Aluminium alloys continued oxide film

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. 

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... 

---