Cr2O3 Scale Research Articles

Behavior of plasma sprayed Cr coatings and FeCrAl coatings on Zr fuel cladding under loss-of-coolant accident conditions

To fabricate the oxidation resistance accident tolerance fuel cladding, Cr layer and FeCrAl alloys were deposited on Zircaloy-4 using atmospheric plasma spraying technology. The specimens were exposed under a simulated loss-of-coolant accident condition. XRD, SEM and EDS techniques were carried out to explore their high temperature behavior. According to the analysis results, both Cr coating and FeCrAl coating were successfully prepared by atmospheric plasma spraying system. Cr coating had superior oxidation resistance. Compact Cr2O3 scale forming on Cr coating acted as an oxygen diffusion barrier, whereas FeCrAl coating degraded due to inter-diffusion, leading to its poor protection for the cladding. Difference in standard Gibbs free energy of formation of Fe and Zr caused the prior oxidation of Zr. Accordingly, Zr-rich and Zr-depleted zones occurred in FeCrAl-coated specimens.

Effect of Ti and Ta on Oxidation Kinetic of Chromia Forming Ni-Base Superalloys in Ar-O2-Based Atmosphere

AbstractIn the present study, two commercially available Ni-base superalloys, namely Rene 80 which contains Ti and IN 792 containing Ti and Ta, were investigated during isothermal oxidation at 1,050°C in Ar–20% O2 gas up to 50 h. It was found that IN 792 showed a slightly lower oxidation rate as compared to Rene 80. Also differences in oxide scale microstructures and thicknesses were found. Both alloys formed qualitatively similar oxide scales consisting of outer TiO2 on top of a Cr2O3 scale and internally oxidized Al2O3. The IN 792 formed a thinner oxide scale in general as well as a thinner outer TiO2 scale. Also the depth of the internally oxidized Al2O3 precipitates is smaller in case of IN 792. These findings were correlated with the formation of TiTaO4, which in turn hampered outer Ti and inward oxygen flux.

Cyclic carburization-oxidation behavior of Hastelloy-X at 1000°C

Abstract Cyclic corrosion, consisting of carburization in Ar+4% CH4+3% H2 for 1 h and oxidation in air for 1 h, of Ni-base in a wrought alloy, Hastelloy-X, was investigated at 1000°C to understand the significant metal loss occurring in fuel injection nozzles of gas turbine combustor operated using natural gas. The mass gain of cyclic corrosion was relatively low up to about eight cycles of corrosion by formation of a Cr2O3 scale in each oxidation stage, but it increased rapidly due to formation of a Fe- and Ni-rich oxide scale as the number of corrosion cycles increases. The Cr content in the subsurface region rapidly decreased under the cyclic corrosion condition compared with a continuous single oxidation or carburization, which resulted in the formation of non-protective Fe- and Ni-rich oxide scale in a small number of corrosion cycles. The formation of a Cr3C2 scale from the protective Cr2O3 scale in the carburization stage was detrimental for the oxidation resistance of Hastelloy-X and was considered to have a similar effect to spallation of a Cr2O3 scale; therefore, it accelerated Cr consumption from the alloy subsurface region.

