On the oxidation of iron in CO2 + CO mixtures. III: Coupled linear parabolic kinetics

High-purity iron has been oxidized at 1000–1200° C in CO₂ and in CO₂ + CO with different compositions and at different total gas pressures (0.1–1 atm.). The experimental work has comprised thermogravimetric reaction rate measurements and characterization of the wüstite scales by metallography and x-ray diffraction. The overall results have been analyzed in terms of a classical model for coupled linear/parabolic kinetics, where it is assumed that the surface of growing wüstite scales has exactly the same defect structure and defect concentrations as that of bulk wüstite equilibrated in the same gaseous atmospheres. Important discrepancies are found between the predicted and the experimentally observed reaction behavior. Thus, both the linear and parabolic rate constants are found to be dependent on the partial pressure of CO₂ and the total gas pressure of the CO₂ + CO gas mixtures, and furthermore, the reaction in CO₂ + CO is slower than in O₂ and in H₂O + H₂ with the same oxygen activity. In order to explain the experimental results, it is suggested that CO and CO₂ molecules interact with the wüstite surface and thereby affect the defect structure and defect concentrations in a thin surface layer, and that this, in turn, affects both the linear and parabolic reaction rates.     

The Effect of Auxiliary Gas Solubilities in Scales upon Parabolic Oxidation Kinetics

In studies concerned with the oxida-tion of metals various gas mixtures are used to establish the chemical potential of non-metallic components in crystals. CO/CO₂ mixtures e. g. are used to estab-lish oxygen potentials at 1000°C; in the case of manganese it is found that the presence of such gas mixtures does not produce a change of the reaction rate which might be attributed to the dissolution of carbon in MnO. On the other hand sulfidation experiments with iron in H₂SH₂ at 800°C show that the aux-iliary gas produces a pronounced in-crease in the number of point defects -a fact in agreement with theoretically derived relations. It is therefore important prior to the interpretation of experimental high temperature corrosion results to establish the extent to which the particular auxiliary gas is soluble in the corrosion products being formed.     

Role of carbon dissolution during manganese oxidation in CO-CO2 Atmospheres

The role of carbon produced from the decomposition of CO-CO₂ gas mixtures traditionally is ignored during oxidation experiments, and only the oxygen potential established by the gas mixture is generally considered. Accordingly, to overcome this problem, a model which takes into account both carbon and oxygen dissolution into a p-type scale has been developed and tested. Two distinct cases of the model were tested at 1002°C. In the first case, variable CO/CO₂ ratios were used to determine the parabolic kinetics. Thus, both variable oxygen and carbon potentials were studied. In the p-type region of MnO, the developed model appeared to hold quite well. In the second case, constant CO-CO₂ ratios diluted with helium were utilized to maintain a constant oxygen potential with a variable carbon activity. The developed model appeared to hold again for the p-type region of MnO. Metallographically it was noted that the MnO scale developed exhibited an outer compact region and an inner layer that was somewhat porous.     

Decarburization and internal oxidation in a commercial-grade nickel

A commercial-grade nickel containing small amounts of carbon, manganese, and silicon was exposed to air for periods up to 288 hr at 1050°C to study the effect of oxidation on the formation of oxides of these impurity elements. Exposure of nickel to air led to decarburization. The maximum amount of decarburization occurred during the initial period of air exposure and the loss in carbon was more in the metal with a smaller section size. Decarburization in the metal produced voids in the oxide scale due to the formation of CO₂ gas. It has been shown further that CO and/or CO₂ gas bubbles, which form in high purity nickel on grain boundaries during exposure to air at elevated temperatures, cannot exist in commercial-grade nickel where manganese is present as an impurity. Instead, oxides of manganese form in the grain boundaries as well as in the matrix. This is because manganese oxide is more stable than CO or CO₂ gas.     

Corrosion of 9Cr Steel in CO2 at Intermediate Temperature II: Mechanism of Carburization

In parallel to the formation of a duplex oxide scale, 9Cr–1Mo steel carburizes strongly under CO₂ at 550?°C and this carburization accelerates with time. It is observed that an increase of the total CO₂ pressure in the environment from 1 to 250 bars induces a higher carbon deposition in the inner Fe–Cr rich spinel oxide layer. In order to explain this phenomenon, modelling of the carburization process was carried out. A mechanism involving gas diffusion of CO₂ and CO through the oxide layer, the Boudouard reaction and carbon diffusion through the metallic substrate is proposed.     

