Environmental Engineering Reference
In-Depth Information
3.
Stress corrosion cracks generally originate at the surface, whereas hydrogen
embrittlement failures originate internally.
4.
HSC usually produces sharp singular cracks in contrast to the branching of
cracks observed in SCC.
Hydrogen Environment Embrittlement
Hydrogen environment embrittlement refers to the embrittlement encountered in
an essentially hydrogen-free material when plastically deformed or mechanically
tested in gaseous hydrogen. This phenomenon has been observed in ferritic steels,
nickel alloys, aluminum alloys, titanium alloys, and in some metastable stainless
steels in hydrogen gas pressures ranging from 35 to 70 MPa. Embrittlement ap-
pears to be most severe near room temperature. The degree of embrittlement is
maximum at low strain rates and when the gas purity is high. These characteristics
are comparable with those observed for HSC. Therefore, there is marked dis-
agreement as to whether this should be treated as a separate class of embrittle-
ment. There is, however, a major exception: nickel alloys are very susceptible
to hydrogen environment embrittlement, whereas they are relatively insusceptible
to HSC.
Embrittlement Due to Hydride Formation
Embrittlement and cracking of a number of transition, rare earth and alkaline
earth metals, such as titanium, zirconium, tungsten, vanadium, tantalum, niobium,
uranium, thorium, and their alloys, result from hydride formation. Significant
increases in strength and large losses in tensile ductility and impact strength are
encountered. The brittleness is associated with the fracture of the hydride particle
or its interface.
The solubility of hydrogen in these metals is 10
3
-10
4
times greater than that
of Fe, Ni, Cu, and Al and increases with decrease in temperature. The solubility
tends toward saturation at low temperatures and at atmospheric pressure the com-
position of the solution approaches that of a finite compound hydride or a pseudo-
hydride. Then either the crack may get arrested at the ductile matrix or its growth
may continue by ductile rupture of the regions between the hydrides. Hydride
formation is enhanced for some metal-hydrogen systems by the application of
stress. In such cases, the stress-induced hydride formation at the crack tip leads
to a continued brittle fracture propagation. Titanium and zirconium are known
to form stable hydrides under ambient conditions when hydrogen is absorbed in
excess of about 150 ppm. The size of the hydride particles is directly related to
the kinetics of the nucleation process. Slow cooling from higher charging temper-
atures and high supersaturation tend to precipitate large particles. Absorption of
hydrogen by these metals increases dramatically once the protective oxide film
normally present on the metal is damaged through mechanical abrasion or chemi-
cal reduction. Surface contaminants, e.g., iron smears, enhance hydrogen intake
