Use of solid-electrolyte Galvanic cells to determine the activity of CaO in the CaO-ZrO 2 system and the standard Gibbs
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AGfl/J mol-' = -25,200 ( + 150) - 17.58 (_+0.085) T(1633 to 1873 K)
I.
INTRODUCTION
U N T O L D ultraclean steel is deoxidized by the addition of aluminum, followed by desulfurizing and deoxidizing by the addition of calcium and zirconium. As a result of the deoxidation and desulfurization of molten steel, considerable oxide and sulfide inclusions, such as AI203 CaO. ZrO2, 12CaO 9 7A1203, MnS, and CaS, are produced. The shape and composition of the inclusions as well as the decreasing of their amount and size should be controlled and the primary crystallization of MnS during solidification can be suppressed because of improvement of the mechanical properties of steel. Nagata et al.Vl found that A1, Ca, and Zr can control the composition of inclusions in sour pipe steel. In this case, the primary crystallization of MnS during solidification can also be suppressed.t2.3I The formation of inclusions can be thermodynamically determined by the activities of Ca, AI, Zr, and Mn as well as oxygen and sulfur potentials and temperature in molten steel. Then, the activities of CaO, A1203, and ZrO 2 in the inclusions of CaOAI:O 3 and CaO-ZrO2 systems are very important. Nagata et al.I4I measured the activities of CaO and A1203 and the standard Gibbs free energies of calcium aluminates in the CaOA1203 system from oxides by the mean of the Galvanic cell using 4CaO 9 PzO5 as a solid electrolyte. Pizzinl and MorlottiISI and Levitski et al.t6I measured tlae activity of CaO in the CaO-ZrO 2 system at 1173 to 1373 K and 1300 to 1550 K, respectively, with Galvanic cells using CaF2 as a solid electrolyte. The CaF2 solid electrolyte exhibits anionic Frenkel disorder (V~ and F ' i ) [7] and has the melting point of 1646 K. At temperatures greater than 1633 K, CaF 2 as solid electrolyte becomes precarious because CaF2 undergoes an interface reaction with CaO at 1633 K. In the present study, the Galvanic cell method with 4CaO 9 P205 as a solid electrolyte was employed for measuring the activities of CaO and the standard Gibbs free en-
JUN TANABE, Assistant Professor, is with the Department of Mechanical Engineering, Faculty of Engineering, Nippon Institute of Technology, Saitama 345, Japan. KAZUHIRO NAGATA, Professor, is with the Department of Metallurgical Engineering, Faculty of Engineering, Tokyo Institute of Technology, Tokyo 152, Japan. Manuscript submitted January 6, 1995. 658--VOLUME 27B, AUGUST1996
ergy of formation CaZrO3 from oxides, because the 4CaO 9 P205 compound is a Ca-'* conductor with a high melting point of 1983 K and is in equilibrium with CaO. II.
EXPERIMENTAL
The CaO-ZrO2 phase diagramtSI shows that C a Z r O 3 exists below about 2523 K. The ordered phase of CarZrwO44 (~b2) forms by a peritectoid reaction at 1628 K ( _ 15 K) and a similar peritectoid reaction forms the CaZraO9 (~bt) phase at 1508 K. At 1413 K and 17 mol pct CaO, the cubic phase (Css) transforms with a eutectoid reaction to a tetragonal phase (Tss) and ~b~. At 1273 K and 4 mol pct CaO, Tss transforms with a eutectoid reaction to monoclinic phase (
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