Synthesis of chromium carbide by reduction of chromium oxide with methane

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Abstract

In the present work, production of the chromium carbide was investigated by reduction of chromium oxide with methane-containing gas mixture. The experiments were conducted on the chromium oxide powder and methane gas at different temperatures, times, and gas mixtures. X-ray diffraction (XRD) analysis was used to characterize the products at different stages of reduction. The morphology of the starting chromium oxide powder and Cr3C2 were studied by electron microscopy technique. The results showed that the minimum temperature and time for carbide formation in 30%-methane gas mixture is about 850 °C and 20 min, respectively. X-ray diffraction analysis showed that Cr3C2 is the only carbide product. The formation of chromium carbide in 30%-methane gas mixture was completed at 1000 °C and 60 min.

Introduction

The chromium carbides belong to the transition metals of VI group of the periodic table. These carbides have properties such as high melting points, good strength and a good corrosion resistance. Chromium carbide films are used for protecting steel and other alloys from chemical attacks. There are three forms of chromium carbides (Cr3C2, Cr7C3, and Cr23C6) with different atomic ratios of carbon to chromium element. Of these carbides, Cr3C2 has a good set of properties of high strength, very good coverage resistance, low density, and good chemical stability. This compound has a very high resistance against corrosion and oxidation up to 900 °C, and it dissociates only at very high temperature of 1813 °C [1].

The main applications of chromium carbide include: thermal spray powder for corrosion and wear resistance coatings [2], [3], hard facing welding electrodes, special tools with maximum chemical resistance, additives as reinforcement in composite materials, substitute for hard chromium coating [4] and coating on carbon fibers for enhancement of wetting with metals [5].

The chromium carbides are produced by using numerous methods such as melting and rapid solidification [6], thermit process [7], carburizing of chromium metal and chromium oxide [8], combustion synthesis [9], mechanical–thermal synthesis [10] and aluminothermy reduction [11]. Chromium carbide coatings can be created by chemical vapor deposition (CVD), physical vapor deposition (PVD), reactive evaporation, and sputtering [12], as well.

Reduction with methane applies for metal and metal carbide production from oxide compounds. The advantages of this method include shorter time and lower temperatures for chromium carbide formation than other methods. In comparison with other chromium carbide production methods, raw materials are inexpensive and never need any post-treatment for separation and refinement, in a later method. Production of iron carbide from iron ore [13], manganese and its carbide from manganese oxide [14], reduction of cobalt oxide to metallic cobalt [15] and titanium oxy-carbide production from titanium oxide were investigated by reduction with methane.

Although application of reduction with methane for production of different carbides has improved during recent years, application of this method to produce chromium carbide still needs to be investigated. The kinetics of reduction of chromium oxide with methane gas at isothermal conditions was studied by Khoshandam [16].

The aim of the present work is to define the conditions for reduction of pure chromium oxide with methane containing natural gas. For this purpose, the reduction process was carried out at different temperatures, times, and gas mixtures.

Section snippets

Experimental

Cr2O3 powder (Merck, 99.96% purity) was used as the precursor material, compressed into flat pellets (diameter: 10 mm, thickness: 1 mm) with 50 kg/cm2 pressing pressure. The porosity of pellets was 63% ± 2. Fig. 1 shows the scanning electron microscopy of chromium oxide powder. As can be seen, the particle size varied from 0.2 to 2 μm, and the mean particle size is about 0.8 μm. The particles have no specific geometrical shapes.

The reducing gas was prepared by mixing high-purity argon (99.9%) with

Results and discussion

X-ray diffraction patterns of samples at various stages of reduction of Cr2O3 by 30%-methane gas mixture (at constant time of 60 min) at different temperatures are shown in Fig. 2. The diffraction pattern of sample at 800 °C shows only chromium oxide peaks indicating no reduction. Considering the following reaction and its standard Gibbs free energy, the equilibrium temperature at standard conditions is about 835 °C:Cr2O3+13/3CH4→2/3Cr2C2+3CO+26/3H2ΔG°=1097519.4-989.79T(J)

On the other hand, XRD

Conclusions

It was observed in the present investigation that production of chromium carbide from chromium oxide by reduction with methane is possible. Minimum temperature and time of reaction progress was obtained about 850 °C (for 60 min) and 20 min (at 1000 °C), respectively. Optimum content of methane in reducing gas was assessed about 30%. Below this limit, chromium oxide was not reduced to chromium carbide completely. Also with higher content of methane, soot formation prevents reaction progress. X-ray

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