ELECTROCHEMICAL WEAR OF GRAPHITE ANODES DURING ELECTROLYSIS OF BRINE

Graphite electrode samples measuring 30 × 30 × 6 mm were machined from an industrial synthetic graphite plate. These were anodically polarized in sodium chloride solution under galvanostatic and potentiostatic conditions. Electolyses experiments were performed within the current density of 25–50 mA/cm−2 for periods ranging from 20–160 hours. The effects of chloride ion concentration, bath temperature, brine pH value and current density on the wear rate of the graphite anodes were investigated. The results obtained reveal that corrosion rate of the graphite anodes during electrolysis of brine takes place 1. (1) due to the neucleophilic attack of water, and 2. (2) via the formation of a chloride-graphite compound having the chemical formula: The compound was isolated and identified. It decomposes to carbon slimes, carbon monoxide, carbon dioxide, chlorine gases, and water. A model for corrosion of graphite anodes has been suggested which assumes that chloride ions are adsorbed on the anode blanketing its surface from the brine attack, and hence inhibit corrosion rate. The overall formation and decomposition reactions of the isolated compound are an endothermic process whereby wear rate is enhanced with rise in temperature. Increasing the current density imparts a corresponding increase in positively charged carbon atoms maintained on the anode surface leading to an increase in wear rate. The latter attains its maximum at neutral rather than in acidic or alkaline solutions. Dense electrode material having mesopores is less corroded than electrodes having pores with pore diameter ⩾5,000 nm.