Thermodynamics of the oxygen solutions in iron-nickel melts

The oxygen solutions in Fe-Ni melts containing chromium, manganese, vanadium, carbon, silicon, titanium, or aluminum are studied thermodynamically. The equilibrium constants of the deoxidation of the melts by these elements are determined, and the activity coefficients for infinite dilution and the interaction parameters in alloys of various compositions are found. The oxygen solubilities in the alloys are calculated as a function of the nickel and deoxidizer contents. The deoxidizer contents at the minima in the oxygen solubility curves for the melts are determined, and the corresponding minimum oxygen concentrations are calculated. As the nickel content in the system increases, the deoxidizing capacities of chromium, manganese, and silicon are shown to increase substantially, and the deoxidizing capacity of carbon increases most strongly. As the nickel content in the melt increases, the deoxidizing capacities of vanadium and titanium first decrease insignificantly and then increase substantially. As the nickel content in the melt increases to 50%, the deoxidizing capacity of aluminum first decreases and then increases; in pure nickel, it is identical to that in pure iron.     

Thermodynamics of the oxygen solutions in Fe-Ni-Ti melts

The oxygen solutions in Fe-Ni melts containing up to 3% titanium are analyzed thermodynamically. The results of the works that determined the fields of the oxide phases in iron and nickel deoxidized by titanium are generalized. The proposed calculation model is shown to adequately describe the titanium deoxidation of iron-nickel alloys. The deoxidizing capacity of titanium decreases as the nickel content in the melt increases to 40% and, then, increases sharply as the nickel content increases further. The oxygen solubility curves pass through a minimum, whose position changes from 0.5644% Ti for pure iron to 0.6332% Ti for pure nickel. The points of equilibrium between the TiO₂, Ti₃O₅, and Ti₂O₃ oxide phases are determined for six alloy compositions at 1873 K. The titanium deoxidation of Fe-40% Ni melts is experimentally studied, and the calculated and experimental results are in good agreement.     

Thermodynamics of the oxygen solutions in niobium-containing Fe-Ni melts

A thermodynamic analysis is performed for the oxygen solutions in niobium-containing Fe-Ni melts. The deoxidizing capacity of niobium in iron??nickel melts is shown to be low. It decreases slightly as the nickel content in a melt increases to 40% and then increases insignificantly as the nickel content increases to 60%; a further increase in the nickel content leads to a marked increase in the deoxidizing capacity. The solubility curves of oxygen in iron??nickel melts passes through a minimum, whose position shifts toward higher niobium concentrations with increasing nickel content. Subsequent niobium additions increase the oxygen concentration in the melt. The equilibrium constants of the reactions of niobium deoxidizing of iron??nickel melts, the activity coefficients, and the interaction parameters characterizing Fe-Ni-Nb-O melts are determined.     

Thermodynamics of oxygen solutions in Fe-Ni-V melts

The oxygen solutions in Fe-Ni melts with up to 5% V are analyzed thermodynamically. The results of the works in which the fields of the vanadium-deoxidized oxide phases in iron and nickel were determined are generalized. The thermodynamic model developed for the calculation of the deoxidation of iron-nickel alloys with vanadium is shown to be adequate. The deoxidizing capacity of vanadium decreases insignificantly as the nickel content in the melt increases to 20% and increases substantially as the nickel content increases further. The oxygen solubility curves pass through a minimum, whose position changes from 2.3192% V for pure iron to 0.7669% V for pure nickel. We determined the equilibrium point [V]* between the (Fe, Ni)V₂O₄ and V₂O₃ oxide phases for alloys of six compositions at 1873 K. In nickel, [V]* is almost 200 times lower than in iron. The deoxidation of the Fe-40% Ni melt with vanadium is studied experimentally, and the experimental results agree satisfactorily with the calculated data.     

Effect of zirconium on the oxygen solubility in liquid nickel and Ni–Fe melts

The oxygen solubility in liquid nickel containing zirconium is studied experimentally for the first time at 1873 K. The equilibrium constants of the reaction of interaction between zirconium and oxygen dissolved in liquid nickel, the interaction parameters characterizing these solutions, and the zirconium activity coefficient in nickel at infinite dilution are found. The equilibrium constants of the reaction of interaction between zirconium and oxygen dissolved in the melt, the Gibbs energy of the reaction of interaction between zirconium and oxygen, and the interaction parameters characterizing these solutions are calculated at 1873 K for a wide composition range of Ni–Fe alloys. The oxygen solubility in various Ni–Fe melts containing zirconium is found at 1873 K. The deoxidizing capacity of zirconium increases as the iron content increases to 30% and decreases at higher iron content in the melt. This can be explained by the fact that an increase in the iron content lead to, on the one hand, a strengthening of the bonding forces of oxygen atoms in a melt and, on the other hand, to a significant weakening of the bonding forces of zirconium atoms with the base metal.     

