Oxygen and carbon sensing in Fe—O—C and Fe—O—C—Xn melts at elevated carbon contents

In the present study on solid electrolyte probes, the attempt was made to measure accurate and reproducible EMF values representing the extremely low oxygen activities in Fe—O—C melts at varying C contents. Various types of sensors were designed and successfully tested in laboratory experiments. Reliable oxygen activities were measured in Fe—O—C melts up to 4 wt. % C under pure CO gas, and f_o and f_c values were derived as a function of C content at 1 400 to 1 600°C. Further measurements were made in Fe—O—4 wt. % C—X_n melts at various contents of X_n (S_n = Si, Mn, Cr). Moreover, a solid oxide electrolyte probe with a CO gas channel to measure oxygen and carbon activities was developed and successfully tested in high-carbon iron melts.     

Equilibria between cerium or neodymium and oxygen in molten iron

Autoradiographs show that there is an obvious reaction between Ce and Nd in liquid iron and the MgO/CaO crucible wall. For reaching the Ce-O, Nd-O equilibrium, a long melting period and the addition of rare earth elements (RE’s) in several batches were needed to ensure the full reaction between the RE’s in the melt and crucible wall. The dissolved Ce or Nd content in iron was measured by means of radioactive isotopes¹⁴¹Ce or¹⁴⁷Nd and electrolysis in the organic electrolyte. The oxygen activity was measured by solid electrolyte sensors made of ZrO₂(MgO) tube. The relationships between the equilibrium constants and the temperatures for reactions Ce₂O₃ (s) = 2[Ce] + 3[O], CeO₂ (s) = [Ce] + 2[O], and Nd₂O₃ (s) = 2[Nd] + 3[O], as well as the corresponding thermodynamic parameters, have been determined. Formerly Graduate Student at the University of Science and Technology Beijing     

Activity of titanium in Fe-Cr melts

The activity of titanium in Fe-Cr melts was measured using a galvanic cell technique. By measuring the activity of oxygen in equilibrium with the molten metal and solid titanium oxide, in conjunction with the analysis of the samples for titanium, the activity coefficient of Ti was measured at 1873 K. Oxygen cells, comprised of a thoria-yttria electrolyte with a Cr-Cr₂O₃ reference electrode and an Al₂O₃ (ZrO₂)-Mo cermet tipped molybdenum lead wire, were used to take the electromotive force (EMF) measurements. The measured values of γ°_Ti and e _Ti ^Cr were 0.018 and 0.1032, respectively.     

Low oxygen activities in steel melts – Possibilities and limits of the solid electrolyte measuring technique

Review on the application of solid oxide electrolyte cells for the determination of low oxygen activities at about 0.0001 to 0.01 in steel melts. Investigation and discussion of possible error sources, such as partial electronic conductivity and oxygen ion transport in the electrolyte, reaction layers on the electrolyte surface, chemical reduction of the electrolyte and the use of inaccurate thermochemical data for the evaluation of oxygen activities, in the EMF measurement and interpretation of results. Comparison of properties of ZrO₂-and ThO₂-based electrolytes.     

New reference materials for electrochemical on-line sensors to measure steel bath composition

Further efforts have to be made to develop reliable sensors for on-line measurements in steel melts. Accurate performance of conventional sensors with a Cr-Cr₂O₃ reference over extended periods is disturbed by polarization effects mainly at the solid electrolyte/oxygen reference interface. A promising way to minimize or eliminate this problem is to reduce the flux of ionic oxygen across the solid electrolyte by applying oxygen references featuring oxygen potentials close to the actual oxygen potential of the steel melt. In search of oxygen references with 's lower than that of Cr-Cr₂O₃ mixtures, EMF measurements were made to determine the equilibrium oxygen pressures of the systems Cr-CaO-CaCr₂O₄, Cr-MgO-MgCr₂O₄, Cr-Y₂O₃-Y₂Cr₂O₆, and Nb-NbO. It is shown that the use of these reference materials enables correct on-line measurements in fully-killed steel baths over periods of 4 hours at least.     

