Hydrogen-Induced Cracking (HIC) Crack Ratio Determination: CLR, CTR and CSR Testing Explained

Hydrogen-induced cracking (HIC) crack ratio determination is a standardized laboratory method that quantifies the resistance of pipeline and pressure vessel steels to internal cracking in wet hydrogen sulfide (sour) environments. By measuring the crack length ratio (CLR), crack thickness ratio (CTR), and crack sensitivity ratio (CSR) in accordance with NACE TM0284, GB/T 8650, and the material requirements of ISO 15156 (NACE MR0175), steelmakers, fabricators, and end users can verify that materials are fit for sour service. This article outlines the testing principle, sample types, measured parameters, test procedure, calculation formulas, acceptance criteria, and typical applications of HIC crack ratio testing.

Technical Overview

Hydrogen-induced cracking, also known as stepwise cracking (SWC), is a brittle cracking mechanism that occurs in carbon and low-alloy steels exposed to wet hydrogen sulfide (H2S) environments, without requiring any externally applied stress. Corrosion of the steel surface generates atomic hydrogen, and a portion of this hydrogen diffuses into the metal, where it becomes trapped at elongated non-metallic inclusions such as manganese sulfides, oxide stringers, centerline segregation bands, and hard microstructural zones. The trapped hydrogen atoms recombine into molecular hydrogen, building up high internal pressure that initiates cracks parallel to the rolling direction. Adjacent cracks may link up in a characteristic staircase pattern, and hydrogen accumulation near the surface can also produce visible blisters.

HIC crack ratio determination quantifies the severity of this internal damage in a reproducible and comparable way. Test coupons are immersed for 96 hours in a solution saturated with H2S, after which metallographic cross sections are examined under an optical microscope at 100x magnification. The measured crack dimensions are converted into three dimensionless ratios, namely CLR, CTR, and CSR, which allow objective comparison between heats, products, and suppliers, and provide the technical evidence required for sour service qualification under ISO 15156-2 (NACE MR0175) or project-specific purchase specifications.

Testing Samples

Line Pipe Steels

Seamless and welded line pipe intended for sour gas gathering and transmission systems, typically supplied to API 5L in grades such as X52, X60, X65, and X70. Specimens are taken at full wall thickness to represent the as-delivered product condition, including the inside surface that will contact the sour fluid.

Pressure Vessel and Boiler Plate Steels

Carbon and low-alloy steel plates for pressure vessels, separators, heat exchangers, and storage tanks operating in wet H2S service, commonly supplied to ASTM A516, ASTM A537, or equivalent normalized and fine-grain specifications.

Fittings, Flanges, and Forgings

Elbows, tees, reducers, flanges, and forged components for sour service piping systems. Sampling is performed to capture the most heavily worked and potentially segregated regions of the product.

Welded Joints

Weld cross sections can be evaluated to assess crack susceptibility of the weld metal and heat-affected zone (HAZ) relative to the base metal, which is important for welded pipe and fabricated components.

Specimen Preparation Requirements

In accordance with NACE TM0284 and GB/T 8650, each test uses three coupons measuring 100 mm (length) by 20 mm (width) by full product thickness, with the thickness limited to a maximum of 30 mm. All surfaces are ground to a finish of 600 grit or finer, deburred, degreased, and clearly identified before exposure.

Testing Items

The core deliverables of a HIC crack ratio determination are the three standardized ratios measured on metallographic sections after the sour exposure:

• Crack Length Ratio (CLR): the sum of all individual crack lengths projected on the section width, expressed as a percentage of the specimen width.
• Crack Thickness Ratio (CTR): the sum of all individual crack thicknesses (heights), expressed as a percentage of the specimen thickness.
• Crack Sensitivity Ratio (CSR): a combined area-based index calculated from the products of individual crack lengths and thicknesses, expressed as a percentage of the total cross-sectional area.
• Qualitative observations: presence of hydrogen blisters, stepwise crack linkage, crack location relative to mid-thickness segregation, and cracking in welds or HAZ.
• Environmental verification data: initial and final solution pH, H2S concentration, solution type, and test temperature, recorded to demonstrate validity of the exposure.

It should be noted that HIC testing is distinct from sulfide stress cracking (SSC) testing, which evaluates cracking under applied tensile stress in accordance with NACE TM0177. The two tests are frequently performed together as part of a complete sour service material qualification program.

Testing Methods

Test Environment

The exposure is conducted in one of two standard environments defined by NACE TM0284 and GB/T 8650. Solution A consists of synthetic seawater prepared per ASTM D1141 and saturated with H2S, while Solution B consists of 0.5% glacial acetic acid in distilled water, likewise saturated with H2S. Solution B is more acidic and represents a more aggressive screening condition. The solution temperature is maintained at 25 °C ± 3 °C throughout the test.

