Experimental studies and modeling of the oxidation of multiphase niobium-base alloys

A three-phase oxidation model has been developed for study the oxidation behavior of multiphase alloys. This model has been utilized to identify the oxidation behavior of three-phase Nb-base alloys and successfully predicted the critical value for the exclusive formation of CrNbO₄. It has shown that the exclusive formation of CrNbO₄ on the Nb-base alloys can be achieved by the volume fraction increasing and the particle size decreasing for NbCr₂ phase. Moreover, the barrier effect of Nb₅Si₃ phase on the diffusion of Cr in Nb-base alloys has also been deduced in this work.     

Degradation kinetics at 650 °C and lifetime prediction of Ni2Al3/Ni hybrid coating for protection against high temperature oxidation of creep resistant ferritic steels

The degradation kinetics of the Ni₂Al₃/Ni hybrid coating formed on creep resistant ferritic steel is studied by prolonged isothermal annealing experiments at 650 °C. The sequence of formation and subsequent consumption of all the intermediate phase layers are determined by intermittent analysis of the specimen at various annealing intervals. It is shown that the Ni₅Al₃ phase layer starts to form only when the outer Ni₂Al₃ phase layer is completely consumed and the growth kinetics of each of the intermediate layers differs from that of its subsequent consumption. A model is subsequently formulated to predict the lifetime of the coating studied.     

A simple model for the oxidation of carbon-containing composites

The isothermal oxidation mechanism of the carbon-containing composites has been investigated based on the experimental data reported in the literature. The results showed that the oxidation kinetics was affected not only by temperature and time but also by carbon content and the sample shape. For the oxidation kinetics, a series of quantitative kinetic models have been developed based on the controlling step. In this model, the effects of carbon content, sample size and temperature on the reaction fraction have been especially discussed. Incorporation of the experimental data into the new model indicates that a good agreement has been reached.     

Model of oxidation of SiC microparticles at high temperature

The oxidation kinetics of SiC microparticles has been studied from both experimental data and theoretical aspects based on recent research work. The effects of temperature, especially the particle size on the oxidation kinetics are explained not only qualitatively but also quantitatively based on our theory and model. The results show that our new model and the calculated results can reach a good agreement. It may also be seen that the important role of particle size in oxidation of SiC powder, especially in the nano era for SiC material.     

On the oxidation mechanism of Ni–Pt alloys at high temperatures

The oxidation mechanism of Ni–Pt alloys has been studied as a function of alloy composition, oxygen pressure and temperature. It has been found that the oxidation rate of all the alloys follows the parabolic rate law, being thus diffusion controlled. In agreement with Wagner’s theory, the slowest step of the overall oxidation rate of alloys with higher nickel content (?40 at.%) is determined by the outward diffusion of nickel cations in the growing NiO scale. On the other hand, the oxidation rate of alloys with a lower nickel content (<40 at.%) is governed by the solid state diffusion in the metallic phase.     

Platinum-catalyzed high temperature oxidation of metals

Samples of Al, Cr, Ni, and Zr were sputter-coated with porous Pt-films with a particle size of 20-30 nm. Thermal oxidation of these samples was studied by gas phase analysis (GPA) and secondary ion mass spectrometry (SIMS). SIMS analysis on partly Pt-coated samples of Al, Cr, Ni, and Zr at different oxide depths in areas with Pt and in areas away from Pt indicates an enhanced inward oxide growth in the Pt area and mm-ranged distance from Pt-area. Weight gain measurements on Pt-coated Ni samples show a reduced or increased oxidation rate depending on the amount of porous Pt-coating. Pt has two effects on the thermal oxidation of metals and the overall effect of Pt on the oxidation of metals depends on the mechanism of oxide growth in the absence of Pt.     

Oxidation protection of C/SiC coated carbon/carbon composites with Si–Mo coating at high temperature

To protect carbon/carbon (C/C) composites against oxidation, a Si–Mo coating was prepared on C/SiC-coated C/C composites by a simple slurry method. The microstructure of the coating was characterized by X-ray diffraction, scanning electron microscopy and Raman spectra. Results showed that the coating was mainly composed of SiC, MoSi₂ and Si. It could protect C/C composites from oxidation at 1873 K in air for 300 h and withstand 13 thermal cycles between room temperature and 1873 K. The excellent oxidation and thermal shock resistance of the coating was attributed to the formation of dense SiO₂ glass at high temperature. The volatilization of MoO₃ and SiO₂ at 1873 K was the main reason of the weight loss of the coated C/C composites.     

