Aluminium continued oxide film

The reactivity of massive metal such as that used commonly in the form of plates, sheets or profiles is in no way comparable with what can be deduced from certain thermodynamic data. The reason is simple when aluminium is put in contact with the oxygen contained in air, it will be covered immediately by a continuous oxide film consisting of alumina, the thickness of which, between 5 and 10 nm, is sufficient to slow down and even annihilate the reactivity of aluminium with many products (see Section B.1.8), including air and oxygen, even at high temperatures. 

Continuous (barrier, passivation) films have a high resistivity (106Q cm or more), with a maximum thickness of 10 4cm. During their formation, the metal cation does not enter the solution, but rather oxidation occurs at the metal-film interface. Oxide films at tantalum, zirconium, aluminium and niobium are examples of these films. 

The initial reaction results in the formation of a continuous film of oxide that is firmly attached to the metal surface. The rate of growth of the film is controlled by the slow diffusion of the Cu ions. However, no corrosion could occur without the transport of electrons, as the mechanism depends on electron transport. The electronic conductivity of the film is therefore of major importance. The reason why both aluminium and chromium appear to be corrosion-resistant lies in the fact that, although oxide films form very rapidly in air, the films are insulators and prevent reaction from continuing. As the thin films are also transparent, the metals do not lose their shiny appearance. 

Most of the work that has been carried out using porous membranes as hard templates was done on porous alumina (anodic aluminium oxide, AAO) or track-etched polycarbonate membranes. The general principle of nanotube formation relies on a continuous polymer film being formed on the inside of the pores that remains as a tubular structure once the template is removed. There are several possibilities as to how this polymer film can be formed inside the pore. A film can be obtained by wetting the pore with a polymer melt or a polymer solution. Alternatively, the polymerization of monomers... 

This behaviour is due to the fact that aluminium, like all passive metals, is covered with a continuous and uniform natural oxide film corresponding to the formula AI2O3, which is formed spontaneously in oxidising media according to the reaction ... 

These reactions take place because alcohol (or phenol) dehydrates the protective natural oxide film and modifies its stmcture. Moreover, since the resulting alcoholate (or the phenolate) is soluble in this medium, aluminium is no longer protected and will, therefore, be continuously attacked. The attack will proceed as pitting. 

In the case of alloys having one constituent considerably more reactive to oxygen than the others, conditions of temperature, pressure and atmosphere may be selected in which the reactive element is preferentially oxidised. Price and Thomas used this technique to develop films of the oxides of beryllium, aluminium, etc. on silver-base alloys, and thereby to confer improved tarnish resistance on these alloys. If conditions are so selected that the inward diffusion of oxygen is faster than outward diffusion of the reactive element, the oxide will be formed as small dispersed particles beneath the surface of the alloy. The phenomenon is known as internal oxidation and is of quite common occurrence, usually in association with a continuous surface layer of oxides of the major constituents of the alloy. 

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 presence of water vapour adversely affects the selective oxidation of elements such as aluminium and chromium from iron-base and nickel-base ° alloys. This is illustrated in Figure 7.2 for a Ni-Cr-Al alloy. Exposure in dry air results in a continuous external alumina film while exposure in moist air results in profuse internal oxidation of the Al. It has also been found that water vapour affects the selective oxidation of aluminium from TiA1.2i... 

The porous film of the aluminium oxide that appears is not a barrier for the reactions of A1 with constituents of the refractory. Molten aluminium and magnesium will continue to penetrate inside the refractory through pores (which will not... 

Pinel M.R., Bennett J.E., Voluminous oxidation of aluminium by continuous dissolution on a wetting mercury film, Journal of Materials Science, vol. 7, 1972, p. 1016-1026. 

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