Parabolic Rate Research Articles

Microstructure evolution formed during early liquefaction stage between pre-oxidized Zircaloy 4 with SUS316 stainless steel at 1573 K

ABSTRACT The reaction between Zircaloy and stainless steel is a key interaction in the core degradation process during a severe accident in a light water reactor because this combination leads to eutectic liquefaction. In this study, the reaction mechanism and rate-limiting process were experimentally investigated at 1573 K where the eutectic liquefaction was observed dominantly. The diffusion couple of pre-oxidized Zircaloy4 and SUS316 stainless steel were annealed for various holding times, and the interface microstructure was metallographically examined. The reaction layer consisted of five phases: the α-(Fe,Cr,Ni) phase, the metastable (Fe,Cr,Ni)23Zr6 phase, the Laves Zr(Fe,Cr,Ni)2, α-Zr(O), and the liquid phase. The reaction layer thickness in the Fe-rich side consisting of α-(Fe,Cr,Ni) and (Fe,Cr,Ni)23Zr6 obeyed the time parabolic rate law, while the ones involving the liquid phase formation followed the saturation-type convection-controlled function. A formula in a combination of diffusion and convection process was introduced for estimation of reacted volume, which showed good agreement with the experimental results. It was newly realized that a model to provide the mass transfer coefficient regarding the convection-controlled process would be required for improvement of the core degradation model.

An in-situ oxide-dispersion-strengthened AlCoCrFeNiY high-entropy alloy composite coating prepared by AC-HVAF with superior oxidation and spallation resistance

We report an in-situ oxide-dispersion-strengthened AlCoCrFeNiY high-entropy alloy composite coating (ODS-HEA composite coating) with exceptional oxidation and spallation resistance at 1100 °C and 1150 °C. The coating is manufactured by activated combustion-high velocity air fuel (AC-HVAF) and has an ultrafine-grained microstructure that comprises γ, β and nanoscale oxide particles (Al2O3, Cr2O3 and Y2O3) formed in-situ in coating processing. The thermally grown oxides (TGOs) on the coating at 1100 °C and 1150 °C are exclusively α-Al2O3 with a columnar grain structure. The parabolic rate constant of the coating is about half of that of the conventional NiCoCrAlY coating (CNA coating) at both 1100 °C and 1150 °C, respectively. The lower TGO growth rate is primarily attributed to its coarser TGO grains that reduce the efficient oxygen diffusivity. Compared to the CNA coating, a higher TGO/coating interfacial toughness is achieved via inhibiting the formation of interfacial pores for ODS-HEA composite coating. The low TGO growth rate decreases the driving force for TGO spallation and the high interfacial toughness increases the TGO adhesion, thus contributing to the strong spallation resistance for the ODS-HEA composite coating. The superior oxidation and spallation resistance of the ODS-HEA composite coating at 1100 °C and 1150 °C make it a candidate bond coat for advanced thermal barrier coatings.

Microstructure, mechanical properties and lead-bismuth eutectic corrosion behavior of FeCrAlTiNb high-entropy alloy coatings

The FeCrAlTiNb high-entropy alloy coatings are fabricated by magnetron sputtering method to prevent the lead-bismuth eutectic (LBE) corrosion of structural components used in lead-cooled fast reactors, and their microstructure, mechanical properties and LBE corrosion behavior are investigated systematically. The coatings exhibit significantly improved hardness, creep and wear resistance compared with ferritic/martensitic (F/M) steel. Moreover, the coatings show promising LBE corrosion resistance with parabolic oxidation rate constant 3–5 orders of magnitude lower than other candidate steels, which is ascribed to their intrinsic amorphous structure and the formation of protective FeCr2O4/Cr2O3 double-layer. Finally, the growth of oxide scale is discussed in detail.

