Chromia-forming Alloys Research Articles

Promoting effect of CO2 on NiCr oxidation: Atomistic origins based on first principles.

The adsorption and dissociation of CO2 on both perfect and oxygen-deficient α-Cr2O3 (0001) surfaces, alongside the subsequent incorporation of the resulting C into the oxide lattice and its impact on oxide growth, are investigated using first-principles calculations. Our findings reveal that oxygen vacancies significantly enhance CO2 adsorption and promote its stepwise decomposition into C and O atoms. The resulting C can spontaneously dissolve into the oxide lattice through the oxygen vacancies. The presence of bulk dissolved C in the Cr2O3 lattice substantially enhances the formation, migration, and clustering of oxygen vacancies in the bulk. These results provide an atomic-level understanding of how CO2 accelerates the oxidation of chromia-forming alloys, offering microscopic insights for controlling oxide growth and mitigating oxidation-induced degradation of high-temperature alloys.

What If the Protection against Oxidation of Chromia-Forming Alloys Was Not Always Due to the Chromia Layer?

What If the Protection against Oxidation of Chromia-Forming Alloys Was Not Always Due to the Chromia Layer?

Atomic origin of CO2-promoted oxidation dynamics of chromia-forming alloys

The development of atomic imperfections within oxide films from high-temperature oxidation of heat-resistant alloys significantly limits the self-protectiveness of the surface oxide, contributing to the failure of energy generating system components such as turbines, engines, and heat exchanges. Directly probing the dynamics of such atomic defects is challenging because of the extreme thermochemical conditions of high-temperature oxidation. Using environmental transmission electron microscopy observations, here we directly capture atomic-scale dynamics of vacancies in growing Cr2O3 film during high-temperature oxidation of NiCr alloy in CO2. Coordinated with theory modeling, we delineate the atomistic mechanisms associated with the effect of interstitial carbon derived from CO2 on promoting the formation, migration and clustering of atomic vacancies to result in the enhanced alloy oxidation. The identified oxidation mechanism can find broader applicability in utilizing the atmosphere to tune the formation and evolution of atomic-scale defects, thereby affecting the mass transport properties of the growing oxide film.

Long-term oxidation and chromium evaporation behavior of Al2O3-forming austenitic stainless steel for 900 °C balance-of-plant components applications in solid oxide fuel cells

The long-term operation of balance-of-plant (BoP) components in solid oxide fuel cells (SOFCs) relies on the presence of a stable and durable oxide layer. In this study, we investigate the oxidation and chromium (Cr) evaporation behaviors of two developmental alumina-forming austenitic (AFA) alloys compared to chromia-forming alloy 625 at 900 °C in air with 10% water vapor. Transpiration tests, weight gain tests, X-ray diffraction, scanning electron microscopy with energy dispersive X-ray analysis, and scanning transmission electron microscopy with energy dispersive X-ray analysis are employed to evaluate the oxidation and Cr evaporation behaviors. Our findings reveal that alloy 625 exhibits significantly higher rates of Cr evaporation compared to OC11 (Y and Hf additions) and OC11LZ (Y and Zr additions), with evaporation amounts ∼56 and ∼28 times greater, respectively. The observed differences between OC11 and OC11LZ can be attributed to variations in the formed oxide scales during long-term operation. Furthermore, we examine the influence of Hf and Zr reactive elements on the long-term oxidation and chromium evaporation behaviors, providing insights into the role of these elements in enhancing the performance and stability of the alloys.

Influence of Ti on the oxidation behaviour at 1250°C of chromia-forming alloys based on nickel and/or cobalt

ABSTRACT Six {Ni,Co}-based alloys containing 25wt-%Cr, 0.4wt-%C and 1.6wt-%Ti were cast and subjected to metallographic characterisation before and after exposure for 70 h in air at 1250°C, the highest temperature at which these alloys may be used under low applied stresses. The alloys based mainly on nickel contain principally chromium carbides while TiC is the principal carbide phase in the alloys mainly based on cobalt. The oxidation resistance is the best for the alloys richer in nickel than in cobalt, but titanium has the same effect for all alloys, whatever the base element: after oxidation it is present as an external TiO2 layer covering chromia. The presence of this outermost more (Ni-based alloys) or less thick (Co-based alloys) TiO2 scale on the outer face of the external chromia scale, is expected to have a protective role against chromia volatilisation, phenomenon which can be very important at so high temperature.

