Environmental Engineering Reference
In-Depth Information
Figure 8.15
Schematic representation of hydrogen diffusion and recombination at in-
ternal voids.
Theoretical analyses have shown [10] that voids can become pressurized if hydro-
gen is transported by dislocations into voids faster than hydrogen escapes from
voids, but there is no experimental evidence that such an effect occurs.
8.4.2 Surface Adsorption Theory
Surface adsorption theory is based on the well-known Griffith criterion for frac-
ture in ideally brittle solids, which relates the energy for fracture to the energy
required to produce new surfaces and is represented by
σ
c
(2
E
γ
s
/
π
c
)
1/2
(8.3)
where
σ
c
is the fracture stress necessary to cause the propagation of an elliptical
crack of length 2
c, E
is Young's modulus, and
γ
s
is the surface energy. The
fracture stress is proportional to the square root of the surface energy and any
reduction in the term would increase the brittleness. The mechanism proposed
by Petch and Stables [11] suggests that the reduction in surface energy is caused
by the adsorption of hydrogen on the walls of microcracks. The fracture stress
is thereby reduced and this enhances crack propagation at stress levels below
those typically experienced for a particular alloy in an inert environment. In met-
als, however, the Griffith microcracks are not inherently present but are produced
by plastic deformation. Accordingly, the crack should propagate when
(
σ
P
)
nb
2(
γ
s
γ
p
)
(8.4)
