Hydrolysis of Elemental Sulphur in Water and its Effect on the Corrosion of Mild Steel

Abstract

The acidity developed by elemental sulphur when mixed with neutral water has been studied by measurements of the variaiion of pH with changes in the ratio of the solid sulphur to water and the effect of the presenece of metallic iron. The results obtained have been shown to extend the data reported at high temperature and high pressuee (145°c to 288°c and 100 MN.m⁻², respeciively) to ambient temperatures. Complete agreement was found between the values of the concentration of H₂S and H₂SO₄ calculated ror low temperature and atmospheric pressure and the values reported from chemical analysis. The hydrolysis of sulphur in water consists of the disproportionation of the octa-atomic sulphur (S₈) into oxidised and reduced compounds in the ratio H₂S/H₂SO₄ = 3/1. The activation energy for the hydrolysis of sulphur in water over a wide range of temperature (15° to 288°c) is in the range, 40–52 kJ. mol⁻¹. Elemental sulphur in the presenee of water acts as a ‘hydrogen ion carrier’, with the tendency to produce hydrogen ions concentrated at the surface of solid particles. A study of the corrosion of iron by sulphur–water mixtures has shown that an induction stage exists where pH increases as corrosion occurs with an increase in the disproportionation of elemental sulphur, stimulated by the removal of HS ions reacting with the ions produced by the dissolving surface.