Hydrogen Induced Cracking (HIC) assessment is a critical materials evaluation for carbon and low-alloy steels exposed to wet hydrogen sulfide (H2S) environments in pipelines, refineries, and petrochemical plants. Accredited third-party laboratories conduct HIC testing in accordance with NACE TM0284, ISO 15156, and GB/T 8650 to quantify the crack sensitivity ratio (CSR), crack length ratio (CLR), and crack thickness ratio (CTR) of steel products. This technical article explains the HIC mechanism, sampling requirements, test procedures, evaluation criteria, and industrial applications of hydrogen induced cracking assessment.
Technical Overview
Hydrogen Induced Cracking (HIC), also referred to as stepwise cracking (SWC), is a hydrogen damage mechanism that affects carbon and low-alloy steels exposed to corrosive environments containing wet hydrogen sulfide, commonly known as sour service. During sulfide corrosion, atomic hydrogen is generated on the steel surface. Because H2S acts as a recombination poison, a significant portion of this atomic hydrogen diffuses into the steel instead of combining into molecular hydrogen at the surface. The absorbed hydrogen becomes trapped at metallurgical discontinuities, such as elongated manganese sulfide inclusions, oxide stringers, and centerline segregation bands, where it recombines into molecular hydrogen and builds up internal pressure. This pressure initiates small internal cracks and blisters that can link up in a characteristic stepwise pattern roughly parallel to the rolled surface.
A defining feature of HIC, in contrast to sulfide stress cracking (SSC), is that it does not require applied or residual tensile stress. Susceptibility is governed primarily by steelmaking quality, including sulfur content, inclusion morphology, calcium treatment practices, centerline segregation, banded microstructures, and hardness distribution. HIC assessment is therefore performed on unstressed test coupons that are exposed to a standardized sour environment and subsequently examined metallographically for internal cracking. The principal standardized procedures are NACE TM0284 and its Chinese equivalent GB/T 8650, with material selection requirements defined in NACE MR0175 / ISO 15156-2 for petroleum and natural gas industries.
- Damage form: internal blistering and stepwise cracking parallel to the rolling direction
- Driving force: atomic hydrogen absorption from wet H2S corrosion reactions
- Key susceptibility factors: sulfur level, inclusion shape control, segregation, microstructural banding
- Evaluation outputs: crack length ratio (CLR), crack thickness ratio (CTR), and crack sensitivity ratio (CSR)
- Governing standards: NACE TM0284, NACE MR0175 / ISO 15156, GB/T 8650, API 5L
Testing Samples
Standard HIC coupons measure 100 mm in length by 20 mm in width, with the thickness equal to the full product wall thickness up to a maximum of 30 mm. The test faces are machined parallel to the original rolled or pipe surfaces so that planar inclusions and segregation bands remain representative of the product condition. In a typical test program, three coupons are exposed per lot, sampled from the locations prescribed by the applicable standard, such as the mid-width position of plate or designated orientations on pipe.
Line Pipe Steel Plates and Coils
Hot-rolled plates and coils intended for API 5L line pipe, from Grade B through high-strength grades such as X70 and X80, are the most frequently tested products. Coupon orientation is selected to intersect centerline segregation and inclusion alignments where stepwise cracking typically initiates.
Seamless and Welded Pipes
Coupons are removed from the pipe body of seamless pipe and from the parent metal of welded pipe, including SAWL, SAWH, and HFW products. Where specified by the purchaser, additional coupons may be oriented to examine crack behavior near the weld seam and heat-affected zone.
Pressure Vessel and Boiler Steels
Carbon and low-alloy pressure vessel plates, such as ASTM A516 and A537 grades, together with forgings, flanges, fittings, and castings, are tested when the equipment is designed for sour service or when residual elements raise cracking concerns.
Weldments and Fabricated Components
Welded joints from fabricated spools, vessels, and repair welds can be evaluated using modified coupon configurations. These tests are particularly relevant for detecting stress-oriented hydrogen induced cracking (SOHIC), a related mechanism that develops adjacent to welds under tensile residual stress.
Testing Items
The core deliverables of an HIC assessment are the three crack ratios defined by NACE TM0284 and GB/T 8650, supplemented by morphological observations of the exposed coupons.
