Environmental Engineering Reference
In-Depth Information
the surface of the metal as atomic hydrogen, whereas gaseous hydrogen absorbs
in molecular form. Molecular hydrogen must dissociate to diffuse into the metal.
At the same time, desorption of the loosely bound molecular hydrogen is also
relatively easy. Consequently, the hydrogen buildup inside the metal varies sub-
stantially in the two processes for the equal hydrogen activities at the surface.
8.3 TYPES OF HYDROGEN DAMAGE
Hydrogen damage encountered under different conditions has been described by
numerous terminologies. The specific types of hydrogen damage may be catego-
rized as follows:
1.
Hydrogen embrittlement, which may be further subdivided as
a)
Loss in tensile ductility
b)
Hydrogen stress cracking
c)
Hydrogen environment embrittlement
d)
Embrittlement due to hydride formation
2.
Hydrogen blistering
3.
Flakes, fish-eyes, and shatter cracks
4.
Hydrogen attack
8.3.1 Hydrogen Embrittlement
Loss in Tensile Ductility
Loss in tensile ductility is one of the earliest recognized forms of hydrogen
damage. The entry of hydrogen into the metal results in significant decreases in
elongation and reduction in area without the formation of any visible defects,
chemical products, or cracking. The loss of ductility is only observed during
slow-strain rate testing and conventional tensile tests. A drop of ductility in such
tests from 42% to 7% has been reported for a carbon steel [3]. Tensile strength
is also affected, but there is no loss in impact strength. As such, impact tests
do not indicate susceptibility and are not recommended to determine whether
embrittlement exists. The extent of loss in tensile ductility is a function of hydro-
gen content of the material, as shown in Fig. 8.1.
The embrittling effect is caused by hydrogen atoms collecting interstitially
between metal atoms causing local distortion of the metal lattice. The mobility
of the dislocations is thus restricted; hence the ability of the lattice to deform.
Hydrogen atoms diffuse preferentially along grain boundaries and zones where
the lattice has already been distorted by cold working or hardening. The solubility
of hydrogen in metals obeys Sievert's law, with the concentration being directly
proportional to the square root of the pressure or fugacity.
