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Mullite as an electrochemical probe for the determination of low oxygen activity in liquid iron
Steel Research International
Abstract
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 commerical 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/Al2O3 equilibrium and those estimated from analyzed oxygen contents. Polarization effect due to electrochemical oxygen transport was observed in a commerical tube-type ZrO2-based probe. Supersaturation in liquid iron deoxidized with aluminium was also measured at 1873 K.
... Stability diagrams are an important tool to investigate the deoxidation equilibrium and formulate the strategies of controlling inclusions in the steelmaking and refining process. Extensive investigations were carried out focusing on systems such as Fe-Al-O [1][2][3][4][5][6][7][8][9], Fe-Mg-O [10][11][12], Fe-Al-Mg-O [13][14][15], Fe-Al-Ti-O [16][17][18][19][20][21], Fe-Al-Ca-O [22], and Fe-Al-Mg-Ca-O [23,24], and the effects of crucibles and slags on the formation of inclusions in these systems were also discussed [25,26]. These stability diagrams have been extended and successfully applied to industrial production to control the type of inclusions in molten steel [27][28][29][30][31][32][33][34]. ...
... Considering only the first-order interaction coefficients is incorrect according to the curve's shape, which shows that aluminum with content higher than 2% can barely deoxidate. Many experimental results proved that considering both the first-order and second-order interaction coefficients was more accurate [1][2][3][4][5][6][7][8][9]. 8) and (9). ...
... , the presence of slags, as well as their compositions, has a significant impact on the equilibrium concentrations of [Al] and [O][4,6,7]. Slags affect the Al deoxidation equilibrium in two ways. ...
Stability diagrams are of great importance for investigating deoxidation equilibrium and controlling inclusions in molten steel. Combined with programming for calculation and drawing, the three-dimensional stability diagrams of MgO, MgO·Al2O3, and Al2O3 in the Fe-Al-Mg-O system were successfully drawn, which can visually explain the meaning of the iso-oxygen contours and the borderlines among different phases. They also offer straight evidence that the borderline method is imperfect. The three-dimensional stability diagrams can visually show more vital details that cannot be illustrated by the iso-oxygen contours. The most significant advantage of three-dimensional stability diagrams is not only judging the most stable phase in the specific component area but also visually showing the transformation trend among the phases from a three-dimensional perspective, especially when the equilibrium surfaces of these phases are relatively close. Besides, an interesting transformation of inclusions in the Fe-Al-Mg-O system was found. When the magnesium content is higher than 0.0065% and the aluminum content is lower than 0.21%, the equilibrium products of aluminum-magnesium composite deoxidation may be transformed from MgO to MgO·Al2O3 and Al2O3 in turn, which can be directly observed from the three-dimensional stability diagram based on the calculations.
... However, the acid-insoluble Ce could not be determined due to the dissolution of Ce-containing inclusions by the acid. Acid-insoluble Al contents for a given total Al level tend to be slightly higher than those in previous experiments [5] of the Fe-0.005 to 0.21 mass pct Al alloys, which are represented by a shaded area. Total Al (M) content represents the sum of the acid-soluble and acid-insoluble Al (M) concentrations, which correspond to the dissolved Al (M) in metal and Al (M)-containing inclusions, respectively. ...
... The validity of this assumption is explained elsewhere. [6] In previous equilibrium experiments, [5] the Fe-10 mass pct Al alloy was added to the iron melt containing the initial oxygen of 200 to 300 mass ppm, which is far above the critical supersaturation value. In this case, therefore, alumina spontaneously precipitated. ...
... The supersaturation ratio, , with respect to alumina SЊ Al O 2 3 precipitation is defined as [9] ϭ C, Te, Mn, Cr, and Si) alloys are plotted against the contents of alloying elements in Figure 13. The error bar at [mass pct M] ϭ 0 was calculated from the data in present and previous [5] experiments, as shown in Figure 2. These values decrease with the addition of the alloying elements, and in the range of [mass pct M] ≥ 0.2 to 0.5, no supersaturation with respect to the precipitation of Al 2 O 3 is observed. ...