Comparative study of oxidation kinetics for pure nickel oxidized under tensile and compressive stress

The growth of oxide scales on pure nickel oxidized under tensile and compressive stress at 973 K was investigated. The objective was to understand the effect of mechanical loading on the oxidation kinetics. The oxidation kinetics curves gained by thermogravimetric analysis (TGA) indicate that the oxidation rate of pure nickel was accelerated by the external stress. Compared with the tensile stress, the effect of compressive stress on the oxidation kinetics was more pronounced. Surface morphologies examined by scanning electron microscopy (SEM) reveal that granular oxides formed on the stressed specimen, which increase the short-circuit diffusion.     

High temperature oxidation of AlCrFe complex metallic alloys

Isothermal oxidation of Al₆₅Cr₂₇Fe₈ and Al₈₀Cr₁₅Fe₅ was studied in the 600–1080 °C range. Formation of transient alumina layers is obtained up to 900 °C. On Al₆₅Cr₂₇Fe₈ transient to α-phase transformations occur when performing oxidation at 1000 °C, together with the possible appearance of (Al_0.9Cr_0.1)₂O₃. At 1080 °C, direct formation of α-alumina is obtained. On Al₈₀Cr₁₅Fe₅, spallation of the oxide layer during the cooling stage is observed following oxidation at 800 and 900 °C, revealing thermal etching of the underneath alloy surface. At 1050 °C the α-Al₂O₃ scale is directly formed but plastic deformation and recrystallization of the underneath alloy into several intermetallic phases is observed.     

Effect of cooling rate on oxidation resistance and powder ignition temperature of AM50 alloy with addition of yttrium

This paper focused on the effect of cooling rate on oxidation resistance and ignition temperature (T_i) of AM50 alloy. Y addition of 0.0 wt%, 0.15 wt%, 0.28 wt%, 0.45 wt% and 1.00 wt%, respectively was added to the AM50 alloy. The result showed that the oxidation resistance was directly affected by the microstructure. Rapid solidification (RS) had a positive effect on improving the oxidation resistance. It is noticeable that no Al₂Y intermetallic compound was found in the microstructure after RS. Elemental Y dissolved in the solid solution increased with increasing Y addition after RS. It is confirmed that Y addition dissolved in the solid solution and phase distribution were key factors for improving the oxidation resistance.     

Early stages of oxidation observed by in situ thermogravimetry in low pressure atmospheres

A highly sensitive set-up for in situ mass-gain measurements during high temperature reaction is presented. By removing the inert gas components of a corrosive atmosphere and keeping the flows and partial pressures of the reactive species constant, the experiment is preformed in a low-pressure environment. A better signal-to-noise ratio which increases sensitivity down to 10 μg/cm² is achieved. The random behaviour of the mass-gain signal, caused by turbulences in the gas stream, could be reduced by a factor of 10.Compared to theoretical simulations, the mass-change during oxidation in binary iron-based model alloys is slower than expected from pure bulk diffusion behaviour.     

Kinetics and mechanisms of non-isothermal oxidation of graphite in air

The non-isothermal oxidation kinetics and mechanisms of the graphite in air was studied by TG and SEM. The oxidation rate increases rapidly with the increase of temperature before 30% weight loss, then turns to steady, and decreases sharply with the increase of temperature after 85% weight loss. SEM observation shows that the oxidation starts from pores both in surface and interior, and the oxidation extent is nearly uniform through the whole body in the initial stage. Kinetics results represent the activation energy strongly depends on the weight loss. Combining SEM and kinetics analysis, the controlling mechanisms and activation energy of non-isothermal oxidation of graphite are obtained.     

High temperature oxidation behaviour of Ni–Fe–Co anodes for aluminium electrolysis

The oxidation behaviour of ternary Ni_xFe_yCo_z alloys (where x/y (wt) = 1 and 1.85; z = 0, 10, 30 and 50 wt.%) was studied in air at 800 °C. Alloys were found to follow complex oxidation kinetics, with the highest oxidation rates observed for alloys having 50 wt.% Co. Significant improvements in oxidation resistance were achieved by addition of 10 and 30 wt.% Co to the Ni–Fe system. The decrease in oxidation rate was associated with suppression of Fe₂O₃ formation in preference for (Co,Ni)_xFe_3−xO₄ growth. The results were discussed in light of the materials requirements for inert anodes for aluminium electrolysis.     