The Oxidation and Carburisation of a 20/25/Nb Steel in Carbon Dioxide,in Carbon Monoxide and in Carbon Dioxide-Carbon Monoxide Mixtures

Abstract

The oxidation and carburisation of a 20% Cr/25% Ni/Nb austenitic steel has been investigated between 600° and 850° in carbon dioxide, carbon monoxide and their mixtures at pressures within the range 0·04–760 cm.

Under all conditions the oxide formed markedly inhibits the attack of the steel. In carbon dioxide, the major reaction is M + CO₂ = MO + CO with the reaction 2M + CO₂ = 2MO + C contributing less than 7% to the total weight gain. All the carbon deposition appears to take place within the first few hours of the oxidation. Detailed studies show that the deposited carbon diffuses into the steel.

In carbon monoxide, the reaction is essentially M + CO = MO + C, the carbon passing through the oxide film into the steel. Carbon is also deposited in varying amounts on the surface of the oxide by the reaction 2CO = C + CO₂. This carbon is present as a filamentary growth and diffuses only slowly into the steel.

In carbon dioxide-carbon monoxide mixtures containing up to 75 vol.-% carbon monoxide, the steel reacts mainly with the carbon dioxide at 650° and 725°. At 800°, however, significant reaction occurs between the steel and the carbon monoxide.

The distribution of the carbon which enters the steel depends upon the temperature of oxidation. Below 750° the carbon concentration steadily decreases as the penetration into the steel increases. At 750° and above, the carbon concentration-penetration curves are anomalous: a maximum carbon concentration is obtained within the body of the steel.     

Metallographische Untersuchungen über den Aufbau des Zunders nach Oxydation von Eisen zwischen 700 und 900 °C in CO/CO2-Gemischen mit kleinen Gehalten an COS,H2S oder SO2

Metallographic investigations into the structure of the scale after oxidation of iron between 700 and 900°C in CO/Co₂ mixtures with minor additions of COS, H₂S or SO₂ The results of metallographic investigations are in agreement with kinetic measurements published recently (Werkstoffe u. Korrosion 21 [1970] 925) and with analytical investigations of the scale. When because of the CO/CO₂ ratio only a reaction with the sulfur compound is possible, pure sulfide layers are formed in gases containing COS and H. S. When, however, in addition to the reaction with the sulfur compound. A reaction with CO₂ is feasible, an inti-mate mixture of FeO and FeS is formed according to a linear scaling law. When the scale is high in oxide, the FeS particles are embedded in linear shape in a FeO matrix. When the scale has medium contents of oxides and sulfides a perlitic structure is formed consisting of FeO and FeS lamellae in parallel arrangement, the location changing from one grain to the other. With the transition to a parabolic law, i.e. with the transition to rate controlling diffusion of iron ions and electrons through the scale layer, only thermodynamically stable FeS is formed. Under certain conditions needles, predominantly of pure FeS, grow out from the compact scale layer. These needles have diameters between 5 and 20 lm, and may attain lengths up to 600 pn. With the transition to the parabolic law they probably grow in thickness and finally form a coherent FeS layer. In CO/CO₂ mixtures containing SO₂ essentially the same structures are formed, in this context it must be noted, that SO₂ may supply not only sulfur but also oxygen. This is why such FeO/FeS lamellae are formed in such gas mixtures where CO₂, cannot supply oxygen. Higher SO₂ and CO₂, contents in the gas the FeO/FeS lamellae “degenerate” form a coarser mixture of sulfide and oxide.     

Theoretical investigation of the CO oxidation on Al<Subscript>12</Subscript>Zr Cluster

Equilibrium geometries of Al₁₂Zr cluster were systematically studied on the basis of density functional theory with generalized gradient approximation. To gain insights into high catalytic activity we use the CO oxidation as a benchmark probe. In Al–Zr bimetallic clusters, Zr site is the catalytically active centre, the adsorption of CO and O₂ on the same site respectively (single-site mechanism), a Langmuir-Hinshelwood (LH) mechanism is proposed, which proceed via two steps, CO + O₂ → CO₂ + O and CO + O → CO₂. Two CO oxidation mechanisms of two CO₂ molecules as product have been simulated. For the later mechanism, the key step is the O–O bond scission in the OCOO* intermediate, which is significantly accelerated due to the attack of the neighboring CO molecule. The calculated barriers for the later reactions are lower compared with the former reaction. Detailed reaction paths corresponding to this case are calculated. Our study suggests that the CO oxidation catalyzed by Al₁₂Zr cluster is likely to occur at the room temperature.     

