Neutron Scattering and Other Nuclear Techniques for Hydrogen in Materials by Helmut Fritzsche Jacques Huot & Daniel Fruchart

Author:Helmut Fritzsche, Jacques Huot & Daniel Fruchart
Language: eng
Format: epub
Publisher: Springer International Publishing, Cham

7.4.1 In Situ Diffusion Measurements

As an example, the process of measuring a diffusion coefficient of hydrogen within a duplex stainless steel is described. Such Cr-Ni-Mo steel consists of a ferritic and an austenitic phase with a volume fraction of approximately 50 % of each phase. Both crystallographic phases differ largely in their hydrogen solubility and in their hydrogen diffusion coefficient. Neutron radiography experiments [21, 22] have been performed in order to measure an overall effective hydrogen diffusion coefficient. This hydrogen transport process is called effusion and includes, besides the volume diffusion towards the surface, desorption of hydrogen gas from the surface into the surrounding atmosphere.

Small coupons with the dimensions 45 × 5 × 2 mm3 were cut from rolled plates and the surfaces were cleaned and polished. The hydrogen was introduced by electrochemical charging for 48 h in a 0.1 M H2SO4 solution. An amount of 0.25 g/L NaAsO2 served as surface recombination inhibitor to increase the rate of inward diffusing atomic hydrogen. The current density at the cathode, i.e., at the coupons, was 10 mA/cm2. The coupons were stored in liquid nitrogen to prevent hydrogen losses due to effusion of the highly mobile hydrogen. The coupons were mounted in an infrared furnace just before the measurement at the neutron imaging facility ANTARES of the research neutron source Heinz Maier-Leibnitz (FRM II) in Garching, Germany. Hydrogen-charged and hydrogen-free coupons were each packed in stacks of five in order to obtain a reasonable image contrast between the charged sample stack and the uncharged sample stack as reference (see Fig. 7.8).

Fig. 7.8Neutron transmission image and corresponding intensity profile of duplex stainless steel (EN 1.4462) coupons with hydrogen (middle) and without hydrogen (left and right)

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