Wear of graphite anodes during electrolysis of add sulphate solutions

Abstract

Graphite electrodes were prepared by mixing calcined coke and coal tar pitch. They were pressed under 250 kg cm⁻² and heat treated up to 2800° C. Rectangles measuring 70 mm x 40 mm x 8 mm were anodically polarized under galvanostatic and potentiostatic conditions. Electrolyses were conducted at 10–50 mA cm⁻² for periods ranging from 10–120 hours in Na₂SO₄ solutions acidified with sulphuric acid to various pH values. The wear of graphite anodes increased with decreasing bath temperature, increasing acid concentration, decreasing pH of the electrolyte and increasing current density. A model is suggested which assumes that corrosion takes place via the formation of a lamellar crystal compound with the formula (C ⁰₈ O)(OH)₃HSO ⁻₄ ·2H₂SO₄.

The compound is unstable at higher temperatures when corrosion is effected by oxidation of graphite by atomic oxygen. The formation of the carbon ions was found to be a necessary precondition for the formation of the complex.