Hydrogen Induced Cracking (HIC) length measurement is a critical metallographic evaluation used to quantify the susceptibility of pipeline and pressure vessel steels to cracking in wet hydrogen sulfide (H2S) environments. Performed in accordance with NACE TM0284, ANSI/NACE MR0175/ISO 15156, and GB/T 8650, the test determines the Crack Length Ratio (CLR), Crack Thickness Ratio (CTR), and Crack Sensitivity Ratio (CSR) required to qualify materials for sour service. Accredited third-party laboratories rely on these measurements to certify linepipe, steel plates, and welded components for oil, gas, refinery, and petrochemical applications.
Technical Overview
Hydrogen Induced Cracking (HIC), also referred to as stepwise cracking (SWC), is a brittle fracture mechanism that affects carbon and low-alloy steels exposed to wet hydrogen sulfide service. Atomic hydrogen generated by corrosion reactions on the steel surface diffuses into the metal and accumulates at elongated non-metallic inclusions, such as manganese sulfide stringers, and at other microstructural discontinuities. The accumulated hydrogen recombines into molecular hydrogen, builds internal pressure, and initiates small cracks that progressively link up in a characteristic stepwise pattern. Unlike sulfide stress cracking (SSC), HIC does not require an externally applied or residual tensile stress.
HIC length measurement is the quantitative stage of HIC resistance testing. After a standardized exposure of test specimens to an H2S-saturated solution, metallographic cross sections are examined under a microscope, and the crack dimensions are measured and converted into the three internationally recognized severity indices: Crack Length Ratio (CLR), Crack Thickness Ratio (CTR), and Crack Sensitivity Ratio (CSR). These values provide an objective, reproducible measure of the material's resistance to hydrogen damage and form the technical basis for sour service material qualification.
- NACE TM0284 — Standard Test Method: Evaluation of Pipeline and Pressure Vessel Steels for Resistance to Hydrogen Induced Cracking
- ANSI/NACE MR0175 / ISO 15156 — Petroleum and natural gas industries: Materials for use in H2S-containing environments in oil and gas production
- GB/T 8650 — Chinese national standard for evaluating the HIC resistance of pipeline and pressure vessel steels
- API Spec 5L / ISO 3183 — Line pipe specifications referencing HIC testing requirements for sour service grades
- ASTM D1141 — Standard Practice for the Preparation of Substitute Ocean Water (used for Test Solution A)
Testing Samples
Line Pipe and Pressure Vessel Plate Base Metal
For plate, strip, and line pipe products, HIC test pieces are typically removed from the parent metal in the rolling direction so that the full product wall thickness is represented. For pipe products, test pieces are generally cut from positions distributed around the circumference, and for welded pipe, additional specimens centered on the seam weld are included. The chemical composition, rolling practice, inclusion morphology, and centerline segregation of the steel strongly influence HIC behavior, so sampling must reflect the actual product condition.
Welded Joints and Heat-Affected Zones
Welded components intended for sour service require evaluation of the weld metal and heat-affected zone (HAZ), where local microstructural changes can create additional hydrogen traps. Specimens are sectioned with the weld seam located along the centerline of the test piece, allowing the post-exposure metallographic sections to pass through the weld, the fusion line, and the HAZ for individual crack measurement.
Specimen Dimensions and Preparation
The standard HIC test piece per NACE TM0284 measures 100 mm ± 1 mm in length and 20 mm ± 1 mm in width, with a thickness equal to the full product wall thickness up to a maximum of 30 mm; thicker products are machined down to 30 mm from one surface. All identification markings are applied only to the end regions. Prior to exposure, specimen surfaces are ground to a finish of 0.8 µm or finer, degreased, and cleaned to remove scale, oil, and grinding residue that could interfere with the corrosion reaction.
Testing Items
The core deliverables of HIC length measurement are the crack severity ratios calculated from the measured crack dimensions on each metallographic cross section:
- Crack Length Ratio (CLR) — the ratio of the sum of all crack lengths to the section width, expressing the extent of cracking parallel to the plate surface
- Crack Thickness Ratio (CTR) — the ratio of the sum of all crack thicknesses to the specimen thickness, expressing the extent of cracking through the wall
- Crack Sensitivity Ratio (CSR) — the ratio of the summed crack areas to the total cross-sectional area, combining length and thickness into a single severity index
- Individual crack length (a) and crack thickness (b) measurements for each crack on each section
- Documentation of crack morphology, including stepwise linking, blister-associated cracking, and near-surface hydrogen damage
Testing Methods
Sour Environment Exposure
Test specimens are immersed in one of the two standard solutions defined by NACE TM0284. Test Solution A is synthetic seawater prepared in accordance with ASTM D1141, and Test Solution B is a brine containing 5.0 wt% sodium chloride and 0.5 wt% glacial acetic acid in distilled or deionized water with an initial pH of approximately 2.7. The solution volume is maintained at the minimum volume-to-surface-area ratio required by the standard, and the test temperature is controlled at 25 °C ± 3 °C.