Oxygen solubility in silicon-containing Fe-Co melts

Thermodynamic analysis of the oxygen solutions in silicon-containing Fe-Co melts is performed. The equilibrium constant of silicon deoxidation of iron-cobalt melts, the activity coefficients for infinite dilution, and the interaction parameters for melts differing in composition are determined. The dependences of the oxygen solubility in the melts under study are calculated for different cobalt and silicon contents. The deoxidizing capacity of silicon increases substantially as the cobalt content in a melt increases. The curves of oxygen solubility in Fe-Co melts have a minimum; the minimum oxygen solubility shifts to a low silicon content as the cobalt content in the melts increases. The silicon contents for the minima in the curves of oxygen solubility and the minimum oxygen concentrations corresponding to the silicon contents are determined.     

Thermodynamics of the oxygen solutions in boron-containing Fe–Co melts

A thermodynamic analysis of the oxygen solutions in boron-containing Fe–Co melts has been performed. The equilibrium constant of reaction between boron and oxygen, which are dissolved in iron–cobalt melts; the activity coefficients at infinite dilution; and the interaction parameters for melts differing in composition have been determined. The oxide phase formed in the Fe–Co melts containing boron and oxygen comprises FeO and CoO along with the B₂O₃ phase. The oxide phase compositions over Fe–Co–B–O melts are calculated. As the cobalt and boron contents in the melts increase, the mole fraction of boron oxide increases; in the case of pure cobalt, it is close to unity. The dependences of the oxygen solubility on the cobalt and boron contents in the melts are calculated. The deoxidizing capacity of boron substantially increases as the cobalt content in a melt increases. The composition dependences of the oxygen solubility in boron-containing Fe–Co melts have a minimum, which shifts to a low boron content as the cobalt content in the melts increases. The boron contents corresponding to the minimum in the oxygen solubility curves and the minimum oxygen concentrations corresponding to the boron contents are determined.     

Thermodynamics of Oxygen Solution in Fe–Ni Melts Containing Boron

Fe–Ni alloys are widely used in engineering today. They are sometimes alloyed with boron. Oxygen is a harmful impurity in Fe–Ni alloys. It may be present in dissolved form or as nonmetallic inclusions. The presence of oxygen in Fe–Ni alloys impairs their performance. Research on the thermodynamics of oxygen solutions in Fe–Ni melts containing boron is of considerable interest in order to improve alloy production. The present work offers a thermodynamic analysis of solutions of oxygen in Fe–Ni melts containing boron. The equilibrium constant of the reaction between boron and oxygen dissolved in the melt in such systems is determined. The activity coefficients at infinite dilution and the interaction parameters in melts of different composition are also calculated. When boron reacts with oxygen in Fe–Ni melts, the oxide phase contains not only B₂O₃ but also FeO and NiO. The mole fractions of B₂O₃, FeO, and NiO in the oxide phase are calculated for different boron concentrations in Fe–Ni melts at 1873 K. For iron melts with low boron content, the mole fraction of boron oxide is ~0.1. With increase in the nickel and boron content in the melts, the boron-oxide content in the oxide phase increases. Its mole fraction is close to one for pure nickel. The solubility of oxygen in Fe–Ni melts is calculated as a function of the nickel and boron content. The deoxidizing ability of the boron improve significantly with increase in nickel content in the melt. The curves of oxygen solubility in Fe?Ni melts containing boron pass through a minimum, which is shifted to higher boron content with increase in nickel content in the melt. The boron content at the minima on the curves of oxygen solubility are determined, as well as the corresponding minimum oxygen concentrations.     

