Hydrogen Induced Cracking (HIC) Testing in Hydrogen Sulfide Environments

Hydrogen Induced Cracking (HIC) testing in hydrogen sulfide (H2S) environments is a critical corrosion evaluation used to qualify steels for sour service in the oil, gas, and petrochemical industries. Performed in accordance with internationally recognized standards such as NACE TM0284, ISO 15156-2 (NACE MR0175), and GB/T 8650, this laboratory test quantifies a steel's susceptibility to stepwise cracking caused by atomic hydrogen absorbed during wet H2S exposure. This article provides a comprehensive technical overview of HIC test specimens, evaluated parameters, experimental procedures, acceptance criteria, and typical industrial applications.

Technical Overview

Hydrogen Induced Cracking (HIC), also referred to as stepwise cracking, is a degradation mechanism that occurs in carbon and low-alloy steels exposed to wet hydrogen sulfide environments. During electrochemical corrosion in an H2S-containing medium, atomic hydrogen is generated on the steel surface. Unlike molecular hydrogen, atomic hydrogen can diffuse into the steel matrix, where it becomes trapped at internal discontinuities such as manganese sulfide inclusions, voids, and laminations. At these trap sites, hydrogen atoms recombine into molecular hydrogen, generating extremely high internal pressure that initiates and propagates cracks parallel to the rolling direction. Linkage of adjacent cracks through the thickness produces the characteristic stepwise crack morphology.

HIC is distinct from sulfide stress cracking (SSC) in that it does not require an applied or residual tensile stress; it is driven primarily by internal hydrogen pressure and is strongly influenced by steel cleanliness, inclusion morphology, and centerline segregation. The HIC test therefore evaluates the inherent metallurgical quality of the steel product rather than its response to mechanical loading. Standardized laboratory evaluation is typically conducted per NACE TM0284 (Evaluation of Pipeline and Pressure Vessel Steels for Resistance to Hydrogen Induced Cracking) or the equivalent Chinese national standard GB/T 8650, with material qualification requirements defined in NACE MR0175 / ISO 15156-2 for sour service applications. Accredited third-party laboratories routinely perform these tests to support material certification and project procurement specifications.

Testing Samples

Line Pipe Steels

Seamless and welded line pipe steels, including grades such as API 5L X52 through X70 and equivalent specifications, constitute the largest category of HIC test samples. Specimens are normally cut from the pipe body at the 3 o'clock and 9 o'clock positions relative to the weld, with the full wall thickness retained up to a maximum of 30 mm. Welded pipe samples are oriented so that the weld, base metal, and heat-affected zone are all captured within the test coupon.

Pressure Vessel and Plate Steels

Carbon and low-alloy steel plates used in pressure vessels, separators, heat exchangers, and storage tanks are tested in the as-received or post-weld heat-treated condition. Specimens are taken from quarter-width and mid-width locations of the plate to account for centerline segregation, which is a primary driver of HIC susceptibility in continuously cast products.

Welded Joints and Fabricated Components

Weld procedure qualification for sour service frequently requires HIC evaluation of the weld cross-section, including weld metal, fusion line, and heat-affected zone. Forgings, fittings, flanges, and other fabricated components may also be sampled where the purchasing specification imposes HIC requirements.

Specimen Preparation Requirements

Standard specimens per NACE TM0284 measure approximately 100 mm in length by 20 mm in width, with thickness equal to the product wall thickness (capped at 30 mm). Test surfaces are ground to a finish of 600 grit or finer to remove scale and deformed material, while the identification markings are applied in a manner that does not contaminate the test surface. Three specimens are typically tested per material condition.

Testing Items

The HIC test quantifies cracking through three standardized crack sensitivity ratios measured after the exposure period:

• Crack Length Ratio (CLR): the ratio of the sum of crack lengths to the total section width, expressed as a percentage, characterizing crack extension along the rolling direction.
• Crack Thickness Ratio (CTR): the ratio of the sum of crack thicknesses to the total section thickness, expressed as a percentage, characterizing crack growth through the wall.
• Crack Sensitivity Ratio (CSR): the ratio of the total crack area to the total section area, expressed as a percentage, providing a composite measure of cracking severity.
• Hydrogen blistering assessment: visual and metallographic examination of external surfaces for blister formation.
• Stepwise crack morphology characterization: metallographic evaluation of crack interlinkage, inclusion association, and centerline segregation features.
• Environmental verification: monitoring of test solution pH, H2S concentration, and temperature throughout the exposure to confirm test validity.

