The rate of decarburization of liquid iron by CO2 and H2

The rate of decarburization of liquid iron in CO-CO₂ mixtures and hydrogen at 1800 K has been investigated. The effect of sulfur on the rate in CO-CO₂ was also determined. Two experimental techniques were employed, one with the gas flow parallel to the surface of the melt, the other with gas flow perpendicular to it. The rate of decarburization in both CO-CO₂ mixtures and hydrogen at high carbon contents is controlled primarily by diffusionsion in the gas film boundary layer near the surface of the liquid. The presence of 0.3 wt pct sulfur reduced the rate of decarburization in CO-CO₂ by about 10 pct indicating that a slow chemical reaction on the surface is effecting the rate slightly when the surface is covered with sulfur atoms. The rate of decarburization at low carbon contents in CO-CO₂ is controlled primarily by carbon diffusion in the metal. The mass transfer relationships for the experimental geometries employed were investigated by measuring the rate of oxidation of graphite in CO-CO₂ mixtures. Previous work in which it was concluded that a chemical reaction was controlling the rate were re-examined and it was concluded that gas phase mass transfer was in fact controlling the rate of the reaction.     

Kinetics of chromite ore reduction from MgO-CaO-SiO2-FeO-Cr2O3-Al2O3 slag system by carbon dissolved in high carbon ferrochromium alloy bath

Abstract

Kinetic experiments were performed in an induction furnace to investigate the reduction of chromite ore by carbon dissolved in a high carbon ferrochromium alloy melt under conditions of varying Cr₂O₃ concentration, slag basicity, and temperature. The results obtained show that chromite reduction by dissolved carbon in slag systems of the type MgO-CaO-SiO₂-FeO-Cr₂O₃- Al₂O₃ occurs principally by a stagewise process encompassing an intermediate reaction in which the divalent chromium oxide species is involved. During the fast period, Cr₂O₃ reduction is controlled by the diffusion of oxygen species in the slag for which a mass transfer coefficient of 0·003 cm s^-1 was calculated. An activation energy value of 117 kJ mol ^-1 obtained for the reduction of Cr₂O₃ implies the rate controlling step is mass transfer of Cr₂O₃ from the slag to the slag/metal interface, since activation energies for metal phase control are typically <70 kJ mol ^-1. The second period represents a pseudo-equilibrium condition with respect to Cr₂O₃ reduction that is probably under thermodynamic control by a step or mechanism involving the reduction of divalent chromium oxide to chromium.     

Chromia-Assisted Decarburization of W-Rich Ni-Based Alloys in Impure Helium at 1273 K (1000 °C)

The microstructure and surface stability of two experimental W-rich Ni-based alloys have been studied at 1273 K (1000 °C) in an impure-He environment containing only CO and CO₂ as impurities. The alloy Ni-2.3Al-12Cr-12W contained 0.08 wt pct carbon in solution, whereas the second alloy Ni-2.3Al-3Mo-12Cr-12Co-12W contained M₆C carbides at the same carbon level. Both alloys, which were preoxidized with ~2.3 μm Cr₂O₃ layer, were decarburized completely within 50 hours of exposure to the helium gas mixture at 1273 K (1000 °C) via the following chromia-assisted decarburization reaction: Cr₂O₃ (s) + 3C_alloy (s) → 2Cr (s) + 3CO (g). Microstructural observations, bulk carbon analysis, and microprobe measurements confirmed that the carbon in solid solution reacted with the surface chromium oxide resulting in the simultaneous loss of chromia and carbon. The Cr produced by the decomposition of the Cr₂O₃ diffused back into the alloy, whereas CO gas was released and detected by a gas chromatograph. Once the alloy carbon content was reduced to negligible levels, subsequent exposure led to the uninterrupted growth of Cr₂O₃ layer in both alloys. In the preoxidized alloys, chromia-assisted decarburization rates were slower for an alloy containing carbides compared with the alloy with carbon in solid solution only. The formation of Cr₂O₃ is shown to be the rate-limiting step in the chromia-assisted decarburization reaction. Exposure of as-fabricated alloys to the impure-He environment led to the formation of a thin layer of Al₂O₃ (<1 μm) between the substrate and surface Cr₂O₃ oxide that inhibited this decarburization process by acting as a diffusion barrier.     

