Hydrogen Induced Cracking (HIC) and Stress Oriented Hydrogen Induced Cracking (SOHIC) are damage mechanisms that affect carbon and low-alloy steels in wet H2S (sour) service environments. Both involve atomic hydrogen diffusing into the steel lattice, but the failure mechanism and assessment approach differ.
HIC occurs when hydrogen atoms recombine at internal defects (inclusions, laminations) forming molecular hydrogen, creating internal pressure that causes planar cracking parallel to the plate surface. SOHIC occurs when HIC cracks link through-thickness driven by applied or residual stress, creating a vertical crack network that can lead to rapid through-wall failure.
API 579-1/ASME FFS-1 Part 7 provides the FFS framework for HIC/SOHIC assessments. The Part 7 Level 1 and Level 2 approach uses damage extent ratios — comparing the measured damage parameters (crack length, through-wall extent, density) against allowable limits derived from fracture mechanics analysis. Level 3 invokes FEA for complex crack geometries or when multiple damage zones interact.
Critical inputs for any Part 7 assessment:
- - Ultrasonic inspection mapping of HIC/SOHIC extent, orientation and depth
- - Material specification and PWHT history
- - Applied stress at damage location (hoop stress + secondary stresses)
- - H2S partial pressure and operating temperature history
For sour service vessels in Indian refineries and gas processing facilities, NACE MR0175/ISO 15156 material requirements provide the prevention framework, while API 579 Part 7 provides the FFS assessment basis when damage is already present.
We have a completed HIC FFS case study with the Part 7 Level 2 methodology documented:
HIC and SOHIC in Sour Service - API 579 Part 7 Assessment Approach - Case Study Discussion