Oxygen probes based on calcia-doped hafnia or calcium zirconate for use in metallic melts

Up to the present, fully or partially stabilized zirconia has been used as a solid electrolyte material in probes for the determination of oxygen in metallic melts. In the present study, the ionic conduction behavior of HfO₂ (CaO) solid solutions and the compound calcium zirconate CaZrO₂ have been investigated. Both polarization experiments and EMF measurements on oxygen concentration cells point out that these two highly refractory oxide materials are also most suitable solid electrolytes. Their use is particularly recommended for oxygen probe measurements in deoxidized steel melts where extremely high chemical stability and low partial electronic conductivity of the solid electrolyte is required. In the paper, properties such as crystal structure, free energy of formation, thermal expansivity, ionic and total electrical conductivity are summarized and compared for fully and partially stabilized ZrO₂, calcium zirconate CaZrO₃, HfO₂ (CaO), and ThO₂ (Y₂O₃) solid solutions.     

Mullite as an electrochemical probe for the determination of low oxygen activity in liquid iron

Oxygen activities in liquid iron deoxidized with aluminium were measured at 1873 K, using a mullite electrolyte having a Cr-Cr₂O₃ reference mixture. These results were compared with those obtained using commercial tube-type ZrO₂-8 mol% MgO and plug-type ZrO₂-9 mol% MgO probes, along with oxygen activities calculated from analysis. Oxygen activities from EMF measurements using mullite and plug-type probes were found to be in agreement with those calculated from the Al /AI₂O₃ equilibrium and those estimated from analyzed oxygen contents. Polarization effect due to electrochemical oxygen transport was observed in a commercial tube-type ZrO₂-based probe. Supersaturation in liquid iron deoxidized with aluminium was also measured at 1873 K.     

Control of the chromium-oxygen reaction in pure iron melts

Oxygen activities in Fe-O-Cr melts, equilibrated with chromium oxide, were determined as a function of chromium content by EMF measurements using ZrO₂(CaO) plug-type sensors with a Cr-Cr₂O₃ reference. Plug-type sensors with or without Cr₂O₃ saturator were used in the experiments. From the experimental results values for the activity coefficient f_O and the interaction parameters and were evaluated. Raoultian chromium activity features a slight positive deviation from ideal solution behaviour in the temperature range from 1550 to 1650°C. Taking into account the calculated interaction parameters, the equilibrium constant K_Cr-O was redetermined at 1550 to 1650°C. In comparison with previous studies a better agreement was obtained with existing thermochemical data.     

Aluminium nitride probes for application in iron melts

Review on high-temperature properties of aluminium nitride AIN. Investigations on aluminium nitride as a possible solid electrolyte at temperatures around 1600°C. Presentation of available data on the electrical conductivity, the thermal conductivity and the thermal expansion coefficient of AIN at elevated temperatures. Experimental study on the oxidation behaviour of AIN in oxidizing gases and oxygen-containing iron melts. EMF measurements on electrochemical cells of the type iron melt ‖ AIN (Al₂O₃) solid electrolyte ‖ reference representing oxygen and aluminium probes in iron melts under long-term conditions.     

Tri-phasic zirconia electrolyte for the in-situ determinations of silicon activities in hot metal

Tri-phasic zirconia electrolyte, which consists of cubic ZrO₂–MgO solid solution, monoclinic ZrO₂ and 2MgO · SiO₂, was applied to solid state electrochemical sensor for rapid determinations of silicon activities in hot metal. The performance of the cell: Mo + MoO₂ / tri-phasic zirconia electrolyte / Fe + Si + C was tested with molten iron at 1723 K. The cell potentials showed good sensitivity to the variation of silicon content in the melt.     

Oxygen activities in Cr-containing Fe and Ni-based melts

Plug-type, ZrO₂-based oxygen sensors have been used for long-term measurements of oxygen activity in Fe–O–Cr and Ni–O–Cr melts. In these melts, equilibrated with chromium oxide, oxygen activities a_O were determined as a function of Cr content. From the experimental results, data were derived for activity coefficients f_O and of 1st and 2nd order interaction parameters e_O^Cr and r_O^Cr. Cr₂O₃ has been identified as the oxide phase in equilibrium with the metal melt at ≥ 5 wt.% Cr in the case of iron and at ≥ 0.2 wt.% Cr in the case of nickel. Oxygen activities and oxygen contents in Cr-containing iron melts are lowered with increasing additions of nickel. Further investigations were directed to a_O determination in Fe–O–Cr–C and Fe–O–Cr–Al melts.     