The effect of the alloying element M (M=C, Te, Mn, Cr, Si, Ti, Zr, and Ce) on the supersaturation of alumina precipitation
in the Fe-0.0017 to 0.41 mass pct Al-M alloys was studied at 1873 K in an Al2O3 crucible based on the contents of analyzed aluminum and oxygen. It was found that the supersaturation ratio with respect
to Al2O3 precipitation,
S_{Al_2 O_3 }^ \circ \{ = (a_{Al}^2 \cdot a_{\underset{\raise0.3em\hbox{}}{O} }^3 )_{obs} /(a_{Al}^2 \cdot a_{\underset{\raise0.3em\hbox{}}{O} }^3 )_{eq} \} S_{Al_2 O_3 }^ \circ
values were found to be independent of the contents of the alloying elements up to the compositions of [mass pct Ti]=1.03,
[mass pct Zr]=0.08, and [mass pct Ce]=0.07.
... Effect of products size on Al-deoxidation equilibrium thermodynamics. The equilibria for Aldeoxidation in liquid iron have been extensively investigated [12][13][14][15][16][17][18][19][20][21][22][23][24][25] . Table 2 lists the experimental conditions and methods of Al-deoxidation equilibrium in liquid iron at 1873 K. ...
... On the other hand, in order to make the Al-deoxidation reaction more close to the final equilibrium state in liquid iron, Rohde et al. 16 , Suito et al. 19 , Paek et al. 25 carried out their experiments with covered CaO-Al 2 O 3 flux on the liquid iron. Meanwhile, the equilibrium oxygen potential in liquid iron was sometimes directly measured by using Electro Motive Force (EMF) technique 15,17,20,21 . It can be seen from www.nature.com/scientificreports/ ...
Products of Al-deoxidation reaction in iron melt are the most common inclusions and play an important effect on steel performance. Understanding the thermodynamics on nano-alumina (or nano-hercynite) is very critical to explore the relationship between Al-deoxidation reaction and products growth in iron melt. In present study, a thermodynamic modeling of nano-alumina inclusions in Fe–O–Al melt has been developed. The thermodynamic results show that the Gibbs free energy changes for the formation of nano-Al2O3 and nano-FeAl2O4 decrease with the increasing size and increase with the increasing temperature. The Gibbs free energy changes for transformation of nano-Al2O3 into bulk-Al2O3 increase with the increasing size and temperature. The thermodynamic curve of nano-alumina (or nano-hercynite) and the equilibrium curve of bulk-alumina (or bulk-hercynite) obtained in this work are agree with the published experimental data of Al-deoxidation equilibria in liquid iron. In addition, the thermodynamic coexisting points about Al2O3 and FeAl2O4 in liquid iron are in a straight line and coincide with the various previous data. It suggested that these scattered experimental data maybe in the different thermodynamic state of Al-deoxidized liquid iron and the reaction products for most of the previous Al-deoxidation experiments are nano-alumina (or nano-hercynite).
Using mullite (3Al2O3·2SiO2)-tube type and ZrO2 (MgO mole fractions of 9 %)-plug type solid electrolyte, the activities of Al in the Fe-15, 29% Ni-0.002 to 0.84% Al (mass contents in%) alloys were measured at 1873 K in an Al2O3 crucible, and those of Si in the Fe-13, 28% Ni-0.13 to 2.3% Si alloys were measured at 1823 K in a mullite crucible with an excess of SiO2. The interaction coefficients of and were determined as −0.0173 ± 0.0052 and 0.00021 ± 0.00007, respectively, and those of and were determined as 0.0094 ± 0.0049 and −0.00008 ± 0.00004, respectively. The oxygen activities obtained by the EMF measurements were compared with those obtained from the contents of analyzed oxygen. The oxygen activities in the Fe-Ni-Al alloys were found to be supersaturated. The interaction coefficients of and were determined as 0.0027 ± 0.0012 and 0.00012 ± 0.00004, respectively, from the contents of analyzed oxygen in Fe-Ni-Si alloys. Die Aluminiumaktivität in Fe-15, 29% Ni-0.002 bis 0.84% Al- (Massenanteil in %) Legierungen wurde bei 1873 K in einem Al2O3-Tiegel mit Festelektrolyten gemessen. Die Aktivität von Silicium wurde ebenfalls ermittelt, hierzu wurden Fe-13, 28% Ni-0.13 bis 2.3% Si-Legierungen bei 1823 K in einem Mullittiegel unter SiO2-Überschuß herangezogen. Man erhielt für die Wechselwirkungskoeffizienten −0.0173 ± 0.0052 bzw. 0.00021 ± 0.00007 für und 0.0094 ± 0.0049 für bzw. −0.00008 ± 0.00004 für . Die aus EMK-Messungen resultierenden Sauerstoffaktivitäten wurden mit den Aktivitäten verglichen, die sich aus der Sauerstoffanalyse ergaben. Fe-Ni-Al-Legierungen erwiesen sich als übersättigt an Sauerstoff. Die Wechselwirkungskoeffizienten und wurden aus dem Sauerstoffanalysengehalt in Fe-Ni-Si zu 0.0027 ± 0.0012 und 0.00012 ± 0.00004 bestimmt.
