Effect of sulphide ions on corrosion behaviour of iron in alkaline solutions

Potential time curves are constructed for Fe electrodes in aerated NaOH solutions devoid of, and containing, increasing concentrations of S2- ions. In sulphide free solutions, the steady state potentials are reached from negative values indicating oxide film thickening. Lower concentrations of S2- ions enhance oxide film thickening and the potential moves more in the noble direction. Increasing the S2- ion content causes the potential to fluctuate in the active and noble directions during a time span, which depends on the S2- ion content. The steady state potential of the iron varies linearly with both the concentration of S2- and pH of solution. A mechanism is proposed for the formation of FeS involving a two electron transfer process.