Inconel®625 oxidation in CO2: Kinetics and reaction mechanism

Inconel®625 oxidizes in CO2 from about 750 °C. The reaction, studied at the beginning, does not depend on the CO2 pressure. Between 900 and 1000 °C, and for weight increases < 0.5 mg/ cm², the oxidation is parabolic forming chromia layers. The components nickel and molybdenum do not oxidize. The apparent activation energy is 310 ± 18 kJ / mol. Unlike what is usually admitted for the oxidation of chromia-forming alloys, the chromia layer does not protect the alloy, the limiting step of the oxidation being the outwards diffusion of chromium inside the alloy, not inside the oxide layer.

High-temperature Corrosion of ~ 30 Pct Porous FeCr Stainless Steels in Air: Long-Term Evaluation Up to Breakaway

In this work, a long-term (up to 6000 hours) corrosion evaluation of three porous (~ 30 pct of initial porosity) ferritic iron-chromium alloys with different Cr contents (20, 22, and 27 wt pct of Cr) was carried out at 600 °C, 700 °C, 800 °C, and 900 °C in air. Mass gain measurements and SEM analyses revealed that at temperatures above 600 °C, all alloys exhibit breakaway corrosion, whereas at 600 °C, none of the alloys were heavily oxidized even after 6000 hours. Based on the results, the diffusion character of the corrosion of porous chromia-forming alloys was identified. The microstructure changes at high temperatures in porous alloys containing 22 wt pct of Cr were determined in detail by transmission electron microscopy. The proposed prediction model indicated that the lifetimes of the Fe20Cr and Fe22Cr alloys were determined as 1250 hours (± 535 hours) and 1460 hours (± 640 hours), respectively. It is in agreement with the long-term oxidation experiment. For the Fe27Cr alloy, the deviation between predicted and observed lifetimes occurs. The proposed model allows for qualitative estimation of the porous alloys’ lifetime with experimentally validated accuracy.

Photothermal performance of three chromia-forming refractory alloys for high-temperature solar absorber applications

Spectral hemispherical emissivity is a crucial material characteristic that determines radiation heat transfer. In high-temperature solar thermal applications, it affects not only the efficiency of the solar energy absorption but also the heat losses caused by thermal radiation and the radiative heat transfer within the receiver. Due to the limitations of the working temperature of existing solar absorber coatings, the spectral hemispherical emissivity of the oxidized surface is a key performance indicator for evaluating the potential of a candidate refractory alloy for high-temperature (> 1000 °C) solar receiver/reactor designs. In this work, we systematically studied the photothermal performances of the oxidized surfaces of three widely used high-performance commercial chromia-forming alloys (Haynes 230, Hastelloy X, and SS 253MA) by analyzing the spectral hemispherical reflectance in the band 0.25–25 μm. The stability of the optical properties of the formed oxide layers have also been studied by exposing the three alloys at 1150 °C in air for three different exposure periods (10 h, 100 h, and 200 h). The results show that the solar absorptivity of all the samples is in the range of 0.800–0.855, with SS 253MA showing the best performances in offering both high and stable solar absorptivity in the range of 0.837–0.855. The evaluation of the photothermal performances suggest the potential of these three alloys in solar-thermal applications.

The Reactive Element Effect

Rationalizing the reactive element effect operative in alumina- and chromia-forming alloys upon oxidation under oxidizing hot gas atmospheres, referring to investigations on zirconium oxide-coated test samples for gas turbine alloys. This retrospective study uses the results of cyclic furnace lifetime tests conducted at 1100 °C on ZrO2-coated Ni-base alloys with Y− or Y+ Hf-doped bond coats and correlates them with the parabolic oxidation rate constant at 1100 °C of binary NiAl alloys doped with Y, Zr, or Hf. Parallel results at higher temperatures allow the respective oxidation processes during the cyclic lifetimes to be assigned to cation-dominated or anion-dominated transport processes. The correlations document the close interrelationship betweenthe refractory element content (Mo, Re, Ta, W) in the substrate alloythe total content of the two reactive elements Y and Zr in the mixed zone of the scale, representing a Me3+ iso-valence valueindividual relative lifetime parameters in pct for EBPVD thermal barrier coating systems associated with cation-dominated transport processes.

