Hot corrosion of B−1900 in CaSO4/Na2SO4 salt mixtures in reducing atmospheres

Hot corrosion of B−1900 in CaSO₄/Na₂SO₄ salt mixtures was performed at 900 ‡C under a host of reducing/sulfidizing atmospheres. Substantial differences were noted as the oxygen pressure was reduced from 10⁻⁷ to 10⁻¹⁵ Pa. In all cases the resulting morphologies were different from those noted under corresponding oxidizing conditions. Substantial alloy sulfidation was noted when thePo ₂ = 10⁻¹⁵ Pa and thePs ₂ = 10⁻³ Pa. Hot corrosion was found to be maximized at higher oxygen potentials,i.e., above 10⁻³ Pa. In all cases studied, the corrosion was less extensive than pure Na₂SO₄ at 900 ‡C in air or in pure O₂. Segregation of CaSO₄ and Na₂SO₄ was noticed; this contributed to the formation of localized regions of extensive hot corrosion while other regions corroded by simple oxidation. S. F. C. STEWART, formerly Graduate Student at Materials Engineering Department, Rensselaer Polytechnic Institute, Troy, NY 12180−3590. S.R. SHATYNSKI, deceased, was Associate Professor of Materials Engineering, Rensselaer Polytechnic Institute, Troy, NY 12180−3590.     

Hot corrosion of B−1900 in CaSO4/Na2SO4 salt mixtures in reducing atmospheres

Hot corrosion of B−1900 in CaSO₄/Na₂SO₄ salt mixtures was performed at 900 ‡C under a host of reducing/sulfidizing atmospheres. Substantial differences were noted as the oxygen pressure was reduced from 10⁻⁷ to 10⁻¹⁵ Pa. In all cases the resulting morphologies were different from those noted under corresponding oxidizing conditions. Substantial alloy sulfidation was noted when thePo ₂ = 10⁻¹⁵ Pa and thePs ₂ = 10⁻³ Pa. Hot corrosion was found to be maximized at higher oxygen potentials,i.e., above 10⁻³ Pa. In all cases studied, the corrosion was less extensive than pure Na₂SO₄ at 900 ‡C in air or in pure O₂. Segregation of CaSO₄ and Na₂SO₄ was noticed; this contributed to the formation of localized regions of extensive hot corrosion while other regions corroded by simple oxidation. S. F. C. STEWART, formerly Graduate Student at Materials Engineering Department, Rensselaer Polytechnic Institute, Troy, NY 12180−3590. S.R. SHATYNSKI, deceased, was Associate Professor of Materials Engineering, Rensselaer Polytechnic Institute, Troy, NY 12180−3590.     

Oxidation of Fe (II) in sulfuric acid solutions with dissolved molecular oxygen

The oxidation of Fe(II) with dissolved molecular oxygen was studied in sulfuric acid solutions containing 0.2 mol · dm⁻³ FeSO₄ at temperatures ranging from 343 to 363 K. In solutions of sulfuric acid above 0.4 mol · dm⁻³, the oxidation of Fe (II) was found to proceed through two parallel paths. In one path the reaction rate was proportional to both [Fe⁻²⁺]² andp _{o 2} exhibiting an activation energy of 51.6 · kJ mol⁻¹. In another path the reaction rate was proportional to [Fe²⁺]², [SO ₄ ^{2 −}], andp _{o 2} with an activation energy of 144.6 kJ · mol⁻¹. A reaction mechanism in which the SO ₄ ^{2 −} ions play an important role was proposed for the oxidation of Fe(II). In dilute solutions of sulfuric acid below 0.4 mol · dm⁻³, the rate of the oxidation reaction was found to be proportional to both [Fe(II)]² andp _{o 2}, and was also affected by [H⁺] and [SO ₄ ^{2 −}]. The decrease in [H⁺] resulted in the increase of reaction rate. The discussion was further extended to the effect of Fe (III) on the oxidation reaction of Fe (II).     