Decarburization of Levitated Fe-Cr-C Droplets by Carbon Dioxide

Experiments have been conducted at 1873 K (1600 °C) to study the kinetics of decarburization of Fe-Cr-C levitated droplets containing 10, 17, and 20 wt pct Cr using argon–carbon dioxide gas mixtures containing up to 30 pct CO₂, at flow rates of 100, 1000, 3000 and 12200 mL min^?1. It was found that chromium did not have a strong influence on the kinetics of decarburization while showing only minor effects on the extent of carbon removal. The results indicate that, for high carbon concentrations in the melt, the decarburization rates were controlled by mass transfer in the gas phase. Conventional formulation of governing mass transport numbers did not adequately describe the experimental observations made in this work. The observed rates are consistently higher than the values predicted using either the Ranz–Marshall correlation or the Steinberger–Treybal equation. A new correlation has been proposed to express the decarburization kinetics of levitated droplets for gas-flows in the range of Reynolds numbers between 2 and 100. The experimentally-derived model was found to be in excellent agreement with rate data derived from studies conducted by other researchers using levitated droplets.     

Reaction mechanism of FeO reduction by solid and dissolved carbon

The reduction reactions of FeO by carbon have been studied in order to be able to understand the fundamental phenomena occurring in smelting reduction process. The reduction of pure FeO by solid carbon proceeds mostly according to the same reaction mechanism as that by dissolved carbon in iron, the rate of which was experimentally determined to be controlled by the interfacial chemical reaction between Fe-C melt and intermediate CO₂ gas. Hence, the reduction rate of pure FeO by solid carbon is also chemically controlled by the Boudouard reaction between the dissolved carbon and CO₂ at the interface of by-product Fe droplet/gas phase, the activation energy of which was found to be about 193.2 kJ/mol. In addition, the reduction reaction of FeO in CaO-SiO₂-Al₂O₃-FeO slags by the dissolved carbon in Fe melt was also investigated over the FeO mass content less than 20 %. The reduction rate shows first order dependence with respect to FeO concentration. The surface active sulphur content in iron does not affect the reduction rate, and the temperature dependence of reduction rate gives the activation energy of 24.78 kJ/mol. Therefore, the reduction rate of FeO in slags by the dissolved carbon can be safely mentioned to be controlled by the liquid phase mass transfer of FeO through the slag phase diffusion-resistant boundary layer over the limited FeO concentration range. The empirical expression for the mass transfer controlled reactioe, deren Aktivierungsenergie ca. 193.2 kJ/mol beträgt. Außerdem wurde die Reduktion von FeO in CaO-SiO₂-Al₂O₃-FeO-Schlacken mit dem in der Eisenschmelze gelöstem Kohlenstoff fär FeO-Massengehalte von weniger als 20% untersucht. Die Reduktionsgeschwindigkeit weist hinsichtlich der FeO-Konzentration eine Abhängigkeit 1. Ordnung auf. Der Anteil an oberflächenaktivemn rate was determined as r = 5.94(±0.07).10^?6.exp(-24780/RT).(%FeOP)^0.96 over the reaction conditions employed.     

Investigation of the Oxidation Kinetics of Fe-Cr and Fe-Cr-C Melts under Controlled Oxygen Partial Pressures

In the current work, oxidation kinetics of Fe-Cr and Fe-Cr-C melts by gas mixtures containing CO₂ was investigated by Thermogravimetric Analysis (TGA). The experiments were conducted keeping the melt in alumina crucibles, allowing the alloy melt to get oxidized by an oxidant gas. The oxidation rate was followed by the weight changes as a function of time. The oxidation experiments were conducted using various mixtures of O₂ and CO₂ with $ P_{{{\text{O}}_{2} }} $ ?=?10^?2 to 10^4?Pa. In order to understand the mechanism of oxidation, the wetting properties between the alumina container and the alloys used in the thermogravimetric analysis (TGA) experiments and the change of the alloy drop shape during the course of the oxidation were investigated by X-ray radiography.The experiments demonstrated that the oxidation rate of Fe-Cr melt increased slightly with temperature under the current experimental conditions, but it is strongly related to the Cr-content of the alloy as well as the oxygen partial pressure in the oxidant gas mixture, both of which caused an increase in the rate. For the Fe-Cr-C system, the oxidation rate has a negative relationship with carbon content, viz. with increasing carbon, the oxidation rate of the alloy melt slightly decreased. The chemical reaction was found to be the rate determining step during the initial stages, whereas as the reaction progressed, the diffusion of oxygen ions through slag phase to the slag?Cmelt interface was found to have a strong impact on the oxidation rate. The overall impact of different factors on the chemical reaction rate for the oxidation process derived from the current experimental results can be expressed by the relationship: $ k_{1} = \frac{{dm}}{{dt}} = \Uplambda {\text C}_{\text{Cr}}^{0. 2 3} {\text{C}}_{{\text{CO}}_{ 2} } ^{ 0. 4 1}{\text{exp}}(\frac{{{{ - E}}_{\text{a}} }}{{{\text{R}}T}} ). $ A model for describing the kinetics of oxidation of Fe-Cr and Fe-Cr-C alloys under pure CO₂ was developed. Simulation of the oxidation kinetics using this model showed good agreement with the experimental results.     