Effects of water vapor on protectiveness of Cr2O3 scale at 1073 K

Fe-Cr alloy is commonly being used as boiler tube’s material. It is subjected to prolonged exposure to water vapor oxidation. The ability to withstand high temperature corrosion can normally be attributed to the formation of a dense and slow growing Cr-rich-oxide scale known as chromia, Cr2O3 scale. However, oxidation may limit the alloy’s service lifetime due to decreasing of its protectiveness capability. This paper is to presents an experimental study of thermo gravimetric and Fourier transform infrared analysis of Cr2O3 at 1073 K in dry and humid environment. Samples were used from commercially available Cr2O3 powder. It was cold-pressed into pellet shape of 12 mm diameter and 3 mm thick with hydraulic press for 40 min at 48 MPa. It then sintered at 1173 K in inert gas environment for 8 h. The samples are cooled and placed in 5 mm diameter platinum pan. It is subjected to reaction in dry and wet environment at 1073 K by applying 100%-Ar and Ar-5%H2 gas. Each reaction period is 48 h utilizing Thermo Gravimetric Analyzer, TGA to quantify the mass changes. After the reaction, the samples then characterized with Fourier Transform Infrared Spectroscopy, FT-IR and Field Emission Electron Scanning Microscopy, FE-SEM. The TGA result shows mass decreasing ratio of Cr2O3 in wet (PH2O = 9.5x105Pa) and dry environment is at a factor of 1.2 while parabolic rate at 1.4. FT-IR results confirmed that water vapor significantly broaden the peaks, thus promotes the volatilization of Cr2O3 in wet sample. FESEM shows mostly packed and intact in dry while in wet sample, slightly porous particle arrangement compare to dry. It is concluded that water vapor species decreased Cr2O3 protectiveness capability.

Communication—Reactive Sintering of a CoFe2O4 Coating on Ferritic Stainless Steels for SOFC Interconnect Application

A mixed Fe+CoO precursor was successfully utilized to synthesize a dense CoFe2O4 spinel coating on the Crofer 22 APU interconnect via reactive sintering at 900°C for 2 h in air without the use of any reduction treatment. Both the uncoated and coated samples were subject to thermal oxidation at 850°C for 500 h in air. Phase constituents, microstructure, and area specific resistance (ASR) of the uncoated and coated samples before and after the oxidation testing were evaluated. The CoFe2O4 layer effectively reduced the Cr2O3 scale growth and lowered the scale ASR.

(Co,Mn)3O4 Spinel Coating for Protecting Metallic Interconnects Thermally Converted from an Electro‐Codeposited Co‐Mn3O4 Composite Coating

Electro‐codeposition was used to prepare the Co and Co‐Mn3O4 precursor coatings on ferritic stainless steel E‐brite. Plated samples were exposed at 800°C in air for different durations (i.e., 10 min and 4 h) for thermal conversion of the deposited layer to a spinel coating. The converted layer was characterized with scanning electron microscopy/energy‐dispersive spectroscopy (SEM/EDS) and X‐ray diffraction (XRD). The results showed that after 10 min heat treatment, the plated Co layer was not fully oxidized and converted into a double‐layer microstructure with an inner CoO layer and outer Co3O4 layer, while for the Co‐Mn3O4 layer, the Mn3O4 particles were not completely dissolved into the oxide layer. After 4 h of exposure, the surface layer was fully converted into the Co3O4 or (Co,Mn)3O4 spinel coating. Though the Mn content in (Co,Mn)3O4 was relatively low, the Mn‐doped spinel coating was advantageous over the pure cobalt spinel coating, as the thermally grown Cr2O3 scale at the coating/substrate interface was more compact and protective. Co diffusion from the deposited layer into the alloy substrate was observed for both coatings.

Atom probe study of impurity segregation at grain boundaries in chromia scales grown in CO2 gas

Model alloys Fe-20Cr and Fe-20Cr-0.5Si (wt.%) were exposed to Ar-20CO2 and Ar-20CO2-20H2O gas mixtures at 650 and 818 °C, forming Cr2O3 scales. Impurities segregated to chromia grain boundaries: carbon at sub-monolayer level, iron as spinel in narrow (10–20 nm) bands alongside the boundaries and silicon as dispersed glassy SiO2 nanoparticles. Thermodynamic driving forces for development of these nanostructures and enabling transport processes are identified. The consequences for grain boundary diffusion supporting scale growth and alloy carburisation are discussed.

