Page 208 - Corrosion Engineering Principles and Practice

182 C h a p t e r 6 R e c o g n i z i n g t h e F o r m s o f C o r r o s i o n 183

to long service. At other times, a plug of graphite develops in the pipe
wall. This pipe may serve for years, but if a pressure surge or water
hammer occurs, the plug may blow out. The hole left by the plug is
often greatly enlarged by the out-rushing water, creating a large
penetration. Finally, the whole wall may graphitize, and if the pipe
is subjected to a heavy earth load or perhaps a washout under a joint,
a circumferential break occurs; hence the common term “water
main break.”

6.3.6 Hydrogen-Induced Cracking
Atomic hydrogen, and not the molecule, is the smallest atom of the
periodic table and as such it is small enough to diffuse readily through
a metallic structure. When the crystal lattice is in contact or is saturated
with atomic hydrogen, the mechanical properties of many metals and
alloys are diminished. Nascent atomic hydrogen can be produced as a
cathodic reaction, either during natural corrosion processes or forced
by cathodic protection, when certain chemical species are present
which act as negative catalysts (i.e., poisons) for the recombination of
atomic to molecular hydrogen as shown in Eq. (6.3).

o
2H → H (g) (6.3)
2
If the formation of molecular hydrogen is suppressed, nascent atomic
hydrogen may diffuse into the interstices of the metal instead of being
harmlessly evolved as a gaseous reaction product. There are many
chemical species which poison this recombination (e.g., cyanides, arsenic,
antimony, or selenium compounds). However, the most commonly
encountered species is hydrogen sulfide (H S), which is formed in many
2
natural decompositions, and in many petrochemical processes [21].
Processes or conditions involving wet hydrogen sulfide, that is,
sour services, and the high incidence of sulfide-induced HIC may
result in sulfide stress cracking (SSC), which has been a continuing
source of trouble in the exploration and exploitation of oil and gas
fields, and the subject of many international standards [22]. However,
similar problems are encountered wherever wet hydrogen sulfide is
encountered (e.g., acid gas scrubbing systems, heavy water plants,
and waste water treatment).
Failures have occurred in the field when storage tank roofs have
become saturated with hydrogen by corrosion and then subjected to
a surge in pressure, resulting in the brittle failure of circumferential
welds. In rare instances, even copper and Monel 400 (N04400) have
suffered of HIC. More resistant materials, such as Inconels and
Hastelloys often employed to combat HIC, can become susceptible
under the combined influence of severe cold work, the presence of
hydrogen recombination poisons, and their presence in a galvanic
couple with a more anodic metal or alloy.