Hydrogen-Induced Cracking (HIC) Sample Testing: Standards, Methods, and Acceptance Criteria

Hydrogen-induced cracking (HIC) sample testing evaluates the resistance of pipeline and pressure vessel steels to internal cracking when exposed to wet hydrogen sulfide (sour) environments. Conducted by accredited third-party laboratories in accordance with ANSI/NACE TM0284, ISO 15156-2, and GB/T 8650, this test quantifies the crack length ratio (CLR), crack thickness ratio (CTR), and crack sensitivity ratio (CSR) to qualify carbon and low-alloy steels for oil and gas applications.

Technical Overview

Hydrogen-induced cracking (HIC), also referred to as stepwise cracking, is a degradation mechanism that affects carbon and low-alloy steels exposed to wet hydrogen sulfide (H2S) service. Corrosion reactions at the steel surface generate atomic hydrogen, and a fraction of this hydrogen diffuses into the metal rather than recombining into harmless molecular hydrogen at the surface. When the atomic hydrogen reaches internal discontinuities, such as elongated manganese sulfide inclusions, centerline segregation zones, or banded microstructures, it recombines into molecular hydrogen at these trap sites. The accumulated internal pressure nucleates cracks that can link up in a characteristic stepwise pattern, sometimes accompanied by hydrogen blisters on the material surface.

A critical distinction is that HIC does not require externally applied or residual tensile stress, which differentiates it from sulfide stress cracking (SSC), a mechanism driven by the combined action of tensile stress and hydrogen in harder microstructures. HIC testing is therefore performed on unstressed coupons, whereas SSC testing uses stressed specimens. The primary test method standard is ANSI/NACE TM0284 (currently maintained by AMPP), which defines specimen geometry, test solution chemistry, H2S exposure procedure, and crack measurement methodology. Material acceptance criteria are specified in ISO 15156-2 (equivalent to NACE MR0175), while GB/T 8650 is the corresponding Chinese national standard. Supplementary requirements for line pipe are also referenced within API Spec 5L for sour service ordering.

Testing Samples

HIC testing is performed on rectangular coupons machined or cut from the material under evaluation. According to ANSI/NACE TM0284, the standard coupon measures 100 mm in length by 20 mm in width, with the full product thickness retained up to a maximum of 30 mm. For products thicker than 30 mm, coupons are taken from each surface to ensure adequate representation. Orientation relative to the rolling direction is essential, since HIC cracks propagate along elongated inclusions and segregation bands.

Plate and Strip Steel Coupons

Plates and strips used for line pipe and pressure vessel fabrication are the most frequently tested product forms. Coupons are typically oriented transverse to the principal rolling direction so that any cracks that form run parallel to the exposed faces, matching the orientation in which HIC develops in service.

Line Pipe Specimens

For seamless and welded line pipe, coupons are extracted from the pipe body, the weld seam, and the heat-affected zone (HAZ) when specified. The specimen length follows the pipe axis, and the full wall thickness is preserved so that cracking through the wall can be assessed, including near the internal surface that faces the sour produced fluid.

Welded Joints and Heat-Affected Zones

Weldments may exhibit locally susceptible microstructures. Coupons spanning the weld metal, fusion line, and HAZ are exposed and sectioned so that cracking associated with the welding thermal cycle can be identified separately from base-metal cracking.

Forgings and Pressure Vessel Materials

Forgings, fittings, flanges, and pressure vessel plates can be evaluated using appropriately located coupons. Sampling locations are agreed between the purchaser and the accredited third-party laboratory, taking into account the directionality of the forging flow lines.

Testing Items

The core deliverables of an HIC test are three crack ratio parameters, measured metallographically after exposure, together with a surface examination for blisters. The parameters are calculated as follows:

• Crack Length Ratio (CLR): the sum of all crack lengths on a polished section divided by the section width, expressed as a percentage.
• Crack Thickness Ratio (CTR): the sum of all crack thicknesses divided by the specimen thickness, expressed as a percentage.
• Crack Sensitivity Ratio (CSR): the sum of the products of individual crack lengths and thicknesses divided by the product of section width and specimen thickness, expressed as a percentage.
• Surface blister examination: visual inspection of exposed coupon surfaces for hydrogen blisters, which are recorded and reported as supplementary evidence of hydrogen damage.
• Crack morphology assessment: metallographic evaluation of crack location, orientation, and linkage, including stepwise cracking paths and cracking associated with inclusions or segregation.

Acceptance criteria most commonly referenced are those of ISO 15156-2 for carbon and low-alloy steels, which limit individual results to CLR of 15% or less, CTR of 5% or less, and CSR of 1.5% or less. Project specifications may impose stricter limits or require averaging across the examined sections.

Testing Methods

HIC testing exposes unstressed coupons to a hydrogen-sulfide-saturated aqueous environment for a fixed duration, after which the specimens are sectioned and examined metallographically. Safety is a paramount consideration because H2S is highly toxic; accredited laboratories perform the test in ventilated enclosures with continuous gas monitoring and trained personnel.