Preparation and characterization of Cr/CrC multilayer on γ-TiAl alloy by the double glow plasma surface alloying technology

Hard ceramic and ductile metal materials combined in multilayered structures provide superior wear and oxidation resistance to the single layer, which has been applied extensively to components subjected to high-temperature oxidation and wear. In this paper, the oxidation and wear properties of Cr/CrC multilayer coating obtained on γ-TiAl alloy by an emerging technology, that is double glow plasma surface alloying technology, were reported. Sliding wear and oxidation tests were performed on Cr/CrC multilayer as well as on chromising reference coating. The friction coefficients of Cr/CrC multilayer at room and high temperature were 0.2 and 0.175 respectively, about only 22.2% and 35% of the γ-TiAl substrate. The Cr/CrC multilayer oxidized approximately parabolically, with lower oxidation rate than the chromising coating. The oxidation mechanism of the Cr/CrC multilayer was mainly dominated by carbide oxidation and dense Cr2O3 scale oxide development.

CuMn1.8O4 protective coatings on metallic interconnects for prevention of Cr-poisoning in solid oxide fuel cells

Cr-poisoning of the cathodes due to the presence of metallic interconnects is detrimental to the performance of intermediate temperature solid oxide fuel cell stacks. Applying a protective coating on the interconnect is an effective solution to preventing Cr-poisoning. In this study, the application of a protective CuMn1.8O4 spinel coating is explored. Dense coatings are deposited on both metallic flat plates and meshes by electrophoretic deposition followed by thermal densification steps. The coating is found to be a mixture of Mn3O4 and cubic spinel phases at room temperature but is a pure cubic spinel phase between 750 °C and 850 °C. A reaction layer between the Cr2O3 scale at the coating/interconnect interface and CuMn1.8O4 coating is found to be a mixture of (Cu,Mn,Cr)3-xO4 cubic spinel phases with Cr-rich precipitates believed to be Cr2O3, indicating that the coating layer acts as a Cr getter. Solubility experiments show that 1 mol of the CuMn1.8O4 phase can getter at least 1.83 mol of Cr2O3 at 800 °C. Electrochemical testing of cells in the presence of coated interconnects show that the CuMn1.8O4 coating getters Cr effectively for 12 days at 800 °C, leading to no performance loss of the cell due to Cr-poisoning.

Determination of the oxide scale growth mechanism using 18O-tracer experiments in combination with Transmission Electron Microscopy and nanoscale Secondary Ion Mass Spectrometry

Two-stage 18O2/16O2 exposures can be used to investigate the effect that alloying elements, secondary phases, or surface treatments have on the high temperature oxidation behaviour of certain materials. During subsequent exposures to 16O2- and 18O2-rich atmospheres, 16O- and 18O-rich layers are formed. Analysis of the layers using Secondary Ion Mass Spectrometry (SIMS) depth profiling allows for conclusions to be drawn about the oxide scale growth mechanism. The conclusions are, however, not entirely unambiguous due to the limited lateral resolution of the technology. Rough surface topography and the thickness variation of the oxide scale over the analysed volume add to the ambiguity of the findings. In this study, an Fe-20%Cr alloy was exposed to both 18O- and 16O-rich environments at 850°C. Two methods were used to analyse the thermally grown Cr2O3 scale: (1) traditional SIMS depth profiling and (2) preparation of a cross-sectional lamellae for Transmission Electron Microscopy (TEM), which, subsequently, was analysed in a NanoSIMS. The NanoSIMS 16O and 18O elemental maps were then superimposed on the TEM image. In comparison with traditional SIMS depth profiling, the nanoSIMS elemental maps reveal detailed information about local oxide growth in different parts of an oxide scale. Moreover, a clear 16O/18O interface can be seen in the nanoSIMS maps, which is not the case in the sputter depth profiles. The findings of this study show that the aforementioned issues associated with sputter depth profiling can be eliminated by mapping a cross-section of an oxide scale using high resolution nanoSIMS.

Simultaneous oxidation and carburization of a Fe-9Cr alloy under different oxygen pressures at 800 °C

A model Fe-9Cr alloy was exposed to various H2CO2CH4 mixtures providing different oxygen pressures and carbon activities at 800°C. Three oxidation modes and three oxygen pressure ranges have been observed. At low pressures (Range I) only thin external Cr2O3 scales formed. An intermediate range (Range II) produced an internal precipitation of Cr2O3 and Cr carbide without an external scale. Higher pressures (Range III) produced external FeOx scales plus an internal oxidation of Cr. Two critical oxygen pressures for the transitions between the three oxidation modes were observed and also calculated in reasonable agreement with the experimental results.

