Emerging mechanisms of FeCrAl(RE) oxide scale formation and permeation from 1st principles
Abstract: Alumina forming alloys are important for high temperature applications due to the high stability of the alpha-Al2O3 scale which forms above 900 C. FeCrAl(RE) alloys are alumina formers with small additions of reactive elements, RE e.g. Y, Zr, Hf and Ce, added to improve among others oxidation behavior and scale adhesion. Cr is added to the binary FeAl system in the role of a 3rd element in order to decrease early Fe, and internal Al oxidation, and promote alpha-Al2O3 formation. As corrosion and oxidation processes deplete the alloy of scale forming metal alloy, the durability suffers. Here, density functional theory, DFT has been employed to study parameters controlling oxide scale growth in general and permeation of oxidants through formed scales on FeCrAl(RE) in particular. The context of oxide growth is given by Wagner theory of oxidation in which diffusion of ionic species; cations, anions, and electrons control oxide growth rate. Inasmuch as alumina is a large band-gap insulator, the conduction band is inaccessible for electron transport. Thus, electron transport utilizing oxygen vacancies has been studied here. Activation energies for electron transport were calculated to be ~0.5 eV rendering electrons mobile. Oxygen vacancy, Vo diffusion barriers range between ~2-5 eV depending on electronic charge of the vacancy. A percolative Vo and electron transport is thus proposed in alumina, rendering both species mobile. The third element effect was given a local meaning at early stages of scale growth in a systematic study comparing Sc, Ti, V, Cr, Mn, Fe, Co, and Ni employed as guest ions in an alumina lattice. Comparing the affinity to oxygen vacancies, Vo only Cr and V displayed ideal intermediate affinities, i.e. intermediate to Fe and Al. V was thus proposed alongside Cr as a third element in the Fe-TM-Al ternary alloy system. Chromia nodules embedded in the protective alumina scale formed on FeCrAl(RE) were observed to permeate nitrogen in a reducing 95% N2, 5% H2, 35 ppm H2O environment. Al in the alloy was shown to reduce the chromia particle upon which a nitrogen permeation channel through said particle is sustained and for which Al acts as nitrogen sink. Enhanced oxidation was observed around surface RE-oxide particles. Here, oxidation of Al by water is understood to be the driving force for incorporating RE into alumina grain-boundaries. This RE decoration retards grains coarsening, leading to a thicker and more adherent early scale. In O2 containing atmospheres, this defect rich "messy" scale eventually becomes oxidized, upon which hydride ions are consumed and RE precipitate. RE(III) are shown to introduce stresses into grain-boundaries leading to a faster precipitation while smaller RE(IV) can maintain enhanced oxidation for longer before precipitation occurs.
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