad in this regard. In ordinary circumstances, aluminium is much better as we showed in the chapters on corrosion. Although the effect of salt on aluminium is bad, heavy anodising will slow down even that form of attack to tolerable... 

The second category was concerned with adhesion to porous or microfibrous surfaces on metals. Aluminium may be anodised to form an oxide surface comprising pores of diameter of tens of nanometers. Electroforming and chemical oxidation can be used to produce microfibrous or needle-like coatings on metals, including copper, steel and titanium. The substrate topography was demonstrated to play an vital part in adhesion to these surfaces [45-48]. 

On the other hand, metals such as Ta, Nb, Ti, Zr, Al, etc. (the valve metals ) do not exhibit transpassive behaviour, and in appropriate electrolyte solutions film growth at high fields rather than corrosion and/or oxygen evolution is the predominant reaction thus aluminium can be anodised to 500 V or more in an ammonium borate buffer titanium can be anodised to about 400 V in formic acid and tantalum can be anodised to high voltages in most acids, including hydrochloric acid. 

By a mechanism similar to that discussed in relation to platinum coating, titanium can function as a conducting jig to support aluminium components and assemblies in conventional anodising baths. In this application the exposed titanium acquires the insulating film, but allows current to pass to the aluminium at the points of contact . 

Fig. 15.2, taken from a paper by Lenz , shows the variation in density with thickness for steam-sealed anodic films produced in sulphuric acid on aluminium of 99-99 7o and 99-5% purity. A mean figure of 2-7 g/cm for sealed, and 2-5 g/cm for unsealed films is accepted by the British Standard for anodised aluminium... 

Thermal expansion The thermal expansion of the film is only about one-fifth that of aluminium, and cracking or crazing is observed when anodised aluminium is heated above 80°C. The fine hair-cracks produced do not seem to impair the protective properties of the coating if anodising conditions have been correct. 

Emissivity Table 15.5 shows the total heat emissivity of various aluminium surfaces, as a percentage of that of a black body. The figures have been recalculated from the data of Hase. The emissivity of anodised aluminium rises rapidly with film thickness up to 3 fim after which the rate of increase diminishes. 

Reflectivity The total and specular reflectivities of an anodised aluminium surface are controlled by both the condition of the metal surface, polished... 

Since the natural passivity of aluminium is due to the thin film of oxide formed by the action of the atmosphere, it is not unexpected that the thicker films formed by anodic oxidation afford considerable protection against corrosive influences, provided the oxide layer is continuous, and free from macropores. The protective action of the film is considerably enhanced by effective sealing, which plugs the mouths of the micropores formed in the normal course of anodising with hydrated oxide, and still further improvement may be afforded by the incorporation of corrosion inhibitors, such as dichromates, in the sealing solution. Chromic acid films, in spite of their thinness, show good corrosion resistance. 

The behaviour of samples under the actual conditions of service is the final criterion, but unfortunately such observations take a long time to collect and assess, and the cautious extrapolation of data from accelerated tests must be relied on for forecasting the behaviour of anodised aluminium in any new environment. 

Table 15.7 Corrosion tests on unprotected and anodised pure aluminium...

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