Degradation of ferritic stainless steels at 1200 °C in air

Three commercial ferritic stainless steels were investigated at 1200 °C by a thermogravimetric analyser (TGA) in air. The oxidation kinetics of the ferritic stainless steels differed significantly. The adhesion of the Cr2O3 scale, the morphology of the SiO2, with or without (Cr, Mn)3O4 spinel on the top can greatly influence the oxidation and the degradation behaviour of the steels. Although the SUS430 steel had less Cr among the ferritic stainless steels (16.2 wt% Cr), it did not show more degradation behaviour than the B443NT steel (21.0 wt% Cr). The spallation of the protective oxide scale on the B443NT steel was caused by vacancy condensation at the scale/substrate interface where the SiO2 particles were and the developed compressive stresses within the oxide scale during oxidation. (Cr, Mn)3O4 spinel on the top of the Cr2O3 scale on the B445J1M steel influenced its evaporation rate. The thermodynamic aspect of the chemical composition and oxidation atmosphere of the Fe–Cr–O system was also discussed.

Evaluation of electroplated copper coating on ferritic stainless steel for solid oxide fuel cells interconnects

A Cu coating was deposited on SUS 430 ferritic stainless steel by a low-cost method of electroplating which was employed for the protection of solid oxide fuel cell metallic interconnects. After oxidation, a two-layer oxide scale consisting of an external layer of CuO and an internal layer of Cr2O3 was primarily formed on the coated steel, and a spinel layer consisting of Cu, Cr, Fe and Mn was formed at the CuO/Cr2O3 interface. The CuO outer layer not only suppressed the Cr2O3 scale growth but also served as an effective obstacle against Cr outward diffusion. The coated steels had lower scale area-specific resistances (ASRs) compared to the uncoated steel. The oxidation mechanism and surface scale electrical properties of the coated steel were discussed.

Strains in Thermally Growing Cr<sub>2</sub>O<sub>3</sub> Films Measured <i>In Situ</i> Using Synchrotron X-Rays

This work takes place in the general context of a better understanding of materials degradation mechanisms in extreme environments. In particular, the aim of the present study was to correlate microstructural elements to growth stress magnitude evolution and stress release mechanisms for thermally grown chromia thin films on NiCr alloys. Strains in thermally grown oxides have been measured in-situ, as the oxides develop and evolve. Data have been acquired from oxides grown for different high temperatures evolutions on NiCr model alloys that form Cr2O3. Using synchrotron X-Ray at the ESRF (Beamline BM02) coupled with an induction furnace, Debye-Scherrer diffraction patterns from the oxidizing specimen were recorded in air at temperature between 700-1000°C and during cooling. The distortion of the diffraction rings was analyzed to yield the in-plane strain. Thermal stresses imposed on Cr2O3/NiCr by abruptly reducing the sample temperature for a period of time, exploiting the thermal expansion difference between oxide and substrate, showed noticeable subsequent stress relaxation by creep. Such a mechanism was monitored using time-dependent in situ measurements of strain relaxation in the oxide. The main results obtained from these experiments are the kinetic of the growth stress from the isothermal measurements (isothermal plateau), and the study of the stress release mechanism after the low-temperature jumps. In complement, the oxide microstructure development during the course of oxidation is also investigated from both the peaks intensity and width evolution. In all cases, the steady stage growth strain was relatively low and compressive. Different degrees of relaxation were also found on cooling depending on scale microstructure through the variation of the initial oxidation conditions. Results are compared with other reports of residual stresses evolution in Cr2O3 scales.

Oxidation behavior of NiCrAlY coatings prepared by arc ion plating using various substrate biases: Effects of chemical composition and thickness of the coatings

NiCrAlY coatings with various Cr and Al contents were prepared on Ni-based superalloy K438G from a single NiCrAlY target by adjusting the substrate bias during high vacuum arc ion plating. The oxidation behavior of the coatings follows the oxide map laws proposed by Wallwork and Hed rather than the guide line based on Cr/Al=4. As the coatings degrade due to Al depletion, formation of Cr2O3 scales beneath the external α-Al2O3 scales was observed. The mechanism for the oxidation of Ni-Cr-Al alloys is discussed.

