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Hydrogen assisted cracking of metals limits the durability of welds and constructions. Still
the failure mechanisms are not fully understood, for instance the role of hydrogen mass
transport during crack initiation. Measuring the hydrogen concentration as a function of
time (and space) would allow to determine diffusion coefficients helping to enlighten this
lack of understanding. Therefore, neutron radiography is a valuable tool for measuring in
situ hydrogen transport in iron and steels [1].
The influence of grain boundaries on the hydrogen diffusion behaviour can be determined
by comparing the diffusion behaviour of samples with e.g. different microstructure. In a
first study we investigated in situ the simplest case, the hydrogen effusion out of pure iron
(ARMCO) using neutron radiography at ANTARES [2]. Samples were charged electrochemically
ex situ with a well defined amount of hydrogen, stored in liquid nitrogen and were then
heat treated during a neutron radiography measurement whilst the change of neutron transmission
was recorded. For analysis the gray values in the pictures were transformed into
concentrations with help of an ex post calibration. Therefore, the experimental temperaturetime
profile of the heat treatment has been repeated with identical samples in the home lab
and the amount of effusing hydrogen was measured by carrier gas hot extraction. In a first
approximation the concentration of hydrogen in the sample can be calculated by subtracting
the hydrogen concentration in the atmosphere from the initial one in the sample. We
were able to measure hydrogen concentrations as low as 65 wt.ppm.
Simultaneously monitoring of hydrogenous
sample and hydrogen-free reference allowed
for in situ measurement of hydrogen effusion.
Sample heating was done by infrared radiation.
Diffusion coefficients for hydrogen effusion
from iron have been calculated. They coincide
fairly with literature data.