Role of promoting oxide morphology dictating the activity of Au/SiO2 catalyst in CO oxidation

The interfacial interaction of gold nanoparticles deposited on either model SiO₂/Si(100) or high surface area amorphous or mesoporous silica with minute amounts of promoter oxide like “active” FeO_x, TiO₂ and CeO₂ has been discussed. The role of the active oxide, its contribution to the perimeter along the gold nanoparticles has been interpreted. The oxide may invoke electronic interaction and simultaneously the defect structure of oxides likely has a key issue in the formation and stabilization of very small Au particles. The activity of the Au/oxide perimeter depends not only on the size of the Au particles, but also on the size and morphology of the oxide component (likely amorphous structure) regardless of whether it is supporting Au nanoparticles or decorating them. The activity in CO oxidation over Au catalysts is strongly affected by the length of the Au/“active” oxide perimeter which is regarded as the “active interface”. The longer length of the perimeter is evidenced by the enhanced CO oxidation activity.     

Kinetic conditions for the simultaneous formation of oxide and sulphide in reactions of iron with gases containing sulphur and oxygen or their compounds

The scaling of pure iron has been investigated in N₂O₂SO₂ and COCO₂COS mixtures between 700 and 900°C. Simultaneous formation of FeO and FeS at the scale/gas phase boundary is observed when the diffusion in the aerodynamic boundary layer or the reaction at the scale/gas phase boundary is the rate-controlling step of the oxidation in O₂N₂ mixtures or of the sulphidation in COCOS mixtures. In those cases the addition of the second oxidant (SO₂ to O₂N₂ mixtures, and an increased CO₂ to COCO₂COS mixtures) increases the rate of the oxidation or sulphidation reactions. When, however, the diffusion of iron ions and electrons through the oxide or sulphide layer respectively, or the reaction at the metal/scale phase boundary are rate-determining, the thermodynamically stable phase (oxide or sulphide) is formed exclusively and the addition of the second oxidant has no influence on the scaling rate. These results may be understood from an evaluation of the equilibria prevailing at the scale/gas phase boundary.     

Die Verzunderung von Eisen zwischen 700 und 900 °C in CO/CO2-Gemischen mit kleinen Zusätzen von COS,SO2 und H2S

Scaling of iron between 700 and 900°C in CO/CO₂mixtures with minor additions of COS, SO₂ and H₂S Scaling of iron in CO/CO, mixtures containing less than 1.6% COS, H₂S or SO₂follows initially a linear kinetic law. The transition from the linear to the parabolic law is displaced toward shorter periods with increasing sulfur contents in the gas and with decreasing temperature. At 800 and 900°C the rate of the reaction between iron and the sul-fur compound in the gas is controlled by the mass transfer in the gas phase. In this conditions the reaction rates with COS and H₂S are practically identical, while the reaction with SO₂yields al-most double the weight increase because in this case not only sulfur, but also part of the oxygen of SO₂ react with iron. At 700°C there is a transition of the control mechanism in CO/CO₂C/S mixtures with increasing COS contents, namely from control by mass transfer in the gas phase to control by the phase boundary reaction. Some consequences concerning the heating of steel in technical furnaces are discussed.     

Kinetics of the formation reaction of manganese carbide under various gases

Carbothermic reduction of manganese oxide under various gases in the temperature range of 1273-1573 K in order to understand the formation reaction of manganese carbide is investigated. The experimental results indicate that the formation reaction of manganese carbide depends on the temperature, the CO partial pressure, and the activity of carbon. The reaction rate for Mn₇C₃ formation is shown to be correlated with the CO gas diffusivity of the atmosphere in that the CO partial pressure acts as the driving force for the formation reaction in question. Furthermore, the results indicate that the reaction rate of Mn₇C₃ formation under H₂ gas is significantly faster than that under He gas; that is because methane gas that is formed by the reaction between H₂ with carbon accelerates the overall reduction process. On the other hand, a high CO partial pressure is shown to retard the formation reaction of Mn₇C₃.     

The Influence of Oxide Layers on the Initiation of Carbon Deposition on Stainless Steel

This paper examines the role played by oxide layers in the deposition of carbon on two 20Cr–25Ni–Nb-stabilized austenitic steels containing either zero or 0.56 wt% Si. Selective preoxidation, at 550°C in Ar/H₂/H₂O, was used to produce regions covered by either chromia or silica or, in the silicon-free alloy, to be bare metal. Deposition was performed, also at 550°C, in a CO₂/1% CO/1000 vppm C₂H₄ gas mixture having an estimated carbon activity very much greater than unity. This gas also had an oxygen potential sufficient to form magnetite, but not nickel oxide. It was found that, even at these high carbon activities, none of the oxides formed could catalyze carbon deposition and that this occurred only when the gas had direct access to the alloy substrate. The carbon filaments formed were found, by high-resolution electron microscopy, to be solid, have a turbostratic structure, and to contain at least one nickel particle at their tips. The source of these nickel particles is the alloy substrate and a mechanism is proposed for their formation.     

Behaviour of Metallic Materials in Simulated Service Environments of CO2/H2O Co-electrolysis Systems for Power-to-X Application

In the present study, the ferritic steel Crofer 22 H as potentially suitable interconnect material for SOEC stacks as well as joints between the steel and Ni- and CuNi contact materials was investigated with respect to the behaviour in simulated service environments of an SOEC system for CO₂/H₂O co-electrolysis. Exposures up to 1000 h at temperatures between 600 and 800 °C were carried out in CO₂/H₂O- and CO/H₂-rich gases, thus simulating conditions at the stack inlet and outlet, respectively. It was found that the steel formed protective surface oxide scales consisting of chromia and/or Cr/Mn spinel in all studied test conditions. No indication of carbon transfer from the gas atmosphere into the steel was found even in the high carbon activity CO/H₂-rich gas simulating stack outlet conditions. However, in the latter gas substantial carbon transfer from the gas to the steel via the Ni- or CuNi-wires resulted in the formation of a carburized zone with substantial M₂₃C₆ and/or M₇C₃ precipitate formation. This effect was more pronounced for the joints of the steel with the Ni-wire than with the CuNi-wire. In the gas simulating the service environment at the stack inlet, only minor carbon transfer was found in case of the Ni/steel joint at 600 °C but not at 800 °C. In case of the CuNi-wires, partial loss of contact between wire and interconnect steel and formation of Kirkendall voids as a consequence of interdiffusion between wire and steel were observed. The experimental results are discussed using thermodynamic considerations involving gas equilibria and stability of possible external and/or internal formation of oxide and carbide phases.

     

A model for mild steel oxidation in CO2

A model for the oxidation of mild steel in CO₂ is proposed, which is an extension of the ideas of Bruckman, Romanski, and Mrowec. A single layer of magnetite forms initially by short-circuit solid-state transport of cations. Lattice vacancies are injected into the underlying metal and eventually cause loss of scale-metal adhesion in some areas. Microchannels develop in the overlying oxide and the scale continues to grow at both oxide-metal and oxide-gas interfaces. In this duplex stage of growth, inner layer oxide nodules form in the vacancy condensation volume produced by departing metal. Their growth is restricted by a build-up of CO released by the oxidation reaction, so that a microporous structure is perpetuated. Breakaway oxidation is the result of local destruction of CO when a catalyst for the Boudouard reaction eventually forms. The inner layer crystals then grow in an atmosphere of higher oxygen potential, and deposited carbon produces a very porous structure.     

Oxidation, carburisation and metal dusting of 304 stainless steel in CO/CO2 and CO/H2/H2O gas mixtures

The oxidation and carburisation behaviour of 304 stainless steel was studied during thermal cycling in CO/CO₂ at 700 °C, and also in CO/H₂/H₂O at 680 °C. Thermal cycling caused repeated scale separation which accelerated chromium depletion from the alloy subsurface regions. The CO/CO₂ gas, with a_C=7 and , caused internal precipitation of oxides and carbides, some surface damage, but no dusting. In contrast, the CO/H₂/H₂O gas, with a_C = 19 and caused rapid graphite deposition and metal dusting. This was accompanied by internal oxidation and carburisation. The internal oxide was identified as spinel, which forms in the short term, but not at long reaction time. Its formation produced a significant volume expansion, which disrupted the material and resulted in surface damage in both gas atmospheres. In CO/H₂/H₂O, however, direct graphite deposition and metal disintegration into dust was the main reaction. The very different reaction morphologies produced by the two gas mixtures are discussed in terms of competing gas-alloy reaction steps.     

