Why this matters
Hydrogen induced cracking (HIC) is one of the most damaging and least visible failure modes in carbon steel pipeline and pressure vessel service. It develops without external load when atomic hydrogen, generated by aqueous H2S corrosion, diffuses into the steel and recombines at internal inclusions or banded microstructure to form internal cracks. By the time HIC is found, the equipment is usually beyond repair.
This guide explains the mechanism, the standard test method (NACE TM0284), the acceptance criteria most projects use, and how to write a HIC-resistant clause into your pipe and fitting PO.
Key technical facts
Mechanism
- Aqueous H2S corrosion at the steel surface releases atomic hydrogen.
- Sulphide ions block hydrogen recombination at the surface, forcing hydrogen atoms to diffuse into the steel.
- Inside the steel, hydrogen atoms collect at MnS inclusions, banded ferrite-pearlite or other discontinuities, recombine into H2 gas and create high internal pressure.
- The pressure forms internal stepwise or planar cracks that propagate without external load.
Test method (NACE TM0284)
NACE TM0284 prescribes a 96-hour immersion of specimens in an H2S-saturated test solution. Two solutions are commonly used:
