Environmental Engineering Reference
In-Depth Information
discussed in the previous section. While these mechanisms are well understood,
controversies exist regarding the mechanism of hydrogen embrittlement, particu-
larly HSC, in steels and in other alloys. The absorbed and dissolved hydrogen
could have a variety of effects, and the various theories proposed (1-4) differ
on this count. Since the critical events associated with HSC occur on an atomic
scale at inaccessible crack tips, the theories cannot easily be verified. Each of
these theories can explain certain observations related to the phenomenon,
whereas the others remain unexplained. The salient features of these theories
have been discussed subsequently.
The phenomenon of hydrogen trapping may be considered in this context.
Diffusion studies of iron and steel have shown lag time for hydrogen diffusion
through these materials before a steady-state diffusivity compatible with that ex-
pected theoretically is achieved. The lag time is attributed to the interaction of
hydrogen with impurities, structural defects, or microstructural constituents in
the metal, which is referred to as ''trapping.'' Hydrogen accumulates at these
internal interfaces, called ''traps.'' These hydrogen traps may be mobile (such
as dislocation and stacking faults) or stationary (such as solute atoms, particle
interfaces, grain boundaries, cracks, and voids). The traps have been classified
as reversible or irreversible [8]. Short-duration trapping of hydrogen is referred
to as reversible, whereas a long residency time for hydrogen characterized by a
high binding energy is termed irreversible trapping. Deep or reversible traps act
as sinks for hydrogen and reduce the population of hydrogen at the crack tip,
thus increasing the resistance to HIC.
8.4.1 Hydrogen Pressure Theory
This is the earliest mechanism of hydrogen embrittlement proposed by Zappfe
and Sims [9]. According to this theory, the atomic hydrogen diffuses through the
metal lattice and accumulates at preexisting voids and other internal surfaces in
the alloy. As the concentration of hydrogen increases at these sites, it recombines
to form molecular hydrogen. A high internal pressure is created that enhances
void growth or initiates cracking. The sequence is represented schematically in
Fig. 8.15.
Pressure theory can successfully explain the phenomenon of hydrogen blis-
tering and also the occurrence of internal defects such as flakes and shatter cracks.
The unusual temperature and strain rate dependency of hydrogen embrittlement
could be explained in terms of the diffusion rate of hydrogen. At high strain rates
or low temperatures, the diffusion of hydrogen to the voids would not be sufficient
and the susceptibility to embrittlement is thereby reduced. However, the theory
has been less successful in explaining the brittle behavior of high-strength steels,
or nonbrittle fracture with loss of ductility found in some low-strength materials.