Carburization susceptibility of chromia-forming alloys in high-temperature CO2

It is generally believed that when a chromia scale forms in high-temperature CO2 environments, carbon uptake of alloys is nearly eliminated, thereby preventing carburization and associated degradation of properties. Herein we quantitively assess this notion through careful examination of several chromia-forming Ni-based and Fe-based commercial alloys after long-term (10,000-h) exposures to CO2 and air at 700 °C. Small but measurable carbon uptake ensued independent of alloy base metal (Ni/Fe), alloy crystal structure (FCC/BCC), and chromia growth rate. The rate for Ni-based alloys was strongly dependent on Si and Mn content, highlighting the need to understand possible long-term effects on mechanical properties.

Influence of the simultaneous processes of scale evaporation and reaction surface reduction on the oxidation kinetics of chromia-forming alloys doped with rare-earth elements

The high-temperature oxidation of FeCr(La) and Cr(Ce) alloys was studied by thermogravimetry. The kinetic dependence of the FeCr(La) mass change has a specific form due to the simultaneous occurrence of scale evaporation and reduction of the reaction surface. The latter is due to the formation of diffusion barriers from lanthanum chromite. The kinetic dependence of the mass change of Cr(Ce) has an identical form. Based on this identity, it was suggested that diffusion barriers can also exist in the Cr(Ce) alloy, as is the case for the FeCr(La) alloy.

High-Temperature Corrosion Kinetics of Selected Nickel-Based and Chromia-Forming Alloys in Air–HF–H2O Atmosphere

High-Temperature Corrosion Kinetics of Selected Nickel-Based and Chromia-Forming Alloys in Air–HF–H2O Atmosphere

The Role of Oxidation Resistance in High Temperature Alloy Selection for a Future with Green Hydrogen

Hydrogen is being considered integral to the future energy landscape but there is limited mechanistic understanding and a lack of predictive models to describe the combined effects of alloy and gas composition, temperature, thermal cycling and water vapor contents on the oxidation behavior of high-temperature materials. Experimental evaluations were combined with coupled thermodynamic-kinetic modeling to investigate the oxidation behavior of five representative Ni-based superalloys in two water vapor contents (10%, 60% H2O) under thermal cycling (1-h, 100-h cyles) conditions at 800°C and 1000°C. The alloy with the highest Ti content demonstrated the poorest cyclic oxidation behavior while the alloy with highest Cr and Al contents was expected to continue to support protective formation of a compact Al2O3 scale. Accelerated degradation of the chromia-forming alloys was observed in the higher water vapor content but the impact on transient oxidation of the alumina-forming alloys needs further investigations.

Molecular-scale investigation of the oxidation behavior of chromia-forming alloys in high-temperature CO2

Current and future power systems require chromia-forming alloys compatible with high-temperature CO2. Important questions concerning the mechanisms of oxidation and carburization remain unanswered. Herein we shed light onto these processes by studying the very initial stages of oxidation of Fe22Cr and Fe22Ni22Cr model alloys. Ambient-pressure X-ray photoelectron spectroscopy enabled in situ analysis of the oxidizing surface under 1 mbar of flowing CO2 at temperatures up to 530 °C, while postexposure analyses revealed the structure and composition of the oxidized surface at the near-atomic scale. We found that gas purity played a critical role in the kinetics of the reaction, where high purity CO2 promoted the deposition of carbon and the selective oxidation of Cr. In contrast, no carbon deposition occurred in low purity CO2 and Fe oxidation ensued, thus highlighting the critical role of impurities in defining the early oxidation pathway of the alloy. The Cr-rich oxide formed on Fe22Cr in high purity CO2 was both thicker and more permeable to carbon compared to that formed on Fe22Ni22Cr, where carbon transport appeared to occur by atomic diffusion through the oxide. Alternatively, the Fe-rich oxide formed in low purity CO2 suggested carbon transport by molecular CO2.

Contribution of modeling to explain the depassivation of chromia-forming alloys in molten glasses

The corrosion of chromia-forming alloys by molten glass is an issue encountered in industrial processes such as glass fiber production or nuclear waste vitrification. A model of the corrosion of such alloys in molten glass media is developed in this work. This simple model predicts the corrosion state of the alloy (passive of active) and the evolution of the Cr2O3 layer thickness with a low number of input parameters. Furthermore, the model allows to explain the depassivation of the alloy by identifying the conditions involving a lack of oxygen supply in the liquid towards the oxide.