The chemical diffusivity of oxygen in liquid iron oxide and a calcium ferrite

Measurements have been made of the chemical diffusion coefficient of oxygen in liquid iron oxide at temperatures from 1673 to 1888 K and in a calcium ferrite (Fe/Ca = 2.57) at temperatures from 1573 to 1873 K. A gravimetric method was used to measure the oxygen uptake during the oxidation of the melts by oxygen or CO₂-CO mixtures. The rate was shown to be controlled by mass transfer in the liquid melt. The chemical diffusivity of oxygen in liquid iron oxide at oxygen potential between air and oxygen was found to be 4.2±0.3 × 10⁻³ cm²/s at 1888 K. That in iron oxide at oxidation state close to iron saturation was established to be given by the empirical expression log D=−6220/T + 1.12 for temperatures between 1673 and 1773 K. For the calcium ferrite (Fe/Ca=2.57) at oxygen potential between air and oxygen, the diffusivity of oxygen was found to be given by log D=−1760/T−1.31 for temperatures between 1673 and 1873 K. This article is based on a presentation made in the “Geoffrey Belton Memorial Symposium,” held in January 2000, in Sydney, Australia, under the joint sponsorship of ISS and TMS.     

The activities of sulfide and oxide components and the solubility of oxygen in copper-iron-sulfur-oxygen mattes at 1300 °C

The activities of iron and copper and the solubilities of oxygen in copper-iron-sulfur-oxygen mattes have been determined by equilibrating mattes with CO−CO₂−SO₂ gas mixtures of fixed partial pressures of oxygen and sulfur and equilibrating a small mass of platinum with the melt. Iron and copper transferred from the matte to form a platinum-iron-copper alloy in which the activities of iron and copper are the same as in the matte. The activities of iron and copper in the matte were then determined from knowledge of the activities of iron and copper in the system platinum-iron-copper. Sulfides ofW _Fe=0.1, 0.3, and 0.5 were studied, whereW _Fe=wt pct Fe/(wt pct Fe+wt pct Cu), and sulfur pressures of 0.005, 0.0158, and 0.025 atm and oxygen pressures of 3.16×10⁻¹⁰, 7.94×10⁻¹⁰, 2.00×10⁻⁹, and 3.16×10⁻⁹ were used. The activity of copper, which varied in the range 0.06 to 0.165, decreases with increasingp _{O 2} at constantW _Fe andp _{S 2} and decreases with increasingp _{S 2} at constantW _Fe and constantp _{O 2}. The activity of iron, which varied in the range 0.002 to 0.06, increases with increasingp _{O 2} at constantW _Fe andp _{S 2} and decreases with increasingp _{S 2} at constantW _Fe andp _{O 2}. The activities of the components Cu₂S, FeS, Cu₂O, FeO, and Fe₃O₄ were calculated from the activities of iron and copper, the partial pressures of oxygen and sulfur, and the approapriate equilibrium constants. The variations of the activities of these components with matte grade, oxygen pressure, and sulfur pressure are presented and discussed. Within the range of experimental conditions studied, the solubility of oxygen in the melts is given by wt pct O=2.59pO₂/0.225pS₂/−0.18 (1+9.0W _Fe)^1.86     

The diffusivity and solubility of oxygen in liquid tin and solid silver and the diffusivity

The diffusivity and solubility of oxygen in liquid tin and solid silver in the temperature range of about 750° to 950°C (1023 to 1223 K) and the diffusivity of oxygen in solid nickel at 1393°C (1666 K) were determined using the electrochemical cell arrangement of cylindrical geometry: Liquid or Solid Metal + O (dissolved) | ZrO₂ + (3 to 4%)CaO | Pt, air The diffusivity and solubility of oxygen in liquid tin are given by:D _O(Sn) = 9.9 × 10⁻⁴ exp(−6300/RT) cm²/s (9.9 × 10⁻⁸ exp − 6300/RT m²/s) andN _O ^S (Sn) = 1.3 × 10⁵ exp(−30,000/RT) at. pct The diffusivity and solubility of oxygen in solid silver follow the relations:D _O(Ag) = 4.9 × 10⁻³ exp (−11,600/RT) cm²/s ( 4.9 × 10⁻⁷ exp − 11,600/RT m²/s) andN _O ^S (Ag) = 7.2 exp (−11,500/RT) at. pct The experimental value for the preexponential in the expression forD _O(Ag) is lower than the value calculated according to Zener’s theory of interstitial diffusion by a factor of 11. The diffusivity of oxygen in solid nickel at 1393°C (1666 K) was found to be 1.3 × 10⁻⁶ cm²/s (1.3 × 10⁻¹⁰ m²/s). Formerly Graduate Student, Department Formerly Graduate Student, Department Formerly Graduate Student, Department This paper is based upon a This paper is based upon a This paper is based upon a This paper is based upon a     