Thermodynamics of the oxygen solutions in manganese-containing Fe-Co melts

Thermodynamic analysis of the oxygen solutions in manganese-containing Fe-Co melts has been performed. The equilibrium constants of deoxidation reaction of iron-cobalt melts with manganese, the activity coefficients during infinity dilution, and the interaction parameters in various melts are found. During the deoxidation of manganese-containing Fe-Co melts, the oxide phase contains FeO and CoO along with MnO. The compositions of the oxide phase above Fe-Co-Mn-O melts are calculated. When the cobalt and manganese contents in the melts increase, the mole fraction of manganese oxide increases, and it approaches 1 in the case of pure cobalt. The dependences of the oxygen solubility in the melts on the cobalt and manganese contents are calculated. The deoxidizing capacity of manganese increases substantially with increasing cobalt content in the melt. The curves of oxygen solubility in Fe-Co melts have minima, whose values shift toward low manganese content in a melt. The manganese contents are determined at the minimum points in the oxygen solubility curves, and the corresponding minimum oxygen contents are found.     

Thermodynamics of the oxygen solutions in the aluminum-containing Ni–Co melts

A thermodynamic analysis of the oxygen solutions in the aluminum-containing Ni–Co melts has been performed. The equilibrium constants of reactions of aluminum deoxidation of nickel-cobalt melts, the activity coefficients at infinite dilution, and the interaction parameters for the melts differing in composition at 1873 K have been determined for the first time. The dependences of the oxygen solubility on the cobalt and aluminum contents in the melts are calculated. The deoxidizing capacity of aluminum slightly varies as the cobalt content in a melt increases to 20%; at higher cobalt contents in the melts, it increases substantially. The aluminum contents at the minima in the oxygen solubility curves and the oxygen contents corresponding to the aluminum contents have been determined.     

Thermodynamics of the oxygen solutions in aluminum-containing Fe-Co melts

Thermodynamic analysis of aluminum-containing Fe-Co melts is performed. The equilibrium constants of the deoxidation of iron-cobalt melts with aluminum, the activity coefficients during infinite dilution, and the interaction parameters in melts with various compositions are determined. The oxygen solubility in the melts under study is studied as a function of the cobalt and aluminum contents. Aluminum is characterized by a very high affinity to oxygen in iron-cobalt melts. The deoxidizing capacity of aluminum substantially increases with the cobalt content in the melt. The curves of the oxygen solubility in aluminum-containing iron-cobalt melts have a minimum, whose position shifts to lower aluminum contents as the cobalt content in the melt increases. Further aluminum additions increase the oxygen concentration in the melt: the higher the cobalt content in the melt, the sharper the increase in the oxygen concentration after the minimum when aluminum is added to the melt. The aluminum contents at the minimum points in the oxygen solubility curves are determined, and the corresponding minimum oxygen concentrations are found.     

Thermodynamics of the oxygen solutions in chromium-containing Ni–Co melts

A thermodynamic analysis of the oxygen solutions in chromium-containing Ni–Co melts at 1873 K has been performed. The equilibrium constants of reactions between chromium and oxygen, the activity coefficients at infinite dilution, and the interaction parameters for melts differing in composition have been determined. The dependences of the oxygen solubility on the cobalt and chromium contents in the melts are calculated. The deoxidizing capacity of chromium slightly decreases as the cobalt content in the melt increases. The composition dependences of the oxygen solubility in the chromium-containing Ni–Co melts have a minimum, which shifts to a high boron content as the cobalt content in the melts increases. The further increase in the chromium addition leads to an increase in the oxygen content in the melt.     

Oxygen Solubility in Fe–Co Melts Containing Carbon

In thermodynamic analysis of solutions of oxygen in Fe–Co melts containing carbon, the equilibrium constants of reactions between carbon and oxygen are determined, as well as the activity coefficients at infinite dilution and the interaction parameters in melts of different composition at 1873 K. The dependence of oxygen solubility in such melts on the cobalt and carbon content is calculated. In iron–cobalt melts, carbon has high oxygen affinity. The deoxidizing ability of carbon increase significantly with increase in cobalt content in the melt. In pure cobalt, it is more than an order of magnitude greater than in pure iron. Deoxidation by carbon produces gaseous oxides: carbon monoxide (CO) and dioxide (CO₂). The reaction of carbon and oxygen dissolved in the melt and hence the deoxidizing ability of carbon depend on the total gas pressure above the melt. Decrease in gas pressure significantly improves the reducing properties of carbon. The minimum oxygen concentration for alloys of the same composition is reduced by practically an order of magnitude with tenfold decrease in the total gas pressure. The gas composition above Fe–Co melts and the equilibrium carbon and oxygen concentrations in the melt are calculated with total gas pressures of 1.0, 0.1, and 0.01 atm. The optimal oxygen concentration (1–10 ppm) in Fe–Co melts is reached at carbon concentrations between 0.01 and 1% depending on the total gas pressure (0.01–1 atm). The solubility of oxygen in iron–cobalt melts containing carbon passes through a minimum, which is shifted to lower carbon content with increase in the melt’s cobalt content. Further additions of carbon increase the oxygen concentrations in the melt. With increase in cobalt content, this increase will be sharper.     

