Influence of chromium and nickel on the dissociation of CO 2 on carbon-saturated liquid iron
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k0 1 kr 1 1 KS aS
where k0 denotes the chemical rate on pure iron, KS is the adsorption coefficient of sulfur, as is the activity of sulfur corrected for Cr, and kr represents the residual rate at a high sulfur level. The rate constants and adsorption coefficient were determined to be: k 0 5 1.8 3 1023 mol/cm2 s atm kr 5 6.1 3 1025 mol/cm2 s atm KS 5 330 5 20 Experiments run at lower carbon contents showed that only a very small quantity of chromium was oxidized, immediately forming a protective layer. However, this oxidation occurred at a higher carbon content (2 pct) than what was expected from the thermodynamics.
I.
INTRODUCTION
FOR Fe-Cr-Ni alloys, the CO2 reaction is important in several metallurgical processes. For instance, in AOD processing, CO2 may be the gaseous intermediary responsible for oxidation of carbon, as it is in the case of steel decarburization by reactions [1] and [2]. CO2 1 C 5 2CO CO 1
1 O 5 CO2 2 2
[1] [2]
Processes for smelting chromite ores similar to iron bath smelting with postcombustion have been proposed. The CO2 from postcombustion may rereact with the metal, hence reducing postcombustion. Finally, if natural gas burners are used to accelerate stainless steel scrap melting in an electric arc furnace, the CO2 could cause excessive chromium oxidation. Sain and Belton[1] have shown that the reaction of decarburization of iron is of the first order with respect to the partial pressure of CO2. They proposed a mechanism in which the rate-limiting step was the dissociation of CO2 at the surface of the metal. They also examined the effect of
C.P. PETIT, formerly Research Associate, Department of Materials Science and Engineering, Carnegie Mellon University, Senior Researcher, IRSID, Metz Cedex, France F-57214. R.J. FRUEHAN, Professor, is with the Department of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, PA 15213. Manuscript submitted July 17, 1995. METALLURGICAL AND MATERIALS TRANSACTIONS B
sulfur on the kinetics of the reactions. Cramb and Belton[3] measured the rate of dissociation of CO2 on liquid iron by the 14CO2-CO isotope exchange reaction and support the conclusion that CO2 dissociation is the rate-limiting step of the decarburization reaction. Mannion and Fruehan[4] have shown the effects of sulfur, tin, phosphorus, and lead on the rate constant of CO2 dissociation. Glaws and Fruehan[5] studied the effect of chromium on the interfacial reaction kinetics between nitrogen and liquid iron, which is controlled by nitrogen dissociation at the surface of the metal. They found that for a constant sulfur activity, the rate of nitrogen reaction increased with increasing chromium concentration. It was speculated that this is because chromium forms a strong nitride, indicating nitrogen dissociation on chromium sites may be faster than on iron sites. Therefore, since chromium forms a more stable oxide than iron, it may also be expected to increase the rate of CO2 dissociation.
II.
EXPERIMENTAL
In order to determine the rate of dissociation of CO2 on F
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