Hydrogen-Induced Cracking (HIC)
Definition
Planar crack arrays that form parallel to the steel rolling direction when high levels of hydrogen are absorbed. Common in pipeline steels exposed to sour wet conditions. Not stress-driven; occurs even at zero applied stress.
- Crack orientation
- Planar, parallel to the steel rolling direction
- Driver
- Hydrogen uptake from wet sour (H2S) service, no applied stress required
- Typical material
- Line-pipe and pressure-vessel steel with banded MnS inclusions
- Linking feature
- Stepwise cracks connect separate planar cracks through the thickness
Common questions
Why does HIC occur without any load on the pipe?+
Hydrogen atoms generated by the sulfide corrosion reaction diffuse into the steel and recombine into molecular hydrogen at internal defects such as inclusions and banding, building internal pressure that splits the metal apart independent of any external stress.
How do inspectors detect HIC in service?+
Ultrasonic testing is the standard method, since the planar cracks reflect sound differently than sound metal; API and NACE standards define acceptance criteria for crack length and density found this way.
Related terms
- Hydrogen-Assisted Cracking (HAC)
- The general term for crack initiation and propagation facilitated by absorbed atomic hydrogen. Encompasses hydrogen embrittlement, sulfide stress cracking, and hydrogen-induced cracking.
- Hydrogen Embrittlement (HE)
- Loss of toughness and ductility in a metal due to absorbed atomic hydrogen, resulting in brittle fracture at stress intensities below the...
- Liquid-Metal Embrittlement (LME)
- Brittle fracture of a normally ductile metal when it is in contact with a specific liquid metal. Not hydrogen-related but mechanistically analogous:...
- NACE MR0175/ISO 15156
- The international standard for materials selection and qualification for sour oil-and-gas service. Sets hardness limits, alloy restrictions, and testing requirements to prevent...
- Sulfide Stress Cracking (SSC)
- Hydrogen-assisted cracking specific to H2S-containing environments. Sulfide ions poison the recombination reaction at the steel surface, forcing more atomic hydrogen to enter...