Synthesis of Pd/α-Fe2O3 nanocomposites for catalytic CO oxidation

Iron oxide and metal doped iron oxide nanocomposites have found attractive and versatile applications in many research areas such as catalysts, sensors, and biomedicine. This work reports a surfactant-free hydrolysis approach for the synthesis of hematite (α-Fe₂O₃) and Pd doped α-Fe₂O₃ nanoparticles with low-temperature catalytic activity. By this method, iron oxide rod-like nanoparticles were achieved by hydrolysis of ferric chloride salt with dilute HCl at a temperature of 100 °C. The Pd/iron oxide nanocomposites were obtained by adding citric acid into a mixture of iron oxide nanoparticles and palladium precursor (Pd(CH₃CN)₂Cl₂) under a reflux heating at 90 °C for 2 h. This method is featured as no use of any surfactants and templates, and no requirement of high-temperature thermal decomposition that are usually required for shape/size control in polar or nonpolar solvents. More importantly, it was found that the Pd/α-Fe₂O₃ nanocomposites exhibited high low-temperature catalytic activity in carbon monoxide (CO) oxidation.     

α‐Fe layer formation during metal dusting of iron in CO‐H2‐H2O gas mixtures

The formation of an α‐Fe layer between cementite and graphite was observed and investigated during metal dusting of iron in CO‐H₂‐H₂O gas mixtures at both 600°C and 700°C. The condition to form this phenomenon is determined by the gas composition which depends on temperature. The iron layer formation was observed for CO content less than 1 % at 600°C and less than 5 % at 700°C. With increasing CO contents, no α‐Fe layer was detected at the cementite/graphite interface by optical microscopy. In this case cementite directly contacts with the coke layer. The morphologies of the coke formed in the gas mixtures with low CO contents were also analysed. Three morphologies of graphite have been identified with 1 % CO at 600°C: filamentous carbon, bulk dense graphite with columnar structure, and graphite particle clusters with many fine iron containing particles embedded inside. At 700°C with 5 % CO the coke mainly consists of graphite particle clusters with some filamentous carbon at the early stage of reaction. Coke analysis by X‐ray diffraction shows that both α‐Fe and Fe₃C are present in the coke. The mechanism of α‐Fe accumulation between cementite and graphite is discussed in this paper.     

Effect of gas composition on cementite decomposition and coke formation on iron

Cementite decomposition and coke formation in the metal dusting process of iron were investigated at 700 °C in CO-H₂-H₂O gas mixtures. The presence of graphite deposited on the surface initiates the decomposition of cementite into iron and graphite. The morphology of the reaction products varies with gas composition. For CO concentrations less than 5 vol%, particles of iron or even closed iron layers have been observed at the cementite/graphite interface. With increasing CO content the amount of iron in the interface decreases. At CO concentrations higher than 30 vol%, iron could not be detected at the interface by optical microscopy. Thermo-gravimetric analysis shows that the rate of carbon take-up increases with increasing CO concentration reaching a maximum at about 60-75 vol%.The morphologies of graphite in the coke layer can be identified as three types: porous graphite clusters with embedded iron-containing particles, compact bulk graphite with a uniform thickness and a columnar layered structure, and filamentous carbon with iron-containing phases at the tip or along its length. For gas mixtures with low CO concentrations, e.g. 5 vol%, porous graphite clusters are the main form of carbon although filamentous carbon can be seen at the early stage of reaction. With increasing CO concentrations to, e.g. 30 vol%, a compact bulk graphite is formed on the top of the surface. Under this compact graphite, there is an inner layer of graphite which is the combination of porous graphite clusters and filaments. These two layers of graphite are clearly distinguishable when CO content reaches more than 75 vol%. In this case, the main form of graphite in the inner layer is filamentous carbon. The compact graphite layer suffers a serious deformation and forms many cracks because of the growth of catalytic filamentous carbon underneath. These filaments grow outside from compact graphite crevices and finally cover the whole surface. The higher the CO content in the gas, the more the tendency of filamentous carbon formation. The interplay between morphologies of carbon formation and metal dusting has been discussed.