Solubility of oxygen in carbon-bearing Fe-Ni melts

Oxygen solutions in carbon-bearing Fe-Ni melts are analyzed thermodynamically. The equilibrium oxygen concentrations in Fe-Ni alloys in the presence of carbon have been determined for the first time over a wide composition range and a wide range of the partial pressures of carbon mono-and dioxides. As the carbon concentration increases, the oxygen concentration decreases in melts of all compositions. As the nickel content in the melt increases, the equilibrium oxygen concentration decreases at the same carbon concentration. The difference in the oxygen concentrations in iron and nickel at the same carbon concentration is almost two orders of magnitude, which can be explained by the substantial weakening of the bonding forces of oxygen in the melt and the less pronounced weakening of the bonding forces of carbon atoms with increasing nickel content. The oxygen solubility curves pass through a minimum, whose position changes with the nickel content from 2.443% C for pure iron to 2.842% C for pure nickel. The solubility of oxygen in a Fe-40% Ni melt is experimentally studied at various carbon contents. The experimental results agree well with the calculated data.     

Surface tension and wettability studies of liquid Fe-Ni-O alloys

Surface tensions of iron-nickel alloys were measured as a function of oxygen potential at 1550 °C using the sessile drop technique. The surface tension of pure liquid nickel and iron-nickel alloys was measured at a total pressure of 1 atmosphere under varying CO₂/CO ratios. An increase in the oxygen potential in the gas phase was found to correspond to a decrease in surface tension of pure nickel and iron-nickel alloys, indicating that oxygen is surface active in both liquid nickel and iron-nickel alloys. At low oxygen potentials, nickel additions to liquid iron were found to cause small decreases in alloy surface tensions; however, at higher oxygen potentials, the surface tension of the alloy exhibited a minimum value as nickel was added to iron. The adsorption coefficients of oxygen in liquid iron-nickel alloys and pure liquid nickel were determined from the surface-tension data using Belton’s analysis, and were found to be similar to those calculated from kinetic studies. Wettability of iron-nickel alloys on an alumina substrate was studied through contact-angle measurements. At a constant alloy nickel content, the contact angle between the alloy and alumina decreased with increased oxygen potential in the gas phase.     

Solubility of oxygen in niobium-bearing iron-nickel melts

The solubility of oxygen in niobium-bearing iron-nickel melts is studied experimentally, for the example of Fe-40% Ni alloy at 1823 K. Niobium reduces the solubility of oxygen in this melt. Values are determined for the equilibrium constant of the reaction between niobium and oxygen dissolved in the given melt (logK _{(1)(Fe-40% Ni)} = ?4.619), the Gibbs energy (ΔG _{(1)(Fe-40%Ni)} ^o = 161210 J/mol), and the interaction parameters (e _{Nb(Fe-40% Ni)} ^O = ?0.630; e _{O(Fe-40% Ni)} ^Nb = ?0.105; e _{Nb(Fe-40% Ni)} ^Nb = 0.010). Over a wide range of concentrations, the Gibbs energy of the reaction between niobium and oxygen dissolved in Fe-Ni melts, the equilibrium constants, and the interaction parameters at 1823 K are determined. The solubility of oxygen in Fe-Ni melts of different composition containing niobium is determined at 1823 K. With increase in nickel content in the Fe-Ni melts, the oxygen affinity of niobium increases significantly, on account of the decrease in oxygen binding forces with increase in nickel content in the melt (γ _O(Fe) ^° = 0.0084, γ _O(Ni) ^° = 0.297).     