A New Silicon Sensor for Hot Metal Measurements

In the present study, an electrochemical sensor has been designed for the direct measurement of silicon content in hot metal. The sensor represents a galvanic silicon concentration cell of the type Ni_sat—Si(1) II silicate electrolyte (1) II Fe—C—Si(1) The tubular silica-glass sensor can be immediately immersed into the metal bath at a response time of 30 to 60 s. A reproducible EMF vs. wt.% Si relationship was obtained for measurements in carbon-saturated Fe—C—Si melts at 1400°C and Si contents from 0.05 to 1.8 wt.%. This relationship is nearly identical with the cell function calculated from thermodynamic data.     

Control of the manganese-oxygen reaction in pure iron melts

EMF sensor measurements using ZrO₂(CaO) or ThO₂(Y₂O₃) plug-type sensors with a Cr-Cr₂O₃ reference were performed to redetermine the Mn-O equilibrium reaction. In these experiments a strong effect of deoxidation of manganese was ascertained at saturation with solid deoxidation products. The measuring interval of the sensors at MnO saturation was limited to about 12 min due to chemical corrosion of the used Al₂O₃ sheaths. In the case of double saturation with solid MnO · Al₂O₃ (MA) and Al₂O₃ (A) the sensors could be used as long as desirable. Moreover, the combined deoxidation of the iron melt with Mn and Al was investigated. An auxiliary electrode at the tip of the sensor ensured rapid saturation of the iron melt with solid MnO or MnO · Al₂O₃ + Al₂O₃.     

Influence of aluminum and chromium on the activity of oxygen in nickel based melts at 1600?C

The activity of oxygen in technically important nickel melts containing 15 wt pct cobalt and 5 wt pct molybdenum has been determined at 1600‡C for various concentrations of chromium ranging from 0 to 30 wt pct and aluminum varying from 0 to 15 wt pct. The activity of oxygen was measured by an electrochemical technique using yttria-doped thoria electrolyte cells. The results obtained are analyzed in terms of activity coefficients of oxygen as a function of aluminum and chromium contents in the melts. Clear positive deviations of the experimentally determined from the calculated activity coefficients of oxygen were found when aluminum was added to Ni-Co-Mo melts with or without chromium. From the results obtained in the range between 1 and 10 wt pct aluminum, the following equilibrium constants for the reaction 2 [Al] + 3 [O] ⇋ Al₂O₃ in the nickel based melts at 1600‡C were calculated: loga ₀ =-2/3 log [pct Al] - 3.94 for 0 pct Cr loga ₀ = -2/3 log [pct Al] - 4.21 for 10 pct Cr loga ₀ = -2/3 log [pct Al] - 4.81 for 20 pct Cr loga ₀ = -2/3 log [pct Al] - 5.06 for 30 pct Cr.     

Thermodynamic properties of oxygen in yttrium-oxygen solid solutions

The oxygen potential in yttrium-oxygen (Y-O) solid solutions was measured by equilibration with titanium-oxygen (Ti-O) solid solutions. Yttrium and titanium samples were immersed in calcium-saturated CaCl₂ melts at temperatures between 1108 and 1438 K, and oxygen levels in the two metals were measured. With the Ti-O system acting as a reference, oxygen potentials in Y-O solid solutions were determined. By this technique, it was possible to make reliable measurements of extremely low oxygen potentials (as low as 10^?44 atm at 1273 K), far beyond the range of solid oxide electrolyte sensors.     

Control of the aluminium-oxygen reaction in pure iron melts

Modified electrochemical oxygen sensors based on CaO-doped ZrO₂ and Y₂O₃-doped ThO₂ were developed and applied to study the aluminium-oxygen reaction in pure liquid iron with special regard to the provision of reliable experimental conditions and the attainment of true chemical equilibrium. The sensors represent valuable tools particularly for EMF long-term measurements over several hours. At 1600 °C, values of the constant and of the interaction parameters and were determined. When Fe-O-Al melts are covered with a slag phase at an activity a(Al₂O₃) < 1, e.g., CaO-saturated CaO-Al₂O₃ slag, the Al-O relationship in the iron may nevertheless be controlled by an equilibrium oxide phase at an activity a(Al₂O₃) = 1, presumably represented by inclusion particles consisting of pure Al₂O₃.     

