Influence of minor element addition

The most spectacular effect on the high temperature oxidation behaviour of metallic materials is observed when reactive elements (RE) are added in small quantities. They decrease the oxidation rate and drastically increase the oxide adherence to the doped metallic substrates. The protective barrier keeps its effectiveness even when submitted to thermal shocks it then guarantees an excellent lifetime of the materials under service conditions. The beneficial effect of reactive elements on the high temperature resistance of alloys has been known for more than 50 years [102,103]. The reasons why 

According to the literature [105], the decrease of the oxidation rate and the improvement of the oxide layer adherence are not the only beneficial effects resulting from the addition of reactive elements they can also  

A tentative list of the beneficial effects ascribed to the introduction of RE in model or commercial alloys will be made in this chapter. 

In comparison to its effect in chromia scales, in which the oxidation rate can be decreased by a factor of 100 in the presence of RE, the introduction of RE has a slight influence on the oxidation rate of alumina-forming alloys (Fig. 10.8 and Table 10.4) [33,34,124-127]. The addition of Zr or Y2O3 decreases the oxidation rate of FeCrAl steels by a factor of only 3 [128]. This is confirmed by the results summarised in Figs 10.3 and 10.9 [29-31,34,56,129-133]. 

The quantity of RE introduced in the alloys plays a considerable role on the oxidation rates. When added over a critical content, the RE leads to negative effects, typically a large increase in the oxidation rate [134,135]. This overdoping effect is probably due to an increase of oxygen diffusion due to the presence of oxygen vacancies in the RE oxides (REO). 

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V. HIGHER-ORDER SYSTEMS AND THE INFLUENCE OF MINOR ELEMENT ADDITIONS... 

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