Hydrogen is often a byproduct of corrosion and electrochemical
processes in aqueous solutions. It may also be a major constituent
of service environments. In aqueous environments, atomic hydrogen
results from the donation of a proton to a hydrogen ion to form
atomic hydrogen on the surface of the material. The effects of
hydrogen on cracking are contrasted to those of local metallic
dissolution in Fig. 1. Depending on the solution and interfacial
characteristics. the hydrogen atoms formed by the corrosion process
may recombine to form molecular hydrogen that can accumulate and
bubble off of the specimen surface. However, under certain circumstances,
such as when hydrogen recombination poisons (e.g., S, P, As, Sn)
are present in the environment, hydrogen recombination is retarded,
which promotes the ability of atomic hydrogen to enter the material.
Once inside the material, hydrogen can affect the mechanical performance
of materials in several ways:
- The formation of internal hydrogen blisters or blister-like
cracks at internal delaminations or at sites of nonmetallic inclusions
in low strength materials. These internal cracks may propagate
by a process called hydrogen-induced cracking (HIC) or hydrogen
blistering. No external stress on test specimens is usually required
to examine this type of cracking. in some cases, however, these
blister cracks may take on an alignment caused by the presence
of residual or applied tensile stresses.
- The process of hydrogen-assisted microvoid coalesce can occur
during plastic straining. This can reduce the ductility of normally
ductile engineering materials while not inducing brittle cracking.
- An extreme case of ductility loss from hydrogen is the brittle
fracture of susceptible materials under applied or residual tensile
stresses. This form of cracking, which typically changes from
transgranular to intergranular with increasing yield strength
and other processing variables, is normally referred to as hydrogen
embrittlement cracking (HEC).
With respect to HE and HEC, most susceptible materials show a
major effect of stress concentration (i.e., notches) and level
of stress intensity and tend to produce failures in a relative
short time (i.e., <1000 h). Therefore. tension, notched, and
precracked specimens and fracture methods are widely utilized
in the evaluation for HEC. Once hydrogen has entered a material,
it can produce delayed failure (i.e., fracture resulting well
after application of a load on the specimen).
Also See Hydrogen Embrittlement in Cracking
