Non-uniform and rough films. Aluminium corrosion

These three regimes are examples of uniform corrosion and can be contrasted with the fourth, i.e. solutions containing aggressive ions such as chloride, where protective layers are not formed, and localised, pitting corrosion takes place. 

Ellipsometry can be used to gain information about the films formed in the three uniform corrosion regimes. Details are given here about two of those cases firstly the barrier or valve metal oxide layers, which optically are nearly ideal films, and secondly alkaline corrosion, which gives some insight into the applications of ellipsometry to non-uniform films and roughened metallic surfaces. 

In calculating a three-phase model signature (superimposed in Fig. 12 as a broken line) there is some doubt as to the correct starting point. It is known that A1 always has an air-formed protective film whose thickness has been estimated by various techniques to be about 2nm. The starting point was estimated by using this value and a film RI of 1-65. 

The agreement between theory and experiment is very good the barrier layer behaves as an almost ideal phase, with a refractive index very close to that of the anhydrous crystalline material. This is confirmed by the A-i/f trajectory for the dissolution of the layer, which, in contrast to the behaviour of the porous hydrated oxide formed in alkaline solution, dissolved uniformly from the outer surface towards the metal. 

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