The effect of sulfur addition on supersaturation observed in aluminum deoxidized liquid iron was studied at 1873 K, by using CaO-Al2O3 slags in an alumina or lime crucible. The contents of supersaturated oxygen for a given aluminum level were not influenced by the addition of sulfur (mass contents of 1×10−5 ∼ 0.42%) in the experiments where the sample was rapidly quenched. However, in the experiments in which the sample was cooled from 1873 to 1723 K at a cooling rate of 2.7 K/min, the effect of sulfur on supersaturation was observed for mass contents of more than 0.01% sulfur. The critical supersaturation for alumina precipitation was discussed based on the variation of the interfacial energy between liquid iron and alumina. Die Wirkung einer Schwefelzugabe auf die Übersättigung in Al-desoxidierten Eisenschmelzen bei 1873K wurde mit CaO-Al2O3-Schlacken in Tonerde- und Kalktiegeln untersucht. Die Gehalte an überschüssigem Sauerstoff für einen bestimmten Aluminiumgehalt wurden nicht von der Schwefelzugabe beeinflußt (Massenanteil von 1×10−5 ∼ 0,42%), wenn die Probe schnell abgeschreckt wurde. In den Versuchen jedoch, in denen die Probe von 1873 auf 1723 K mit einer Abkühlgeschwindigkeit von 2,7 K/min abgekühlt wurde, zeigte sich ein Einfluß des Schwefels auf die Übersättigung bei Massengehalten von mehr als 0,01% S. Die kritische Übersättigung für Tonerdeausscheidungen wird auf der Grundlage der Veränderung der Grenzflächenenergie zwischen Schmelze und Tonerde diskutiert.
The effect of selenium addition on supersaturation observed in aluminium deoxidized molten iron was studied at 1873K, by using Fe-Al alloys and CaO-Al2O3 slags in an alumina crucible. Selenium mass content was varied from 0.0005 to 1.81%. In experiments where the samples were rapidly quenched, the contents of supersaturated oxygen for given aluminium levels were not influenced by the addition of selenium. However, the effect of selenium addition on supersaturation was observed for mass contents of more than 0.009% selenium. The selenium distribution ratio between liquid iron and CaO-Al2O3 slags was determined as a function of the aluminium content of the melt.
Basic properties of muUite such as chemical composition, phase stability, crystalline structure and microstructure are briefly reviewed. Effect of impurities, resulting in the formation of grain boundary glassy precipitates on high temperature properties such as mechanical properties and ionic conductivity is considered. Properties of mullite which is free of the glassy phase are analyzed as an ionic conductor for high temperature oxygen sensors. Electromotive force (EMF) of an oxygen concentration cell, based on mullite (which is free of impurities or involving an excess of silica) as an oxygen conductor, indicates that the electronic conductivity component at high temperatures assumes negligible values especially at very low oxygen activities (below 5ppm).
