Hydrogen-Induced Cracking (HIC) Corrosion Testing: Standards, Methods, and Laboratory Evaluation of

Hydrogen-Induced Cracking (HIC) corrosion testing is a critical laboratory evaluation that measures the resistance of pipeline and pressure vessel steels to internal stepwise cracking in wet hydrogen sulfide (sour) environments. Performed in accordance with internationally recognized standards such as NACE TM0284, NACE MR0175/ISO 15156, and GB/T 8650, the test quantifies crack sensitivity parameters including CLR, CTR, and CSR to qualify materials for sour service. Accredited third-party laboratories conduct these evaluations to help oil and gas operators, refiners, and equipment manufacturers prevent costly and potentially catastrophic material failures.

Technical Overview

Hydrogen-Induced Cracking (HIC), also referred to as stepwise cracking (SWC), is a brittle fracture mechanism that affects carbon and low-alloy steels exposed to wet hydrogen sulfide (H2S) environments, commonly known as sour service. During corrosion in these environments, sulfide species poison the hydrogen recombination reaction on the steel surface, causing large quantities of atomic hydrogen to diffuse into the metal. The hydrogen becomes trapped at metallurgical discontinuities such as elongated manganese sulfide (MnS) inclusions, oxide stringers, and centerline segregation bands, where it recombines into molecular hydrogen and generates extreme internal pressures. These pressures initiate and propagate internal cracks parallel to the rolling plane, and the linkage of adjacent cracks in a staircase pattern can severely compromise structural integrity without any visible external indication.

Environmental and Metallurgical Factors

The susceptibility of a steel to HIC depends on both the severity of the service environment and the quality of the steelmaking process. Key influencing factors include:

• Partial pressure of H2S and the pH of the electrolyte in contact with the steel
• Sulfur content, inclusion morphology, and calcium treatment practice during steelmaking
• Centerline segregation of manganese, phosphorus, and other segregating elements
• Banded ferrite-pearlite microstructures produced by conventional hot rolling
• Steel strength level, hardness, and residual stress state
• Operating temperature, with susceptibility generally highest near ambient conditions
• Presence of chlorides, carbon dioxide, and cyanides that accelerate hydrogen entry

Related Cracking Phenomena

HIC is distinct from sulfide stress cracking (SSC), which requires an applied tensile stress and is evaluated under NACE TM0177. It also differs from stress-oriented hydrogen-induced cracking (SOHIC), a variant in which HIC cracks link in a through-wall direction under tensile stress, typically near welded joints. Hydrogen blistering (HB) describes surface bulging caused by hydrogen accumulation at near-surface defects. A comprehensive sour-service qualification program should address all of these mechanisms as applicable.

Testing Samples

Line Pipe Steels

Seamless and welded line pipe manufactured to API 5L, covering grades from B through X70 and higher grades by agreement. Specimens may be taken from the parent metal, the weld seam, and the heat-affected zone (HAZ). Sour service line pipe is typically supplied under API 5L Annex H or with supplementary HIC requirements as specified by the purchaser.

Pressure Vessel and Plate Steels

Carbon and low-alloy steel plates used in vessels, separators, heat exchangers, and storage tanks handling sour fluids, including ASTM A516, A537, A299, and A203, as well as EN 10028 grades such as P265GH and P355GH.

Forgings, Fittings, Flanges, and Weldments

Pipeline fittings, flanges, forgings, and procedure qualification weld coupons can also be evaluated, since weld metal and HAZ regions frequently exhibit hydrogen damage susceptibility different from that of the base metal.

Specimen Configuration

In accordance with NACE TM0284 and GB/T 8650, standard HIC specimens are rectangular bars measuring approximately 100 mm in length, 20 mm in width, and 30 mm in thickness, with full wall thickness retained when the product wall is thinner. Three specimens per orientation are normally tested, identified with respect to the principal rolling or working direction, and all surfaces are prepared by grinding to remove scale and decarburized layers before exposure.