A fluid-flow approach in oxidation evaluation of SUS 430 steels coated by Fe/Mn-modified MnCo2O4 for solid oxide fuel cell application

The Fe/Mn-modified Mn–Co spinel oxides (MCO) are coated on SUS430 interconnect to protect against oxidation and chromium diffusion by wet spray followed by nitrate solution impregnation for intermediate-temperature solid oxide fuel cell (IT-SOFC) application. 1000 h oxidation tests at 800 °C show that the FeMCO-coated sample have the minimum parabolic rate constant (9.02 × 10−7 mg2cm−4s−1), which is lower than half of the uncoated sample. The area-specific resistance (ASR) of the FeMCO-coated sample is 6.41 mΩcm2 at 800 °C after 1000 h oxidation, which is much lower than the MCO sample (19.38 mΩ cm2) and the uncoated sample (186.84 mΩ cm2). The samples microstructures are examined by scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). Based on a novel approach, the dimensionless fluid numbers are related to the high-temperature conductivity of coated interconnects, and the inward diffusion of oxygen is introduced as the limiting factor for oxide-scale growth of coated interconnects.

Evolution of intermetallics between solid Fe-Cr/Fe-Ni alloys and molten aluminium

Investigations of the evolution of intermetallic compounds (IMCs) at the interface between iron-based alloys and molten aluminium have important technical and scientific significance in many key areas. Most previous studies of IMCs formed at molten aluminium/solid steel interfaces have been focused on relatively long reaction times, but less research has been performed on the evolution of IMCs at the aluminizing interfaces of Fe-Cr and Fe-Ni alloys within a few seconds. In this study, hot-dip aluminizing experiments under natural convection conditions were employed to study the evolution of two interfacial IMCs (Fe-15Cr alloy and Fe-35Ni alloy with molten aluminium) at 700–900°C within 30 s. The IMCs for Fe-15Cr consisted of an Fe4Al13 layer next to the aluminium and an Fe2Al5 layer adjacent to the steel. In contrast, only one layer of Fe4Al13 was detected at the interface between the Fe-35Ni alloy and molten aluminium. The growth of Fe2Al5 was described by a parabolic rate law. An activation energy of 7.53 kJ/mol was determined for the temperature range of 700–900°C. The evolution of Fe4Al13 for the two alloys was similar: their thicknesses decreased anomalously with increasing dipping temperature, and their kinetics were controlled by both diffusion and dissolution. In this study, based on reaction-diffusion theory, growth and dissolution equations of the IMCs were established, and the diffusion coefficients of Al atoms in Fe2Al5 and Fe4Al13 were obtained for the first time. The similarities and differences in the IMCs for the two alloys were explained mainly by the different diffusion coefficients.

Predicting the parabolic growth rate constant for high-temperature oxidation of steels using machine learning models

The parabolic growth rate constant (kp) of high-temperature oxidation of steels is predicted via a data analytics approach. Four machine learning models including Artificial Neural Networks, Random Forest, k-Nearest Neighbors, and Support Vector Regression are trained to establish the relations between the input features (composition and temperature) and the target value (kp). The models are evaluated by the indices: Mean Absolute Error, Mean Squared Error, Root Mean Squared Error and Coefficient of Determination. The steel composition regarding Cr and Ni content and the temperature were the most significant input features controlling the oxidation kinetics.

High-temperature oxidation behavior of Cu64Zr36 metallic glass powders

Cu64Zr36 metallic glass powders were prepared via gas atomization. The high-temperature oxidation behavior of Cu64Zr36 metallic glass powders under the influence of internal factors (powder particle size) and external factors (oxidation temperature and oxidation time) was experimentally investigated by using XRD, DSC, TGA, SEM and TEM. The oxidation kinetics, oxide phase, surface morphology and microstructure of metallic glass powders were analyzed. The results show that Cu64Zr36 metallic glass powders form multiple layers of surface oxides after oxidation and the major components of the surface oxides are t-ZrO2, m-ZrO2, Cu2O and CuO. The oxidation kinetics follows a double-stage parabolic rate law. The oxidation process is dominated by the inward diffusion of O anions and the outward diffusion of Cu cations. Thermodynamic analysis shows that the Gibbs free energy change of the oxidation reaction increases with the decrease of particle size, leading to a more rapid and thorough oxidation of the powder with a smaller particle size.

