Emerging mechanisms of FeCrAl(RE) oxide scale formation and permeation from 1st principles
Doktorsavhandling, 2019
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.
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.
Författare
Vedad Babic
Chalmers, Kemi och kemiteknik, Energi och material
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Metaller och legeringar används i alltifrån småskruvar och konstruktionsmaterial till komponenter i större kraftverk, bränsleceller och motorer. Samtidigt är de utsatta för korrosionsprocesser som underminerar materialens funktioner och lastbärande egenskaper. Samhällets kostnader för korrosion är idag enorma men det är inte enbart av ekonomiska skäl som man strävar efter att utveckla nya material. Miljömässig nytta av forskning uppnås om bl.a. kraftvärmeverk kan köras under längre tid och mer effektivt med mer hållbara material.
Samtidigt och i takt med att datorernas beräkningskapacitet ökar, så blir alltfler komplicerade material tillgängliga för grundläggande studier med kvantkemiska metoder där man kan undersöka deras egenskaper på atomär nivå. Med ökade kunskaper om mekanismerna bakom oxidations- och korrosionsprocesserna blir det möjligt att utveckla skräddarsydda material för specifika tillämpningar. Vi har studerat en klass av legeringar, FeCrAl(RE), som används vid olika högtemperaturtillämpningar så som exempelvis elektriska värmeelement och i valsverk. Vid temperaturer över 900 °C bildar dessa ett tunt skyddande aluminiumoxidskikt som ger bra korrosionsskydd. Dock visar det sig att i mer aggressiva miljöer så är dessa legeringar utsatta för korrosionsangrepp som utarmar legeringen på dess aluminiuminnehåll. Vi har i detta arbete valt att studera hur detta skyddande aluminiumoxidskikt bildas i både syrerika och syrefattiga miljöer. Mer specifikt har vi studerat hur de olika legeringskomponenterna (järn, krom, aluminium) tillsammans med små mängder av tillsatser blir avgörande för det skyddande oxidskiktets tillväxt och vidhäftningsförmåga.
Samtidigt och i takt med att datorernas beräkningskapacitet ökar, så blir alltfler komplicerade material tillgängliga för grundläggande studier med kvantkemiska metoder där man kan undersöka deras egenskaper på atomär nivå. Med ökade kunskaper om mekanismerna bakom oxidations- och korrosionsprocesserna blir det möjligt att utveckla skräddarsydda material för specifika tillämpningar. Vi har studerat en klass av legeringar, FeCrAl(RE), som används vid olika högtemperaturtillämpningar så som exempelvis elektriska värmeelement och i valsverk. Vid temperaturer över 900 °C bildar dessa ett tunt skyddande aluminiumoxidskikt som ger bra korrosionsskydd. Dock visar det sig att i mer aggressiva miljöer så är dessa legeringar utsatta för korrosionsangrepp som utarmar legeringen på dess aluminiuminnehåll. Vi har i detta arbete valt att studera hur detta skyddande aluminiumoxidskikt bildas i både syrerika och syrefattiga miljöer. Mer specifikt har vi studerat hur de olika legeringskomponenterna (järn, krom, aluminium) tillsammans med små mängder av tillsatser blir avgörande för det skyddande oxidskiktets tillväxt och vidhäftningsförmåga.
Metals and alloys are used in everything from small screws and construction materials to components in larger power plants, fuel cells and engines. They are at the same time exposed to corrosion processes that undermine the material functions and load-bearing properties. The costs to society for corrosion are today enormous, but it is not only for economic reasons that one strives to develop new materials. Environmental benefits from research are achieved if e.g. combined heat and power plants can run for longer and more efficiently with more sustainable materials.
Simultaneously as the computational capacity of computers increases, more and more complicated materials become available for basic studies with quantum chemical methods, by which properties at the atomic level can be studied. With increased knowledge of the mechanisms behind the oxidation and corrosion processes, it becomes possible to develop tailor-made materials for specific applications. We have studied a class of alloys, FeCrAl (RE), which are used in various high temperature applications such as electric heating elements and furnace rollers. At temperatures above 900 °C, they form a thin protective alumina layer that provides good corrosion protection. However, in more aggressive environments, these alloys are subject to corrosion attacks which deplete the alloy of its aluminum content. In this work we have chosen to study how this protective alumina layer is formed in both oxygen-rich and oxygen-poor environments. More specifically, we have studied how the various alloying elements (iron, chromium, aluminum) together with small amounts of additives become decisive for the growth and adhesion of the protective oxide layer.
Simultaneously as the computational capacity of computers increases, more and more complicated materials become available for basic studies with quantum chemical methods, by which properties at the atomic level can be studied. With increased knowledge of the mechanisms behind the oxidation and corrosion processes, it becomes possible to develop tailor-made materials for specific applications. We have studied a class of alloys, FeCrAl (RE), which are used in various high temperature applications such as electric heating elements and furnace rollers. At temperatures above 900 °C, they form a thin protective alumina layer that provides good corrosion protection. However, in more aggressive environments, these alloys are subject to corrosion attacks which deplete the alloy of its aluminum content. In this work we have chosen to study how this protective alumina layer is formed in both oxygen-rich and oxygen-poor environments. More specifically, we have studied how the various alloying elements (iron, chromium, aluminum) together with small amounts of additives become decisive for the growth and adhesion of the protective oxide layer.
Ämneskategorier
Oorganisk kemi
Metallurgi och metalliska material
Den kondenserade materiens fysik
Fundament
Grundläggande vetenskaper
Infrastruktur
C3SE (Chalmers Centre for Computational Science and Engineering)
Styrkeområden
Materialvetenskap
ISBN
978-91-7597-877-2
Doktorsavhandlingar vid Chalmers tekniska högskola. Ny serie: 4558
Utgivare
Chalmers