Stress-Oriented Hydrogen-Induced Cracking (SOHIC) Testing: Standards, Methods, and Applications

Stress-Oriented Hydrogen-Induced Cracking (SOHIC) testing is a critical laboratory evaluation used to qualify carbon and low-alloy steels for wet hydrogen sulfide (sour) service in the oil, gas, and petrochemical industries. This testing determines whether steel plates, pipes, and weldments can resist the ladder-like, through-wall cracking caused by hydrogen absorption under tensile stress. In this article, we explain the SOHIC mechanism, applicable standards such as NACE TM0103 and NACE TM0284, sample preparation, test procedures, evaluation criteria, and typical industrial applications.

Technical Overview

Stress-Oriented Hydrogen-Induced Cracking (SOHIC) is a hydrogen damage mechanism that occurs in carbon steel and low-alloy steel exposed to wet hydrogen sulfide (H2S) environments. Atomic hydrogen is generated on the steel surface by sulfide corrosion and absorbed into the metal matrix. Under the influence of tensile stress—whether applied service loads, residual welding stresses, or both—small hydrogen-induced cracks align in stacked arrays perpendicular to the stress direction and link together through ductile shear ligaments. The result is a characteristic ladder-like crack pattern that can propagate through the full wall thickness of a component, potentially causing leakage or catastrophic failure without significant external corrosion damage.

SOHIC is distinguished from two related hydrogen damage mechanisms:

• Hydrogen-Induced Cracking (HIC): blistering and internal cracking driven by hydrogen accumulation at elongated inclusions and segregated zones, occurring without externally applied stress.
• Sulfide Stress Cracking (SSC): cracking of hard, susceptible microstructures (such as weld heat-affected zones) under tensile stress in wet H2S service.
• SOHIC: essentially a stress-driven linkage of HIC-type cracks, combining features of both mechanisms and typically occurring at welds, heat-affected zones, and areas of high residual or applied stress.

Key risk factors for SOHIC include high H2S partial pressure, low pH, the presence of CO2, elemental sulfur or cyanides, poor steel cleanliness, elongated manganese sulfide inclusions, centerline segregation, and hard or susceptible microstructures. SOHIC testing is therefore an essential part of material qualification programs specified by standards such as NACE MR0175 / ISO 15156, NACE TM0103, NACE TM0284, and API RP 571.

Testing Samples

Plate and Strip Steels

Pressure vessel plates and hot-rolled strip intended for sour service are sampled in accordance with NACE TM0103 or NACE TM0284. Test blanks are typically taken from mid-width locations at both the delivered plate surface and the quarter-thickness position to capture the most heavily segregated centerline region. Both longitudinal and transverse orientations are evaluated, since inclusion morphology varies with rolling direction.

Line Pipe Steels

Seamless pipe, electric-resistance welded (ERW) pipe, and submerged-arc welded (SAWL/SAWH) pipe are sampled as flat rectangular blanks cut from the pipe body, or as full-ring (annular) specimens for pipe-specific evaluations. For welded pipe, additional samples are taken to include the weld seam, fusion line, and heat-affected zone (HAZ) at root, mid-wall, and cap positions.

Welded Joints and Weldments

Because SOHIC preferentially initiates at or near welds, qualification programs frequently include cross-weld specimens containing weld metal, fusion boundary, and HAZ. Both as-welded and post-weld heat treated (PWHT) conditions may be tested, since PWHT reduces residual stress and hardness and can significantly improve SOHIC resistance.

Fittings, Flanges, and Forgings

Butt-weld fittings, flanges, forgings, and castings used in sour service piping systems are sampled by agreement between the purchaser and manufacturer, usually following the sampling logic of the applicable product standard and NACE TM0284.

Testing Items

A comprehensive SOHIC test program typically includes the following evaluation items:

• SOHIC resistance under a specified applied tensile or bending stress level
• HIC resistance assessment: Crack Length Ratio (CLR), Crack Thickness Ratio (CTR), and Crack Sensitivity Ratio (CSR)
• Stepwise cracking and blister characterization by metallographic cross-section examination
• Post-exposure ultrasonic examination (C-scan or straight-beam) to detect internal planar defects
• Through-thickness crack detection and pass/fail judgment for the applied stress condition
• Hardness verification of base metal, weld metal, and HAZ to confirm compliance with NACE MR0175 / ISO 15156 hardness limits
• Optional complementary SSC testing per NACE TM0177 (Method A tensile, Method B bent-beam, Method C C-ring, or Method D double-cantilever beam) for a complete sour service qualification package

Testing Methods

Test Environment

Specimens are exposed to one of the standard NACE TM0284 test solutions, both saturated with H2S gas at 25 °C ± 3 °C:

• Solution A: synthetic seawater acidified to pH 5.1–5.4, representing moderately sour, near-neutral environments.
• Solution B: an acetic-acid-buffered solution at pH 3.5, representing a more aggressive acidic sour environment.

The test solution is deaerated with inert gas before H2S saturation, and H2S is bubbled continuously or per protocol throughout the exposure period to maintain saturation.