Effects of boron addition on the high temperature oxidation of MoSi2 alloys

Boron containing MoSi2 is a promising material for applications at high temperature, but the oxidation mechanism is still unclear. In this work, the high temperature (1100 °C) oxidation of B doped MoSi2 in synthetic air has been investigated. A (boro)silicate layer is formed on the surface of the alloy, which features a mixture of amorphous SiO2 and cristobalite. After an initial transient period, the oxidation kinetics follows a parabolic growth rate law. The growth rate constant of the oxide layer is enhanced by the boron in the alloy by 90 % per at.% B. The increase in growth rate is associated with boron mitigating the formation of cristobalite thereby promoting the formation of amorphous SiO2.

High-temperature oxidation resistance of Cu–0.65 wt-% Mg alloy

ABSTRACT A copper 0.65 wt-% magnesium alloy was oxidised in the air from 700 to 950°C, in a TGA/DTA apparatus and a muffle furnace. The alloy oxidises following the parabolic rate law. Oxidation of this material is diffusion-controlled. A second experimental was conducted to determine the extent of oxidation by measuring the thickness of the internal oxidation layer. The amount of magnesium added to the copper prevents its oxidation up to 800°C. At higher temperatures, more magnesium is needed to slow down the rate of oxidation of copper. Furthermore, the oxidation layer of the Mg–Cu alloy can be estimated using a kinetic model designed by us for alloy oxidation.

Ablation behaviors and mechanism of ZrC-SiC-Si/SiC-Si double-layered coatings on C/C composite under plasma flame at 3000 °C

To improve the ablation performance of C/C composite at 3000 °C, a ZrC-SiC-Si/SiC-Si double-layered coatings (ZSS/SS) was prepared by slurry dipping-carbonization and gaseous silicon infiltration. During ablation, a competition between mechanical denudation and oxidation derived thickening occurred. The ZSS/SS coating possessed a superior ablation resistance showing lowest linear ablation rate of 0.53 µm/s in 90 s with a parabolic rate constant Kp (0.053 µm/s3). The excellent performance was attributed to good self-sealing of oxidation derived glassy, thus no micro-cracks were observed. However, the Kp became worse to be 0.131 for long-term ablation because of dominant mechanical denudation.

The Effect of Dopants on Structure Formation and Properties of Cast SHS Alloys Based on Nickel Monoaluminide.

Alloys based on NiAl-Cr-Co (base) with complex dopants (base+2.5Mo-0.5Re-0.5Ta, base+2.5Mo-1.5Re-1.5Ta, base+2.5Mo-1.5Ta-1.5La-0.5Ru, base+2.5Mo-1.5Re-1.5Ta-0.2Ti, base+2.5Mo-1.5Re-1.5Ta-0.2Zr) were fabricated by centrifugal SHS metallurgy. The phase and impurity compositions, structure, mechanical properties, and the mechanism of high-temperature oxidation at T = 1150 °C were studied; the kinetic oxidation curves, fitting equations and parabolic rate constant were plotted. Al2O3 and Co2CrO4 were the major phases of the oxidized layer. Three layers were formed: I-the continuous Al2O3 layer with Co2CrO4 inclusions; II-the transitional MeN-Me layer with AlN inclusions; and III-the metal layer with AlN inclusions. The positive effect of thermo-vacuum treatment (TVT) on high-temperature oxidation resistance of the alloy was observed. The total weight gain by the samples after oxidative annealing decreased threefold (from 120 ± 5 g/m2 to 40 ± 5 g/m2). The phases containing Ru and Ti microdopants, which reduced the content of dissolved nitrogen and oxygen in the intermetallic phase to the values ∑O, N = 0.0145 wt.% for the base+2.5Mo-1.5Ta-1.5La-0.5Ru alloy and ∑O,N = 0.0223 wt.% for the base+2.5Mo-1.5Re-1.5Ta-0.2Ti alloy, were identified by transmission electron microscopy (TEM). In addition, with the significant high-temperature oxidation resistance, the latter alloy with Ti had the optimal combination of mechanical properties (σucs = 1644 ± 30 MPa; σys = 1518 ± 25 MPa).