Testing Methods

Test Environment and Solution Chemistry

The standard exposure per NACE TM0284 and GB/T 8650 utilizes Solution A, consisting of synthetic seawater (5.0% sodium chloride and 0.5% glacial acetic acid in distilled water) saturated with hydrogen sulfide gas, with an initial pH between 2.7 and 3.4. Solution B, a buffered variant containing sodium acetate at a pH near 4.5 to 5.0, may be specified when the simulated service environment is less acidic. The solution volume-to-specimen-surface-area ratio is maintained at a minimum stipulated by the standard to ensure environmental consistency.

Exposure Procedure

Deaeration of the test vessel with inert gas precedes H2S introduction to eliminate dissolved oxygen, which would otherwise alter the corrosion mechanism. Hydrogen sulfide is then bubbled through the solution at a controlled rate for a minimum of one hour at the start of the test. The exposure duration is 96 hours at 25 °C ± 3 °C, during which H2S concentration and pH are periodically verified. All H2S handling is performed inside fume hoods or environmental chambers with continuous atmospheric monitoring, in strict accordance with laboratory safety protocols for toxic gas management.

Post-Exposure Evaluation

After the 96-hour exposure, specimens are visually inspected for blisters, then sectioned transversely at defined intervals. Each cross-section is ground, polished, and etched for metallographic examination. All cracks exceeding the minimum measurable size specified by the standard are measured using calibrated optical microscopy or image analysis, and the CLR, CTR, and CSR values are computed for each section and averaged per specimen. The maximum individual section values are also reported, as purchasing specifications frequently impose limits on both average and maximum results.

Acceptance Criteria

While acceptance limits are ultimately defined by the end user's specification, the commonly applied indicative values cited in NACE MR0175 / ISO 15156-2 are CLR not exceeding 15%, CTR not exceeding 3%, and CSR not exceeding 1.5%. More stringent limits may be specified for severe sour service conditions.

Applications

HIC testing in hydrogen sulfide environments is essential wherever carbon and low-alloy steels contact wet H2S at partial pressures above the sour service thresholds defined in ISO 15156. Key application sectors include:

• Oil and gas production: flowlines, gathering lines, and transmission pipelines transporting sour gas and multiphase fluids containing H2S.
• Midstream processing: separators, scrubbers, degassers, and produced water handling vessels in gas plants and gathering facilities.
• Refining: hydrotreating units, amine regenerators, sour water systems, and overhead condensation circuits exposed to H2S-bearing process streams.
• Offshore and subsea engineering: platform topside piping, risers, subsea manifold components, and pipeline qualification programs.
• Storage and transportation: tanks and vessels holding sour condensate, NGL, and refinery intermediate products.
• Material qualification and procurement: mill certification of HIC-resistant plate and pipe grades, vendor qualification audits, and failure analysis investigations.

FAQ

What is the difference between HIC and SSC testing?

HIC is a stress-independent cracking mechanism driven by internally generated hydrogen pressure at inclusions, evaluated on unstressed coupons. Sulfide stress cracking (SSC), evaluated under standards such as NACE TM0177 / ISO 15156-3, is a stress-driven cracking mechanism affecting materials under tensile loading, and it is particularly sensitive to steel hardness and microstructure. Sour service material programs frequently require both tests.

Why is the standard exposure duration 96 hours?

The 96-hour period specified in NACE TM0284 and GB/T 8650 has been validated to produce crack development representative of longer-term sour service exposure for susceptible steels, while remaining practical for laboratory execution. Extension of exposure is permitted when specified, but the standard duration is the benchmark for acceptance testing.

Can HIC testing be performed on welded materials?

Yes. HIC evaluation of weld cross-sections, including weld metal and heat-affected zones, is common for weld procedure qualification in sour service. The same crack ratio calculations are applied, with cracks assessed separately in each metallurgical region.

What steel features most influence HIC resistance?

Steel cleanliness is the dominant factor: low sulfur content, calcium treatment for inclusion shape control, reduced centerline segregation, and homogeneous microstructure all improve HIC performance. Specifying HIC-resistant grades with verified CLR, CTR, and CSR results is the most effective procurement strategy for sour service equipment.

How should HIC test results be reported?

A compliant test report includes material identification and chemistry, specimen orientation and dimensions, solution chemistry, initial and final pH, H2S concentration verification, exposure temperature and duration, metallographic crack measurements, calculated average and maximum CLR, CTR, and CSR values, and photographic documentation of representative crack morphology, all issued under the scope of accreditation of the performing laboratory.