Influence of Na₂S on the degradation kinetics of CCl₄ in the presence of very pure iron

This paper presents the results of kinetic studies to investigate the effect of FeS film formation on the degradation rate of CCl₄ by 99.99% pure metallic iron. The film was formed by submersing metallic iron grains in an oxygen free HCO₃ ⁻/CO₃ ²⁻ electrolyte solution. When the grains had reached a quasi steady-state value of the corrosion potential, Na₂S₍aq₎ was injected. Upon injection, a μm thick poorly crystalline FeS film formed immediately on the iron surface. Over time, the iron became strongly corroded and both the FeS film and the metallic iron grains began to crack leading to exposure of bare metallic iron to the solution. The effect of the surface film on the degradation rate of CCl₄ was investigated following four periods of aging, 1, 10, 30, and 60 days. Relative to the controls, the 1-day sulfide-aged iron showed a substantial decrease in rate of degradation of CCl₄. However, over time, the rate of degradation increased and surpassed the degradation rate obtained in the controls. It has been proposed that CCl₄ is reduced to HCCl₃ by metallic iron by electron transfer. The FeS film is substantially less conducting than the bulk iron metal or non-stoichiometric magnetite and from the results of this study, greatly decreases the rate of CCl₄ degradation relative to iron that has not been exposed to Na₂S. However, continued aging of the FeS film results in breakdown and stress-induced cracking of the film, followed by dissolution and cracking of the iron itself. The cracking of the bulk iron is believed to be a consequence of hydrogen embrittlement, which is promoted by sulfide. The increase in CCl₄ degradation rate, as the FeS films age, suggests that the process of hydrogen cracking increases the surface area available for charge transfer.