Page 396 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

374 CONSEQUENCES OF CORROSION

Blisters are a form of HIC in which the buildup of hydrogen gas pressure at the ini-
tiated cracks or preexisting laminations results in localized deformation and bulging
of the steel to the closest surface. High hydrogen pressures occur in blisters. Blisters
occur when the hydrogen-induced crack is unable to propagate and is unable to link
up with HIC on adjacent planes in the steel.
Stepwise cracking (SWC) is a form of HIC in which adjacent cracks in different
planes link up in a stepwise manner. This can lead to through-wall cracking and the
loss of vessel integrity. SWC does not require applied or residual stress. The hydrogen
gas pressure in the HIC cracks can be as high as several hundreds of atmospheres and
generate stress at the tips of HIC cracks, leading to plastic deformation of the steel.
The linkup of cracks leads to HE cracking.
The presence of tensile stress in the component may cause individual ligaments
of HIC to form a stack of through-thickness array. This is a necessary precursor
to stress-oriented hydrogen-induced cracking (SOHIC). SOHIC is common in heat
affected zones (HAZs) of welds in low-strength steels.
Examples of SOHIC failure in spiral-welded line pipe are common forms of fail-
ures. Failures were also encountered in electric-resistant welded pipes because of
HIC. Failures of seamless line pipes have also been reported (30). HIC-related dam-
age and failures of 20 line pipes in H S and CO atmospheres have been reported (19).
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Despite the availability of adequately HIC-resistant line pipe since the late 1970s,
failures are still occurring in pipelines constructed since then.
HIC occurs when hydrogen atoms diffusing through a line pipe steel get trapped as
H molecules in the inhomogeneities in the steel. A planar gas-filled defect is created,
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and it grows parallel with the vessel surfaces and traps more H atoms. The defect may
grow to the extent of forming a blister. HIC failure occurs when the defects result in
a blister. HIC failure occurs when the defects or blisters link up inside or outside the
vessel surfaces with the possible linkage by SWC or SOHIC.
Nitrate Stress Corrosion Cracking (Scc) of Carbon Steel in the Heat
Recovery Steam Generators of a Cogeneration Plant The carbon steel cas-
ings of heat recovery steam generators (HRSGs) in a cogeneration plant developed
cracks that originated on the inside surfaces. The HRSGs receive hot exhaust from
the primary GTs, which are fueled with pipeline natural gas. The GTs were initially
operating without any suppression of nitrogen oxides.
After about 2 years of operation, cracks were found to penetrate through
1 / 4-inch-thick casings. Steel samples cut from the casings showed: (i) fine cracks
propagating through the steel from inside surface; (ii) presence of large amounts of
nitrate on the inside surfaces. Nitrate ions originated from the nitric acid on the walls
and the nitric acid formed by the combination of nitrogen oxides and moisture.
The outer surface of a steel plate cut from HRSG casing had a crack running per-
pendicularly to the butt weld. The majority of the cracks associated with welds joining
the plates was perpendicular to the welds and ran through the weld. Repair-welded
areas also cracked. Metallography showed small branched cracks originating at the
inner surface and running across the plate. Highly branched intergranular cracks led
to the conclusion that the cause of the failure is intergranular SCC because of nitrate.