Test Solutions

ANSI/NACE TM0284 defines two standard environments. Solution A consists of synthetic seawater prepared per ASTM D1141 and saturated with H2S, with an initial pH of 4.8 to 5.4, simulating marine and brine conditions. Solution B contains 5.0% sodium chloride and 0.5% glacial acetic acid saturated with H2S, with an initial pH of 3.4 to 3.6, representing a more acidic environment. The test temperature is maintained at 25 °C plus or minus 3 °C, and the solution volume is kept sufficient relative to the specimen surface area as prescribed by the standard.

Exposure Procedure

Coupons are degreased, identified, and placed in the test vessel. The solution is first deaerated with inert gas to minimize dissolved oxygen, after which H2S gas is bubbled through the solution at a controlled rate to achieve and maintain saturation. A positive pressure of H2S is maintained throughout the standard exposure period of 96 hours. After exposure, coupons are removed, cleaned of corrosion products, and visually examined for blisters before sectioning.

Post-Test Metallographic Evaluation

Each exposed coupon is sectioned at the locations prescribed by the standard, typically transverse to the long dimension of the specimen. The cut faces are mounted, ground, and polished, and cracks are measured using optical microscopy. Crack lengths and thicknesses are recorded for every crack exceeding the minimum measurable size, and the CLR, CTR, and CSR values are calculated for each examined section and reported individually and as averages.

Alternative and Supplementary Methods

For screening purposes or where sectioning must be minimized, an ultrasonic examination method described in ANSI/NACE TM0284 can be used to detect internal planar reflectors indicative of HIC after exposure. Autoclave testing under elevated temperature and pressure is available for environments that cannot be represented at ambient conditions, and longer exposure durations can be specified for research or fitness-for-service investigations.

Applications

HIC sample testing is a fundamental qualification step for steels destined for sour oil and gas service. Typical applications include:

• Qualification of line pipe and pipeline fittings for wet H2S transmission and gathering systems, including orders to API Spec 5L with sour service supplementary requirements.
• Material selection and procurement verification for pressure vessels, heat exchangers, and storage tanks in refineries and petrochemical plants handling sour streams.
• Offshore platform piping, risers, and process equipment exposed to produced fluids containing H2S and chloride-bearing water.
• Acceptance testing of HIC-resistant steel grades, demonstrating low sulfur content, calcium treatment for inclusion shape control, and homogeneous continuous casting practice.
• Failure investigations and fitness-for-service assessments of equipment removed from sour service, where crack mapping supports remaining life analysis.
• Research and development programs evaluating the influence of microstructure, inclusions, segregation, and steelmaking practice on hydrogen damage resistance.

FAQ

What is the difference between HIC and SSC testing?

HIC occurs in unstressed material due to internal hydrogen pressure accumulating at inclusions and segregation bands, so HIC coupons carry no applied load. Sulfide stress cracking requires tensile stress acting on a susceptible, usually harder microstructure, and SSC tests are performed on stressed specimens such as bent beams, tensile bars, or C-rings.

How long does a standard HIC test take?

The standard exposure period defined by ANSI/NACE TM0284 is 96 hours. Additional time is required for specimen machining, solution preparation, post-test cleaning, sectioning, metallographic polishing, and crack measurement, so a complete test program typically spans one to two weeks including reporting.

Which solution should be used, Solution A or Solution B?

The choice depends on the intended service environment and the governing material specification. Solution A, based on synthetic seawater, is common for offshore and pipeline applications, while Solution B, the acidified brine, represents more aggressive low-pH conditions. End users and engineering specifications select the environment based on ISO 15156-2 assessment of the production fluid.

What are the typical acceptance criteria?

The widely adopted limits from ISO 15156-2 are CLR not exceeding 15%, CTR not exceeding 5%, and CSR not exceeding 1.5%. Purchase specifications may tighten these limits or require that no visible stepwise cracking or blistering be present.

What specimen dimensions are required?

The standard rectangular coupon is 100 mm long by 20 mm wide with full product thickness up to 30 mm. For thicker products, coupons are taken from each face. The required number of coupons and their orientation relative to the rolling direction are defined by the applicable standard and project specification.

Can HIC testing be performed at elevated temperature and pressure?

Yes. Autoclave-based HIC testing can be conducted with H2S-containing environments at controlled temperature and pressure when service conditions exceed ambient capabilities. Test parameters are agreed in advance, and evaluation of crack ratios follows the same metallographic approach as the standard method.

Why do low sulfur and calcium-treated steels perform better in HIC testing?

Elongated manganese sulfide inclusions are the primary trap sites where hydrogen recombines and cracks initiate. Low sulfur levels reduce the volume of these inclusions, and calcium treatment converts remaining sulfides into rounded, less harmful shapes, both of which lower CLR, CTR, and CSR values in testing.