62 Fundamentals of Corrosion
3.10.2.1.3 Hydrogen Environment Embrittlement
Hydrogen environment embrittlement is the embrittlement encountered
in an essentially hydrogen-free material when it is plastically deformed
or mechanically tested in gaseous hydrogen. This phenomenon has been
observed in ferritic steels, nickel alloys, aluminum alloys, titanium alloys,
and some metastable stainless steels in hydrogen gas, pressures ranging
from 35 to 70 MPa. Embrittlement appears to be most severe at room tem-
perature. The degree of embrittlement is maximum at low strain rates and
when the gas purity is high. These characteristics are the same as those
observed for HSC. Because of this, there is some question as to whether or
not this should be treated as a separate class of embrittlement. However,
there is one exception. While nickel alloys are very susceptible to hydrogen
environment embrittlement, they are relatively unsusceptible to HSC.
3.10.2.1.4 Embrittlement due to Hydride Formation
Embrittlement and cracking of titanium, zirconium, tungsten, vanadium, tan-
talum, niobium, uranium, thorium, and their alloys are the result of hydride
formation. Signicant increases in strength and large losses in tensile ductil-
ity and impact strength are found. The brittleness is associated with the frac-
ture of the hydride particle or its interface.
The solubility of hydrogen in these metals is 10
3
to 10
4
greater than that of
iron, copper, nickel, and aluminum and increases with a decrease in tem-
perature. The solubility tends toward saturation at low temperatures and
at atmospheric pressure. The composition of the solution approaches that
of a nite compound hydride or a pseudo-hydride. The crack gets stopped
at the ductile matrix or continues to grow by ductile rupture of the regions
between the hydrides. For some metal–hydrogen systems, the application of
stress increases hydride formation. In these cases, the stress-induced hydride
formation at the crack tip leads to a contained brittle fracture propagation.
Titanium and zirconium form stable hydrides under ambient conditions
and hydrogen is absorbed in excess of 150 ppm. Absorption of hydrogen by
these metals increases rapidly if the protective oxide lm normally present
on the metal is damaged mechanically or by chemical reduction. Surface
contaminants (e.g., iron smears) enhance hydrogen intake, and the absorp-
tion is accelerated at temperatures exceeding 160°F (70°C). Hydrogen is rap-
idly picked up during melting or welding, and hydride formation takes
place during subsequent cooling. When sufcient hydrogen is present, the
cracking is attributed to the strain-induced formation of hydrides.
3.10.2.2 Hydrogen Blistering
This type of damage is prevalent in low-strength, unhardened steels as a
result of the pressure generated by the combination of atomic hydrogen to
form molecular hydrogen.