Feeder Pipe Oxidation in the Presence of Steam During a Nuclear Reactor Accident

A Canada Deuterium Uranium (CANDU) reactor has hundreds of carbon steel feeder pipes that are connected at one end to the fuel channels at the reactor face and are connected at the other end to the reactor inlet and outlet headers above the reactor. The production of hydrogen gas from metal–water interaction at high temperatures is a major concern during a severe accident in a nuclear reactor. It is generally accepted that the main source of hydrogen gas during a severe accident is the chemical interaction of Zircaloy fuel cladding and the water coolant. However, it has recently been suggested that the amount of hydrogen produced by the oxidation of carbon steel located outside of a CANDU core could exceed that produced by zirconium oxidation. In this work, the linear and parabolic oxidation rate constants of CANDU feeder carbon steel, in the temperature range of 600–1,100°C, were measured to be W/t = 1.806 × 103 exp (− 126,337/RT) and W2/t = 1.879 × 105 exp (− 135,835/RT), respectively. For parabolic steam oxidation beginning at roughly 90 minutes after reaching steady-state heating temperatures, the latter equation should be used. These rates are about a factor of 5–10 greater than oxidation rates measured in 304L stainless steel and in zirconium specimens, meaning that the carbon steel oxidation can be significant in an accident.

Comparative Evaluation of Oxidation Resistance of Detonation Gun-Sprayed Al2O3–40%TiO2 Coating on Nickel-Based Superalloys at 800 °C and 900 °C

High-temperature oxidation is the major degradation mechanism of boiler and gas turbine components. Superalloys are used in these applications because of their superior mechanical properties and creep resistance, but they lack resistance to oxidation under aggressive environment. In the present investigation, Al2O3–40%TiO2 coating was deposited on Superni718 and Superni600 superalloys by a detonation gun spray process. The as-sprayed coating was characterized by SEM analysis. The high-temperature oxidation behavior of Superni718 and Superni600 and Al2O3–40%TiO2 coating on these superalloys was evaluated in air under cyclic conditions at 800 °C and 900 °C for a total duration of 50 cycles. Each cycle consisted of keeping the samples for one hour at the elevated temperature followed by 20 min cooling in ambient air. Oxidation kinetics were studied using a thermo-gravimetric technique and found to follow the parabolic rate law. The coating was successfully formulated using a detonation gun spray process. The coating was found to be adherent to the substrate superalloys. It was found that among the two alloys, uncoated and coated Superni600 superalloys have much better oxidation resistance as compared to uncoated Superni718 superalloy.

High-temperature oxidation and TGO growth behaviors of laser-modified YAG/YSZ double-ceramic-layer TBC

Yttrium—aluminum garnet/yttria-stabilized zirconia (YAG/YSZ) double-ceramic-layer thermal barrier coating (TBC) with NiCoCrAlY bond coating was deposited by atmospheric plasma spraying (APS) on Inconel 738 alloy substrate, and laser modification was carried out on the surface of YAG ceramic coating to improve the high- temperature oxidation resistance of TBC. Then, isothermal oxidation tests were designed to study the high-temperature oxidation and thermally grown oxide (TGO) growth behaviors of the as-sprayed (AS) and the laser modified (LM) YAG/YSZ TBC. Results show that TGO thickness and structure of both AS-TBC and LM-TBC exhibit the similar evolution trend, TGO thickness increases with the increasing of isothermal oxidation time, and TGO structure develops from single Al2O3 layer to the double-layer structure of upper mixed oxides and lower Al2O3. Because of the crucial inhibition effect of the laser modification on oxygen permeation, the appearance time of mixed oxides in LM-TBC is postponed, and the total TGO thickness of LM-TBC is reduced at the middle and final oxidation stages. The Al2O3 layer thickness proportion of total TGO in LM-TBC is always greater than or equal to that in AS-TBC at the same isothermal oxidation time. Compared with the AS-TBC, the parabolic oxidation rate of LM-TBC is decreased by 18.42%. Therefore, YAG/YSZ LM-TBC presents better oxidation resistance and lower TGO growth rate than AS-TBC.

