Hydrogen Induced Cracking (HIC) Test: Operational Analysis and Complete Procedure Guide

Hydrogen Induced Cracking (HIC) testing is a critical corrosion assessment used to evaluate the resistance of carbon and low-alloy steels to internal cracking in wet hydrogen sulfide (sour) service environments. This article provides a detailed operational analysis of HIC testing in accordance with NACE TM0284, GB/T 8650, and ISO 15156, covering sample preparation, test solutions, exposure procedures, evaluation parameters, and acceptance criteria. It serves as a practical technical reference for metallurgists, quality control engineers, and material selection specialists across the oil and gas, petrochemical, and pressure equipment industries.

Technical Overview

Hydrogen Induced Cracking (HIC), also known as stepwise cracking, is a brittle fracture phenomenon that occurs in carbon and low-alloy steels exposed to wet hydrogen sulfide (H2S) environments, commonly referred to as sour service. Unlike sulfide stress cracking (SSC), HIC does not require any external or residual tensile stress; it is driven entirely by internal hydrogen pressure generated during the electrochemical corrosion process. The damage typically manifests as cracks parallel to the rolling plane, which link together in a characteristic stepwise pattern and may propagate toward the surface, producing hydrogen blisters or, in severe cases, through-wall rupture.

The failure mechanism begins when steel corrodes in the sour environment. The cathodic half-reaction generates atomic hydrogen at the steel surface, and sulfide ions inhibit the recombination of atomic hydrogen into harmless molecular gas. This inhibition forces a large flux of atomic hydrogen to diffuse into the steel lattice. The hydrogen atoms migrate to metallurgical trap sites such as elongated manganese sulfide inclusions, oxide stringers, micro-voids, and centerline segregation zones, where they recombine into molecular hydrogen. The molecular hydrogen cannot escape, so internal pressures build up at these interfaces until the local cohesive strength of the steel is exceeded and cracking initiates.

HIC testing reproduces this damage mechanism under accelerated laboratory conditions so that the susceptibility of a heat of steel can be quantified before it is placed in sour service. The dominant test methods and material qualification frameworks include:

• NACE TM0284 – the primary international test method for evaluating pipeline and pressure vessel steels for resistance to hydrogen induced cracking.
• GB/T 8650 – the Chinese national standard, technically aligned with NACE TM0284, for assessing HIC resistance of line pipe and pressure vessel steels.
• NACE MR0175 / ISO 15156 – the materials requirements standard for oil and gas production equipment in H2S-containing environments.
• API Spec 5L and ISO 3183 – line pipe standards that define supplementary HIC acceptance requirements for sour service ordering.
• ASTM D1141 – the specification for the synthetic seawater used in one of the standard HIC test solutions.

Testing Samples

HIC test coupons are unstressed rectangular specimens cut from the delivered product. The standard coupon geometry is 100 mm in length by 20 mm in width, retaining the full material thickness up to a maximum of 30 mm. When the wall or plate thickness exceeds 30 mm, the coupon is machined down to 30 mm from one surface. A complete test normally requires three coupons taken from adjacent locations of the same product, tested simultaneously.

Plate and Strip Samples

For plates and strips, coupons are cut with their long axis perpendicular to the principal rolling direction so that any centerline segregation and elongated inclusions, which are the most probable crack initiation sites, are intersected by the metallographic sections. Coupons are removed from the mid-width region of the plate and are tested in the as-delivered metallurgical condition without any stress-relief heat treatment unless it represents the actual fabrication condition.

Pipe Samples

For seamless and welded line pipe, coupons are cut with the long axis parallel to the pipe axis, with the 20 mm width measured around the circumference. For welded pipe, the coupons are taken approximately 180 degrees from the weld seam to evaluate the base metal, and the parent coupon surfaces are left in the as-processed condition with only degreasing and descaling performed. The full wall thickness, up to the 30 mm limit, is retained so that near-surface and mid-wall hydrogen damage can both be detected.

Welded Joint Samples

When the resistance of the weld zone itself must be assessed, coupons are located so that the weld seam, heat-affected zone, and adjacent base metal are all included within the coupon width. This configuration evaluates the susceptibility of the weld metal and the HAZ to hydrogen charging and is frequently specified for ERW and SAW pipe intended for wet H2S service. Evaluation of stress-oriented hydrogen induced cracking (SOHIC) susceptibility may also be performed on welded coupons using additional standardized procedures.

Testing Items

The core deliverable of an HIC test is a set of quantitative crack ratios measured on metallographic cross-sections after the exposure period. The three standard evaluation parameters are defined as follows:

• Crack Length Ratio (CLR) – the sum of the individual crack lengths measured along the coupon width, expressed as a percentage of the section width. CLR reflects the extent of cracking in the rolling plane.
• Crack Thickness Ratio (CTR) – the sum of the individual crack widths measured through the thickness, expressed as a percentage of the section thickness. CTR reflects the degree of through-thickness crack propagation.
• Crack Sensitivity Ratio (CSR) – the total cracked area expressed as a percentage of the cross-sectional area, calculated as the product of CLR and CTR divided by 100. CSR is a composite severity index.

In addition to the three crack ratios, the test report typically documents the presence of hydrogen blisters on the coupon surfaces, the morphology of stepwise cracking, the initial and final pH of the test solution, the H2S concentration, and the exposure temperature. For sour service line pipe ordered to API Spec 5L / ISO 3183, the commonly applied acceptance limits are CLR no greater than 15%, CTR no greater than 5%, and CSR no greater than 1.5%, although project specifications may impose stricter or modified thresholds.