Dissolution of solid magnetite in Fe-S-O melt

The dissolution rate of solid magnetite in binary Fe-S and ternary Fe-S-O melts was measured at 1493 K using a rotating magnetite rod of 5 mm in diameter. At rotating speed higher than 52 rpm, the flow of sulfide melt was turbulent, and the effect of natural convection on the dissolution rate was negligible. Magnetite dissolution took place without the evolution of SO₂ gas at the rod surface immersed in the interior of the melt. On the other hand, SO₂ gas was evolved from a portion of magnetite rod which was in contact with the melt surface. The overall rate of dissolution was controlled by mass transfer through liquid boundary layer on the rod surface, and the dissolution rate of magnetite decreased with increasing oxygen concentration of the sulfide melt. Mass transfer coefficient was between 1.8 × 10⁻³ and 4.6 × 10⁻³ cm · s⁻¹ at the rotating speed of 156 rpm, and it decreased with increasing oxygen concentration of the melt. Formerly Graduate Student Formerly Graduate Student Formerly Graduate Student     

The leaching of chalcopyrite with ferric sulfate

The leaching kinetics of natural chalcopyrite crystals with ferric sulfate was studied. The morphology of the leached chalcopyrite and the electrochemical properties of chalcopyrite electrodes also were investigated. The leaching of chalcopyrite showed parabolic-like kinetics initially and then showed linear kinetics. In the initial stage, a dense sulfur layer formed on the chalcopyrite surface. The growth of the layer caused it to peel from the surface, leaving a rough surface. In the linear stage, no thick sulfur layer was observed. In this investigation, chalcopyrite leaching in the linear stage was principally studied. The apparent activation energy for chalcopyrite leaching was found to range from 76.8 to 87.7 kJ mol⁻¹, and this suggests that the leaching of chalcopyrite is chemically controlled. The leaching rate of chalcopyrite increases with an increase in Fe(SO₄)_1.5 concentration up to 0.1 mol dm⁻³, but a further increase of the Fe(SO₄)_1.5 concentration has little effect on the leaching rate. The dependency of the mixed potential upon Fe(SO₄)_1.5 concentration was found to be 79 mV decade⁻¹ from 0.01 mol dm⁻³ to 1 mol dm⁻³ Fe(SO₄)_1.5. Both the leaching rate and the mixed potential decreased with an increased FeSO₄ concentration. The anodic current of Fe(II) oxidation on the chalcopyrite surface in a sulfate medium was larger than that in a chloride medium.     

Kinetics of evaporation/oxidation of iridium

Iridium wires self-resistance-heated to temperatures in the range of 1675 to 2260°C (1948 to 2533 K) were oxidized in naturally convected oxygen and air at pressures in the range of 9.9×10⁻⁸ to 1.32 atm (10⁻² to 1.34×10⁵ Pa) and in force convected oxygen and air at pressures in the range of 4.6×10⁻³ to 0.84 atm (470 to 8.5×10⁴ Pa). The experimental results were closely correlated by a theoretical rate equation based on control of the oxidation rates by the rates of evaporation of Ir(g), IrO₂(g) and IrO₃(g) and by the rates of their subsequent diffusion through the gaseous boundary layer. Apparent values were obtained for the standard state free energies of formation of IrO₂(g) and IrO₃(g) and were correlated linearly with temperature. R. T. WIMBER, formerly Professor of Mechanical Engineering, Montana State University, Boseman, MT were all Graduate Research Assistant/M.S. Candidates, Montana State University.     

Acidity, valency and third-ion effects on the precipitation of scorodite from mixed sulfate solutions under atmospheric-pressure conditions

The present study is focused on the precipitation of scorodite from mixed sulfate media at 95 °C under atmospheric pressure. In particular, this study explores the effects of acidity (pH), valency [Fe(II)/Fe(III), As(III)/As(V)], and solution composition (third cation/anion) on the yield, crystallinity, and stability (leachability) of scorodite precipitates. Thus, it was found that the precipitation of crystalline scorodite can be achieved without stringent pH control once the precipitation has started. Nonetheless, the selection of the initial pH is critical to avoid the formation of an amorphous precipitate. A leachability as low as 0.5 mg/L As at pH 5 and 22 °C (TCLP-like test) is obtained when the initial molar ratio Fe(III):As(V) is increased to 3:1, but the precipitation yield is very low. When Fe(II) is used as excess iron, the precipitate solubility drops to 0.2 mg/L As with a yield exceeding 80 pct in 2.5 hours. The stability of the product is not measurably affected by the presence of Cu²⁺, Zn²⁺, Ni²⁺, Co²⁺, Mn²⁺, SO ₄ ²⁻ , and NO ₃ ⁻ . The presence of PO ₄ ³⁻ , however, leads to the formation of crystalline phosphate-containing scorodite precipitates of somewhat reduced stability. In most cases, the TCLP leachability of the precipitate was found to be between 1 and 3 mg/L As, and never exceeded the regulatory limit of 5 mg/L As.     

