Environmentally assisted cracking [EAC] provides a generic reference to four
recognized types of EAC
• stress corrosion cracking [SCC]
• hydrogen induced cracking [HIC]
• corrosion fatigue [CF]
• liquid metal embrittlement [LME]
We confine our brief discussion to stress corrosion cracking, SCC, HIC and its variation,
stress oriented hydrogen induced cracking, SOHIC.
SCC / HIC/ SOHIC / Classification
SCC refers to crack propagation driven by an anodic corrosion reaction at the crack tip.
HIC involves loss of bond strength due to atomic hydrogen at grain boundaries and interstitial
sites and a cathodic reaction at the crack tip. SOHIC is potentially more damaging than
HIC being driven by high levels of residual or applied stress. SOHIC usually appears adjacent
to the weld HAZ, presents as through thickness damage and appearing as arrays of cracks
stacked on top of each other.
A common theme among these cracking modes is the presence of wet H2S where wet
H2S refers to the presence of, among other conditions
• > 50 wppm dissolved H2S in free water
• > 0.05 psia partial pressure of H2S in the gas phase
To preclude or mitigate the onset of this damage mechanism, PWHT, weld hardness limits and use
of "clean" steels is mandated by many industry practices. However, this does not necessarily
eliminate long term crack development but rather, only delays it. This delay is a function
of KI,EAC, a parameter defining the time dependent threshold stress intensity
factor for subcritical crack growth of the material / environment system.
FFS of Refining & Petrochemical Plant Equipment
ASME FFS - 1 / API 579 - 1 fitness for service assessment methodologies are available and require
augmentation to identify the sub-critical crack growth stage, the use of fracture mechanics
to identify conditions for critical crack growth and the need for
rigorous risk assessment of any FFS treatment.
FAD risk assessment mapping
The FAD mapping indicates the uncertainty that is expected in an initial FFS assessment. To
"tighten" the evaluation, appropriate subject matter expertise working within the
FFS assessment team are needed to ensure all essential data are identified
and available to decide a repair / replace / continue to operate decision.
John Aumuller, P. Eng., Ph. D.