Testing Methods

Sample Preparation

Coupons are stamped or tagged for identification, degreased with a suitable solvent, and cleaned to remove scale, rust, and oil so that hydrogen entry is not obstructed. Dimensions and weights are recorded before exposure. Care is taken to avoid introducing residual stress or cold work at the coupon edges during cutting, since severely worked edges can act as artificial crack initiation sites.

Test Solution and Environment Control

Two standard test solutions are defined by NACE TM0284 and GB/T 8650. Solution A is an acidified brine containing 5.0 wt% sodium chloride and 0.5 wt% glacial acetic acid in distilled water, which after saturation with H2S exhibits an initial pH of approximately 2.6 to 2.8. Solution B is synthetic seawater prepared in accordance with ASTM D1141 and saturated with H2S, used when the service environment to be simulated is closer to seawater. For Solution A, the pH at the end of the test must not exceed 4.0, confirming that the environment remained adequately corrosive and hydrogen-charging throughout the exposure. The test temperature is maintained at 25 °C plus or minus 3 °C, and the dissolved H2S concentration must remain at or above the minimum level specified by the standard for the entire test.

Exposure Procedure

The three coupons are placed in a sealed, H2S-resistant test vessel with the solution volume-to-coupon-surface-area ratio kept at or above the minimum required by the standard. The vessel is first purged with inert gas such as nitrogen to remove dissolved oxygen, which would otherwise suppress hydrogen charging. H2S gas is then bubbled through the solution at the purge rate specified in the standard until saturation is achieved, and an H2S atmosphere is maintained above the solution for the duration of the test. The exposure period is 96 hours, after which the gas supply is safely shut down, the solution is vented through an appropriate scrubber, and the coupons are retrieved, cleaned of corrosion products, and visually examined for surface blisters.

Post-Test Metallographic Evaluation

Each exposed coupon is sectioned transversely at the positions specified by the standard, typically including mid-length and near-end sections. The sections are mounted, ground, and polished metallographically, then examined by optical microscopy at approximately 100× magnification. Every crack exceeding the minimum measurable size is recorded, applying the alignment rules of the standard under which closely spaced cracks in the through-thickness direction are treated as a single crack. The individual crack lengths and thicknesses are summed, and CLR, CTR, and CSR are calculated for each section; the reported values are normally the average of all sections for the three coupons. Because hydrogen entry is a surface-sensitive phenomenon, the evaluation is performed on unetched or lightly etched sections to keep crack edges sharply defined.

Safety Considerations

H2S is a highly toxic gas with a very low occupational exposure limit, so the entire operation must be conducted inside a ventilated fume hood or a dedicated gas-handling enclosure equipped with fixed gas detection, alarms, and emergency scrubbing of exhaust gas. Operators must be trained in H2S safety, use appropriate personal protective equipment, and follow strict protocols for cylinder handling, leak checking, purging, and disposal of the contaminated test solution after neutralization.

Applications

HIC testing is a mandatory or contractually specified qualification step for a wide range of steel products and equipment in sour hydrocarbon service. Typical applications include:

• Line pipe for the transmission of natural gas and crude oil containing wet H2S, qualified to API Spec 5L / ISO 3183 sour service supplementary requirements.
• Pressure vessel and heat exchanger plate steels used in refineries and gas processing plants operating in wet H2S service.
• Offshore platform piping, process piping, and structural components exposed to seawater and sour produced fluids.
• Storage tanks and associated weldments handling sour crude, condensate, or produced water.
• Material qualification and vendor surveillance programs performed by accredited third-party laboratories on behalf of engineering contractors and end users.
• Steel mill product development, where HIC test data are used to validate inclusion shape control, calcium treatment, low sulfur practice, and centerline segregation reduction.

FAQ

What is the difference between HIC and SSC testing?

HIC is evaluated on unstressed coupons exposed to a saturated H2S solution, and cracking is driven by internal hydrogen pressure at inclusions. SSC (sulfide stress cracking) is evaluated on actively loaded tensile specimens, because it requires combined tensile stress and hydrogen embrittlement of the microstructure, typically tested under NACE TM0177 or GB/T 4157. The two tests address distinct failure mechanisms and are often specified together for sour service materials.

How long does a complete HIC test take?

The standard exposure period is 96 hours. Including coupon machining, solution preparation, metallographic sectioning, polishing, microscopy, and report preparation, the practical turnaround in most accredited third-party laboratories is typically one to two weeks.

What results are required to pass an HIC test?

The most widely applied acceptance criteria, drawn from API Spec 5L and ISO 3183, are CLR of 15% or less, CTR of 5% or less, and CSR of 1.5% or less. Individual project or purchaser specifications may impose stricter limits, and any observed blistering is normally reported regardless of the calculated ratios.

Which test solution should be selected, Solution A or Solution B?

Solution A, the acidified brine, is the more severe and most commonly specified environment and is the default choice for pipeline and pressure vessel qualification. Solution B, H2S-saturated synthetic seawater per ASTM D1141, is selected when the actual service environment is better represented by near-neutral seawater, such as certain offshore applications. The choice should always follow the governing project specification.

Can HIC occur without any applied stress?

Yes. HIC is a spontaneous internal damage mechanism driven by molecular hydrogen pressure accumulating at inclusions and defects, so it can initiate and propagate in completely unstressed material. This is the fundamental reason why HIC testing is performed on unstressed coupons, and why even low-strength, soft steels can suffer severe HIC damage in sour environments.

How can steel producers improve HIC resistance?

HIC resistance is improved by reducing sulfur content, applying calcium treatment to globularize sulfide inclusions, minimizing centerline segregation through controlled continuous casting, using low phosphorus and low carbon practice, and avoiding banded microstructures. Clean steelmaking combined with homogeneous solidification significantly lowers the density of hydrogen trap sites and directly improves CLR, CTR, and CSR results.