Oxidation-resistant alloys Reactive element effect

Ion implantation is less expensive than VPS or HVOF and can be used in environments that are slightly less corrosive. A concentration of 1016-1018 ions/cm drastically improves corrosion resistance. Rare earth (RE) metals also improve the corrosion resistance. Rare earth metals such as Y improve oxide scale formation on alloys. This improvement is based upon the reactive element effect (REE), which improves resistance in several ways According to Hussey et al. [63], the observed increase in corrosion resistance is due to ... 

OXIDATION-RESISTANT ALLOYS 11.13.1 Reactive Element Effect (REE)... 

In air, carburising atmospheres and sulphidising/oxidising atmospheres all alloys showed excellent corrosion resistance due to the formation of protective oxide scales. It was found that the concentration of aluminium or chromium does not significantly effect the corrosion behaviour in any of these environments. 10 wt% aluminium, probably even less, are sufficient to enable the formation of protective Al203-scales even at temperatures as low as 650°C. Overdoping with reactive elements (mischmetal), however, causes high oxidation rates in air and should be avoided. 

The addition of small amounts of reactive elements such as cerium, yttrium, hafnium, thorium, lanthanum, or their oxide dispersions greatly increases the high-temperature oxidation resistance of Fe-Cr alloys under isothermal or cyclic conditions. [11], Beneficial effects also result from ion implantation of the active element or from surface-applied coatings [11]. The ion implantation work of Bennett et al. [12] concerns the oxidation behavior of a 20Cr-25Ni-Nb stainless steel in CO2 at temperatures in the range of 900 to 1050°C. SIMS can be used to locate the position of the reactive element after oxidation. 

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... 

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