The equilibrium oxygen pressure over the Cr-Y2O3-YCrO3 coexistence measured with the galvanic cell using stabilized ZrO2 solid electrolyte

The equilibrium oxygen pressure over the Cr-Y₂O₃-YCrO₃ coexistence has been measured by the following cells: Cr, Y₂O₃, YCrO₃∥ZrO₂∥Cr, Cr₂O₃ [I] Mn, MnO∥ZrO₂∥Cr, Y₂O₃, YCrO₃ [II] Moreover, the partial electronic conduction parameter,P _e, has been determined simultaneously, as the oxygen partial pressure where then-type electronic and ionic conductivities are equal in the stabilized ZrO₂. The equilibrium oxygen pressure,P _{O 2}, over the Cr-Y₂O₃-YCrO₃ coexistence andP _e are expressed as log (P_{O 2}/atm) = 10.6 − 4.39 × 10⁴1/T ± 0.1 (1385 to 1470 K) log ( P_e/atm = 8.86 − 4.21 x 10₄ 1/T ± 0.3 (1385 to 1470 K) From the equilibrium oxygen pressure and the standard Gibbs energy of formation of Y₂O₃, the standard Gibbs energy of formation of YCrO₃ is calculated as ΔG _o ^f /J mol^™1 = −1.58 x 10⁶ + 2.93 x 10² T ± 7 × 10³ (1385 to 1470 K)     

Oxygen and sulfur solubilities in Ni-Fe-S-O melts

Oxygen and sulfur solubilities were determined in Ni-Fe-S-O melts under the following conditions: 10^-11.50 ≤ P_O2 ≤ 10^-8.50 atm; 10^-3.00 ≤ P_{s 2} ≤ 10^-2.00 atm; 0.19 ≤ Ni/(Ni + Fe) ≤0.85; and 1473 K ≤T ≤ 1573 K. The oxygen solubility was found to increase with increasing partial pressure of oxygen up to a maximum value at oxide saturation and to decrease with increasing equilibrium partial pressure of sulfur. The ferrous metal content enhanced oxygen solubility. The trends in dissolution behavior of sulfur were opposite to those of oxygen with respect to changing P_{O 2} and P_{S 2} and to the Ni/(Ni + Fe) ratio; however, at high matte grades Ni/(Ni + Fe) > approximately 0.5, sulfur solubility appeared to decrease as a function of the Ni/(Ni + Fe) ratio, as did oxygen solubility. The standard Gibbs energy of oxygen dissolution in Ni-Fe-S-O melts Ni/(Ni + Fe) = 0.47 in the temperature range 1473 to 1573 K can be described by ΔG° = −202.5 + 0.0660 T(K) (±1.5 kJ/mol)     

Kinetics of pyrite oxidation in sodium carbonate solutions

The kinetics of pyrite oxidation in sodium carbonate solutions were investigated in a stirred vessel, under temperatures ranging from 50 °C to 85 °C, oxygen partial pressures from 0 to 1 atm, particle size fractions from −150 + 106 to −38 + 10 μm (−100 + 150 Mesh to −400 Mesh + 10 μm) and pH values of up to 12.5. The rate of the oxidation reaction is described by the following expression:−dN/dt = SbkpO ₂ ^0.5 [OH₋]^0.1 whereN represents moles of pyrite,S is the surface area of the solid particles,b is a stoichiometric factor,k is an apparent rate constant, pO```2`` is the oxygen partial pressure, and [OH⁻] is the hydroxyl ion concentration. The experimental data were fitted by a stochastic model for chemically controlled reactions, represented by the following fractional conversion(X) vs time (t) equation: (1−X)^−2/3−1 =k _STt The assumption behind this model,i.e., surface heterogeneity leading to preferential dissolution, is supported by the micrographs of reacted pyrite particles, showing pits created by localized dissolution beneath an oxide layer. In addition to the surface texture, the magnitude of the activation energy (60.9 kJ/mol or 14.6 ± 2.7 kcal/mol), the independence of rate on the stirring speed, the inverse relationship between the rate constant and the initial particle diameter, and the fractional reaction orders are also in agreement with a mechanism controlled by chemical reaction.     

