Hydrogen-Induced Cracking (HIC) Corrosion Assessment: Testing Methods, Standards, and Evaluation Cri

Hydrogen-Induced Cracking (HIC) corrosion assessment is a critical materials integrity evaluation for carbon and low-alloy steels exposed to wet hydrogen sulfide (sour) service environments in the oil and gas, petrochemical, and energy industries. By exposing steel specimens to H2S-saturated solutions and quantifying crack ratios such as CLR, CTR, and CSR in accordance with NACE TM0284, ISO 15156, and related standards, accredited third-party laboratories help manufacturers qualify materials for sour service and prevent costly hydrogen damage failures.

Technical Overview

Hydrogen-Induced Cracking (HIC), also known as stepwise cracking, is a form of hydrogen damage that occurs in carbon and low-alloy steels exposed to aqueous environments containing hydrogen sulfide (H2S). Unlike sulfide stress cracking (SSC), HIC does not require externally applied tensile stress; instead, it is driven by internal hydrogen pressure generated at metallurgical discontinuities within the steel.

The mechanism begins when atomic hydrogen is produced on the steel surface by the corrosion reaction in the wet H2S environment. A portion of this atomic hydrogen diffuses into the steel matrix rather than recombining into molecular hydrogen at the surface. Once inside the steel, the atomic hydrogen becomes trapped at elongated non-metallic inclusions (such as manganese sulfide stringers), centerline segregation zones, and other planar defects. At these trap sites, the hydrogen atoms recombine into molecular hydrogen, generating extremely high internal pressures that initiate and propagate internal cracks. These cracks can link up in a stepwise manner, and in severe cases can reduce load-bearing capacity and lead to through-wall cracking, blistering, or leakage.

The susceptibility of a steel to HIC depends on several metallurgical factors, including sulfur content, inclusion morphology and distribution, centerline segregation, microstructure, banding, and hardness. Modern low-sulfur, calcium-treated steels with controlled rolling practices exhibit significantly improved HIC resistance. The principal governing standards for HIC assessment include ANSI/NACE TM0284, NACE MR0175/ISO 15156 (materials requirements for sour service), GB/T 8363, API 5L, and EFC Publication No. 16.

Testing Samples

Line Pipe and Pipeline Steel Samples

Seamless and welded line pipe materials supplied to API 5L (including grades such as X42 through X70 and above) are the most common materials submitted for HIC assessment. Test specimens are typically cut to dimensions of 100 mm in length by 20 mm in width at full wall thickness, with three specimens per test normally taken from the pipe body at locations corresponding to different positions relative to the rolling direction.

Pressure Vessel and Plate Steel Samples

Plate steels used for pressure vessels, separators, heat exchangers, and storage tanks, such as ASTM A516, ASTM A537, and equivalent EN or GB grades, are routinely evaluated. Specimens are sampled from plate products with defined orientations relative to the principal rolling direction to capture the most adverse inclusion alignment.

Welded Joint Samples

For welded components, specimens may be oriented to include the weld metal, heat-affected zone (HAZ), and base metal so that crack susceptibility across all metallurgical zones can be assessed. This is particularly important for line pipe with longitudinal or spiral seams, where HAZ and weld region behavior under sour conditions must be verified.

Testing Items

A complete HIC corrosion assessment typically includes the following evaluation items, each performed in accordance with ANSI/NACE TM0284 or GB/T 8363:

• Crack Length Ratio (CLR) — the ratio of total crack length to section width
• Crack Thickness Ratio (CTR) — the ratio of total crack thickness to specimen thickness
• Crack Sensitivity Ratio (CSR) — the ratio of total crack area to cross-sectional area
• Hydrogen blistering inspection of exposed surfaces
• Ultrasonic examination for internal blister and delamination detection
• Metallographic cross-section examination and crack measurement
• Test solution pH and H2S concentration monitoring throughout exposure
• Supplementary hardness testing and microstructure verification
• Optional sulfide stress cracking (SSC) testing per ANSI/NACE TM0177 Method A

Testing Methods

Specimen Preparation

Specimens are machined to the standard dimensions of 100 mm × 20 mm × full wall thickness. All surfaces are ground to remove scale, sharp edges, and machining marks that could influence hydrogen absorption or crack detection. Specimens are degreased, identified, and measured before exposure, and three metallographic cross-section locations per specimen are marked in advance to ensure consistent post-test evaluation.

