Studies in Dry Oxidation

In this chapter we look first at an important class of alloys designed to resist corrosion the stainless steels. We then examine a more complicated problem that of protecting the most advanced gas turbine blades from gas attack. The basic principle applicable to both cases is to coat the steel or the blade with a stable ceramic usually Cr203 or AI2O3. But the ways this is done differ widely. The most successful are those which produce a ceramic film which heals itself if damaged - as we shall now describe. 

A considerable quantity of this foreign element is needed to give adequate protection. The best is chromium, 18% of which gives a very protective oxide film it cuts down the rate of attack at 900°C, for instance, by more than 100 times. 

Ceramics themselves are sometimes protected in this way. Silicon carbide, SiC, and silicon nitride, Si3N4 both have large negative energies of oxidation (meaning that they oxidise easily). But when they do, the silicon in them turns to Si02 which quickly forms a protective skin and prevents further attack. 

Obviously this sort of loss is not admissible, but how do we stop it  

Suppose for a moment that our material is chromium. Table 21.2 shows that Cr would lose 0.1 mm in 1600 hours at 0.77 - Of course, we have forgotten about one 

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