Modelling Decarburization in Electrical Steels

A model is presented to predict the decarburization rate of electrical steels during reactive annealing. In a first step, the warm annealing atmosphere composition is calculated as function of the composition of the cold gas containing N₂‐H₂‐H₂O‐CO‐CO₂‐CH₄‐O₂. In a second step, the decarburization kinetics, which is controlled both by the surface reaction and by the diffusion of carbon towards the surface, is calculated. The model is then used to study the balance between surface reaction and the diffusion control of the decarburization process. We could conclude that for low sheet thickness and/or low H₂O/H₂ ratio in the annealing atmosphere, the decarburization is surface reaction controlled, while for commercial thicknesses and industrially applied dew points, the process is diffusion controlled. Furthermore, we looked at the difference in decarburization between complex N₂‐H₂‐H₂O‐CO‐CO₂ atmospheres used in industrial application, and N₂‐H₂‐H₂O atmospheres typically used in lab annealing. We could conclude that the decarburization rate is influenced by the addition of CO and CO₂ and that the final carbon level is increased if CO and CO₂ are added to the gas.     

Thermodynamic estimation on the reduction behavior of iron-chromium ore with carbon

The thermodynamics for reduction of iron-chromium ore by carbon is discussed. The thermodynamic properties of iron-chromium ore were evaluated from our previous work on the activities of constituents in the FeO·Cr₂O₃-MgO·Cr₂O₃-MgO·Al₂O₃ iron-chromite spinel-structure solid solution saturated with (Cr, Al)₂O₃, and those of the Fe-Cr-C alloy were estimated by a sublattice model. The stability diagrams were drawn for carbon reduction of pure FeO · Cr₂O₃, (Fe_0.5Mg_0.5)O·(Cr_0.8Al_0.2)₂O₃ iron-chromite solid solution, and South African iron-chromium ore. The evaluated stability diagrams agreed well with the literature data. It was concluded that the lowest temperature for reduction of FeO · Cr₂O₃ in the iron-chromium ore was 1390 K and a temperature higher than 1470 K would be necessary to reduce Cr₂O₃ in MgO·(Cr,Al)₂O₃ in the prereduction process of iron-chromium ore. The composition of liquid Fe-Cr-C alloy in equilibrium with iron-chromium ore was also estimated under 1 atm of CO at steelmaking temperature. The predicted metal composition showed reasonable agreement with the literature values.     

High-Temperature Oxidation of Alloy 617 in Helium Containing Part-Per-Million Levels of CO and CO<Subscript>2</Subscript> as Impurities

The objective of this study was to determine the mechanisms of carburization and decarburization of alloy 617 in impure helium. To avoid the coupling of multiple gas/metal reactions that occurs in impure helium, oxidation studies were conducted in binary He + CO + CO₂ gas mixtures with CO/CO₂ ratios of 9 and 1272 in the temperature range 1123 K to 1273 K (850 °C to 1000 °C). The mechanisms were corroborated through measurements of oxidation kinetics, gas-phase analysis, and surface/bulk microstructure examination. A critical temperature corresponding to the equilibrium of the reaction 27Cr + 6CO ↔ 2Cr₂O₃ + Cr₂₃C₆ was identified to lie between 1173 K and 1223 K (900 °C and 950 °C) at CO/CO₂ ratio 9, above which decarburization of the alloy occurred via a kinetic competition between two simultaneous surface reactions: chromia formation and chromia reduction. The reduction rate exceeded the formation rate, preventing the growth of a stable chromia film until carbon in the sample was depleted. Surface and bulk carburization of the samples occurred for a CO/CO₂ ratio of 1272 at all temperatures. The surface carbide, Cr₇C₃, was metastable and nucleated due to preferential adsorption of carbon on the chromia surface. The Cr₇C₃ precipitates grew at the gas/scale interface via outward diffusion of Cr cations through the chromia scale until the activity of Cr at the reaction site fell below a critical value. The decrease in activity of chromium triggered a reaction between chromia and carbide: Cr₂O₃ + Cr₇C₃ → 9Cr+3CO, which resulted in a porous surface scale. The results show that the industrial application of the alloy 617 at T > 1173 K (900 °C) in impure helium will be limited by oxidation.     

Reactions of Fe-Cr and Ni-Cr alloys in CO/CO2 gases at 850 and 950 °C

A series of Fe-Cr and Ni-Cr solid solution alloys was reacted at 850 and 950 °C in CO/CO₂ gas mixtures in which FeO and NiO were unstable. The compctitive tendencies toward the carburization and oxidation of the chromium solute, as compared to a graphical thermodynamic "metastability" criterion, were tested experimentally. Relatively good agreement was found between predictions and experiments for the occurrence of Cr carburization beneath Cr₂O₃ internal oxides or external scales. The chromium contents required for the transition from internal oxidation of Cr to the formation of Cr₂O₃ external scales in CO/CO₂ gas mixtures were established for Fe-Cr and Ni-Cr alloys. The Cr₂O₃ external scales formed on Fe-Cr alloys were found to be relatively impervious to carbon penetration for short (12-hour) experiments. No carburization was observed in the Ni-Cr alloys, but the only alloys that were predicted to carburize were the ones that formed external scales. Formerly Graduate Student, The Ohio State University     

Solubility of nitrogen and carbon in CaO-Al2O3 melts in the presence of graphite

CaO-Al₂O₃ slags were melted in graphite crucibles under N₂-CO-Ar gas mixtures at 1600°C. The contents of total nitrogen, cyanide and total carbon of the slags were determined by chemical analyses of quenched samples taken by suction from the melt. The nitrogen is present in the melt as nitride N^3- ion and cyanide CN^-1 ion, and carbon as cyanide and carbide C_2- ion. The equilibrium constants for the respective reactions were evaluated. It is found that the nitride capacity of the melt decreases whereas the cyanide and carbide capacities increase with increasing CaO/Al₂O₃ ratio.     

Solubility of nitrogen and carbon in CaO-Al2O3 melts in the presence of graphite

CaO-Al₂O₃ slags were melted in graphite crucibles under N₂–CO–Ar gas mixtures at 1600°C. The contents of total nitrogen, cyanide and total carbon of the slags were determined by chemical analyses of quenched samples taken by suction from the melt. The nitrogen is present in the melt as nitride N⁻³ ion and cyanide CN⁻¹ ion, and carbon as cyanide and carbide C ₂ ²⁻ ion. The equilibrium constants for the respective reactions were evaluated. It is found that the nitride capacity of the melt decreases whereas the cyanide and carbide capacities increase with increasing CaO/Al₂O₃ ratio.     

Kinetics of oxidation of carbon in liquid iron-carbon-silicon-manganese-sulfur alloys by carbon dioxide in nitrogen

The oxidation of carbon with the simultaneous oxidation of silicon, manganese, and iron of liquid alloys by carbon dioxide in nitrogen and the absorption of oxygen by the alloys from the gas were studied using 1-g liquid iron droplets levitated in a stream of the gas at 1575 °C to 1715 °C. Oxidation of carbon was favored over oxidation of silicon and manganese when cast iron (3.35 pct C, 2.0 pct Si, 0.36 pct Mn, and 0.05 pct S) reacted with CO₂/N₂ gas at 1635 °C. An increase in the flow rate of CO₂/N₂ gas increased the decarburization rate of cast iron. The rate of carbon oxidation by this gas mixture was found to be independent of temperature and alloying element concentrations (in the range of silicon = 0 to 2.0 pct manganese = 0 to 0.36 pct and sulfur = 0 to 0.5 pct) within the temperature range of the present study. Based on the results of a kinetic analysis, diffusion of CO₂ in the boundary layer of the gas phase was found to be the rate-limiting step for the reactions during the earlier period of the reaction when the contents of carbon, silicon, and manganese are higher. However, the limiting step changed to diffusion of the elements in the metal phase during the middle period of the reaction and then to the diffusion of CO in the gas phase during the later period of the reaction when the content of the elements in the metal were relatively low. For the simultaneous oxidation reactions of several elements in the metal, however, the diffusion of CO₂ in the gas phase is the primary limiting step of the reaction rate for the oxidation of carbon during the later period of reaction. Formerly Visiting Assistant Research Scientist, Department of Materials Science and Engineering, University of Michigan, Ann Arbor, MI 48109     

Thermodynamic Studies of the Fe-Cr-C-N System by EMF Measurements

In this work, the Fe-Cr-C-N alloys were synthesized by nitriding the Fe, Cr, and C powder mixtures at 1573 K in the N₂ gas (101 325 Pa). The nitrogen content and phase relationships at 1173 K in the alloys were investigated by the use of an equilibration technique. The thermodynamic activities of chromium in the alloys were studied using the solid-state galvanic cell method with CaF₂ as the solid electrolyte in the temperature range 973 to 1173 K in an atmosphere of N₂ gas (101 325 Pa). The activities of chromium in the Fe-Cr-C-N alloys were calculated and compared with those of the corresponding Fe-Cr-C ternary alloys with pure bcc-Cr as standard state. X-ray diffraction (XRD) and scanning electron microscopy (SEM) methods were used to identify the equilibrium phases and microstructures of the investigated alloys. The experimental results show that a Cr₂N-based nitride was formed during the nitriding procedure in the alloys. The nitrogen content in the alloys decreases with the decreasing chromium content, as well as the increasing temperature. The addition of nitrogen to the ternary Fe-Cr-C alloy was found to have a strong negative impact on the Cr activity in the Fe-Cr-C-N system.     

Aspects of smelting reduction of chromite ore fines in CaO-FeO-Cr2O3-SiO2-Al2O3 slag system by carbon dissolved in high carbon ferrochromium alloy bath

Abstract

Chromite reduction by carbon dissolved in a high carbon ferrochromium alloy melt has been investigated in the temperature range 1580-1640°C using a slag system based on CaO₂-FeO-Cr₂O₃-SiO₂-Al₂O₃. Although the reduction is essentially first order with respect to Cr₂O₃ concentration, it exhibits both zero order and first order reaction kinetics. The zero order period is occupied by the preferential reduction of iron oxide, during which time there is no significant change in the concentration of Cr₂O₃. The predominance of the divalent chromium oxide in the slag phase is seen to provide further evidence that the reduction of chromite occurs by a stagewise process, involving the thermodynamically stable CrO species. While high basicity slags may be recommended to minimise the generation of CrO, and hence improve reaction kinetics and the extent of Cr₂O₃ reduction, there is a limitation imposed by chemical erosion of the alumina crucible as the slag basicity is increased above unity, with the dissolving Al₂O₃ further retarding the reduction kinetics. There is also evidence to suggest the participation of a reductant other than carbon (possibly silicon) in the reduction of chromite.     

Rate of decarburization of iron-carbon melts: Part I. experimental determination of the effect of sulfur

The kinetics of decarburization of iron-carbon melts with CO-CO₂ gas mixtures were investigated at 1700 ° using the levitation technique. The influences of different experimental variables on the decarburization kinetics were determined. It was found that sulfur has a clear and reproducible retarding effect on the decarburization of iron-carbon melts; and this effect is most pronounced at sulfur concentrations in the range of 0 to 0.05 wt pct. The initial carbon concentration has no discernible effect on the decarburization kinetics. Melts containing 2.48 wt pct C and 0.92 wt pct C initially were found to decarburize at virtually identical rates until a substantial portion of the carbon was removed. The decarburization rate of a melt with a specified initial carbon content was found to remain essentially constant until the carbon content fell to a characteristic level below which the rate tended to level off. The partial pressure of CO₂ of the gas mixture has a marked effect on the decarburization kinetics. The flow-rate of the gas mixture has a small but finite effect on the rate of decarburization.     