The test vessel is deoxygenated by purging with inert gas, after which H2S gas is introduced at a rate of at least 100 mL per minute per litre of solution for the initial saturation period. A positive pressure of H2S is maintained throughout the 96-hour exposure, which is the standard test duration specified by NACE TM0284 and GB/T 8650.
Sectioning and Metallographic Examination
After exposure, each test piece is cleaned and sectioned transversely at three evenly spaced locations along the 100 mm length, producing three metallographic cross sections per specimen. For welded test pieces, the sections are prepared through the weld region as specified by the standard. The sections are ground, polished, and lightly etched following standard metallographic practice, taking care that the preparation does not introduce or obscure cracks. Examination is performed under an optical microscope at a magnification of approximately 100×, using a calibrated reticle or calibrated image analysis software to measure crack dimensions.
Crack Measurement Rules and Calculation
All cracks on a given section are measured individually. Cracks separated by less than 0.5 mm are treated as a single crack with combined dimensions, which correctly accounts for stepwise crack linkage. The severity ratios are calculated as follows:
- CLR (%) = (Σa / W) × 100, where Σa is the sum of all crack lengths and W is the section width
- CTR (%) = (Σb / T) × 100, where Σb is the sum of all crack thicknesses and T is the specimen thickness
- CSR (%) = [Σ(a × b) / (W × T)] × 100, where a × b represents the area of each individual crack
The reported results are the average values obtained from the three sections of each test piece.
Acceptance Criteria
The most widely specified acceptance limits for HIC-resistant steel, cited in line pipe specifications such as ISO 3183 and API Spec 5L Annex H as well as European corrosion federation guidance, are CLR ≤ 15%, CTR ≤ 5%, and CSR ≤ 1.5%. Final acceptance values, however, are always defined by the purchaser, the design code, or the end-user specification, and more stringent limits are frequently imposed for critical service conditions such as high H2S partial pressure or heavy-wall equipment.
Applications
HIC length measurement is an essential qualification and quality control tool across the energy and process industries wherever steel contacts wet H2S:
- Qualification of sour service line pipe grades under API Spec 5L and ISO 3183 for gathering, transmission, and distribution pipelines
- Material selection and certification of pressure vessel and storage tank plates for refineries, sour water systems, and hydroprocessing units
- Verification of HIC-resistant steel plate production at steel mills, including centerline segregation and inclusion shape control assessment
- Offshore platform piping, process piping, and wellhead equipment operating in H2S-containing produced fluids
- Weld procedure qualification for sour service fabrications, covering weld metal and heat-affected zone cracking resistance
- Failure analysis and remaining life assessment of equipment removed from wet H2S service
- Research and development of low-sulfur, calcium-treated, and accelerated-cooled HIC-resistant steel grades
FAQ
What is the Crack Length Ratio (CLR) and how is it calculated?
CLR is the percentage of the metallographic section width occupied by hydrogen-induced cracks. It is calculated by dividing the sum of all measured crack lengths on a section by the section width and multiplying by 100. The reported CLR is the average of the three sections per test piece, and it is the primary indicator of the extent of planar cracking.
What are the typical acceptance criteria for HIC testing?
The industry-standard acceptance limits are CLR ≤ 15%, CTR ≤ 5%, and CSR ≤ 1.5%, as referenced in ISO 3183 and related sour service specifications. Purchasers may impose stricter limits, such as CLR ≤ 5%, for severe sour environments or safety-critical equipment.
How long does a standard HIC test take?
The exposure period specified by NACE TM0284 and GB/T 8650 is 96 hours in H2S-saturated solution at 25 °C ± 3 °C. Including specimen machining, exposure, sectioning, metallographic preparation, and microscopic crack measurement, the complete test cycle typically requires five to seven working days.
How many specimens are required for one HIC test?
NACE TM0284 generally requires three test pieces per test, sampled to represent the product. For line pipe, specimens are taken from positions distributed around the circumference, with welded products requiring additional specimens through the seam weld. Test frequency and sampling plan are defined by the applicable product specification or purchase order.
What is the difference between HIC and SSC testing?
HIC testing evaluates cracking driven solely by internally absorbed hydrogen under no applied stress, whereas sulfide stress cracking (SSC) testing applies a sustained tensile load to a stressed specimen, typically per NACE TM0177, to evaluate susceptibility to environmentally assisted cracking of high-strength or hardened materials. Sour service materials often require both evaluations as complementary qualification tests.
Can HIC testing be performed at higher H2S partial pressures or custom pH conditions?
Yes. While the standard test uses ambient pressure H2S saturation at 1 bar, test protocols can be adapted to higher H2S partial pressures, lower pH, or process-specific brine chemistries to simulate actual field conditions. Such tests should be designed and documented carefully, and the conditions must be clearly reported along with the CLR, CTR, and CSR results.