Basic considerations on the design of oxygen probes for continuous measurements in steel melts

Review on general aspects of oxygen probes with solid reference materials for long-term measurements in steel melts. Discussion of polarization effects influencing the EMF at extended measuring intervals. Largely limited application of tubular solid electrolyte sensors in continuous oxygen monitoring. Development of improved plug-type sensors with optimized cell geometry, modified ceramic sleeves and reliable reference contact leads. Desciption of the test performance of these sensors under long-term operation up to two hours.     

Gibbs energy of formation of nickel chromite

The equilibrium partial pressure of oxygen for a system containing coexisting NiCr₂O₄, Cr₂O₃, and Ni was determined at 1000 to 1300 K using the electrochemical technique. Yttria-stabilized zirconia was employed as a solid-state electrolyte in the open-circuit electromotive force (EMF) measurements. Results were compared to the reported data in the literature. The standard Gibbs energy of NiCr₂O₄ at 1000 to 1300 K was determined to be ΔG°_f(NiCr₂O_{4)=-1327.2+0.314T±2.874 (kJ/mole)}     

Oxygen permeability of calcia-stabilized zirconia

Oxygen permeability of commercial calcia-stabilized zirconia has been measured at 1673 to 1823 K by the following cell; O₂ ZrO₂(CaO) N₂ - O₂ (P’tO₂ = 1 atm) solid electrolyte (P″O₂ = 0.39 - 10^10-3 atm). Oxygen permeability of calcia-stabilized zirconia is proportional to (1 - P″O₂ ^1/4). From the permeability measurement, the conduction properties of the electrolyte were log σ-_? ^b+ = 0.28- 5100/T and logP_b+ =-0.87 + 15,400/T where σ-_? ^b+ is the þ-@#@ type electronic conductivity at P_{O 2} = 1 atm, and P_b+ is the oxygen activity at which the þ-@#@ type electronic conductivity and the ionic conductivity are equal.     

Cathodic deposition of copper in the presence of aqueous sulfurous acid,bivalent aquacobalt ion,or both using a stainless steel cathode and a graphite anode

A study on cathodic deposition of copper in acidic aqueous sulfate solution has been carried out using a stainless steel cathode and a graphite anode. The individual and the combined effects of added [H₂SO₃·aq] and [Co²⁺·aq] on cathode potential, current efficiency, crystal orientations, and deposit morphology have been investigated and are compared. The maximum decrease of ≿50 pct in cathode potential is more pronounced in the presence of ≿10.25 g/L of H₂SO₃ alone in the electrolyte than that (≿30 pct) in the presence of ≿100 ppm of added Co²⁺ (aq) alone; however, the presence of added Co²⁺ (aq) along with H₂SO₃ (aq) does not cause further decrease in cathode potential in comparison to that observed in the presence of only H₂SO₃ (aq) in the electrolyte. The current efficiency is found to decrease in the presence of [H₂SO₃·aq] in the range of 1.32 to 30.75 g/L or in the presence of added [Co²⁺·aq] in the range of ≿10 ppm to 600 ppm, while the decrease of about 4 pct in current efficiency is more pronounced in the presence of only H₂SO₃ (aq) in the electrolyte, it is about 2 pct in the presence of only added Co²⁺ (aq) in the same electrolyte. The addition of Co²⁺ (aq) to the electrolyte containing H₂SO₃ (aq) does not alter the current efficiency (94 pct) of copper at the cathode. The linear sweep voltammetry (LSV) method was used to study the effect of added [H₂SO₃·aq], [Co²⁺·aq], or both, on the copper deposition at the cathode. The presence of each of these two additives or both causes a depolarization effect; the extent of the depolarization depends on the concentration of H₂SO₃ (aq), Co²⁺ (aq), and the current density. X-ray diffraction (XRD) data suggest that there is a change in the order of the preferred crystal orientations (viz., from the (220) plane in the absence of added H₂SO₃ (aq) and Co²⁺ (aq) to the (111) plane in the presence of added H₂SO₃ (aq) and Co²⁺ (aq) in the electrolyte solution) due to a change in the preferred plane of relative crystal growth. Results of scanning electron microscopy (SEM) indicate that cathode deposits of better surface morphology due to small-sized crystallites are found in the presence of added H₂SO₃ (aq)+Co²⁺ (aq) in the electrolyte solution. 1000 ppm=1.0 g/L=1.0 × 10⁻³ kg dm⁻³