Page 99 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

ENVIRONMENTALLY INDUCED CRACKING (EIC) 77

the steel and a major portion will evolve on the external surface. But some of the
atomic hydrogen may diffuse into a void and combine to form molecular hydrogen.
Molecular hydrogen builds up pressure in the voids. The pressure of molecular hydro-
gen in contact with steel is several hundred thousand atmospheres, which can cause
rupture of any known engineering material. Filiform corrosion can cause blisters, and
hydrogen evolution under paint can result in swelling, but the mechanisms of forma-
tion of these similar defects in appearance are totally different from that of hydrogen
blistering.

1.8.10.8 Hydrogen Blistering Hydrogen blistering occurs because of hydrogen
atoms diffusing through the steel and accumulation at hydrogen traps such as voids
around inclusions. Hydrogen atoms combine and form molecular hydrogen in the
trap. The hydrogen pressure builds up in the trap resulting in HIC and formation of
blisters. Blisters occur in low-strength steels of yield strength <80 ksi or 535 MPa
yield strength. The blisters are generally formed along elongated nonmetallic inclu-
sions or laminations in the steel pipe (102, 103).
Two types of HIC cracks, namely, centerline cracks and blister cracks are shown
in Figure 1.21. Blister cracks are related to the type and distribution of nonmetallic
inclusions in the steel. MnS inclusions as well as planar arrays of other inclusions
are generally initiation sites for cracking. The cracks propagate along a longitudinal
direction, which is also the direction of alignment of the inclusions.
The steels that have high concentrations of manganese and sulfur lead to the forma-
tion of MnS inclusions. Rolling of steel leads to elongation of the inclusions followed
by an increase in surface area of hydrogen traps. Low sulfur bearing steels are not
necessarily HIC resistant as alloying for inclusion shape control, reduced centerline
segregation, and reduction of nitrides and oxides are also necessary. High sulfur bear-
ing steels are not necessarily susceptible to HIC as microsegregation and inclusion
shape are more important factors than bulk sulfur content of the steel (105).

1.8.10.9 Hydrogen Sulfide Damage The most extensively studied failures in prac-
tice is the pipeline steel used in the petroleum industry. Cracking failure in pipeline
steel exposed to sour gas in an oil medium is commonly encountered.

1.8.10.10 SOHIC This form of corrosion is caused by atomic hydrogen dissolved
in steel combining to form molecular hydrogen. The molecular hydrogen collects
at defect sites in the metal lattice similarly to HIC. Because of applied or residual
stresses the trapped molecular hydrogen produces microfissures that align and inter-
connect in the through wall direction. Although SOHIC can propagate from blisters
caused by HIC and SSC, and from prior weld defects, neither HIC nor SSC is a
precondition for SOHIC (106, 107).
Just as in the case of HIC, the primary cause of SOHIC is probably atomic hydro-
gen produced at the steel surface by wet acid gas corrosion (108).
SOHIC tends to occur in the base metal adjacent to hard weldments in pipe
and plate steels where cracks may initiate by sulfide stress cracking. SOHIC is
characterized by interlinking microscopic cracks oriented both in the direction