Surface oxide reduction by hydrogen permeation through iron foil detected using a scanning Kelvin probe

The interaction of atomic hydrogen with the air-formed surface oxide film of an iron foil, following cathodic hydrogen evolution on the reverse face, is shown to produce significant Volta potential changes which can be mapped using a scanning Kelvin probe (SKP). The magnitude of the Volta potential depression decreases with holding time following hydrogen charging and exhibits a dependence upon the oxygen partial pressure of the holding environment. It is proposed that the Volta potential depression observed in the area of hydrogen emergence results from a change in the ratio of FeIII/FeII states within the surface oxide.

► Hydrogen diffusion through iron foil is studied by scanning Kelvin probe.
► Local Volta potential changes are detected in the region of hydrogen emergence.
► A dependence on oxygen partial pressure is observed.
► Atomic hydrogen at the metal–oxide interface reduces Fe(iii) to Fe(ii).
► Fe(ii) is re-oxidised at the oxide–air interface, maintaining a local equilibrium.