Hydrogen Induced Cracking (HIC) test standards define the standardized procedures used to evaluate the resistance of pipeline and pressure vessel steels to hydrogen damage in wet hydrogen sulfide (H2S) environments. This technical article provides a comprehensive overview of the major international and national standards—including NACE TM0284, ISO 15156, and GB/T 8650—covering test specimens, exposure conditions, evaluation parameters, and acceptance criteria essential for sour service qualification.
Technical Overview
Hydrogen Induced Cracking (HIC) is a hydrogen damage mechanism that affects carbon and low-alloy steels exposed to wet hydrogen sulfide (H2S) environments, commonly referred to as "sour service." Atomic hydrogen generated by corrosion reactions on the steel surface diffuses into the metal and accumulates at internal discontinuities such as manganese sulfide inclusions and segregation zones. The recombination of atomic hydrogen into molecular hydrogen at these sites generates internal pressure, which propagates stepwise cracks parallel to the rolling direction. Under sustained tensile stress or residual stress, adjacent cracks may link up in the through-thickness direction, producing a more severe form of damage known as Stepwise Cracking (SWC).
Several standardization organizations have published test methods and material requirements to assess HIC resistance. The most widely recognized standards include NACE TM0284, which specifies the laboratory test procedure; NACE MR0175/ISO 15156 (Part 2), which defines material requirements for sour service; GB/T 8650, the Chinese national standard for evaluating pipeline and pressure vessel steels; and ISO 7539, which addresses stress corrosion testing principles. These standards harmonize specimen preparation, test solution chemistry, exposure duration, and crack ratio calculations, enabling consistent material qualification across the oil, gas, and petrochemical industries.
- NACE TM0284 – Standard Test Method for Evaluating Resistance to Hydrogen Induced Cracking of Pipeline and Pressure Vessel Steels (current designation: ANSI/NACE TM0284 / ASTM G146 family context for related testing)
- NACE MR0175 / ISO 15156-2 – Petroleum and natural gas industries – Materials for use in H2S-containing environments in oil and gas production
- GB/T 8650 – Chinese national standard for evaluation of pipeline and pressure vessel steels for resistance to hydrogen induced cracking
- ANSI/API Spec 5L – Line pipe specification incorporating sour service supplementary requirements (Annex H)
- EFC Publication No. 16 – Guidelines on materials requirements for carbon and low alloy steels for H2S-containing environments
Testing Samples
Pipeline Steel Samples
Seamless and welded line pipe materials intended for sour gas transmission are the primary subjects of HIC testing. Standard practice requires three test specimens per heat or per pipe, each cut with approximate dimensions of 100 mm (length) × 20 mm (width) × full wall thickness, with a maximum thickness of 30 mm. Specimens are taken at defined locations around the pipe circumference—typically at positions 90°, 180°, 270°, and 360° relative to the weld—to account for directional variation in inclusion distribution.
Pressure Vessel and Plate Steel Samples
Plate materials used in pressure vessels, heat exchangers, and storage tanks are tested using specimens of the same nominal dimensions, sampled in accordance with product specification requirements. Because HIC susceptibility is closely linked to through-thickness segregation, plate samples must preserve the full thickness condition whenever practical.
Welded Joint Samples
For welded pipes and fabricated components, specimens may include the weld seam, heat-affected zone (HAZ), and base metal. Post-weld heat treatment (PWHT) conditions must be documented, since residual stresses and microstructural changes in the HAZ can significantly influence crack initiation and propagation behavior.
Material Grade Considerations
- Carbon steels and carbon-manganese steels for sour service (e.g., API 5L PSL2 grades with supplementary requirement S)
- Low-alloy steels with controlled sulfur content and calcium treatment for inclusion shape control
- Normalized, quenched-and-tempered, or controlled-rolled delivery conditions
- Clad plate and overlay-welded materials for severe sour environments
Testing Items
The HIC test evaluates quantifiable crack indicators measured after a fixed exposure period. The principal evaluation items defined by NACE TM0284 and GB/T 8650 are:
- Crack Length Ratio (CLR) – The sum of individual crack lengths measured on each metallographic cross-section divided by the specimen width, expressed as a percentage
- Crack Thickness Ratio (CTR) – The sum of individual crack thicknesses divided by the specimen thickness, expressed as a percentage
- Crack Sensitivity Ratio (CSR) – The sum of individual crack areas divided by the total cross-sectional area of the examined section, expressed as a percentage
- Stepwise cracking assessment – Qualitative evaluation of crack linking through the thickness direction
- Hydrogen blistering observation – Visual examination of the exposed specimen surfaces for blister formation
- Surface and cross-sectional metallography – Sectioning at defined intervals (typically 13 mm from each end and mid-length) followed by grinding, polishing, and etching for microscopic crack measurement
Acceptance criteria are typically specified by the end user or derived from NACE MR0175/ISO 15156-2 guidance. Widely applied industry limits are CLR ≤ 15%, CTR ≤ 5%, and CSR ≤ 0.5%, although more stringent project-specific requirements are common for critical service conditions.
Testing Methods
Test Solution Preparation
NACE TM0284 specifies two standard test solutions. Solution A consists of synthetic seawater (5.0% sodium chloride and 0.5% acetic acid in distilled or deionized water) saturated with H2S at 25 °C ± 3 °C, with an initial pH of 2.7 to 3.4. Solution B is a buffered solution with an initial pH of 3.5 to 4.0. The choice between solutions A and B depends on the intended service environment and is defined by the material specification or end-user requirement.