Long Term (4 Years) Performance of Co/Ce Coated 441 for SOFC Interconnect Applications

Ferritic stainless steel interconnectors are widely used in solid oxide fuel cells (SOFC) due to a combination of low cost, compatible thermal expansion properties and ease of manufacturing. However, their viability is hindered by some key technical hurdles. Most stainless steels suggested for SOFC applications rely on the formation of a fairly protective Cr2O3 scale. However, in air side environments Cr species evaporation leads to material failure and insufficient lifetimes. Coatings offer a way to mitigate Cr evaporation and in some cases decrease oxide scale growth. Numerous research papers have investigated different coating compositions and deposition techniques, however, the most commonly suggested material is (Mn,Co)3O4 (MCO). Instead of depositing MCO metallic Co can be applied. During operation Co is converted into Co3O4 and subsequently enriched in Mn from the underlying steel substrate. It has been shown that this type of coating effectively impedes Cr volatilization, even when only very thin layers (600 nm) are applied. Moreover the major advantage of metallic Co coatings are cost savings, since Co can be applied in a large scale roll-to-roll process before stamping the interconnect into its final shape. Furthermore, our previous work has shown that an additional layer of 10 nm Ce in combination with the Co coating results in the aforementioned mitigation of volatilization of Cr species, and also effectively reduces oxide scale growth. Oxide scale growth is detrimental for two reasons. First, a depletion of oxide scale forming element (Cr) can result in the formation of poorly protective iron oxide. Second, even a protective Cr2O3 scale grows with time. Since Cr2O3 is only a moderate electronic conductor this results in an increase of Area Specific Resistance (ASR) as the oxide scale grows in thickness. This eventually creates prohibitively high resistances. Both effects can be mitigated by addition of a thin Ce layer. In the present work Ce/Co (10/600 nm) coated AISI 441 has been exposed for 4 years (35 000 h) at 800 °C. The mass gain of the samples over the course of exposure has been recorded at selected intervals. After 35 000 h a mass gain of 3.76 mg/cm2 has been measured. Over the entire exposure parabolic mass gain kinetics are observed indicating the absence of a poorly protective iron oxide. After 35 000 h the samples were moved to a different setup to measure Cr(VI) evaporation. It was found that despite the fact that the coating was only 600 nm in thickness, it effectively inhibits volatilization of Cr even after 4 years of exposure. Furthermore, samples were removed at selected intervals to measure ASR.

Isothermal oxidation behavior and mechanism of a nickel-based superalloy at 1000°C

The oxidation behavior of a nickel-based superalloy at 1000°C in air was investigated through X-ray diffraction, scanning electron microscopy, and energy-dispersive spectroscopy analysis. A series of oxides, including external oxide scales (Cr2O3, (TiO2 + MnCr2O4)) and internal oxides (Al2O3,TiN), were formed on the surface or sub-surface of the substrate at 1000°C in experimental still air. The oxidation resistance of the alloy was dependent on the stability of the surface oxide layer. The continuity and density of the protective Cr2O3 scale were affected by minor alloying elements such as Ti and Mn. The outermost oxide scale was composed of TiO2 rutile and MnCr2O4 spinel, and the growth of TiO2 particles was controlled by the outer diffusion of Ti ions through the pre-existing oxide layer. Severe internal oxidation occurred beneath the external oxide scale, consuming Al and Ti of the strength phase γ′ (Ni3(Al,Ti)) and thereby severely deteriorating the surface mechanical properties. The depth of the internal oxidation region was approximately 35 μm after exposure to experimental air at 1000°C for 80 h.

