What causes High-temperature hydrogen attack? Atomic
hydrogen (formed in corrosion processes or by dissociation
of molecular hydrogen in a gas stream at the steel surface)
diffuses into steel. At grain boundaries, crystal
imperfections, inclusions, discontinuities and other
defects, the atomic hydrogen reacts with the dissolved
carbon or with the metal carbides, forming methane:
8Н+C+Fe3C=2CH4+3Fe (eq.1) Because of the pressure build-up
of the methane in the steel, this results in the formation
of intergranular cracks (refer to the micrograph above),
fissures and blisters, often extending to the surface of the
steel. This form of hydrogen damage sometimes resembles the
low-temperature hydrogen blistering. Moreover, the
decarburization process leads to the loss of carbon in the
steel and hence a reduction in tensile strength and an
increase in ductility and creep rate. Interestingly, the
reverse process (Eq.1 above), carburization, can also occur
in hydrogen-hydrocarbon mixtures such as that encountered in
petroleum- refining operations. Nelson curves are commonly
used to select the various grades of steels and the saf
temperature and hydrogen partial pressure.
