Hydrogen permeation behaviour and associated phase transformations in annealed AISI304 stainless steels

The effective diffusion coefficient and subsurface concentration of hydrogen in annealed AISI304 austenitic stainless steels have been measured by the electrochemical permeation method. The effects of different cathodic current densities on the effective diffusion coefficient, hydrogen concentration beneath the cathodic surface and steady state permeation current density have been studied. The value of the effective diffusion coefficient for the permeated specimens increases with increasing charging current density. The hydrogen subsurface concentration and steady state permeation current density first increase with increasing charging current density, then decrease with increasing current density. X-ray diffraction analyses were used to investigate the phase transformation during hydrogen charging. The results revealed that cathodic charging resulted in the formation of a considerable amount of ε and α′ martensites, which will increase with charging time.