Oxidation properties of self-propagating high temperature synthesized niobium disilicide

NbSi₂ monoliths were prepared by self-propagating high temperature synthesis (SHS) and hot pressing (HP) and their oxidation behavior was investigated at various temperatures (823–1123 K) in air. The combustion mode of SHS reaction was steady state combustion, and the combustion product was single-phase NbSi₂. Oxidation studies show that the highest mass gain was 0.95675 kg m⁻² at 1023 K. In cyclic oxidation, the oxidation rate was reduced and the mass gain was only 0.15507 kg m⁻². A dense protective amorphous SiO₂ scale formed at 823 K and 923 K whereas a porous multilayer SiO₂ and α/β-Nb₂O₅ oxide scales formed at and above 1023 K and spalled off. Pest oxidation of NbSi₂ monoliths was not observed in hot pressed NbSi₂ monoliths.     

The oxidation kinetics of multi-walled carbon nanotubes

The kinetics of isothermal oxidation of multi-walled carbon nanotubes (MWNTs) in air has been investigated in the experimental range of 573-823 K using thermogravimetry. The results show that the oxidation of MWNTs is both chemically controlled and diffusion controlled depending on the temperature range. Chou’s model has been applied to treat the oxidation behavior of MWNTs and the comparison of the experimental data with the theoretical ones validates the applicability of the model. This model offers a quantitative expression for the weight loss as a function of time at constant temperature conditions. The possible mechanism for oxidation has also been discussed briefly.     

pH prediction in concentrated aqueous solutions under high pressure of acid gases and high temperature

An extended model for pH prediction in oil and gas environments has been developed. Accurate pH calculations for high pressure and high temperature applications depends mainly on CO₂ and H₂S partial pressures, the ionic strength, the chemical composition of the solution, and the temperature. Accounting for the non-ideal behaviors of liquid and gas phases allows pH calculations up to 200 °C, 2000 bar total pressure, and ionic strengths up to 5 mol L⁻¹. The results are consistent with experimental measurements and with other models reported in the literature.     

SiC nanowire-toughened MoSi2-SiC coating to protect carbon/carbon composites against oxidation

To protect carbon/carbon (C/C) composites against oxidation, a SiC nanowire-toughened MoSi₂-SiC coating was prepared on them using a two-step technique of chemical vapor deposition and pack cementation. SiC nanowires obtained by chemical vapor deposition were distributed random-orientedly on C/C substrates and MoSi₂-SiC was filled in the holes of SiC nanowire layer to form a dense coating. After introduction of SiC nanowires, the size of the cracks in MoSi₂-SiC coating decreased from 18 ± 2.3 to 6 ± 1.7 μm, and the weight loss of the coated C/C samples decreased from 4.53% to 1.78% after oxidation in air at 1500 °C for 110 h.     

The effect of temperature on the corrosion of steel in concrete. Part 2: Model verification and parametric study

A comprehensive model for predicting the corrosion rate of steel in concrete has been developed using the concept of simulated polarization resistance experiments. This model is developed by carrying out a nonlinear regression analysis on data obtained from numerical experiments that are based on the solution of Laplace’s equation in a domain determined by the polarized length of the rebar. This part of the paper provides a comprehensive verification of the developed model and illustrates the application of the model to investigate the coupled effects of parameters affecting corrosion of steel in concrete. The results of the verification study show that the model predictions are in good agreement with the experimental data.     

Experimental and theoretical research on interfacial reaction of solid Co with liquid Al

Interfacial reaction between solid Co and liquid Al was investigated with immersion tests and theoretical modeling. The microstructure characterization indicated that a Co₂Al₅ intermetallic layer was formed at solid–liquid interface during the immersion test. The modeling results indicated that the corrosion rate of a solid metal in a liquid metal was controlled by both the formation and dissolution of the intermetallic layer. In Co–Al reaction system, the formation and dissolution of the Co₂Al₅ layer reached an equilibrium state in a very short time, and the corrosion of the Co matrix was mainly dominated by the dissolution of the Co₂Al₅ layer.     

The isothermal and cyclic oxidation behaviour of two Co modified aluminide coatings at high temperature

The isothermal and cyclic oxidation behaviour of two Co modified aluminide coatings together with the simple aluminide coating were performed at 1000 °C and 1100 °C. All the three coatings show a much lower oxidation rate compared with the bare alloy. Results also indicate the addition of Co to the aluminide coating decreases the oxidation resistance slightly. It can be ascribed to that Co is easier to be oxidized than Ni at high temperature, and the Cr(W) rich phases which could act as a diffusion barrier are less in the coating with higher Co content.