High-temperature alloys are crucial to many important technologies that underpin our civilization. All these materials rely on forming an external oxide layer (scale) for corrosion protection. Despite decades of research on oxide scale growth, many open questions remain, including the crucial role of the so-called reactive elements and water. Here, we reveal the hitherto unknown interplay between reactive elements and water during alumina scale growth, causing a metastable 'messy' nano-structured alumina layer to form. We propose that reactive-element-decorated, hydroxylated interfaces between alumina nanograins enable water to access an inner cathode in the bottom of the scale, at odds with the established scale growth scenario. As evidence, hydride-nanodomains and reactive element/hydrogen (deuterium) co-variation are observed in the alumina scale. The defect-rich alumina subsequently recrystallizes to form a protective scale. First-principles modelling is also performed to validate the RE effect. Our findings open up promising avenues in oxidation research and suggest ways to improve alloy properties.
The FeCrAl alloy Kanthal APMT TM was exposed to N 2 -5%H 2 at 900°C. Trace oxygen in the gases supplied an oxygen activity which was sufficient to render alumina and chromia thermodynamically stable. The exposures revealed that the oxide scale was penetrated by nitrogen exclusively at chromia domains. Microscopic analyses of the oxide scale did not reveal micro-cracks that could serve as points-of-entry for nitrogen. Instead it is suggested that nitrogen is transported through a dense chromia layer. Density functional theory was employed to investigate decisive nitrogen surface chemistry and transport properties in chromia and alumina. The study was used to validate that the complex redox chemistry of Cr 3?as opposed to Al 3? is a sufficient discriminating factor between alumina and chromia, facilitating N 2 dissociation and mobility of N in chromia.
Oxidation of FeCrAl(Re), when exposed to ∼35 ppm of water as sole supply of oxygen in predominantly nitrogen atmosphere, has two characteristic signatures. One is the internal nitridation owing to chromia nodules acting windows toward nitrogen permeation locally short-circuiting the protective α-Al 2 O 3 scale. The second remarkable feature is the growth of thick, apparently defect-rich alumina scale under yttria-rich nodules. Hence, one part of the present study comprises exploratory DFT calculations to discriminate between the impacts of chromia and yttria viz. nitrogen permeation. The second part concerns boundary conditions for apparent rapid growth of alumina under yttria nodules. Yttriaassociated surface energy stabilization of defect-rich alumina in presence of water was argued to involve hydrolysis-driven hydroxylation of said interface. Subsequent inward growth of the alumina scale was associated with outward diffusion of oxygen vacancies to be accommodated by the remaining proton producing a hydride ion upon surfacing at yttria-decorated alumina interfaces. The latter comprises the cathode process in a quasi-Wagnerian context. Two fates were discussed for this surface ion. One has H − -H + recombination to form H 2 at the interface in conjunction with OHaccommodation upon hydration, while the second allows hydrogen to be incorporated at V O sites in hydroxylated grain boundaries of the growing alumina scale. The latter was taken to explain the experimentally observed rapid oxide growth under yttria-rich nodules.Space charge due to proton reduction was proposed to cause transient inward cationic drag. Keywords High temperature alloy . FeCrAl . Yttria . Alumina . Oxidation by water . Hydride in oxide . Oxygen vacancy . Hydrogen evolution . Confinement effect . Defects . Oxide growth . Low partial pressure of oxygen . N 2 atmosphere . H 2 reducing conditions * Itai Panas
A mechanistic perspective on the growth of protective oxides on high temperature alloys at elevated temperatures is provided.
Reactive elements-REs-are decisive for the longevity of high-temperature alloys. This work joins several previous efforts to disentangle various RE effects in order to explain apparently contradicting experimental observations in alumina forming alloys. At 800-1000 °C, "messy" aluminum oxy-hydroxy-hydride transients initially formed due to oxidation by H 2 O which in turn undergo secondary oxidation by O 2. The formation of the transient oxide becomes supported by dispersed RE oxide particles acting as water equivalents. At higher temperatures, electron conductivity in impurity states owing to oxygen vacancies in grain boundaries (GBs) becomes increasingly relevant. These channels are subsequently closed by REs pinning the said vacancies. The universality of the emerging understanding is supported by a comparative first-principles study by means of density functional theory addressing RE(III): Sc 2 O 3 , Y 2 O 3 , and La 2 O 3 , and RE(IV): TiO 2 , ZrO 2 , and HfO 2, that upon reaction with water, co-decorate a generic GB model by hydroxide and RE ions. At 100% RE coverage, the GB model becomes relevant at both temperature regimes. Based on reaction enthalpy ΔH r considerations, "messy" aluminum oxy-hydroxyhydride transients are accessed in both classes. Larger variations in ΔH r are found for RE(III)-decorated alumina GBs as compared to RE(IV). For RE(III), correlation with GB width is found, increasing with increased ionic radius. Similarly, upon varying RE(IV), minor changes in stability correlate with minor structural variations. GB decorations by Ce(III) and Ce(IV) further consolidate the emerging understanding. The findings are used to discuss experimental observations that include impact of co-doping by RE(III) and RE(IV).
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