Thermodynamics of Zr deoxidation equilibrium in liquid iron has been studied at 1 873 K by using ZrO(2)-9mol% MgO and mullite (3Al(2)O(3)center dot 2SiO(2)) electrolytes coupled with the quantitative alalysis of soluble Zr and insoluble Zr as ZrO(2) by using a potentiostatic electronic extraction method. The soluble 0 content for a given soluble Zr content is lower than the previously reported values in the plot of soluble O vs. soluble Zr contents, and the oxygen activity for a given soluble Zr content is higher than the previously reported values. The free energy change of solid Zr dissolution into iron melt: Zr(s)=Zr at 1 873 K is obtained as Delta G degrees=-153 +/- 10 kJ. The interaction coefficients for e(O)(Zr), r(O)(Zr), r(O)(Zr,O) are estimated as -70 +/- 15, 901 +/- 131 and 10300 1 500, respectively. However, the following interaction coefficients are represented by using a continuous function of logarithms of X, where X=[%sol.Zr]+5.7[%sol.O]+17.1[%sol.Zr].[%sol.O]/(2[%sol.Zr]+5.7[%sol.O]): [GRAPHICS]
Aluminum deoxidation equilibrium of molten Fe-Ni alloy was determined by chemical equilibrium method at temperature of 1 873 to 1 973 K. Also, critical point in molten Fe-40mass%Ni alloy that coexisted with Al(2)O(3) and FeO center dot Al(2)O(3) was determined experimentally at 1 973 K. Solvent extraction method was applied to improve the analytical accuracy of the low aluminum content in molten Fe-Ni alloy. The relation between aluminum and oxygen contents in molten Fe-Ni alloy equilibrated with Al(2)O(3) or FeO center dot Al(2)O(3) was estimated in the whole alloy composition range at 1 873 to 1 973 K by utilizing the metallic solution model based on
The supersaturation ratio with respect to alumina precipitation, S-Al2O3 (=(partial derivative(Al)(2) .partial derivative(O)(3))(ss)/(partial derivative(Al)(2) .partial derivative(O)(3))eq), in Fe-0-0.008 similar to 0.090mass%Al melt was electrochemically measured at 1873K in an Al2O3 crucible by blowing CO2 gas or adding an Fe-2mass%Al alloy intermittently, using plug-type ZrO2-9mol%MgO and tube-type mullite probes. The S-ZrO2 (=(partial derivative(Zr).partial derivative(O)(2))ss/(partial derivative(Zr).partial derivative(O)(2))eq) value in Fe-O-0.04mass%Zr melt was also measured at 1873 K in a ZrO2-11mol%CaO crucible by adding an Fe-4mass%Zr alloy. As a result, the critical values for log S-Al2O3 and log S-ZrO2 were 3.5 and 1.3, respectively, No supersaturation for the precipitation of SiO2 was observed. The presence of solute element M (M=C, Mn, Cr, Si, and Ti) in an Fe-O-0.148 similar to 0.158mass%/Al melt was found to significantly influence the supersaturation phenomenon.
We have investigated thermally induced cation surface segregation in mullite/zirconia composites with zirconia content ranging from 0% to 100% using x-ray photoelectron spectroscopy (XPS). As a result of thermal annealing at 1000°C, slight surface Si enrichment with respect to Al was found in the pure mullite sample (0% zirconia). In contrast to the mullite sample, surface segregation in Si, Na, Al and Y was observed in the pure zirconia sample. The segregation behaviour of all the composite samples (10-94 mass% zirconia) is dominated by the zirconia phase. For samples with 10-100 mass% zirconia, segregation leads to the formation of silicates and sodium oxides at the surface.
This document is part of Volume 11 `Ternary Alloy Systems: Phase Diagrams, Crystallographic and Thermodynamic Data', Subvolume D `Iron Systems', of Landolt-Börnstein - Group IV `Physical Chemistry'. It contains crystallographic and thermodynamic data about the ternary alloy system: Al-Fe-O
Electrical conductivities of various mullite/zirconia composites, as well as monolithic mullite and zirconia, were measured using AC impedance spectroscopy from 100 Hz to 10 MHz at temperatures ranging from 150 to 1300 °C. The impedance spectra of monolithic zirconia and mullite/zirconia composites showed two semicircles because of the contributions from grains and grain boundaries, while those of monolithic mullite had one semicircle due to the predominant contribution from grains. This indicates that the conductivities of the mullite/zirconia composites increased with zirconia content. The activation energies of electrical conduction in mullite and zirconia were about 65 and 79 kJ/mol, respectively, and those of mullite/zirconia composites were between 65 and 79 kJ/mol. While the conductivities of various composites at 1 MHz were fitted by Lichtenecker’s rule, the general mixing equation could be applied to the conductivities measured at 1 kHz.
Using a mullite (3Al2O3 · 2SiO2)-tube and ZrO2-9 mol pct MgO-plug type solid electrolyte galvanic cells, the activities of supersaturated oxygen in Fe-0.0017 to 0.41 mass
pct Al-M (M=C, Te, Mn, Cr, Si, Ti, Zr, and Ce) alloys were measured as a function of total Al or M contents at 1873 K in an alumina
crucible. Based on these results, the effects of alloying elements on the supersaturated oxygen activity with respect to alumina
precipitation were studied. In the Fe-Al-M (M=C, Te, Mn, Cr, and Si) alloys, the supersaturated oxygen activities for a given Al level approach the equilibrium values
with increasing contents of alloying elements. However, the oxygen activities for a given Al level are still supersaturated
in the Fe-Al-M (M=Ti, Zr, and Ce) alloys even in the presence of considerable amounts of the alloying elements.
A coulometric titration technique was used for the determination of the parameterP
, defined as the oxygen partial pressure at which the ionic conductivity and n-type electronic conductivity of the electrolyte are equal, for mullite and 9 mol% MgO stabilized zirconia electrolytes in the temperature range 1674–1921 K. The results ofP
for mullite and ZrO2-9 mol% MgO were log(P
/101 325 Pa)=54.32–137000/T and log (P
/101 325 Pa)=27.16–75500/T, respectively. The results for low oxygen activities in liquid iron are discussed on the basis of the present results forP
values.
The metallurgical industry faces challenges in the development and production of new products in response to rapidly changing technologies that demand materials with widely different properties and increasingly stringent quality control. Such materials include semiconductors, ultrahigh-purity metals, chemical-vapor-deposited metallic films, high-performance intermetallics, metallic superconductors, and metal-based composites. In view of this, the author propounded the establishment of value-addition metallurgy as a subdiscipline of extractive and process metallurgy. Such a subdiscipline would cover the principles and practice involved in the production of these value-added advanced materials based on metals. In this respect, this annual review article now includes a section covering papers concerned with these topics.
The effect of tellurium addition on supersaturation observed in aluminum deoxidized liquid iron was studied at 1873 K, using
CaO-Al2O3 slags in an alumina crucible. Supersaturated oxygen content for a given aluminum level decreased with the addition of tellurium
as a result of lowering the interfacial energy between liquid iron and alumina and promoting the elimination of precipitated
alumina by flotation. Furthermore, the degree of lowering of oxygen content was found to increase with a decrease in cooling
rate. Tellurium distribution ratio between liquid iron and CaO-Al2O3 slags was determined as a function of aluminum content.
The method for the determination of oxygen in iron containing aluminum was studied by use of the inert gas fusion-infrared absorptiometry as a function of flux, temperature and heating pattern. The double graphite crucibles containing Sn, more than four times of the sample weight, were first degassed at 3000 K and the temperature was reduced to 2630-2760 K. Then the sample enclosed by a nickel capsule was dropped into the tin bath and fused for 30 s. This analytical procedure made the oxygen determination in an Fe-10∼15 mass%Al alloy possible without observation of the “gettering effect” due to aluminum. The recovery of oxygen was confirmed to be almost 100% by the analysis of alumina powder. The results for the oxygen in a Ti-25∼51 mass%Al alloy was also discussed.
Aluminium-oxygen equilibria between CaO-Al2O3 melts and liquid iron were studied in the temperature range of 1 823 to 1 923 K, using an alumina and lime crucibles. The equilibrium constant K for the reaction: 2Al+3O= Al2O3 was discussed with reference to the supersaturation for nucleation of alumina, and compared with the previous experimental and thermodynamic data. The supersaturation was observed in the experiments in which an Fe-Al alloy with low concentrations of initial oxygen was used under the condition of no stirring. Supersaturation increased with a decrease of oxygen content in liquid iron. Two types of alumina inclusions with a size of 2-5 μm and below 0.1 μm were observed by SEM.
The solubility and activity of oxygen in Fe−Al and Fe−Ti melts at 1600°C were measured. The activity was measured electrochemically
using the following galvanic cells: Cr-Cr2O3(s) ⋎ ThO2(Y2O3) ⋎ Fe-Al-O(l), Al2O3(s) Cr-Cr2O3(s) ⋎ ThO2(Y2O3) ⋎ Fe-Ti-O(l, saturated with oxide) Cr-Cr2O3(s) ⋎ ZrO2(CaO) ⋎ Fe-Ti-O(l, saturated with oxide) Aluminum and titanium decrease the solubility of oxygen in liquid iron to a minimum of 6 ppm at 0.09
wt pct Al and 40 ppm at 0.9 wt pct Ti, respectively. The value of the interaction coefficients ε0(Al) and ε0(Ti) are −433 and −222, respectively. the activity coefficient of aluminum at infinite dilution in liquid iron is 0.021, while
that of titanium is 0.038. The value of the aluminum equilibrium constant, the solubility product at infinite dilution, is
5.6×10−14 at 1600°C. The ThO2(Y2O3) electrolyte exhibited insignificant electronic conductivity at 1600°C down to oxygen partial pressures of 10−15 atm, which corresponds to about 0.3 ppm O in unalloyed iron.
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