Testing Items

The principal quantitative outputs of an HIC corrosion test are the crack ratios determined by metallographic examination after exposure. Typical testing items include:

• Crack Length Ratio (CLR): the sum of crack lengths divided by the sum of section widths, expressed as a percentage
• Crack Thickness Ratio (CTR): the sum of crack thicknesses divided by the sum of section thicknesses, expressed as a percentage
• Crack Sensitivity Ratio (CSR): the sum of crack areas divided by the total cross-sectional area, expressed as a percentage
• Measurement of individual crack length and width at each prescribed metallographic section
• Detection and characterization of hydrogen blistering and stepwise crack interlinkage
• Optional pre-test and post-test ultrasonic examination of internal reflectors and near-surface discontinuities
• Supporting chemical composition analysis and hardness verification for full material qualification

Testing Methods

HIC Exposure Test per NACE TM0284 and GB/T 8650

Ground specimens are fully immersed for 96 hours at a controlled temperature of approximately 25 degrees Celsius in a test solution continuously saturated with H2S gas. Two standard environments are specified: an acidified brine consisting of 5.0 weight percent sodium chloride and 0.5 weight percent glacial acetic acid in distilled or deionized water, and synthetic seawater prepared in accordance with ASTM D1141, both saturated with H2S. The selection between the two solutions reflects whether the component will be exposed to acidic process brines or to seawater environments. Because H2S is highly toxic and flammable, all exposure work must be performed by trained personnel in ventilated enclosures equipped with gas detection and exhaust gas scrubbing, as practiced by accredited third-party laboratories.

Post-Exposure Metallographic Evaluation

Following the 96-hour exposure, each specimen is sectioned at the prescribed intervals, mounted, ground, polished, and etched. Crack dimensions are measured under an optical microscope, typically at approximately 100 times magnification, on every required section of all three specimens. The measured crack lengths and thicknesses are then used to calculate CLR, CTR, and CSR, and the results are reported together with photomicrographs documenting the cracking morphology.

Acceptance Criteria

NACE TM0284 is a test method and does not itself define pass or fail limits; acceptance values are established by the purchaser based on the guidance of NACE MR0175/ISO 15156 and end-user specifications. Industry-typical acceptance limits are CLR no greater than 15 percent, CTR no greater than 5 percent, and CSR no greater than 0.5 percent, although stricter limits are frequently imposed for critical service or higher-strength materials.

Complementary Sour Service Qualification

For complete sour-service material qualification, HIC testing is often combined with sulfide stress cracking testing per NACE TM0177 (Method A tensile, Method B bent-beam, Method C-ring, or Method D double cantilever beam), weld hardness surveys against the limits of NACE MR0175/ISO 15156-2, and SOHIC assessment for susceptible plate materials in restrained weldments, guided by established industry references such as EFC Publication 16.

Applications

HIC corrosion testing is applied throughout the oil and gas value chain and adjacent process industries wherever wet H2S service is present, including:

• Qualification of line pipe for sour crude oil and natural gas gathering and transmission systems, including API 5L Annex H requirements
• Refinery pressure vessels, sour water systems, amine treating units, and hydrotreating equipment
• Offshore platforms, subsea flowlines, and topside processing modules designed to NACE MR0175/ISO 15156
• Sour gas processing plants, gas sweetening units, and sulfur recovery facilities
• Storage tanks and terminals handling sour crude, intermediates, and condensates
• Failure analysis, remaining life assessment, and fitness-for-service evaluation of existing equipment
• Procurement quality assurance, mill qualification programs, and research into clean-steel manufacturing practices

FAQ

What is the difference between HIC and SSC?

HIC is a form of internal cracking driven solely by hydrogen pressure at metallurgical trap sites and does not require any externally applied stress. Sulfide stress cracking (SSC), by contrast, requires sustained tensile stress and typically affects higher-strength or harder microstructures such as weldments, and it is evaluated using different methods under NACE TM0177.

How long does a standard HIC test take?

The immersion exposure itself lasts 96 hours. Including specimen machining, exposure, sectioning, metallographic preparation, evaluation, and reporting, a complete HIC test typically requires two to four weeks of laboratory turnaround time.

What sample sizes are required for testing?

Three rectangular specimens of approximately 100 mm by 20 mm by 30 mm are required for each orientation, with full wall thickness retained for thin-wall products. Additional material may be needed for chemical composition analysis and complementary tests.

Which steels are most susceptible to hydrogen-induced cracking?

Conventionally rolled carbon steels with high sulfur content, elongated manganese sulfide inclusions, pronounced centerline segregation, and banded microstructures are the most susceptible. Modern clean-steelmaking practices, including low sulfur levels, calcium treatment for inclusion shape control, and controlled cooling, substantially improve HIC resistance.

When is HIC testing mandatory?

When the H2S partial pressure of the service environment exceeds approximately 0.05 psi (0.3 kPa), the environment is generally classified as sour under NACE MR0175/ISO 15156, and the selection and qualification of HIC-resistant materials, supported by HIC testing, is required by the standard or by end-user specifications.