The effect of a constant tensile load on the hydrogen diffusivity in dual phase steel by electrochemical permeation experiments

This work considers the effect of a constant load on the hydrogen diffusion characteristics studied by electrochemical permeation experiments. Dual phase steel and Armco iron are used for this purpose. Different degrees of load, both in the elastic and plastic regime, are applied on the sample during the permeation experiment when hydrogen is migrating through the material. The resulting transients show that applied elastic tensile stresses increase the hydrogen diffusion coefficient of both materials. Due to the elastic stress, the crystallographic lattice expands which increases hydrogen diffusivity. From an applied constant load of 100% of the yield strength and higher, the diffusivity decreases again. This is associated to the formation of lattice defects such as vacancies, vacancy dusters and dislocations. The strain state of the specimen in the constant loading device at different loading conditions is obtained by digital image correlation. This revealed that the in-situ instrumentation is key and confirmed that a homogeneous strain distribution is present in the zone of interest for the permeation experiments.