- Crack Length Ratio (CLR): the sum of individual crack lengths divided by the section width, expressed as a percentage
- Crack Thickness Ratio (CTR): the sum of individual crack thicknesses divided by the coupon thickness, expressed as a percentage
- Crack Sensitivity Ratio (CSR): the sum of individual crack areas divided by the cross-sectional area, expressed as a percentage
- Surface blister survey: presence, size, and distribution of hydrogen blisters on all coupon faces after exposure
- Crack morphology classification: straight, stepwise, blister-associated, or stress-oriented (SOHIC-type) cracking
- Supplementary metallography: inclusion rating to ASTM E45, centerline segregation assessment, and microstructural examination
- Optional hardness verification against the limits of NACE MR0175 / ISO 15156, typically 22 HRC or 248 HV10 for carbon steels
Testing Methods
The standard procedure follows NACE TM0284 and GB/T 8650. Test coupons are degreased, weighed, dimensioned, and immersed in one of two standard solutions: Solution A, a synthetic seawater prepared in accordance with ASTM D1141, or Solution B, a 0.5 percent acetic acid solution. The test vessel is deoxygenated with inert gas, then saturated with H2S gas at essentially atmospheric pressure. The exposure is maintained at 25 °C plus or minus 3 °C for 96 hours, with the solution pH verified before and after the test to confirm compliance with the standard.
After exposure, each coupon is visually inspected for surface blisters and then sectioned at the transverse planes specified by the standard. The sections are mounted, ground, and polished, and internal cracks are measured by optical microscopy at approximately 100x magnification. Individual crack lengths and thicknesses are recorded, the CLR, CTR, and CSR values are calculated for each section using the standard formulas, and the results are averaged over all measured sections of all coupons to produce the reported values.
Typical acceptance criteria referenced by NACE MR0175 / ISO 15156 and most project specifications are CLR of 15 percent or less, CTR of 5 percent or less, and CSR of 2 percent or less, although tighter limits may be imposed by the purchaser or the applicable product specification. Where service conditions are more severe than the standard environment, customized test parameters, such as elevated H2S partial pressure, lower pH, longer duration, or higher temperature, can be applied to simulate the intended service envelope in line with ISO 15156-2 severity guidance. The final test report documents material identity, heat number, chemical composition, coupon orientation and dimensions, solution chemistry, exposure conditions, individual crack measurements, calculated ratios, and representative photomicrographs.
Applications
HIC assessment is a mandatory qualification and quality control activity across the hydrocarbon processing value chain wherever wet H2S can contact carbon or low-alloy steel.
- Qualification of HIC-resistant plate, coil, and line pipe grades for sour gas and sour crude transmission pipelines
- Acceptance testing of seamless and welded pipe to API 5L supplementary sour service requirements
- Materials selection and procurement verification for refinery units handling sour water, amine, and sour hydrocarbon streams
- Pressure vessels, storage tanks, heat exchangers, and process piping in hydrogen sulfide bearing service
- Offshore platforms, subsea manifolds, and flowline components exposed to produced fluids containing H2S
- Steel mill development and certification of low-sulfur, calcium-treated, inclusion-shape-controlled steel grades
- Failure analysis and fitness-for-service evaluation of equipment suspected of in-service hydrogen damage
- Research programs studying hydrogen trapping, cleanliness requirements, and hydrogen embrittlement resistance
FAQ
What is the difference between HIC and SSC?
HIC is driven by hydrogen pressure accumulation at internal inclusions and occurs without external stress, producing cracks parallel to the rolled surface. Sulfide stress cracking (SSC) is a tensile stress driven cracking mechanism evaluated under applied load using methods such as NACE TM0177, and it typically produces transverse cracks in hard microstructures such as weld heat-affected zones.
When is HIC assessment required?
Assessment is generally required when a service environment is classified as sour, which per NACE MR0175 / ISO 15156 means a water phase in equilibrium with a gas or liquid containing an H2S partial pressure above approximately 0.3 kPa (0.05 psi), or when specified by the purchaser, design code, or end-user material specification.
How long does a standard HIC test take?
The standard exposure itself lasts 96 hours, while the complete workflow, including coupon machining, solution preparation, exposure, sectioning, metallographic preparation, measurement, and reporting, typically requires two to three weeks in accredited third-party laboratories.
Which test solution should be used, Solution A or Solution B?
Solution B, the dilute acetic acid solution saturated with H2S, is the more commonly applied default for pipeline and pressure vessel steels and provides an acidic, moderately severe environment. Solution A, the synthetic seawater solution, is selected when the actual service environment is near-neutral brine or when the purchase specification directs its use.
What sample dimensions and quantities are needed?
Each coupon is 100 mm long by 20 mm wide with thickness equal to the product wall thickness up to 30 mm, and three coupons per lot or heat are normally tested in accordance with NACE TM0284 and GB/T 8650. Sampling location and orientation should be agreed with the laboratory before material is cut.
Can welded joints be evaluated for HIC?
Yes. Modified coupon configurations that include the weld seam and heat-affected zone can be exposed using the same environmental conditions, and the results are examined for both stepwise cracking and stress-oriented hydrogen induced cracking adjacent to the weld.
What happens if a material exceeds the acceptance criteria?
If reported CLR, CTR, or CSR values exceed the specified limits, the material is normally rejected for the intended sour service application. Corrective actions include remelting or improved steelmaking practices, stricter sulfur and inclusion control, normalization heat treatment, or reselection of a proven HIC-resistant grade.