Hydrogen-induced cracking is a stepwise internal cracking in carbon and low-alloy steel pipelines and vessels exposed to wet H2S service, in which atomic hydrogen accumulates at inclusions and recombines, generating high internal pressure that causes blister-like cracks.
Definition
HIC is described in API 571 as a sour-service damage mechanism. HIC steel is qualified per NACE TM0284. HIC is detected by UT and identified by its characteristic step-like crack pattern oriented parallel to the rolling plane.
How this damage forms
Atomic hydrogen entering the steel — from corrosion in wet sour service, from cathodic protection, or from welding — recombines into molecular hydrogen at internal discontinuities such as elongated inclusions. Molecular hydrogen cannot diffuse back out, so pressure builds and drives planar cracks parallel to the plate surface, which may then step between planes.
How it is detected
Ultrasonic examination is the practical method: straight-beam scanning finds the mid-wall planar separations, and time-of-flight or phased array characterises the stepwise linking. It is generally not surface-breaking, so surface methods are of little use.
Why it gets missed
Visual and surface methods find nothing until the damage reaches a surface or causes blistering. Radiography is poor at it because the cracks are tight and lie across the beam in most practical geometries.
What causes it, and what prevents it
Susceptibility is governed by steel cleanliness, and HIC-resistant steels are specified for wet sour service on that basis. Where existing equipment is affected, monitoring the extent over time matters more than a single detection.
Codes that govern assessment
NACE/AMPP TM0284 for HIC resistance testing of plate; NACE MR0175 / ISO 15156 for materials in H2S service; API 571 describes the damage mechanism and where it appears.
The common reporting error
Treating it as ordinary corrosion. Wall thickness readings can be entirely acceptable while the plate is internally laminated by hydrogen damage — thickness is the wrong measurement for this mechanism.
Where Hydrogen-Induced Cracking fits in an inspection programme
A term is only useful when it connects to a decision. Hydrogen-Induced Cracking appears in written procedures, in technique sheets, and in the records an owner or accreditation body reviews afterwards — which means the way it is defined in your documentation has to match the way it is applied on site. Where the two drift apart, audits find it. Atlantis writes and reviews procedures against the governing codes, trains inspection personnel to apply them, and builds the record-keeping that makes the evidence retrievable years later. Procedure development and code consulting · NDT training and certification · Ask us about your programme.
Related terms
Stress Corrosion Cracking (SCC) — Stress corrosion cracking is the brittle cracking of a normally ductile material under the combined action of tensile stress and a specific corrosive environment, propagating along grain boundaries (intergranular) or through grains (transgranular).
Sulfide Stress Cracking (SSC) — Sulfide stress cracking is hydrogen-assisted brittle cracking of high-strength steels in wet H2S environments, principally affecting hardened steels above approximately 22 HRC such as fasteners, valve internals, and high-strength pipe.
Hydrogen Embrittlement — Hydrogen embrittlement is the loss of ductility and load-bearing capacity of a metal caused by the absorption of atomic hydrogen, typically affecting high-strength steels, titanium alloys, and some nickel alloys.
API 571 Damage Mechanisms — API 571 is the recommended practice describing damage mechanisms affecting refining and petrochemical equipment, including general/uniform corrosion, localized corrosion, environmentally assisted cracking, metallurgical degradation, mechanical damage, and high
Terms like this one appear in three places that matter commercially: the written practice that governs how your personnel are qualified, the procedures and technique sheets that define how an examination is actually performed, and the evidence an auditor or client asks for when they want to know why an inspection was accepted. Getting the terminology right is the easy part; being able to produce the qualification record, the calibration traceability and the procedure revision that applied on the day of the inspection is the part that decides audits.
Hydrogen induced cracking is planar cracking that forms inside the wall of carbon steel in wet hydrogen sulphide service. Atomic hydrogen produced by the corrosion reaction diffuses into the steel and recombines into molecular hydrogen at elongated inclusions, where the pressure it generates drives cracks parallel to the plate surface. No applied stress is required.
That last point is what separates HIC from the other wet H2S damage mechanisms and it governs where inspection effort goes. Sulphide stress cracking needs applied or residual tensile stress and hard microstructure, so it concentrates at welds and hard heat-affected zones. HIC does not — it forms wherever the steel contains the inclusion population to nucleate it, which in practice means older, higher-sulphur, segregated plate anywhere in the wetted wall. Individual cracks lie in the rolling plane and, as they grow, adjacent cracks at different depths link through the wall thickness in short shear steps, producing the stepwise appearance that gives stepwise cracking its name. Stress-oriented HIC is the hybrid case: arrays of small HIC cracks stacked and linked in the through-wall direction under tensile stress, typically adjacent to welds, and it is more threatening than classical HIC because it develops a through-wall path.
Source: API RP 571 Damage Mechanisms Affecting Fixed Equipment in the Refining Industry; NACE MR0175 / ISO 15156 for materials in H2S-containing environments; NACE TM0284 for HIC test method; NACE SP0296 for evaluation of cracking in wet H2S refinery equipment; API 579-1 / ASME FFS-1 Part 9 for crack-like flaw assessment.
