Behaviour of hydrogen in gas nitrided iron studied by electrochemical permeation and desorption techniques

Hydrogen permeation through and desorption from cathodically charged iron membranes with nitrides γ′, (ε + γ′) and ε layers have been studied. On the basis of the nonstationary permeation, the effective diffusion coefficients of hydrogen for the nitrided membranes, and hence the real diffusion coefficients in each nitride layer were evaluated. After cessation of charging, desorption rate of hydrogen was measured at both sides of the membrane. An analysis of the desorption rates enabled to distinguish the diffusible (mobile) hydrogen from the reversibly trapped one. Hydrogen trapping was enhanced by the γ′ precipitates and especially by the nitrided compound layers.

► Nitriding layers on iron can effectively reduce hydrogen absorption.
► The nitrided layers impede the entry and transport of hydrogen, but they enhance hydrogen trapping.
► This effects strongly depend on the chemical composition and the phase structure of nitrided layers.
► Nitriding may be an effective method to prevent hydrogen embrittlement.