Low-cost surface modification of a biomedical Zr-2.5Nb alloy fabricated by electron beam melting

• Thermal oxidation is an effective way to prepare oxide layer on a Zr-2.5Nb alloy fabricated by electron beam melting (EBM). • The morphology, phase constituents, and surface color of oxide layer change in the oxidation process. • Grain refinement significantly delays the breakaway oxidation. • A dense and black oxide layer with a hardness of 902 HV is prepared on a EBM- built Zr-2.5Nb alloy by oxidation at 600°C for 3 hours. The Zr-2.5Nb alloy with a fine microstructure consisting of α laths was successfully prepared by electron beam melting (EBM). The thermal oxidation behaviors and kinetics of the as-built, and the EBM-built and hot isostatically pressed (HIPed) Zr-2.5Nb materials in a temperature range of 450–600°C were investigated and compared with those of the alloy prepared by conventional casting and forging. It was found that the oxidation kinetics of the as-built and the forged materials followed the parabolic rate law during isothermal oxidation at 550°C, but the HIPed materials exhibited a parabolic-to-linear kinetic transition, suggesting that the larger grain sizes enhanced the oxidation. The oxide layers of all materials were composed of a large fraction of monoclinic zirconia phase ( m -ZrO 2 ) and a small fraction of tetragonal zirconia phase ( t -ZrO 2 ), and transformed from t -ZrO 2 to m -ZrO 2 with increasing oxidation time. The surface hardness of the as-built, the forged and the HIPed materials increased from 215, 204, and 188 HV before oxidation to 902, 1070, and 1137 HV after oxidation, respectively. The cross-sections of the materials showed the presence of micropores and microcracks inside the oxide layers with thicknesses ranging from 4 to 8 µm. With the oxidation temperature of 600°C and oxidation time duration of 3 h, a dense black m -ZrO 2 oxide layer with smooth surface and 902 HV hardness was obtained on the EBM as-built Zr-2.5Nb materials.

Fabrication of Ni-Co film for enhancing the high-temperature corrosion resistance of interconnects in solid oxide fuel cells (SOFCs)

ABSTRACT Low-cost Ni-Co alloy coating was performed by electroplating technique to improve the high-temperature corrosion resistance of stainless steel interconnects in solid oxide fuel cells (SOFCs). The effect of plating on the surface morphology, microstructure and composition of Ni-Co alloys was investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD) and atomic force microscopy (AFM). The Ni-Co coatings were fabricated at thickness of around two microns and compared at different Ni-Co molar ratios. Corrosion performance of Ni-Co coating was evaluated in a muffle furnace under ambient atmosphere at 800℃ for 110 h. Co2NiO4 spinel oxides and Cr2O3 were found on the Ni-Co coated samples after the oxidation test. Results indicated that steel substrate corrosion protection improved with a low parabolic rate constant of 20 times after Ni-Co alloy electrodeposition coating. A smooth coating layer containing Co2NiO4 oxide with fewer defects promoted high oxidation resistance of the steel samples.

Oxidation and Scale Adhesion of a Type 430 Stainless Steel in Ar–CO2 Gas Mixtures at 800 °C

Biogas is an alternative source of fuel potentially used to run solid oxide fuel cells (SOFCs). It mainly consists of CH4 and CO2 which can degrade the SOFC interconnect, which is typically made of stainless steel. To investigate the effect of each gas constituent, we focussed here on the effect of CO2 on high-temperature oxidation behavior of and scale adhesion on the stainless steel interconnect, Type 430 stainless steel. The samples studied were oxidised in CO2 at contents of 5–100% at 800 °C. The oxidation kinetics were found to be parabolic with the rate constant increasing when the CO2 content increased. The scale adhesion was assessed using a tensile-test method. The scale formed in the atmosphere containing higher CO2 content exhibited poorer scale adhesion, as indicated by a lower strain initiating the first spallation and a larger spallation percentage after the first spallation took place. The worsened scale adhesion relates to pores formed at the scale/steel interface. The adhesion energies were further quantified giving the values of about 40–100 J m–2. Oxidation mechanisms were suggested based on the dependence of the parabolic rate constant on the oxygen partial pressure and the inward diffusion of carbon-bearing species.

