Inorganic coatings anodizing

For some metals an intrinsic inorganic coating can be produced by anodic oxidation and the formation of a thick surface oxide layer. One example is aluminum oxidation by the Eloxal process. 

As explained in Chap. 14, anodizing is a widely use technique to produce a protective inorganic coating of some engineering materials such as aluminum, magnesium, titanium and a few other metals and alloys by the application of an anodic potential that would be normally quite corrosive if it was not for the barrier created by the process itself. Of all metals that are routinely anodized, aluminum alloys are... 

Barrier coatings are of four types - anodic oxides, inorganic coatings, inhibitive coatings and organic coatings. 

The most important parts of the coating system are the preparation of the surface and the selection and application of the anti-corrosive medium (various anodic metaUic coatings, prefabrication primers, organic or inorganic corrosion-inhibiting primers, conversion coatings, anodizing). Undercoats are only for improvement of appearance. 

Xhe main two types of inorganic coatings are conversion coatings and anodized coatings. 

In general, there are two types of surface contamination (1) organic contamination—such as oils, greases, paint coatings etc. and (2) inorganic contamination —such as rust, oxide films, corrosion products, scale, anodic films etc. Although these two types of contaminant can be removed simultaneously, it is simpler to consider the cases separately. 

Finally the synthesis of inorganic-polymer composite membranes should be mentioned. Several attempts have been made to combine the high permeability of inorganic membranes with the good selectivity of polymer membranes. Furneaux and Davidson (1987) coated a anodized alumina with polymer films. The permeability increased by a factor of 100, as compared to that in the polymer fiber, but the selectivities were low (H2/O2 = 4). Ansorge (1985) made a supported polymer film and coated this film with a thin silica layer. Surprisingly, the silica layer was found to be selective for the separation mixture He-CH4 with a separation factor of 5 towards CH4. The function of the polymer film is only to increase the permeability. No further data are given. 

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