The influence of sulfides on the oxidation behavior of nickel-base alloys

The oxidation of presulfidized chromium, Ni?Cr, and Ni?Al alloys, and complex nickel base alloys was studied at 1000°C in 1.0 atm of oxygen. Sulfur-rich surface layers were produced in the pretreatment by using H₂S?H₂ mixtures. Presulfidized chromium oxidized at a rate similar to that of sulfur-free chromium. The oxidation rate of presulfidized Ni?Cr alloys was affected by sulfur only when liquid nickel sulfide was present which accelerated the oxidation rate by creating rapid diffusion paths through the Cr₂O₃ scale. The oxidation behavior of presulfidized Ni?Al alloys, with aluminum contents sufficient for the formation of a protective Al₂O₃ layer in the sulfur-free condition, was influenced by sulfur only when aluminum sulfide was formed in the presulfidation treatment which caused the Al₂O₃ scale to be porous. The oxidation behavior of nickel-base alloys containing both chromium and aluminum was insensitive to the presence of sulfides when the concentration of aluminum in the alloy was such that a protective Al₂O₃ scale was formed during oxidation of the sulfur-free alloy and aluminum sulfide was not formed in the presulfidizing treatment.     

Solubility of carbon in CaO-Al2O3 melts

Carbon distribution ratios between CaO-Al₂O₃ slags and Fe-0.0003 to 0.8 mass pct Al-0.2 to 5.6 mass pct C alloys were measured at 1873 K in an Al₂O₃, CaO, or graphite crucible. The carbon distribution ratios were dependent on the oxygen potential, determined by theAl/(Al₂O₃) equilibrium, not by theC/CO (P _co = 1 atm) equilibrium. The (mass pct C)/a _c ratios were proportional to the activity of Al in logarithmic form with a slope of 2/3, indicating that carbon in slag is dissolved as C^2? ion. Solubilities of carbon in CaO-Al₂O₃ slags were also measured at 1873 K under the CO-CO₂-Ar gas mixtures in an Al₂O₃ or graphite crucible. It was found that C^2? ion is present in the range of log $P_{O_2 } $ (atm) < ?15 and CO ₃ ^2? ion in the range of log $P_{O_2 } $ (atm) > ?7.     

Formation of metallic phase on reducing-gas injection in multicomponent oxide melt. Part 2. Density and surface properties

The density and surface tension of melts of ferronickel (0–100% Ni) and oxidized nickel ore are measured by the sessile-drop method, as well as the interface tension at their boundary in the temperature range 1550–1750°C. The composition of the nickel ore is as follows: 14.8 wt % Fetot, 7.1 wt % FeO, 13.2 wt % Fe₂O₃, 1.4 wt % CaO, 16.2 wt % MgO, 54.5 wt % SiO₂, 4.8 wt % Al₂O₃, 1.5 wt % NiO, and 1.2 wt % Cr₂O₃. In the given temperature range, the density of the alloys varies from 7700 to 6900 kg/m³; the surface tension from 1770 to 1570 mJ/m²; the interface tension from 1650 to 1450 mJ/m², the density of the oxide melt from 2250 to 1750 kg/m³; and its surface tension from 310 to 290 mJ/m². The results are in good agreement with literature data. Functional relationships of the density, surface tension, and interphase tension with the melt temperature and composition are derived. The dependence of the alloy density on the temperature and nickel content corresponds to a first-order equation. The temperature dependence of the surface tension and interphase tension is similar, whereas the dependence on the nickel content corresponds to a second-order equation. The density and surface tension of the oxide melt depend linearly on the temperature. The results may be used to describe the formation of metallic phase when carbon monoxide is bubbled into oxide melt.     

Kinetics of the Oxidation of CaS

The kinetics of the oxidation of dense pellets of CaS by the reaction:

$$2CaS(s) + 3O_2 (g) \to 2CaO(s) + 2SO_2 (g)$$

was examined by continuous thermogravimetric analysis. The experiments covered a temperature range of 1673 to 1853 K, Ar-O₂-SO₂ mixtures varying from 1 to 40 pct O₂ and 0 to 20 pct SO₂, and gas velocities ranging from 20 to 57 cm/s at the reaction temperature. Analysis of the data shows that the initial rate of the reaction is controlled by the mass transfer of O₂ through the gaseous boundary layer of the pellet and is subsequently controlled by the diffusion of O₂ through the porous layer of the reaction products. To verify these results, the permeation rates of SO₂ through the reaction products were measured at room temperature and compared to values calculated from the thermogravimetric data.