Wet H2S damage mechanisms distinguished by driver, location and detection
Mechanism
Requires applied stress
Where it forms
Detection approach
HIC / stepwise cracking
No
Mid-wall, in the rolling plane, at inclusion stringers
Straight-beam and angle-beam ultrasonics from the accessible surface
SOHIC
Yes
Stacked arrays through wall, usually near welds
Angle-beam ultrasonics, TFM or phased array; surface methods only once it breaks through
Sulphide stress cracking
Yes
Hard heat-affected zones and hard weld metal
Wet fluorescent magnetic particle after surface preparation; hardness survey
Hydrogen blistering
No
Sub-surface laminations near the surface, raising a visible blister
Visual, confirmed by ultrasonic thickness and mapping
Because HIC does not require stress, a scope that inspects only welds and heat-affected zones will not find it.
Why the inclusion population decides susceptibility
Atomic hydrogen entering the steel diffuses until it reaches a trap. Elongated manganese sulphide stringers and planar non-metallic inclusions, both aligned by rolling, make excellent traps, and once hydrogen recombines into molecular form inside one it cannot diffuse back out. Pressure builds until the surrounding metal cracks in the plane of the inclusion.
That mechanism explains why susceptibility tracks steelmaking practice more closely than it tracks strength or grade designation. Plate produced to low sulphur levels with calcium treatment for inclusion shape control resists HIC because the traps are absent or spheroidised, while older plate of nominally the same grade can be highly susceptible. Two vessels of the same specification, built a decade apart, can behave completely differently in the same service.
It also explains why HIC is not found by hardness surveys. Hardness governs sulphide stress cracking; it says very little about the inclusion content that governs HIC.
Detecting it, and the scoping error that misses it
HIC is mid-wall and planar in the rolling plane, which makes it a volumetric ultrasonic problem. Straight-beam scanning detects the laminar reflectors directly; angle-beam, phased array or total focusing method inspection characterises the through-wall linking that determines severity. Automated or encoded scanning is strongly preferable because HIC is rarely a single flaw and the extent of the affected area matters as much as any individual crack.
The scoping error that misses it is inspecting only welds. That instinct comes from sulphide stress cracking and SOHIC, both of which are weld-associated because both need stress. HIC needs no stress, so it forms in base metal anywhere the inclusion population supports it, and a weld-only scope in a susceptible vessel can return clean while the shell plate is extensively cracked.
Surface methods have a narrow role here. Wet fluorescent magnetic particle finds SOHIC and sulphide stress cracking once they break the surface, and finds classical HIC only after it has linked all the way through — which is far too late to be useful as a screening result.
What to do once it is found
HIC in a vessel is not automatically a repair or replacement decision. API 579-1 / ASME FFS-1 provides a fitness-for-service route for crack-like flaws and for HIC damage specifically, and a great deal of HIC-affected equipment runs safely under an assessed and monitored regime.
What the assessment needs is extent, not just presence: the size and through-wall position of the affected zones, the degree of linking, and remaining ligament. That is a mapping deliverable rather than a spot examination, which is the practical reason encoded scanning is worth the setup time on this mechanism.
Recurrence control is a materials and process question. Where the service cannot be changed, replacement plate specified as HIC-resistant with controlled sulphur and inclusion shape treatment, tested to the relevant NACE test method, removes the susceptibility rather than managing it.
What causes hydrogen induced cracking?
Atomic hydrogen generated by the wet hydrogen sulphide corrosion reaction diffuses into the steel and recombines into molecular hydrogen at elongated inclusions. The gas pressure that builds inside those traps cracks the surrounding metal in the rolling plane. Susceptibility is governed by inclusion content from steelmaking practice, not by strength or grade.
How is HIC different from sulphide stress cracking?
HIC requires no applied stress and forms in base metal wherever susceptible inclusions exist. Sulphide stress cracking requires tensile stress and hard microstructure, so it concentrates at welds and hard heat-affected zones. The two demand different inspection scopes, and a weld-focused scope will miss HIC entirely.
What is stepwise cracking?
The through-wall linking of adjacent HIC cracks lying at different depths. Individual cracks form in the rolling plane, and as they extend they join through short shear steps between planes, producing a staircase profile. Linking is what converts a population of laminar flaws into a potential through-wall leak path.
Which NDT method detects HIC?
Ultrasonics. Straight-beam scanning finds the laminar reflectors and angle-beam, phased array or TFM characterises through-wall linking and extent. Encoded or automated scanning is preferred because the affected area matters as much as any single flaw, and surface methods detect HIC only once it has already linked through.
Does HIC always require repair or replacement?
No. API 579-1 / ASME FFS-1 provides an assessment route for HIC damage and crack-like flaws, and substantial HIC-affected equipment operates safely under an assessed and monitored regime. The assessment needs mapped extent, through-wall position and remaining ligament rather than a simple presence-or-absence result.
Which steels resist hydrogen induced cracking?
Plate made to low sulphur levels with calcium treatment for inclusion shape control, so the elongated sulphide stringers that trap hydrogen are absent or spheroidised. Resistance is verified by test to the applicable NACE method rather than inferred from the grade, because two plates of the same specification can differ substantially.
Frequently asked
Can HIC be detected before it links through the wall?
Yes, and that is the point of ultrasonic screening on susceptible equipment. Individual laminar cracks are detectable well before linking, which is what allows an assessed and monitored operating regime rather than a reactive repair.
Is post-weld heat treatment a defence against HIC?
It reduces residual stress and so helps against SOHIC and sulphide stress cracking, but it does not change the inclusion population that drives classical HIC. Materials selection is the control for HIC; stress relief is the control for the stress-driven mechanisms.