Using the electrochemical permeation technique and a model for hydrogen diffusion in a metal, the fluctuations of the concentration, ΔC(t), of hydrogen absorbed in the first atomic layers of an Armco iron membrane, under cathodic polarization and at the corrosion potential in sulfuric acid solution, were measured. The fluctuations of the electrode potential, ΔE(t), and of the electrolyte resistance, ΔR_e(t), induced by bubble evolution were also simultaneously recorded. Under cathodic potential, ΔE(t) and ΔC(t) are clearly induced by the evolution of big hydrogen gas bubbles. However, at the corrosion potential, another source of ΔE(t) and ΔC(t) must be proposed. It has been shown that this difference is related to the influence of an intermediate reaction species which partly blocks the hydrogen absorption under cathodic polarization and disappears at the corrosion potential.