Development of Robust Metal‐Supported SOFCs and Stack Components in EU METSAPP Consortium

AbstractThe potential of MS‐SOFCs was demonstrated through the previous EU METSOFC project, which concluded that the development of oxidation resistant novel metal‐supported solid oxide fule cell (MS‐SOFC) design and stack is the requirement to advance this technology to the next level. The following EU METSAPP project has been executed with an overall aim of developing advanced metal‐supported cells and stacks based on a robust, reliable and up‐scalable technology. During the project, oxidation resistant nanostructured anodes based on modified SrTiO3 were developed and integrated into MS‐SOFCs to enhance their robustness. In addition, the manufacturing of metal‐supported cells with different geometries, scalability of the manufacturing process was demonstrated and more than 200 cells with an area of ∼150 cm2 were produced. The electrochemical performance of different cell generations was evaluated and best performance and stability combination was observed with doped SrTiO3 based anode designs. Furthermore, numerical models to understand the corrosion behavior of the MS‐SOFCs were developed and validated. Finally, the cost effective concept of coated metal interconnects was developed, which resulted in 90% reduction in Cr evaporation, three times lower Cr2O3 scale thickness and increased lifetime. The possibility of assembling these cells into two radically different stack designs was demonstrated.

Effect of the batch annealing atmosphere on the oxide scale of hot-rolled AISI 430 stainless steel

Abstract The main purpose of this experiment is to study the effect of batch annealing atmospheres on oxide scales by annealing in air, hydrogen and nitrogen at 1123 K for 72, 90 and 108 ks. Grazing incidence x-ray diffraction (GIXRD) and energy dispersive X-ray (EDX) were used to characterize oxide scales after batch annealing process. The morphology and oxide scale of the studied specimens were investigated by scanning electron microscopy (SEM) and optical microscope. Result obtained by GIXRD showed Fe3O4 and spinel FeCr2O4 oxide scales on the as-received hot-rolled specimens. After batch annealing in air, Fe2O3 and (Fe,Cr)2O3 oxide scales were found. Whereas after batch annealing in hydrogen, Fe3O4 and spinel FeCr2O4 oxide scales were found. In addition, after batch annealing in nitrogen, Fe3O4, (Fe,Cr)2O3 and spinel FeCr2O4 oxide scales were seen. Oxide scales after batch annealing in air are thicker than the scales after batch annealing in hydrogen, however thinner than those scales after batch annealing in nitrogen. The oxide scales thickness after batch annealing in hydrogen is quite equivalent to that found in the as-received hot-rolled oxide scales. Cracks were observed perpendicular to the rolling direction. Weight change after batch annealing in air is higher than that after batch annealing in nitrogen and hydrogen, respectively.

Isothermal Oxidation Comparison of Three Ni-Based Superalloys

Ni-based superalloys are used for high-temperature components of gas turbines in both industrial and aerospace applications due to their ability to maintain dimensional stability under conditions of high stress and strain. The oxidation resistance of these alloys often dictates their service lifetime. This study focuses on the isothermal oxidation behavior of three Ni-based superalloys, namely, polycrystalline cast IN738LC, single-crystal N5, and a ternary Ni-Fe-Cr (TAS) powder metallurgy alloy. The isothermal oxidation tests were conducted at 900 °C in the static air up to 1000 h, and the specific aspects studied were the oxidation behavior of these chromia-forming and alumina-forming alloys that are used extensively in industry. In particular, the behavior of oxide scale growth and subsurface changes were analyzed in detail using various techniques such as SEM, EDS, and AFM. From the isothermal oxidation kinetics, the oxidation rate constant, k p, was calculated for each alloy and found to be; k p = 2.79 × 10−6 mg2 cm−4 s−1 for IN738LC, k p = 1.42 × 10−7 mg2 cm−4 s−1 for N5 and k p = 1.62 × 10−7 mg2 cm−4 s−1 for TAS. Based on a microstructural analysis, IN738LC exhibited a continuous dense outer scale of Cr2O3 and discontinuous inner scale of Al2O3, whereas N5 and TAS showed a dense outer scale of Al2O3 alone. The results suggested that the N5 and PM-TAS alloys are more oxidation resistant than the IN738LC under these conditions.