Thermodynamic properties of S-Fe-Co-Ni and Fe-Co-Ni systems
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I.
INTRODUCTION
D I S S O L V E D sulfur is generally an undesirable impurity in metals and alloys. The removal of sulfur is a difficult process that requires a basic understanding of thermodynamics of small concentrations of sulfur in liquid metallic phases. The activity of sulfur in liquid Fe, Co, Ni, and their binary alloys was presented in a previous paper by the authors, where it was concluded that the existing theories of solutions were able to present only qualitative trends in the behavior of dilute solutions of metalloids in alloys. The purpose of this investigation was to correct, summarize, interpret, and present an unpublished set of data 2 on the activity of sulfur in ternary alloys of Fe, Co, Ni. Complete interpretation of the results required critical selection of thermodynamic data for the liquid binary alloys of metals from which the corresponding data for Fe-Co-Ni were obtained. The method used for determination of sulfur activities was equilibration of dissolved sulfur in a melt with mixtures of hydrogen and hydrogen sulfide, i.e., H2(g) + I S in melt] = H2S(g);
K" =
PH,S
PH2 " XI
[1]
where K[ is the equilibrium product for sulfur in a liquid phase "i", P is the pressure in atmospheres, and X~ is the mol fraction of sulfur. For a given solvent, K,! is independent of sulfur concentration when XI is less than 0.05. The activity of sulfur is a~ = 7~ XI, where a~ is the activity, and Y~ is the activity coefficient, and the reference state for Yl is the dilute solution of S in pure Fe; hence, KFe =
Pros -
PH2 " 7 1 X ] 2)
=
PH,~S = K~; PH2 ~ X ] 2) (X] 2) < 0 . 0 5 ;
71 =
1)
[2]
where X]2) is the mol fraction of sulfur in Fe as indicated by the superscript 2, and further for XI 2) less than 0.05, a~ = X]2) and KF~ = K~e. Actually, the tool fraction of sulfur in the present investigation was kept to less than 0.015 to assure that al can be taken as X] 2) for liquid iron. The values of K[ vary with the solvent alloy composition, and the activity coefficient y~ is such that it makes K [ / y l the N. A. GOKCEN is Research Supervisor, United States Bureau of Mines, Albany, OR 97321. M.R. BAREN is Associate Professor, College of Engineering and Architecture, Temple University, Philadelphia, PA 19122. Manuscript submitted May 11, 1984. METALLURGICAL TRANSACTIONS A
same as Kve because S-Fe is taken to be the reference state. Since KFe = K~/7~, then the activity coefficient 71 is obtained from 7t = K~/KFc. This choice of reference state for the activity is equivalent to the selection of an arbitrarily fixed value of P.2s/P. 2, or a corresponding pressure of gaseous diatomic sulfur as the standard state, which was discussed previously by the authors.L II. EXPERIMENTAL P R O C E D U R E AND RESULTS
The experimental procedure was presented previously in detail.~'3 Briefly, it consisted of the equilibration of a given mixture of H2S(g) and H2(g) with a liquid metal phase at 1825 K in a resistance furnace. The gas was bubbled through the melt to assure rapid attainment of equilibrium. The inlet com
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