Thermodynamic investigations of the C-Cr-Fe system by galvanic cell technique
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and the individual electrode reactions as
I. INTRODUCTION
THE knowledge of the thermodynamics of the C-Cr-Fe system is very important in designing heat- and corrosionresistant steels. Over the years, there have been several experimental and theoretical investigations on the equilibrium phases at various temperatures in this particular system.[1–10] The reliability of the data, however, has been discussed in detail in a recent publication by Klekamp.[11] It is also important to point out that, to the knowledge of the present authors, no experimental data are available on the activity of chromium in the system in question below 1300 K. In the literature, it has been discussed that the electromotive force (EMF) technique is especially suited for determination of the thermodynamics of metal-carbon alloys.[11,12] It is a well-known fact, however, that chromium has a strong affinity to oxygen, which will lead to serious problems with oxidation of chromium during the experimental measurement. Based on the authors’ experience in performing EMF measurements, as well as the use of the standardized inert gas purification system developed at the Department of Materials Science and Engineering, Royal Institute of Technology (KTH), the thermodynamic activities of chromium in different C-Cr-Fe alloys have been successfully studied. During the present EMF measurements, single crystals of calcium fluoride (CaF2) were used as the solid electrolyte. II. EXPERIMENTAL The solid-state galvanic cell method involving CaF2 electrolyte was employed to investigate the activity of chromium in several different C-Cr-Fe alloys. The galvanic cell arrangement used, as well as the experimental setup for EMF measurements, was similar to that described earlier by the present authors.[13] The galvanic cell used to determine the electromotive force can be presented as follows: () Pt, Cr CrF2 CaF2//CaF2//CaF2 CrF2 alloy-Cr, Pt ()
[1]
L.D. TENG, Doctor, R.E. AUNE and M. SELLEBY, Doctors, and S. SEETHARAMAN, Professor, are with the Royal Institute of Technology, 10044 Stockholm, Sweden. Manuscript submitted June 19, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS B
Cr 2F 4 CrF2 2e
(left)
[2]
CrF2 2e 4 Cr-alloy 2F
(right)
[3]
Cr 4 Cr-alloy
(overall)
[4]
A. The Raw Materials and Electrode Preparation Four groups of C-Cr-Fe alloys with different carbon contents were selected for the present EMF measurements. In each group, the mass ratio of C/(C Cr) was kept constant, and the Fe content were changed from 20 to 80 mass pct. The nominal compositions of the investigated alloys are presented in Table I (CCF denotes the C-Cr-Fe system). The isothermal phase diagram of the C-Cr-Fe system, originally assessed by Andersson in 1988[4] and here calculated by using the TCFE3 database[14] available with the ThermoCalc software,[15] is presented in Figure 1. The composition of the present investigated alloys has been marked in Figure 1. Sample preparations were made in 20-g quantities with fine powders of Cr (99.95 pct pure, suppli
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