Page 97 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

ENVIRONMENTALLY INDUCED CRACKING (EIC) 75

(i) Hydrogen environment embrittlement occurs during the plastic deformation of
alloys in contact with hydrogen-bearing gases or a corrosion reaction and is
hence strain rate dependent. Some examples of this are the degradation of the
mechanical properties of ferritic steels, nickel-based alloys, titanium alloys, and
metastable austenitic stainless steel when there is low strain rate and pressure of
pure hydrogen is high.
(ii) Hydrogen stress cracking or hydrogen-induced cracking (HIC) is typified by a
brittle fracture under sustained load in the presence of hydrogen. This cracking
mechanism depends on: (i) hydrogen fugacity; (ii) strength of the material; (iii)
heat treatment/microstructure; (iv) applied stress; and (v) temperature. For many
steels, a threshold stress exists below which hydrogen stress cracking does not
occur, but this is not material property as it depends on the strength of the steel
and the specific hydrogen environment. This is sometimes known as stepwise
cracking (SWC).

Hydrogen stress cracking usually produces sharp, singular cracks in contrast to
the extensive branching in SCC. Experimental evidence supporting a HE mechanism
is that immersion in a cracking solution before stress application produces a fracture
similar to a SCC fracture. The effect because of preimmersion in a cracking solution is
reversed by vacuum annealing. Testing in gaseous hydrogen produces results similar
to the crack characteristics in solutions. SCC occurs at crack velocities at which the
adsorbed hydrogen is present at the crack tip.
A critical minimum stress exists below which delayed cracking will not occur.
The value of critical stress decreases with increasing hydrogen concentration. These
effects are seen in SAE steel (0.4% C) charged with hydrogen by cathodic polarization
in sulfuric acid, followed by cadmium plating to retain hydrogen and finally subjected
to a static stress (8, 22).

1.8.10.4 Formation of Metallic Hydrides The precipitation of brittle metal
hydride at the crack tip results in considerable loss in strength, ductility, and tough-
ness of metals such as Mg, Nb, Ta, V, Th, U, Zr, Ti, and their alloys in hydrogen
environments. Alloys that form hydrides fail by ductile fracture.
Nickel and aluminum alloys may form unstable hydrides leading to hydrogen dam-
age. Some alloys may fail in hydrogen by other mechanisms.

1.8.10.5 Acceleration by Ions It has been reported that hydrogen damage is
accelerated by species such as hydrogen sulfide (H S), carbon dioxide (CO ), chlo-
2
2
+
−
−
ride (Cl ), cyanide (CN ), and ammonium ion (NH ). Some of these ions produce
4
severe hydrogen charging of steel equipment leading to HIC and stress-oriented
hydrogen-induced cracking (SOHIC), which can cause a failure. It is helpful to
know the cracking severity of the environments so that either the environment
can be modified or more crack-resistant materials are chosen. Cracking requires
the presence of nascent hydrogen atoms at the steel surface as in the presence of
H S-containing solution
2
Fe 2+ + H S → FeS + 2H
2