The equilibrium partitioning of titanium between Ti3+ and Ti4+ valency states in CaO-SiO2-TiO x slags

The redox behavior of titanium in CaO-SiO₂-TiO _x melts was investigated using a slag-gas equilibrium technique. Titanium partitioning between Ti³⁺ and Ti⁴⁺ valency states and the ratio of activity coefficients of TiO_1.5 and TiO₂ were determined as functions of oxygen partial pressure, temperature, and slag composition. The equilibrium experiments were carried out at temperatures between 1783 and 1903 K under CO-CO₂-Ar gas atmosphere with oxygen partial pressure ranging from 10⁻¹² to 10⁻⁷ atm (1.01×10⁻¹⁰ kPa to 1.01×10⁻⁵ kPa). The slags had CaO/SiO₂ ratios between 0.55 and 1.35 and total titanium oxide concentrations from 7 to 50 mass pct. Experimental results showed that the Ti³⁺/Ti⁴⁺ ratio in CaO-SiO₂-TiO _x slags, containing up to 50 mass pct TiO _x , increased with decreasing oxygen partial pressure and decreased with increasing CaO/SiO₂ ratio and decreasing temperature. Measured variation of the redox ratio Ti³⁺/Ti⁴⁺ with oxygen partial pressure closely followed the ideal behavior. Increasing the CaO/SiO₂ ratio increased the ratio of activity coefficients of TiO_1.5 and TiO₂. The effect of total titania content on this ratio was more complex and in accord with Raman spectroscopy data.     

Oxidation kinetics of niobium in the temperature range of 873 to 1083 K

Oxidation kinetics of niobium (columbium) in oxygen are investigated in the temperature range of 873 to 1083 K. The observed abnormal dependence of the linear oxidation rate on temperature is suggested to be due to the relative amount of NbO₂ in the oxide scale. The amount of oxygen dissolved in the metal is calculated and it constitutes a small fraction of the total oxygen uptake. From hardness measurements, the oxygen diffusion coefficient in Nb can be expressed as:D = 2.72 × 10⁻³ exp ( − 95.4/R T) with the activation energy in kjoule/mole.     

Activities of As,Sb, Bi,and Pb in copper mattes— Effect of O,Ni, and Co

The effects of oxygen, nickel, and cobalt on the activity coefficients of As, Sb, Bi, and Pb in copper mattes were measured at 1200 °C (1473.15 K) using the transportation method. The transportation experiments concerning the effect of oxygen were carried out as a function of the SO₂ content (1 to 100 vol pct) in the carrier gas and using high- and low-grade matte samples, ≈80 and ≈40 wt pct Cu, respectively. The prevailing sulfur and oxygen partial pressures were evaluated on the basis of matte and carrier gas compositions. The effect of the SO₂ pressure on the activity coefficients was found to be very small compared with the effect of the sulfur pressure, whereas the effect of the SO₂ partial pressure on the vaporization behavior, especially of As, was very significant, due to the additional vaporization of As as AsO gas molecules, which caused an increase in the As removal rate. At a higher oxygen partial pressure than 10^−8.5 atm (3.2·10⁻⁴ Pa) a noticeable decrease in the Sb activity coefficients was observed due to the oxidation. This did not, however, decrease the Sb removal rate, since the relative proportion of the oxide gas molecules in the gas phase increased simultaneously. The interactions between dissolved Ni or Co and the impurity elements were investigated by doping (1 wt pct) the high grade (Cu ≈75 wt pct) matte samples with Ni or Co. At stoichiometric and sulfur-deficient matte compositions, Ni and especially Co decreased the activity coefficients of As and Sb, but did not have any effect on the activity coefficients of Bi and Pb, compared with the corresponding sulfur content in the Ni- and Co-free mattes. For mattes of higher sulfur content Ni and Co did not show any marked effect on the activity coefficients of As, Sb, Bi, and Pb. A. ROINE, formerly with Institution of Process Metallurgy, Helsinki University of Technology, SF-02150, ESPOO, Finland     

