Hydrogen-Assisted Cracking (HAC)
Definition
The general term for crack initiation and propagation facilitated by absorbed atomic hydrogen. Encompasses hydrogen embrittlement, sulfide stress cracking, and hydrogen-induced cracking.
- Definition
- Cracking facilitated by absorbed atomic hydrogen
- Umbrella scope
- Covers hydrogen embrittlement, sulfide stress cracking, hydrogen-induced cracking
- Category
- Failure mechanism in metallurgy
- Relevance
- Root-cause analysis in forensic engineering failures
Common questions
How does HAC differ from ordinary fatigue cracking in a failure investigation?+
Fatigue cracking develops from repeated cyclic loading and typically shows characteristic beach marks or striations, while HAC results from atomic hydrogen weakening the metal's grain boundaries or lattice, often producing more sudden, brittle-appearing failure that can occur under static load without cyclic stress.
What environmental or process conditions typically introduce the hydrogen behind HAC?+
Common sources include electroplating, pickling in acid, welding without adequate preheat or hydrogen control, cathodic protection systems operating outside design limits, and exposure to hydrogen sulfide or other sour service environments.
Why does the umbrella term matter rather than just naming the specific subtype?+
Different HAC subtypes have distinct root causes, for example sulfide stress cracking is tied specifically to hydrogen sulfide exposure while classic hydrogen embrittlement can arise from plating processes, so correctly identifying which subtype occurred is necessary to trace the failure back to its actual originating process.
Related terms
- 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...
- Hydrogen-Induced Cracking (HIC)
- Planar crack arrays that form parallel to the steel rolling direction when high levels of hydrogen are absorbed. Common in pipeline steels...
- 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...