Kinetics Of Internal Oxidation Research Articles

Effect of Hydrogen on the Internal Oxidation of a Pd–Cr Alloy in Dual-Atmosphere Conditions

The effect of hydrogen on oxygen permeability has been studied in a diluted Pd–Cr alloy in dual- and single- atmosphere conditions between 600 and 950 °C. The 0.3 mm thick Pd–1.5Cr foil was exposed in dry and humid air as well as in dual-atmosphere conditions, with one sample surface being exposed to air and one to hydrogen, as encountered in solid oxide fuel cells. At all temperatures, Cr oxidized internally forming internal oxidation zones which were measured in metallographic cross sections. Below 800 °C, an external layer of PdO formed on the surface decreasing the internal oxidation kinetics. No measurable effect of hydrogen on the internal oxidation of Cr in Pd has been detected.

Time–Temperature–Precipitation Relations for Nitrides and Evaluation of Internal Oxidation Theory for Nitridation of Austenitic Stainless Steel

Internal nitridation kinetics were determined for a UNS N08810/800H alloy using a general model of the form $$x^{n}=kt$$ . Nitridation behavior was studied at service-relevant temperatures 800 °C to 1000 °C in a 95 pct $$\hbox {N}_2/5\,{\text{pct}}$$ $$\hbox {H}_2$$ atmosphere for times 50 to 750 hours. Optical and scanning electron microscopy were used for microstructural characterization and measurement of nitride penetration. AlN, $$\hbox {Cr}_2\hbox {N}$$ , and CrN were formed, and the experimentally observed precipitation sequence was consistent with equilibrium calculations for this alloy using Thermo-Calc. A combination of diffusivity data determined using DICTRA and experimentally verified equilibrium calculations showed that Wagner’s analysis for internal oxidation kinetics was valid for AlN penetration. Parabolic kinetics closely approximated measured AlN penetration. This suggests that extension of AlN penetration models to other temperatures and Fe-Ni-Cr-Al alloy systems is reasonable. $$\hbox {Cr}_2\hbox {N}$$ penetration did not conform to Wagner’s analysis. Deviation from parabolic behavior was evident, and general model penetration predictions for $$\hbox {Cr}_2\hbox {N}$$ were experimentally validated. Using the experimentally determined models, time–temperature–precipitation diagrams for AlN and $$\hbox {Cr}_2\hbox {N}$$ penetration were constructed.

Mechanism of the transition from external oxide scales to internal oxidation for Fe-Si alloys in the atmosphere with very low oxygen partial pressure

For silicon steels surface defect is rather common as compared with that of the plain carbon steels due to the incorporation of Si. Researcher have studied oxidation behavior of silicon steels in reheating stage a lot while that of cooling stage after coiling was seldom discussed. In the present work, hot rolled Fe-Si alloys containing 0.75 and 1.5 wt% Si, which possessed the external oxide scales in the thickness of approximately 10 μm, were used to study the transition behavior from external oxide scales to internal oxidation. The hot rolled specimens were heat-treated at 600 °C–1150 °C in the atmosphere of pure Argon at 1 atm for 2 h. It has been observed that internal oxidation zone (IOZ) was formed in the temperature range from 700 to 1150 °C in Fe-0.75Si alloy, while the IOZ was formed in the temperature range from 1000 to 1150 °C in Fe-1.5Si alloy. By using the Wagner theory for internal oxidation kinetics and taking the equilibrium oxygen partial pressure for FeO into account, the internal oxidation depth has been calculated. After modification with reheating and cooling processes, the calculated values are in good agreement with the measured values. For Fe-0.75Si alloy, after the heat treatment at 800 °C, the calculated value after modification was obtained to be about 5.7 μm, close to the measured value of 6.1 μm, which indicates that the decomposition of oxide scale should be the main driving force for the growth of internal oxidation. In the atmosphere of pure Ar at high temperatures, Fe3O4 transformed to FeO, which is relatively thermodynamically stable. After the transformation, FeO may be decomposed to Fe and O2- due to the depletion of oxygen at the interface between oxide scale and steel substrate. O2− ions are dissolved and diffused in the steel substrate, which react with Si to form internal oxides of SiO2.

Internal Oxidation of Fe\u2013Mn\u2013Cr Steels, Simulations and Experiments

A multi-element and multi-phase internal oxidation model that couples thermodynamics with kinetics is developed to predict the internal oxidation behaviour of Fe–Mn–Cr steels as a function of annealing time and oxygen partial pressure. To validate the simulation results, selected Fe–Mn–Cr steels were annealed at 950 °C for 1–16 h in a gas mixture of Ar with 5 vol% H2 and dew points of − 30, − 10 and 10 °C. The measured kinetics of internal oxidation as well as the concentration depth profiles of internal oxides in the annealed Fe–Mn–Cr steels are in agreement with the predictions. Internal MnO and MnCr2O4 are formed during annealing, and both two oxides have a relatively low solubility product. Local thermodynamic equilibrium is established in the internal oxidation zone of Fe–Mn–Cr steels during annealing and the internal oxidation kinetics are solely controlled by diffusion of oxygen. The internal oxidation of Fe–Mn–Cr steels follows the parabolic rate law. The parabolic rate constant increases with annealing dew point, but decreases with the concentration of the alloying elements.

Internal oxidation process in diluted Fe–Cr alloys: a transmission Mössbauer spectroscopy study

The room-temperature study of Fe0.97Cr0.03 and Fe0.94Cr0.06 alloys was performed using the transmission Mossbauer spectroscopy (TMS). The TMS measurements allow to determine changes in chemical composition and content of iron oxides in the bulk of the studied samples during exposure to air at 570, 870 and 1070 K. The results indicate that the chemical composition of samples changes significantly, and it is strongly dependent on temperature at which the studied samples are exposed to air. The iron internal oxidation kinetics as well as the activation energies for the oxidation of iron atoms in studied alloys were calculated.

Comparison of the Oxidation Process in Pure Iron and Diluted Fe\u2013Si Alloys: A Transmission M\xf6ssbauer Spectroscopy Study

The room temperature study of polycrystalline iron, Fe0.99Si0.01 and Fe0.97Si0.03 alloys, was performed using the 57Fe transmission Mössbauer spectroscopy (TMS). The TMS measurements allow us to determine changes of chemical composition and content of iron oxides in the studied samples during exposure to air at 570, 870 and 1070 K. The results indicate that the chemical composition of samples changes significantly and it is strongly dependent on the temperature at which the studied samples are exposed to air. Moreover, it was shown that the addition of few atomic percent of Si impurities to iron leads to a significant slowdown of the internal oxidation process. The iron and silicon internal oxidation kinetics for Fe0.99Si0.01 and Fe0.97Si0.03 alloys were calculated using the Deal-Grove model. The activation energies EA for the internal oxidation of iron atoms in studied Fe–Si alloys were estimated: EA = 1.46 (3) eV for Fe0.99Si0.01 and EA = 1.62 (6) eV for Fe0.97Si0.03.

Internal Oxidation of Ternary Alloys Forming a High Oxygen Conductive Oxide

Five ternary alloys consisting of a noble base metal (Ni, Co, Fe, Cu) and two reactive metals (Zr + Y, Ce + Gd) being able to form a high oxygen ion conductive oxide were internally oxidized under low oxygen partial pressures. All alloys developed either a continuous yttria-stabilized zirconia phase or a continuous gadolinia-doped ceria phase behind the front of internal oxidation. A Ni–Ce–Gd alloy showed extraordinarily high internal oxidation rates of up to 120 µm2/s at 900 °C. High internal oxidation rates in these ternary alloys were not limited to low concentrations of the reactive metals. The type of the internal oxide phase was found to be more important for the internal oxidation kinetics than the noble base metal.

