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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 for HIC damaged equipment=

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.