Exposure Procedure
- Specimens are degreased, identified, and measured before testing, then placed in the test vessel with all surfaces fully exposed to the solution
- The test solution volume must maintain a minimum ratio of solution volume to specimen surface area (typically ≥ 5 mL/cm² and ≥ 3 mL/cm² minimum) to ensure adequate corrosive conditions throughout the exposure
- The solution is deoxygenated by purging with inert gas (nitrogen or argon) before H2S introduction
- H2S gas is bubbled through the solution for a minimum initial saturation period (typically 1 hour) at a rate of at least 100 mL/min per liter of solution
- Test duration is 96 hours of continuous exposure with H2S saturation maintained; pH and H2S concentration are verified at the start and end of the test
- Test temperature is maintained at 25 °C ± 3 °C throughout the exposure period
Post-Test Evaluation
After exposure, specimens are cleaned and visually inspected for blisters. Each specimen is then sectioned at the prescribed transverse planes, mounted, ground, polished, and etched (typically with nital) for examination under optical microscopy at magnifications of approximately 100×. All cracks exceeding 1 mm in length and 0.1 mm in thickness are measured, and the CLR, CTR, and CSR values are calculated for each section. The reported results represent the average of the measured sections for each specimen, along with individual values for engineering review.
Complementary Test Methods
- NACE TM0177 (Method A – tensile) – Sulfide Stress Cracking (SSC) testing for materials under tensile stress in sour environments, often performed alongside HIC testing for full sour service qualification
- NACE TM0284 Solution B testing – Simulates environments with lower acidity, such as those containing carbonates and bicarbonates
- SOHIC evaluation – Stress-Oriented Hydrogen Induced Cracking assessment for components under high residual or applied stress
Applications
HIC testing per these standards is a mandatory qualification requirement across multiple industry sectors where steel equipment contacts wet H2S service:
- Oil and gas pipelines – Qualification of line pipe (seamless, ERW, SAWL, and SAWH) for sour gas and sour crude transmission per API 5L Annex H and ISO 3183
- Pressure vessels and reactors – Material selection verification for hydroprocessing units, amine regenerators, and sour water separators in refineries
- Storage tanks and process equipment – Assessment of plate steels for tanks handling H2S-containing hydrocarbons
- Offshore and subsea facilities – Qualification of flowline, riser, and structural materials exposed to produced fluids with elevated H2S partial pressure
- Downhole and wellhead equipment – Evaluation of tubular goods and components per NACE MR0175/ISO 15156 material requirements
- Gas processing and petrochemical plants – Material verification for amine units, acid gas removal systems, and sulfur recovery units
Manufacturers, engineering contractors, and accredited third-party laboratories rely on HIC test results to demonstrate compliance with purchase specifications, support fitness-for-service assessments, and prevent catastrophic in-service failures caused by stepwise cracking in wet sour environments.
FAQ
What is the difference between HIC and SSC testing?
HIC (Hydrogen Induced Cracking) occurs without externally applied stress and is driven by internal hydrogen pressure at inclusions, whereas SSC (Sulfide Stress Cracking) is a cracking mechanism requiring tensile stress and affects materials through a hydrogen embrittlement mechanism. HIC is tested per NACE TM0284 using unstressed rectangular coupons, while SSC is tested per NACE TM0177 using stressed specimens. Sour service materials often require qualification against both mechanisms.
How long does a standard HIC test take?
The standard exposure duration specified by NACE TM0284 and GB/T 8650 is 96 hours of continuous H2S saturation. Including specimen machining, pre-test preparation, post-exposure metallographic sectioning, measurement, and reporting, the complete laboratory workflow typically requires one to two weeks. Extended-duration tests of 168 hours or longer may be specified for particularly critical applications.
What are the standard acceptance criteria for HIC testing?
The commonly accepted industry criteria, referenced in NACE MR0175/ISO 15156-2 and many end-user specifications, are CLR ≤ 15%, CTR ≤ 5%, and CSR ≤ 0.5%. However, acceptance limits are ultimately defined by the purchaser or project specification, and stricter values may be required for severe sour service or higher-strength materials.
Which steelmaking practices improve HIC resistance?
HIC resistance is enhanced through low sulfur content (typically ≤ 0.003%), calcium treatment for inclusion shape control, reduced centerline segregation achieved through continuous casting process control, low phosphorus and hydrogen levels, and appropriate rolling and heat treatment practices. Ultrasonic examination of plate and pipe for internal discontinuities is also specified in many purchase orders.
Is HIC testing mandatory for all pipeline projects?
HIC testing is required when the service environment is classified as sour—defined by H2S partial pressure and pH conditions described in NACE MR0175/ISO 15156-1. For sweet service (non-H2S environments), HIC testing is generally not required, although some operators retain it as a quality assurance measure for critical pipelines.
Can HIC testing be performed at independent laboratories?
Yes. HIC testing is routinely conducted by accredited third-party laboratories operating under ISO/IEC 17025 quality management systems, with proper fume handling infrastructure for H2S gas. Independent testing provides objective verification of material compliance for manufacturers, purchasers, and certifying authorities.