Solutions of iron oxides in molten cryolite

All the iron oxides (FeO, Fe₃O₄, Fe₂O₃, and FeAl₂O₄) dissolve in cryolite-alumina melts to give solutions containing both Fe(II) and Fe(III). The factor controlling the Fe(II)/Fe(III) ratio is the oxygen pressure, and experimental results are interpreted on that basis. Predictions are made of the variation of solubility with oxygen pressure, and the standard potential of the Fe²⁺/Fe³⁺ redox couple is calculated. The anode and anode gas of an industrial Hall-Heroult cell appear to be insufficiently oxidizing to cause significant conversion of Fe(II) to Fe(III). An anomaly in the liquidus diagrams for FeF₂ – Na₃AlF₆ and FeO – Na₃AlF₆ is accounted for in terms of solid solution of FeF₂ in cryolite. This article is based on a presentation made at “The Milton Blander Symposium on Thermodynamic Predictions and Applications” at the TMS Annual Meeting in San Diego, California, on March 1–2, 1999, under the auspices of the TMS Extraction and Processing Division and the ASM Thermodynamics and Phase Equilibrium Committee.     

On the kinetics of oxidation of liquid copper and copper-sulfur alloys by carbon dioxide

The rates of transfer of oxygen between CO₂-CO gas mixtures and liquid copper and copper-sulfur alloys have been studied by a steady-state electrochemical technique. For sulfur-free stagnant copper, and under the conditions of the experiments, the rates are shown to be controlled by the diffusion of oxygen in the metal. The resulting diffusivities are in close accord with the bulk of the previous determinations. At high sulfur concentrations, the rate is found to be controlled by an interfacial reaction which is first order with respect to the pressure of CO₂ and inversely proportional to the sulfur concentration. The rate constant, in mole (at. pct)cm⁻² s⁻¹ atm⁻¹, is approximately 8 × 10⁻⁹ at 1146°C. Formerly a Graduate Student.     

Studies on role of oxygen in the adsorption of Au(CN) 2 − and Ag(CN) 2 − onto activated carbon

Comparative studies of the adsorption of Au(CN) ₂ ⁻ , Ag(CN) ₂ ⁻ , and Hg(CN) ₂ ⁻ onto activated carbon (Norit R2020) have suggested that oxygen and oxygen containing surface functional groups play a role in the adsorption process of Au(CN) ₂ ⁻ and Ag(CN) ₂ ⁻ but not in the adsorption of Hg(CN) ₂ ⁻ . Adsorption of Au(CN) ₂ ⁻ and Ag(CN) ₂ ⁻ on carbon degassed at 950 °C under 10⁻⁵ torr (1.33 × 10⁻³ P) vacuum is decreased by 50 pct compared with the adsorption on normal activated carbon. However, in the presence of oxygen in solution, the degassed carbon adsorbs Au(CCN) ₂ ⁻ to the same extent as normal carbon. The effect of organic solvents and the variation in the potential of the two types of carbon upon adsorption of Au(CN) ₂ ⁻ were also investigated. These results indicate that activated carbon behaves like an ion-exchange resin but is capable of oxidizing cyanide and cyanide complexes by chemisorbed oxygen. A dual mechanism for the adsorption of Au(CN) ₂ ⁻ and Ag(CN) ₂ ⁻ onto activated carbon is therefore proposed, in which cyanide complexes adsorb on carbon by anion exchange with OH⁻ followed by partial oxidative decomposition of Au(CN) ₂ ⁻ or Ag(CN) ₂ ⁻ to insoluble AuCN or AgCN. N. TSUCHIDA, formerly Postgraduate Student at Murdoch University,     

Enthalpies of formation of chromium carbides

Adiabatic oxygen combustion calorimetry has been used to determine the enthalpies of combustion of the chromium carbides Cr₂₃C₆, Cr₇C₃ and Cr₃C₂ to be—15,057.6±12.4 kJ ·mole⁻¹,—4985.3±3.8 kJ ·mole⁻¹ and—2400.5±0.9 kJ ·mole⁻¹ respectively. The products of combustion in all cases were Cr₂O₃ and CO₂. Using standard data for Cr₂O₃ and CO₂, the enthalpies of formation of the carbides have been calculated to be:fΔH ₂₉₈ ^o Cr₂₃C₆=−290.0±27.6 kJ·mole⁻¹ fΔH ₂₉₈ ^o Cr₇C₃=−149.2±8.5 kJ·mole⁻¹ fΔH ₂₉₈ ^o Cr₃C₂=−81.1±2.9 kJ·mole⁻¹