The kinetics and interface microstructure evolution in the internal oxidation of Ag–3at.%Sn alloy

Based on theoretical analysis of internal oxidation kinetics, the local oxygen partial pressure within the internal oxidized Ag–3at.%Sn alloy was derived as a function of the ambient oxidization parameters firstly. Further combined with the interface structure stability diagram, the evolution of Ag/SnO2 interface structures in internal oxidization of Ag–3at.%Sn alloy has been quantitatively clarified. The prediction results indicates that the stoichiometric Ag/SnO2 interface dominates in internal oxidation of Ag–3at.%Sn alloy in air atmosphere at 973K, while a gradient structures with both O-rich and stoichiometric interface were formed at the condition of 3atm pure oxygen atmosphere at 873K.

Kinetics of Internal Oxidation of Mn-Steel Alloys

Internal oxidation of three Mn-steel alloys with 1.7, 3.5 and 7.0 wt% Mn concentration at 950 °C in a gas mixture composed of nitrogen, hydrogen and water vapor with a dew point of +10°C was evaluated. For these alloys, the kinetics of internal oxidation are diffusion-controlled and obey parabolic growth rate law. The diffusion coefficient of oxygen and manganese determined from the observed internal oxidation kinetics are 3.35 × 10−7 and 4.14 × 10−12 cm2/s at 950 °C, respectively. The formed internal oxide precipitates are mainly composed of MnO. The solubility product of MnO in an austenitic iron matrix is estimated to be (7.66 ± 0.18) × 10−9 mol fraction2 at 950 °C. The numerical simulation of concentration depth profiles of precipitated oxygen is in agreement with depth profiles determined with image analysis and X-ray microanalysis. Validity of the numerical simulation in case of the phase transformation was also tested. When a 1.7 wt% Mn-steel alloy is oxidized at 850 °C (instead of 950 °C) with a dew point of +12 °C partial phase transformation from austenite to ferrite takes place due to the Mn depletion. The associated precipitated oxygen concentration depth profile can be predicted accurately with numerical simulation.

Internal oxidation with significant contribution of oxygen diffusion through the oxide phase

Internal oxidation kinetics of alloys is often controlled by the diffusion of oxygen through the metallic matrix. However high rates of internal oxidation of binary Ni–Al and Ni–Zr alloys can only be explained taking oxygen diffusion along the metal/oxide interfaces into account. In this work ternary Ni–Zr–Y alloys are investigated in H2/H2O atmospheres under isothermal conditions from 800 to 1000°C. Internal oxidation rates for these ternary alloys are about three orders of magnitude higher than for binary Ni–Zr alloys. These extremely high rates are attributed to fast oxygen diffusion through the oxide phases which are forming during internal oxidation.

Kinetics of internal oxidation in Ag-6Sn-2In-1Te-0.2Ni and Ag-6Sn-0.6Cu-1Bi-0.2Ni type alloys

Internal oxidation process was followed in Ag-6Sn-2In-1Te-0.2Ni and Ag-6Sn-0.6Cu-1Bi-0.2Ni alloy types in free air and oxygen at 650 °C and 680 °C. It was found that the Cu and Bi bearing alloy had a higher rates of internal oxidation in all of the tested conditions, whish was due to Bi effect accelerating the internal oxidation. In the early stage of internal oxidation of the Ag-6Sn-2In-1Te-0.2Ni alloy, an inhomogeneous structure revealed, which changed to a homogenous pattern after longer oxidation times. Such a structure was suppressed in the Bi bearing alloy. The applied 6 bar oxygen atmosphere brought on a 5–6 times higher rate of internal oxidation, with a homogenous structure compared to the free air oxidation. In free air after long time oxidation (200 h–250 h), the rate of internal oxidation was found to have almost doubled in both alloys.

Temperature and gas composition dependence of internal oxidation kinetics of an Fe10%Cr alloy in water vapour containing environments

AbstractRecently it was proposed, that the hampered formation of external protective chromia scales on FeCr‐alloys in water vapour containing, low‐pO2 gases is correlated with enhanced internal oxidation of chromium. In the present study the internal oxidation kinetics of Fe10Cr (in mass%) during isothermal oxidation in ArH2H2O mixtures at temperatures in the range 800–1050 °C has been investigated. It was found that the tendency for Cr to become internally oxidized decreased with decreasing temperature. At the higher test temperatures the internal oxide precipitates consisted of Fe/Cr‐spinel. With decreasing temperature the precipitates near the oxidation front gradually exhibited increasing amounts of chromia. At 900 °C the oxidation morphology in the ArH2 base gas mixture changed from exclusive internal oxidation of Cr at a water vapour content of 2% towards a combined internal Cr oxidation and external Fe‐oxide formation at higher water vapour partial pressures.