Exposure Procedure

The prepared coupons are placed in the test vessel, which is then deaerated with inert gas to remove dissolved oxygen. H2S gas is subsequently bubbled through the solution until saturation is reached, with the H2S concentration monitored and typically maintained at a minimum of 2300 ppm at the start of the test. A continuous slight positive pressure of H2S is maintained over the solution for the full 96-hour exposure period. The pH of the solution is measured at the beginning and end of the test, and end-of-test values must fall within the limits specified by the standard for the test to be valid.

Post-Exposure Metallographic Evaluation

After exposure, the coupons are cleaned and sectioned transverse to their length at the locations prescribed by the standard, producing multiple metallographic cross sections for examination. The sections are mounted, ground, polished, and lightly etched to reveal the microstructure. Crack measurement is performed under an optical microscope at 100x magnification using a calibrated graticle. Each individual crack is measured for its length (a) and thickness (b), counting only genuine planar cracks approximately parallel to the rolled surface, in accordance with the exclusion rules of the standard.

Calculation of Crack Ratios

The three ratios are calculated from the individual crack measurements as follows:

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

In these formulas, Σa is the sum of all crack lengths on the section, Σb is the sum of all crack thicknesses, W is the section width (20 mm), and T is the specimen thickness. The ratios are calculated for each examined section, averaged for each specimen, and reported for all three specimens together with the mean values.

Acceptance Criteria

ISO 15156-2 does not impose a single mandatory acceptance limit; the criteria are agreed between the equipment user and the material supplier. The most widely applied industry criteria are CLR ≤ 15%, CTR ≤ 5%, and CSR ≤ 2%. For more severe sour service conditions, stricter requirements such as CLR ≤ 5%, CTR ≤ 1.5%, and CSR ≤ 0.5% are frequently specified in purchase orders.

Safety Considerations

Because H2S is a highly toxic gas, all charging, sampling, and venting operations must be performed in closed systems within fume hoods, with exhaust gases passed through appropriate scrubbers and with calibrated gas detection and emergency procedures in place.

Applications

HIC crack ratio determination is applied across the oil and gas, petrochemical, and steel manufacturing sectors, including:

• Qualification of line pipe, plates, fittings, and flanges for wet H2S service in accordance with ISO 15156-2 (NACE MR0175) and project specifications.
• Quality control at steel mills for HIC-resistant plate and strip grades, supporting clean-steel practices such as low sulfur content, calcium treatment, and controlled rolling.
• Procurement acceptance testing for EPC projects, offshore platforms, and sour gas field development programs.
• Refinery integrity management for sour water systems, amine units, hydroprocessing units, and sour storage tanks covered by API RP 571 damage mechanism assessments.
• Failure analysis and root cause investigation of in-service blistering and stepwise cracking, providing quantitative evidence for fitness-for-service evaluation.
• Research and development of inclusion control, continuous casting parameters, rolling schedules, and heat treatment practices for improved sour service performance.

FAQ

What standards govern HIC crack ratio determination?

NACE TM0284 and GB/T 8650 define the test method, specimen preparation, exposure conditions, and crack measurement procedure. Material selection and application requirements for sour service are covered by ISO 15156-2 (NACE MR0175), while ASTM D1141 is referenced for the preparation of synthetic seawater used in Solution A.

What are the typical acceptance criteria?

The most common industry acceptance limits are CLR ≤ 15%, CTR ≤ 5%, and CSR ≤ 2%. Stricter limits, for example CLR ≤ 5%, CTR ≤ 1.5%, and CSR ≤ 0.5%, may be specified for severe service. The applicable values should always be confirmed in the purchase specification.

How is HIC different from SSC?

HIC is caused by internally generated hydrogen pressure and occurs without applied external stress, producing cracks parallel to the rolled surface. Sulfide stress cracking (SSC) requires a tensile stress acting on a susceptible microstructure and is evaluated by different methods such as those in NACE TM0177. Many sour service qualifications require both tests.

How long does the test take?

The H2S exposure itself lasts 96 hours. Including specimen machining, exposure, sectioning, metallographic preparation, crack measurement, and reporting, a complete test is typically completed within one to two weeks.

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

The choice depends on the service environment being simulated and the governing purchase specification. Solution A, based on synthetic seawater, represents mildly sour brine environments, while Solution B, based on acidified water, provides a more aggressive screening condition.

Can welded pipes and components be evaluated?

Yes. Coupons can be oriented and sectioned so that the weld metal and heat-affected zone are examined, allowing the crack susceptibility of the welded joint to be compared with that of the base metal.