Evaluation of Short-Term Oxidation Mechanism of Laser Cladded Ni–Cr–C Coating on Titanium Aluminide Substrate

In the present investigation, a NiCr–Cr3C2 powder mixture was deposited on titanium aluminide substrate by laser cladding. The kinetics and mechanism of oxidation of the substrate and coating were studied at 800–1100 °C using XRD, SEM, ToF–SIMS and XPS characterization techniques. The results demonstrated that the Ni–Cr–C coating had improved oxidation resistance, as compared to the substrate. The oxidized surface of the coating was covered with Cr2O3 and NiCr2O4 phases at the early stages of the oxidation. The formation of NiCr2O4 could be attributed to the reaction of NiO (an oxidation product of the NiCr phase) and Cr2O3 (an oxidation product of the chromium carbide phase) during oxidation. Increasing the oxidation time at 900 °C led to the partial formation of CrO3 as a result of the oxidation of Cr2O3. It was also found that the parabolic rate constant ratio of $$\left( {K_{{\text{p}}}^{{{\text{substrate}}}} /K_{{\text{p}}}^{{{\text{coating}}}} } \right)$$ increased with the temperature, which showed, comparatively, the better oxidation resistance of the coating at higher temperatures.

Oxidation behavior of Ti2AlC MAX-phase foams in the temperature range of 600–1000 °C

The isothermal oxidation behavior of Ti2AlC gel-cast foams with an almost completely open porosity of 87 vol% was investigated in the temperature range of 600–1000 °C, in static air, and for exposure times of up to 6.5 h. It was found that the oxidation process obeyed the parabolic rate law. The determined parabolic oxidation rate constants were low, indicating that the material’s resistance to oxidation at high temperatures was very good, especially given its highly porous form. The nature of the scale formed on the strut surface of Ti2AlC foams changed significantly with increasing oxidation temperature. After oxidation at 600 °C, gaps/cracks were observed between Ti2AlC layers on the strut surface, and the oxidation products were mixed Ti and Al oxides. At 700–800 °C, the scales were integrated and composed of Ti and Al oxides. At 900–1000 °C, the oxidation products were composed of large elongated plates of TiO2 (rutile) and small, needle-like Al2O3 (corundum) grains.

Superior oxidation resistance of the chemically complex but structurally simple Ti-Al-Ta-Ce-Si-La-B-nitride

Alloying (Ti,Al)N with Ta, CeSi2 or LaB6 is beneficial for both hardness and thermal stability. Here we show that their different mechanisms allow for a cumulative improvement when alloyed together, which is especially pronounced for the oxidation resistance. During isothermal oxidation treatments in ambient air at 900 °C (for up to 25 h), Ti0.44Al0.44Ta0.12N allows for a parabolic scale growth rate constant kp of 6.039 × 10-5 µm2/s. This is reduced to 2.074 × 10-5 and even 0.399 × 10-5 µm2/s when alloying with 2 mol% CeSi2 respectively CeSi2 + LaB6 (1 mol% each). The oxide scale growth kinetics for the latter can be even better described by a logarithmic or cubic law. In the as-deposited state the CeSi2 and CeSi2 + LaB6 alloyed (Ti,Al,Ta)N are single-phase fcc structured providing an indentation hardness of 32.6 ± 1.5 and 37.8 ± 1.5 GPa, combined with an indentation modulus of 496 ± 22 and 496 ± 14 GPa, respectively. After vacuum-annealing at 1000 °C, their hardness is still 33.0 ± 1.6 and 34.8 ± 1.1 GPa, and noticeable formation of hexagonal AlN and TaNx phases can only be detected by X-ray diffraction when annealed at temperatures above 1200 °C.Using CeSi2 and LaB6 instead of their elemental form is furthermore beneficial for the target production itself, as Ce and La are highly reactive.