Microstructural Changes During Internal Oxidation of a Ag-Sn Alloy

AbstractElectrical contact materials are expected to combine improved mechanical and physical properties with good corrosion resistance at increased temperatures. One of the most widely used groups of contact materials are internally oxidized dispersion hardened silver alloys. The principle of their production involves selective oxidation of less noble solute elements (usually Mg, Al, Sn, Cd and Zn) forming the fine dispersion of oxide particles in the solvent silver matrix. In this study the internal oxidation of dilute silver alloy containing 2 at. % of tin was extensively investigated. The morphological evolution of the alloy and the internal oxidation kinetics were determined by several metallographic examinations and in-situ electrical resistance measurements in the air atmosphere and in the temperature range from 600°C to 800°C. The results of experiments and their analyses show that the microstructural changes during internal oxidation of Ag-Sn alloy are strongly dependent on the annealing temperature. At the highest temperatures the perfectly dispersive distribution of the oxide particles SnO2 in the silver matrix was found, while at lower temperatures the formation of inner oxide bands and distribution of oxide particles along the grain boundaries is predominate.

Kinetic equation for internal oxidation of Cu-Al alloy spheres

The kinetics of internal oxidation of Cu-Al alloy spheres, containing up to 2.214% mole fraction Al was investigated in the temperature range 1 023 K to 1 273 K, and the depth of internal oxidation was measured in the microscopy. A kinetic equation was derived to describe the internal oxidation of Cu-Al alloy spheres, which was checked experimentally by means of oxidation depth measurements. The results show that the derived equation is exact enough to describe the kinetics of internal oxidation of Cu-Al alloy spheres. Based on this equation and the oxidation depth measurements, the permeability of oxygen in solid copper has been obtained. Investigation also shows that in the process of internal oxidation, there is no evidence for preferential diffusion along grain boundaries.

In-situ monitoring of internal oxidation of dilute alloys

This paper presents a non-destructive measurement method that enables identification and characterization of phenomena during internal oxidation of metallic materials as well as monitoring the kinetics of internal oxidation using “in-situ” electrical resistance measurements. A special laboratory device, based on the unique measurement cell, for the electrical resistance measurements at high temperatures and the model for electrical resistance transformation into an instantaneous microstructure were developed. To accomplish this, the process of internal oxidation was divided into the sequence of the key partial reactions that were presented in the model as the parallel and/or serial connected time variable resistors in the electrical circuit. The validity of the transformation model was experimentally confirmed by internal oxidation of Ag–Sn (2 at.% Sn) alloy at different oxidation temperatures in air atmosphere. The comparison of the results obtained by “in-situ” electrical resistance measurements with those obtained by metallographic analysis and Wagner’s theory shows that the novel method presents a more effective tool for monitoring of internal oxidation kinetics. The method identifies also the microstructural defects and their influences on the kinetics of internal oxidation.

Kinetics of internal oxidation of Cu−Al alloy plate

The kinetics of internal oxidation of dilute Cu−Al alloys, containing up to 2.214% molar fraction Al, was investigated over the temperature range of 1023 K to 1273 K, and the depth of internal oxidation was measured by microscopies. A modified rate equation was derived to describe the kinetics of internal oxidation of Cu−Al alloy plate. Based on the derived equation, the permeability of oxygen in solid copper was obtained from the internal oxidation measurements. The experimental results show that the depth of the internal oxidation is a parabolic function of time, there is no evidence for preferential diffusion along grain boundaries and an outer layer of pure copper was formed on the external surface of samples.

