Validity of the Critical Thickness of Steel for Volume Controlled Diffusion During Measurement of Electrochemical Hydrogen Permeation

Abstract

In the electrochemical hydrogen permeation measurement, the rate-determining step can be governed by either bulk diffusion or surface reaction depending primarily on the thickness of steel membrane. In order to validate the critical thickness for the volume-controlled hydrogen atom transport in the permeation test under cathodic polarization condition, the sheet-type thin steel membrane with various thicknesses in the range from 0.5 mm to 2 mm is evaluated. The experimental results demonstrate that the permeation flux evaluated under cathodic polarization is inversely proportional to the steel thickness down to 0.5 mm Based on the verified membrane thickness for the volume-controlled diffusion, a comparison of hydrogen diffusivity with respect to microstructure of the ferritic steel is discussed. It clearly indicates that the increase in fraction of pearlite and bainite in the microstructure contributes effectively to slower diffusion kinetics due mainly to the hydrogen trapping at various trap sites in the microstructures.