Fates of Hydrogen During Alumina Growth Below Yttria Nodules in FeCrAl(RE) at Low Partial Pressures of Water

Electrocatalysis DOI 10.1007/s12678-017-0368-8 ORIGINAL ARTICLE Fates of Hydrogen During Alumina Growth Below Yttria Nodules in FeCrAl(RE) at Low Partial Pressures of Water Vedad Babic 1 & Christine Geers 1 & Bo Jönsson 1,2 & Itai Panas 1 # The Author(s) 2017. This article is published with open access at Springerlink.com Abstract 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-Al2O3 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 H2 at the interface in conjunction with OH– accommodation upon hydration, while the second allows hydrogen to be incorporated at VO 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. * Itai Panas 1 Department of Chemistry and Chemical Engineering, Chalmers University of Technology, SE-412 96 Göteborg, Sweden 2 Sandvik Heating Technology AB, Hallstahammar, Sweden 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 . N2 atmosphere . H2 reducing conditions Introduction The fate of hydrogen is of central concern in solid oxide fuel cell applications [1]. Processes competing with the generic property of the solid electrolytes, i.e., as proton conductors, are essential for the efficiency and longevity of the cell. The Wagnerian decomposition of corrosion phenomena into anode and cathode processes [2–5] makes possible a conceptual transferability between the two technologically, highly relevant fields of research. Utilization of this analogy is realized in dual-atmosphere fuel cell interconnection, which has hydrogen permeation through the steel as an important process causing breakaway corrosion on the air side [6]. In the present study, the impact of low-partial pressures of water during oxidation of high-temperature alloys is taken to serendipitously offer an opportunity to explore the diverse roles of hydrogen in the oxidation process, while the main driving force in this endeavor is in fact the internal nitridation [7]. Indeed, very low oxygen activities in predominantly N2(g) environment have unique corrosive impact at elevated temperatures. Alloys, which display near-ideal protective properties under normal corrosive conditions, suddenly exhibit surprising vulnerability. FeCrAl is one such alloy for which chromia particles of possible carbide origin embedded in α-Al2O3 scale were proven to short-circuit the protective oxide scale by offering a Electrocatalysis window for nitrogen ingress, thus causing rapid internal nitridation and degradation. The detrimental impact of nitrogen once arriving at the oxide/alloy interface originates from its significant solubility in the alloy. This property distinguishes the N2 atmosphere from, e.g., O2/H2O; in that, the low solubility of oxygen in FeCrAl is in fact an essential reason for its excellent performance under oxidizing conditions. The resulting internal nitridation of aluminum, avoided under oxidizing conditions, disallows outward aluminum diffusion to form a protective oxide scale, and this in turn has catastrophic impact on the alloy component. While the driving force for the internal nitridation is the formation of stable aluminum nitride in the alloy, a necessary condition for this to happen in the first place is the existence of chromium ions occupying coordinatively unsaturated surface sites (CUSs) at the gas/oxide interface, where N2 is able to dissociate [7]. Thus, it was demonstrated that internal nitridation can result from formation of transient chromium oxynitrides comprising solid solution of oxygen ions, nitride ions, and oxygen vacancies in the chromia matrix. Access to the alloy is achieved by co-diffusion of nitrogen and oxygen through the transient chromium oxynitride. This is triggered by the chromia particle becoming reduced due to aluminum oxidation in a metathesis reaction. Subsequently, the reduced chromia is reoxidized by nitrogen acting oxidant, resulting in the chromia effectively constituting a window for nitrogen permeation. Indeed, it was shown in [7] that coordinatively unsaturated Cr site CUSs on chromia are able to support N2 reduction. Improved long-term corrosion properties have been achieved by introducing additives such as zirconium and yttrium in the alloy enhancing transport in the resulting alumina oxide grain boundaries by single Zr4+ or Y3+ ion decoration [8]. Here, the initial impact of yttria particles on the corrosion process is addressed, i.e., prior to their dissolution. Thus, one objective of the present study is to provide experimental evidence and theoretical rationale for selective oxygen permeation associated to yttria particles penetrating the alumina scale, while simultaneously shutting the nitrogen out. Indeed, facilitated oxidation at the metal/oxide interface is observed, i.e., at a competitive pace when compared to the internal nitridation. Microscopy reports (vide infra) formation of a defect-rich Al2O3 at the yttria/alloy interface, rendering this region a sustained sink for aluminum. Consequently, depletion of Al in the alloy in the vicinity of the yttria particles is observed to cause an AlN-free halo in spite of nearby chromium-rich particles. Necessary prerequisites for sustaining any N2 dissociation are explored here, arriving at the concentration of CUS at the gas/yttria interface as being the single most important condition for N2 dissociation. Relevance of this condition is put in question when taking into account hydrolysis and rapid transport of water equivalents along the yttria/alumina rim. Such hydrolysis offers one possible mea (...truncated)