Applied Stress Configurations

Unlike plain HIC testing, SOHIC testing superimposes a sustained tensile stress on the specimen while it is exposed to the sour environment. Common configurations include:

• Three-point or four-point bent-beam loading per NACE TM0103, in which rectangular specimens are deflected to a target outer-fiber stress (commonly a specified percentage of the actual yield strength, such as 90% or 100% of SMYS) and held in the test solution.
• Dead-weight or proof-ring loaded tensile specimens adapted from SSC practice, used when a direct uniaxial stress is required.
• Full-ring or ring-bend evaluations for line pipe geometries, applying circumferential stress representative of service conditions.

The standard exposure duration is 96 hours for HIC-type evaluation, with extended durations (168 hours or longer) frequently specified by end users for SOHIC qualification, followed by detailed crack evaluation.

Evaluation and Acceptance Criteria

After exposure, specimens are ultrasonically scanned and sectioned at defined positions for metallographic examination. Crack dimensions are measured, and the following ratios are calculated:

• CLR (Crack Length Ratio): total crack length as a percentage of section width.
• CTR (Crack Thickness Ratio): total crack thickness as a percentage of specimen thickness.
• CSR (Crack Sensitivity Ratio): the area-based product of CLR and CTR, reflecting overall crack severity.

Typical acceptance limits widely specified by purchasers, consistent with common industry practice referenced alongside NACE TM0284, are CLR ≤ 15%, CTR ≤ 5%, and CSR ≤ 2%. For SOHIC evaluation under applied stress, acceptance is generally based on the absence of any through-thickness crack linking the stacked HIC arrays. Specific criteria are always defined by the applicable purchase specification or by NACE MR0175 / ISO 15156 requirements agreed between manufacturer and end user.

Safety and Quality Requirements

All testing must be performed by accredited third-party laboratories or qualified in-house facilities using closed, ventilated H2S exposure rigs with continuous gas monitoring, scrubber systems, and trained operators, since hydrogen sulfide is highly toxic at low concentrations. Test vessels, gas handling, and analytical procedures follow the controls defined in the governing NACE/ASTM test standards.

Applications

SOHIC testing is applied wherever carbon and low-alloy steel equipment operates in wet H2S service, including:

• Upstream oil and gas production facilities, flowlines, and gathering systems handling sour produced fluids
• Sour gas and crude transmission pipelines qualified to NACE MR0175 / ISO 15156
• Refinery sour water systems, amine units, hydroprocessors, and delayed coker equipment identified in API RP 571 damage mechanism reviews
• Offshore platforms, topside piping, and subsea hardware exposed to H2S-containing fluids
• Gas processing plants, sour acid gas injection systems, and CO2 capture units with H2S contamination
• Storage tanks, pressure vessels, and heat exchangers in sour service
• Procurement qualification of plates, pipes, fittings, and fabricators for projects with sour service specifications such as those referenced in ISO 3183 and API 5L annexes

Test results support material selection, procurement specifications, welding procedure qualification, fitness-for-service assessments, and mechanical integrity programs under inspection codes such as API 510, API 570, and API 653.

FAQ

What is the difference between SOHIC and HIC?

HIC is internal cracking caused by hydrogen accumulation at inclusions without any applied stress, whereas SOHIC is the stress-driven linkage of stacked HIC-type cracks that propagates through the wall thickness. SOHIC therefore requires the combined presence of hydrogen uptake and tensile stress, and it typically occurs near welds or highly stressed regions.

Which standards govern SOHIC testing?

The primary standards are NACE TM0103 for evaluation of SOHIC resistance of plate steels and NACE TM0284 for the HIC test environment and evaluation methodology. Material qualification requirements are defined in NACE MR0175 / ISO 15156, SSC testing is covered by NACE TM0177, and damage mechanism guidance is provided by API RP 571.

How long does a SOHIC test take?

The standard exposure is 96 hours in H2S-saturated Solution A or Solution B. Many end-user specifications extend the exposure to 168 hours or longer for SOHIC qualification. Including machining, pre-test inspection, exposure, and metallographic evaluation, the complete program typically takes two to four weeks.

What are typical acceptance criteria?

Commonly specified limits are CLR ≤ 15%, CTR ≤ 5%, and CSR ≤ 2%, together with no through-thickness cracking under the applied stress condition. The exact limits must be confirmed against the governing project specification or the requirements of NACE MR0175 / ISO 15156 agreed between purchaser and supplier.

Can welded joints be tested for SOHIC?

Yes. Cross-weld specimens containing weld metal, fusion line, and heat-affected zone are routinely tested, and SOHIC resistance is often a decisive qualification criterion for welded pipe and vessel fabrication. Post-weld heat treatment condition is normally included in the evaluation because it strongly influences residual stress and hardness.

What specimen orientations should be tested?

Both transverse and longitudinal orientations relative to the rolling or forming direction should be evaluated, since elongated inclusions and centerline segregation vary with orientation and directly affect crack susceptibility.