Page 232 - Gas Purification 5E

21 8 Gas PuriJication

HIC (Hydrogen-Induced Cracking)

When hydrogen atoms dissolved in carbon steel meet a non-metallic inclusion, e.g., a sul-
fide or oxide particle, a slag inclusion, a lamination, or other discontinuity, they often com-
bine irreversibly to form molecular hydrogen. The molecular hydrogen, unlike atomic hydro-
gen, cannot escape; therefore, it accumulates and builds up high pressure inside the metal.
Eventually, the pressure causes the metal-inclusion interface to separate, resulting in crack-
ing or blistering. The blisters are parallel to the steel surface because the carbon steel lamina-
tions or inclusions are typically elongated parallel to the carbon steel surface when the steel
is rolled during manufacture. Figure 3-15 shows HIC blistering in a steel pipe. HIC rarely
occurs in product forms other than plate or plate products.
HIC is also called hydrogen blistering cracking or stepwise cracking. HIC depends on
steel cleanliness and composition. It has been found primarily in the bottom of absorber tow-
ers, in the amine regenerator overhead system, and in the top section of the amine regenera-
tor tower (Gutzeit, 1990). These locations suggest that the principal cause of HIC is wet acid
gas corrosion. HIC may be avoided by using specially manufactured “clean steel” plates that
are more HIC-resistant than conventional carbon steel. HIC-resistant carbon steel is made by
ladle treating with either calcium or a rare earth metal for residual sulfide-inclusion shape
control. A recently published NACE International Technical Committee Report reviews the
manufacturing and test methods for HIC-resistant steel (NACE, 1994A). Since hydrogen
induced cracking depends on the cleanliness of the carbon steel and its method of manufac-
ture, HIC cannot be prevented by PWHT (Buchheim, 1990).
SOHIC (Stress-Oriented Hydrogen-Induced Cracking)

As in HIC, SOHIC is caused by atomic hydrogen dissolved in the carbon steel lattice com-
bining irreversibly to form molecular hydrogen. The molecular hydrogen collects at imper-
fections in the metal lattice, just as in HIC. However, due to either applied or residual stress-
es, the trapped molecular hydrogen produces micro-fissures which align and interconnect in
the through-wall direction. SOHIC can propagate from blisters caused by HIC, SSC, and
from prior weld defects (Gutzeit, 1990, Bucbheh, 1990). For example, Figure 3-16 shows
SOHIC propagating from a blister caused by HIC, and Figure 3-17 shows SOHIC propagat-
ing from SSC (API, 1990). However, neither HIC nor SSC are preconditions for SOHIC
(Buchheim, 1990). In amine systems, SOHIC has been found mostly in the upper section of
the amine regenerator tower, in the amine regenerator overhead system, and in the bottom
section of the absorber below the bottom tray (Gutzeit, 1990). As with HIC, these locations
suggest that the primary cause of SOHIC is probably atomic hydrogen produced by wet acid
gas corrosion (see Figure 3-1). PWHT improves the resistance of carbon steel to SOHIC,
but does not totally eliminate it (Buchheim, 1990). In recent years, many users have speci-
fied HIC-resistant carbon steels, with PWHT, for SOHIC resistance. However, under very
corrosive laboratory conditions even HIC-resistant steels have been shown to be susceptible
to SOHIC (Cayard et al., 1994). Therefore, carbon steel plate clad with austenitic stainless
steel has been used to eliminate the risk of SOHIC. Since SOHIC is most prevalent in the
amine regenerator overhead system, cladding this area, as shown in Figure 3-1, can prevent
both SOHIC and HIC.