Hydrogen Diffusivities and Concentrations in 520M Carbon Steel under Cathodic Protection in 0.5M NaCl and the Effect of Added Sulphite, Dithionite, Thiosulphate, and Sulphide

Hydrogen diffusion experiments in 520M carbon steel were performed with the Devanathan- Statchurski permeation cell at 22°C in 0.5M NaCl at pH 7 and -800, -900 or -1050 mV SCE. The hydrogen diffusivity and subsurface concentration (C0) were evaluated by fitting a fourier series expansion of the diffusion equation to the permeation data. The hydrogen diffusivity in the steel was found to be approximately 2×10-7 cm2/s. Furthermore, the influence of sulphite, dithionite, thiosulphate and sulphide on hydrogen uptake in 520M were studied at -800, -900, and -1050 mV at two different concentrations (10 or 100 mM in 0.5M NaCl) and pH 7 in the charging cell solution. Generally, sulphur components were found to increase the hydrogen permeation rate, basically with one exception. The hydrogen permeation rate was found to be reduced at -800 mV when thiosulphate was present in the solution. The increased hydrogen permeation rate is assumed to be caused by an increase in C0, which was found to increase when the sulphur compounds were present in the charging cell solution. The highest increase in C0 was found for solutions containing sulphide. The sulphur species are believed to poison the steel surface which increases the chemical potential of adsorbed hydrogen on the surface.