H2S Exposure Test

The test is performed in one of two standard environments specified by ANSI/NACE TM0284. Solution A is synthetic seawwater prepared per ASTM D1141, saturated with H2S at ambient temperature and pressure. Solution B is a buffered solution of 5% sodium chloride and 0.5% glacial acetic acid, also saturated with H2S, which provides a more aggressive low-pH environment (typically pH 3.1 to 3.5 at test start). The specimens are exposed for 96 hours while H2S saturation is maintained, and the solution pH is monitored and recorded at defined intervals. Test temperature, purge rates, and safety protocols follow the standard requirements for handling toxic H2S gas.

Post-Test Evaluation

After exposure, the specimens are visually examined for hydrogen blisters and screened ultrasonically for near-surface and internal discontinuities. Each specimen is then sectioned at the pre-marked locations, and the cross-sections are ground, polished, and examined under an optical microscope. The dimensions of all cracks are measured, and the three standard ratios are calculated as follows:

• CLR (%) = (Σa / W) × 100
• CTR (%) = (Σb / T) × 100
• CSR (%) = (Σ(a × b) / (W × T)) × 100

In these formulas, a is the measured crack length, b is the measured crack thickness, W is the section width, and T is the specimen thickness. Reported values are averaged across the sections examined per specimen and per lot.

Acceptance Criteria

Acceptance criteria are normally agreed between the purchaser and manufacturer, but the widely applied industry benchmark, consistent with EFC Publication No. 16 and common sour service specifications, is CLR ≤ 15%, CTR ≤ 5%, and CSR ≤ 1.5%. Materials meeting these limits are generally classified as HIC-resistant and suitable for wet H2S service provided the hardness and microstructure requirements of NACE MR0175/ISO 15156 are also satisfied.

Applications

HIC corrosion assessment supports material qualification and asset integrity across a wide range of demanding industries and service conditions:

• Sour service pipelines and flowlines transporting fluids containing wet H2S
• Refinery equipment, including sour water systems, amine units, hydrotreaters, and crude distillation overheads
• Pressure vessels, separators, and heat exchangers in upstream and midstream facilities
• Storage tanks and tanker systems handling sour crude and condensates
• Offshore platforms and subsea hardware exposed to seawater and H2S-bearing production fluids
• Mill qualification and quality control for plate, pipe, and fitting manufacturers supplying HIC-resistant grades
• Remaining life assessment and fitness-for-service evaluation of aging assets in sour environments

FAQ

What is the difference between HIC and SSC?

HIC is a cracking mechanism driven by internally generated hydrogen pressure at inclusions and does not require applied or residual tensile stress. Sulfide stress cracking (SSC), by contrast, is a stress-driven cracking mechanism affecting high-strength or high-hardness steels under tensile stress in wet H2S environments. HIC is evaluated per ANSI/NACE TM0284, while SSC is typically evaluated per ANSI/NACE TM0177.

How long does a standard HIC test take?

The standard exposure duration specified by ANSI/NACE TM0284 is 96 hours in an H2S-saturated solution. Including specimen preparation, exposure, sectioning, metallography, and reporting, the total turnaround for a full assessment at an accredited third-party laboratory is generally one to two weeks.

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

Solution A (synthetic seawater per ASTM D1141) is appropriate when the intended service environment is neutral or mildly acidic brine, such as offshore conditions. Solution B (acidified brine at pH 3.1–3.5) is more severe and is often specified for refinery or pipeline service where condensed water can be highly acidic. The selection should reflect the anticipated field conditions or follow the requirements of the purchase specification.

What results indicate that a material passes the HIC test?

A material is generally considered HIC-resistant when the evaluated crack ratios do not exceed CLR of 15%, CTR of 5%, and CSR of 1.5%, with no significant surface blistering. Some project specifications impose stricter limits, so the applicable acceptance criteria should always be confirmed before testing.

Can welded pipes be tested for HIC resistance?

Yes. Specimens can be oriented to include the weld seam, heat-affected zone, and adjacent base metal so that crack susceptibility is assessed across all zones. This practice is commonly required for electric resistance welded (ERW) and submerged arc welded (SAW) line pipe intended for sour service.

Does HIC testing require external loading of the specimen?

No. HIC testing per ANSI/NACE TM0284 is conducted on unstressed specimens immersed in the H2S-saturated solution. The cracking is caused entirely by hydrogen absorbed during the corrosion reaction and trapped at internal discontinuities, which distinguishes it from stress-based sour service cracking tests.