Utilisation of Carbon Dioxide for Electro-Carburisation of Mild Steel in Molten Carbonate Salts

Abstract

The CO 2 gas was utilised as a source of carbon for electro-carburisation of mild steel in carbonate containing molten salts at 800 • C. In the process, the mild steel to be carburised was made the cathode. An inert anode of SnO 2 was used to ensure oxygen gas as the by-product. Two molten salt baths, i.e. Na 2 CO 3 -NaCl (molar ratio = 4:1) and Li 2 CO 3 -K 2 CO 3 (molar ratio = 1:1), were investigated as the electrolyte and also the medium for CO 2 absorption. Microstructural changes in the electro-carburised samples, as revealed by either optical or scanning electron microscopy, were featured by the increase of the carbon rich cementite phase (Fe 3 C) at the expense of the original ferrite phase near the surface of the samples. Micro-hardness profiles measured from the surface to the centre of the electro-carburised sample presented clear evidence of carbon penetration as a function of the electrolysis voltage, and the activity of carbonate ions in the molten salts. The carbon-hardened case was up to 0.60 mm in thickness with the carbon content in the near surface region reaching saturation (Fe 3 C, 6.69 wt.%). The current efficiency of electro-carburisation depended on the cell voltage, and possible causes are discussed with the aid of a simple model correlating the hardness and carbon content.

Influence of CO2 Gas in the Electro-Carburisation Process of Mild Steel

“…The experimental procedures are described by the authors detail elsewhere [5], [6] and covered briefly as follows; Na 2 CO 3 , Li 2 CO 3 , K 2 CO 3 , and NaCl (purity 99.0% purity; Fisher Sci.) were used for preparing two carbonate containing molten salt electrolytes; Na 2 CO 3 -NaCl (4:1 mole ratio) and Li 2 CO 3 -K 2 CO 3 mixture (1:1 mole ratio).…”

“…were used for preparing two carbonate containing molten salt electrolytes; Na 2 CO 3 -NaCl (4:1 mole ratio) and Li 2 CO 3 -K 2 CO 3 mixture (1:1 mole ratio). Salt selection and composition is discussed by the author Influence of CO 2 Gas in the Electro-Carburisation Process of Mild Steel Nancy Julius Siambun, Daniel A. Jewell, and George Z. Chen elsewhere [5], [6]. A mild steel rod (0.1~0.2 wt.% C; 5 mm diam., Unicorn Metals) was made the cathode and an ELR SnO 2 rod (98.5% purity; 10 mm diam.…”

“…This is also observed in a previous study [13] of electrolytic carburisation in an open furnace. Therefore, a continuous supply of CO 2 gas into the system during electrolysis allows reaction (1) to occur and ensures a continuous supply of carbon for the carburisation during the electrolysis process [5], [6].…”

“…The molten salt electro-carburisation process that was developed by authors, whose mechanism has been described in detail elsewhere [5], [6] had successfully carburised mild steel in CO 2 gas environment, and eliminate the usage of toxic cyanide molten salt in the conventional molten salt carburisation process [7], with no formation of thick crust on salt surface and the need for continuous addition of replenishment material as source of carbon that was reported by other researchers to replace the toxic cyanide by carbonate salt [8]- [10]. The influence of CO 2 gas concentration to metal case hardening however was not studied and reported.…”

Influences of graphite anode area on electrolysis of solid metal oxides in molten salts

“…The O 2− ion can react with the CO 2 molecule generated at the anode to form the CO 3 2− ions, i.e. reaction (9), which then can dissolve in the molten salt and transport back to the cathode to be reduced to carbon, and contaminate the cathode product [18][19][20]. In theory, at sufficiently high temperatures (>887°C), the CO 3 2− ion is unstable and hence does not exert a great impact.…”