Rate of coke formation of centrifugally cast austenitic chromia-forming and alumina-forming alloys in coking conditions

The coking kinetics and the long-term coking performance of an alumina-forming alloy against a traditional chromia-forming alloy was studied in a hydrogen-ethane coking atmosphere. After 1000 h of exposure, the alumina-forming alloy had 91% less mass gained compared to that of the chromia-forming alloy. Additionally, microstructural analysis showed that the carburisation attack in the chromia-forming alloy was more severe, especially at a higher temperature and at a higher exposure time. Results indicate that a better performance in coking conditions can be attained when implementing alumina-forming alloys.

Supercritical-CO2 corrosion behavior of alumina- and chromia-forming heat resistant alloys with Ti

Short-term corrosion behavior of the alumina- and chromia-forming Fe-Ni-based heat resistant alloys with Ti for γ’-Ni3(Al,Ti) precipitation were evaluated in supercritical-CO2 (sCO2) environment at 650 °C and 20 MPa for 457 h. The corrosion test showed development of thinner oxides with some occasional Fe-rich oxide nodules in alumina-forming alloy A (16Cr-4.5Al-3Ti). However, chromia-forming alloy C (20Cr-2Al-2Ti) exhibited several occurrences of Fe-rich oxide nodules and spallation of thinner oxides. Meanwhile, the pre-oxidation treatment resulted in improved corrosion resistance in sCO2 environment by preventing the Fe-rich oxide nodule formation and oxide spallation, especially for alloy A with formation of α-Al2O3.

Corrosion behavior and lifetime prediction of VM12, Sanicro 25 and Inconel 617 in supercritical carbon dioxide at 600 °C

The corrosion behavior of VM12, Sanicro 25, and Inconel 617 in supercritical carbon dioxide at 600 °C and 15 MPa was investigated. Chromia-forming alloys Sanicro 25 and Inconel 617 had better corrosion resistance than VM12, which formed Fe-oxide rich scales on its surface. In all materials, internal carburization zones were found underneath the oxide scales. Based on the results that the growth of corrosion layers of the investigated materials followed a parabolic corrosion law, the stability of the Cr2O3 scale of Sanicro 25 and Inconel 617 was evaluated and the corrosion allowances of the investigated materials were calculated.

Microstructure influence on creep properties of heat-resistant austenitic alloys with high aluminum content

This work focusses on the link between microstructure and creep properties for heat-resistant austenitic alloys with high aluminum content (3–5 wt %). An emphasis was put on the coupling of thermodynamic simulations, microstructural characterizations by scanning electron microscopy and transmission electron microscopy, and creep testing. The phase predictions performed by the calculation of phase diagrams method are in good agreement with the observed microstructure after creep at 1000 °C and 1050 °C. Correlation between creep properties and microstructure characterizations at 1000 °C and 1050 °C revealed that NiAl and α’ (chromium-rich base centered cubic phase) phases are deleterious for the creep properties at service temperature. Several high Al-content alloys are selected in order to replace the chromia-forming alloys standardly used in cracking furnaces.

Effect of interconnect coating procedure on solid oxide fuel cell performance

Chromium (Cr) species vaporizing from chromia-forming alloy interconnects is known as a source of degradation in solid oxide fuel cell (SOFC) stacks called “cathode poisoning”. (Mn,Co)3O4 spinel coatings offer good protection against Cr evaporation during operation. In this study, Crofer 22 APU steel interconnects were electrophoretically deposited in different mediums to obtain high packing of green coating layer. The optimized sample was then sintered in air-atmosphere or dual-atmospheres (reducing-oxidizing). Anode-supported cells were exposed from cathode side to bare or coated interconnects for 100 h at 800 °C. The electrochemical performances of aged cells were compared at 800 °C. Due to higher density coating obtained by dual-atmospheres sintering process, the exposed cell showed performance similar to non-poisoned cell (0.95–0.98 W/cm2). The cell exposed to air-atmosphere sintered coating interconnect showed lower output power (0.75 W/cm2) due to coating lower density. The cell exposed to uncoated interconnect exhibited considerable degradation and the lowest power (0.66 W/cm2).