Determination of the hydrogen fugacity during electrolytic charging of steel

•Thermodynamic relationships were established between hydrogen activity and potential.•Permeability transients were used to measure the critical parameters.•Cathodic pre-charging conditioned the entry side of the permeability specimen.•The hydrogen fugacity during electrochemical charging conditions was determined.•The most severe hydrogen fugacity was ∼6500 atm.

This work studied the determination of the hydrogen fugacity during electrolytic charging. With a virgin surface, there were irregular permeation transients, attributed to irreproducible surface conditions. Cathodic pre-charging conditioned the entry side to a stable state. Permeability transients were used to measure the critical parameters in the thermodynamic relationship between hydrogen activity and electrochemical potential. At the same overpotential, the hydrogen fugacity in the pH 12.6 0.1 M NaOH solution was higher than that in the pH 2 0.1 M Na2SO4 solution, attributed to differences in (i) the hydrogen evolution reaction, (ii) the surface state, and (iii) the true surface area.