Thermodynamics of Oxygen Solutions in Ni–Cr Melts Containing Aluminum

The influence of aluminum on the solubility of oxygen in Ni–Cr melts is considered on the basis of thermodynamic analysis. Very low aluminum concentrations have practically no influence on the oxygen concentration in the melt, which is determined by the chromium content. When the aluminum content exceeds 0.01%, it determines the solubility of oxygen in the melt, in all cases. The minimum oxygen concentration corresponds to about 0.2% Al. With increase in chromium content in the melt, the minimum oxygen concentration increases. It is 2 × 10^–3%, 7 × 10^–3%, and 10^–2% for nickel alloys with 10%, 20%, and 30% Cr, respectively.     

Solubility of oxygen in Fe–Co melts containing titanium

Solutions of oxygen in Fe–Co melts containing titanium are subjected to thermodynamic analysis. The first step is to determine the equilibrium reaction constants of titanium and oxygen, the activity coefficients at infinite dilution, and the interaction parameters in melts of different composition at 1873 K. With increase in cobalt content, the equilibrium reaction constants of titanium and oxygen decline from iron (logK(FeO · TiO₂) =–7.194; logK(TiO₂) =–6.125; logK(Ti₃O₅) =–16.793; logK(Ti₂O₃) =–10.224) to cobalt (logK(CoO · TiO₂) =–8.580; logK(TiO₅) =–7.625; logK(Ti₃O₅) =–20.073; logK(Ti₂O₃) =–12.005). The titanium concentrations at the equilibrium points between the oxide phases (Fe, Co)O · TiO₂, TiO₂, Ti₃O₅, and Ti₂O₃ are determined. The titanium content at the equilibrium point (Fe, Co)O · TiO₂ ? TiO₂ decreases from 1.0 × 10^–4% Ti in iron to 1.9 × 10^–6% Ti in cobalt. The titanium content at the equilibrium point TiO₂?Ti₃O₅ increases from 0.0011% Ti in iron to 0.0095% Ti in cobalt. The titanium content at the equilibrium point Ti₃O₅ ? Ti₂O₃ decreases from 0.181%Ti in iron to 1.570% Ti in cobalt. The solubility of oxygen in the given melts is calculated as a function of the cobalt and titanium content. The deoxidizing ability of titanium decline with increase in Co content to 20% and then rise at higher Co content. In iron and its alloys with 20% and 40% Co, the deoxidizing ability of titanium are practically the same. The solubility curves of oxygen in iron-cobalt melts containing titanium pass through a minimum, whose position shifts to lower Ti content with increase in the Co content. Further addition of titanium increases the oxygen content in the melt. With higher Co content in the melt, the oxygen content in the melt increases more sharply beyond the minimum, as further titanium is added.     

Thermodynamics of oxygen solutions in nickel melts containing aluminum and titanium

Thermodynamic analysis of oxygen solutions in nickel melt shows that, as aluminum and titanium are added to the melt, the solubility of oxygen decreases. However, after reaching 0.205% Al and 0.565% Ti, the oxygen concentration in the melt begins to rise with increase in the Al and Ti content. The minimum oxygen concentrations in the reduction of nickel melt by aluminum (1.44 × 10^–4% O) and titanium (2.98 × 10^–4% O) are determined. On that basis, we may propose the optimal approach to alloying nickel melts with aluminum and titanium. First, the melt is reduced by adding sufficient aluminum to minimize the oxygen concentration in the melt (~0.2% Al). Then the oxide formed is removed, so as to prevent repeated oxidation of the melt. Finally, the melt is alloyed with aluminum and titanium to obtain the required alloy composition.     

The solubility of aluminum in cryolite-alumina melts and the mechanism of metal loss

The solubility of aluminum in cryolite-alumina melts has been determined in laboratory experiments by analyzing rapidly-quenched samples of the melt after equilibration with metal at temperatures between 960 and 1060°C. The solubility in pure cryolite increases from 0.085 at 1020°C to 0.12 wt pct Al at 1060°C. The addition of alumina decreases the solubility at 1020°C to 0.081 with 5 pct A1₂O₃ and to 0.073 wt pct Al in melts saturated with alumina. Quenched samples have been taken from operating 130 kA prebake cells at different heights above the metal pad, both in the center channel and beneath the anodes. Within about 10 mm of the cathode the metal content is close to the equilibrium value obtained in the laboratory but above this level it decreases rapidly. It is suggested that oxidation occurs in a central zone of the electrolyte. Mechanisms of metal loss and implications for current efficiency are discussed.