The Internal Oxidation of Ternary Alloys. IV: The Internal Oxidation of the Most-Reactive Component Beneath External Scales of the Component Having Intermediate Reactivity

The kinetics of internal oxidation of the most-reactive component C of ternary A–B–C alloys in the presence of external scales of the oxide of the component of intermediate reactivity B, BOν, are examined using convenient approximations. The precipitation of the most-stable oxide leaves behind a matrix composed of a mixture of the two most-noble components, A and B, whose composition changes with depth due to the consumption of B to form the external scale. For the calculation of the parabolic rate constant use is made of approximate expressions for the concentration of oxygen dissolved in the binary A–B metal matrix within the zone of internal oxidation and for the diffusion coefficient of oxygen as functions of the alloy composition. Numerical calculations of the parabolic rate constant of internal oxidation are carried out for different combinations of values of the various parameters involved. The results obtained for the ternary alloys are compared with the corresponding data calculated for the binary A–C and B–C alloys under the same conditions.

Internal Oxidation of a Ag-1.3 at.% Se Alloy. Part II: Kinetics of Internal Oxidation

The kinetics of internal oxidation of a two-phase Ag-1.3 at.% Se alloy were studied at 750, 800 and 830°C in pure oxygen. The parabolic rate law was followed to a reasonable extent for the first 2 hr of oxidation. The rate of internal oxidation was much faster than expected from the classical Wagner model. Three major factors affected this: (i) the non-uniform distribution of Ag2Se particles in the as-cast alloy caused non-uniform internal oxidation of the alloy; (ii) the formation of a liquid phase during in-situ oxidation of Ag2Se particles accelerated the diffusional processes and lowered the activation energy of the oxidation reactions (a sort of “catastrophic” internal oxidation); (iii) gradual oxidation of the alloying element in the form of second-phase particles caused the extension of the single internal-oxidation front (IOF) to the internal-oxidation volume (IOV) between the inner and outer IOFs. The IOV requires a much lower oxygen quantity for its advancement than the (single) IOF thereby enabling much faster internal oxidation of two-phase alloys that oxidize in the in situ (diffusionless) mode. The kinetics are, also discussed according to the treatment which was proposed for internal oxidation of two-phase alloys by Gesmundo.7

The internal oxidation of ternary alloys I: The single oxidation of the most-reactive component under low oxidant pressures

The internal oxidation of the most-reactive component C of ternary A-B-C alloys by a single oxidant is examined assuming a gas-phase oxidant pressure below the stability of the oxides of the other two components. The precipitation of the most-stable oxide leaves behind a matrix composed of a binary alloy of the two less-reactive components, whose composition affects the solubility and diffusivity of the oxidant within the region of internal oxidation, with an effect on the reaction kinetics. Approximate relations between these properties are proposed and used to predict the kinetics of internal oxidation of C under the assumption of parabolic rate law. The results obtained for the ternary alloys are compared with the behavior of binary A-C and B-C alloys with the same C content. A new important factor in establishing the difference between the internal oxidation in ternary A-B-C alloys and in binary A-C and B-C alloys under a fixed gas-phase oxygen pressure and C content is the ratio between the concentrations of A and B in the bulk ternary alloy.

Internal and External Oxidation of Pt(Al) Solid Solution at Elevated Temperatures

High temperature oxidation of Pt(Al) solid solution and Pt(Al) with was investigated in order to prepare a Pt wire with an external film for a hot‐wire technique to measure thermal conductivities of fused metals. At temperatures from 1373 to 1673 K in air, Pt‐4.6 mol % Al (solid solution) indicated internal oxidation. Continuous scales developed on Pt‐9.4 mol % Al (solid solution) and Pt‐13.1 mol % (the two phase alloy). External scales formed on those alloys after high temperature oxidation under low oxygen pressure of gas mixture. The oxidation rate increased with increasing Al content and oxygen partial pressure. The Al concentration at the transition from internal to external oxidation was determined by using oxygen permeability through Pt‐Al alloys calculated from internal oxidation kinetics. © 2000 The Electrochemical Society. All rights reserved.