Hydrogen-induced cracking (HIC) area determination is a laboratory evaluation that quantifies the resistance of carbon and low-alloy steels to internal cracking in wet hydrogen sulfide (sour) service. Following recognized procedures such as NACE TM0284, EN 10229, and GB/T 8650, accredited third-party laboratories expose standardized coupons to H2S-saturated solutions, then section and measure them microscopically to calculate the crack length ratio (CLR), crack thickness ratio (CTR), and crack sensitivity ratio (CSR). This article explains the sampling, exposure, metallographic measurement, calculation, and acceptance criteria involved in HIC area determination.
Technical Overview
Hydrogen-induced cracking (HIC), also known as stepwise cracking (SWC), is a degradation mechanism that affects carbon and low-alloy steels exposed to wet hydrogen sulfide (H2S) environments, commonly referred to as sour service. Atomic hydrogen generated by sulfide corrosion on the steel surface diffuses into the metal and accumulates at elongated non-metallic inclusions, manganese sulfide stringers, and internal voids. Within these trap sites, atomic hydrogen recombines into molecular hydrogen, which cannot diffuse back out; the resulting internal pressure nucleates cracks that can link up in a characteristic stepwise pattern across the wall thickness, even in the absence of externally applied stress.
HIC area determination is a quantitative post-exposure evaluation method in which standard-sized coupons are exposed to a controlled sour environment, cross-sectioned, metallographically prepared, and examined under a measuring microscope. Every crack indication is measured and converted into standardized crack ratios that express the cracked area as a percentage of the examined cross-section. These ratios provide an objective, reproducible basis for qualifying steel grades for sour service pipelines, pressure vessels, and process equipment.
The principal standards governing HIC area determination include:
- NACE TM0284 — Evaluation of Pipeline and Pressure Vessel Steels for Resistance to Hydrogen-Induced Cracking
- NACE MR0175 / ISO 15156 — Materials for use in H2S-containing environments in oil and gas production
- EN 10229 — Determination of the resistance to hydrogen induced cracking (HIC)
- GB/T 8650 — Chinese national standard for evaluating pipeline and pressure vessel steel resistance to HIC
- API Spec 5L and ISO 3183 — Line pipe specifications with supplementary sour service requirements
- ASTM E3 and ASTM E407 — Metallographic specimen preparation and etching practices used during evaluation
Testing Samples
Plate and Strip Steel Coupons
For plate material, three test coupons are typically taken with dimensions of approximately 100 mm in the rolling (principal working) direction, 20 mm in width, and up to 30 mm in thickness (or the full product thickness when thinner). Coupons are sampled from agreed plate locations, such as the quarter and mid-width positions, and identified clearly with product direction so that orientation-dependent cracking can be properly evaluated.
Line Pipe Coupons (Seamless and Welded)
For line pipe, coupons are cut with the long axis parallel to the pipe axis, approximately 100 mm long and 20 mm wide in the circumferential direction, at full wall thickness. Three coupons are removed at locations spaced around the pipe circumference (commonly approximately 120 degrees apart). For welded pipe, sampling is arranged so that the weld seam, heat-affected zone, and base metal can all be assessed.
Pressure Vessel and Storage Tank Steels
Plates intended for pressure vessels, separators, heat exchangers, and storage tanks are qualified using coupon configurations equivalent to those for plate products. Project specifications may additionally require evaluation after forming or post-weld heat treatment to reflect the actual delivered metallurgical condition.
Welded Joints and Clad Products
Weldment qualification coupons traverse the weld metal and heat-affected zone. For clad or overlay products, coupon design follows the applicable specification so that cracking at the clad-to-base-metal interface and the underlying base metal is captured during sectioning.
All coupons must be clean, free of mill scale, grease, and machining residues, and must not be coated or pickled in a manner that pre-charges the steel with hydrogen before testing.
Testing Items
The core deliverables of an HIC area determination program are:
- Crack Length Ratio (CLR) — the sum of crack lengths projected onto the section width, expressed as a percentage
- Crack Thickness Ratio (CTR) — the sum of crack thicknesses projected onto the section thickness, expressed as a percentage
- Crack Sensitivity Ratio (CSR) — the summed crack areas relative to the total examined cross-sectional area, expressed as a percentage
- Individual crack length and thickness measurements on each metallographic section
- Crack morphology characterization, including stepwise cracking, straight cracks, blister-linked cracking, and near-surface cracking
- Optional post-test microstructure examination and hydrogen blister surface documentation
Testing Methods
Test Environment Preparation
Two standard environments are defined in NACE TM0284. Solution A is an acidified brine (5% sodium chloride and 0.5% acetic acid in distilled or deionized water) that simulates moderately aggressive sour produced fluids. Solution B is a buffered solution (0.5% acetic acid and 0.25% sodium acetate) used when closer pH control is required. The selected solution is deaerated and saturated with H2S gas before and during coupon immersion; the choice of solution is normally dictated by the end-user specification or the applicable material qualification standard.
Exposure Procedure
Coupons are fully immersed in the H2S-saturated test solution in a sealed test vessel for a standard duration of 96 hours at 25 °C ± 3 °C. H2S purging is applied initially at a sufficient rate to achieve rapid saturation and then maintained at a low continuous flow to keep the environment saturated and to prevent oxygen ingress. Solution pH is verified at the start and monitored at the end of exposure; out-of-range pH values can invalidate a test and require retesting.
Sectioning and Metallographic Preparation
After exposure, each of the three coupons is sectioned transversely into three evaluation sections, yielding nine polished cross-sections for measurement per test. The sections are ground, polished, and lightly etched in accordance with metallographic practices such as ASTM E3 and ASTM E407 so that crack contours are clearly resolved. When two cracks are closely spaced and aligned within approximately 0.5 mm of each other, they are treated as a single continuous crack for calculation purposes.
Measurement and Calculation of Crack Ratios
Each section is examined at approximately 100× magnification with a calibrated measuring microscope or image analysis system. For every section, the length (a) and thickness (b) of each crack are recorded, together with the section width (W) and section thickness (T). The crack ratios are then calculated as follows:
- CLR (%) = (Σa / W) × 100
- CTR (%) = (Σb / T) × 100
- CSR (%) = Σ(a × b) / (W × T) × 100
Results are reported as the average of all nine sections, and individual maximum values are also documented because many specifications impose limits on both average and single-section results.
Acceptance Criteria
HIC susceptibility limits are set by the purchasing specification rather than the test method itself. The most widely applied industry criteria for average values are CLR ≤ 15%, CTR ≤ 5%, and CSR ≤ 2%, which correspond to the acceptance guidance embedded in sour service material requirements. Stricter limits may be specified for severe sour service, high-pressure pipelines, or heavy-wall equipment.
Applications
HIC area determination is a mandatory or commonly specified qualification test wherever wet H2S corrosion is credible. Typical applications include:
- Qualification of HIC-resistant line pipe grades (for example, API 5L grades with supplemental sour service requirements) for gas and oil transmission pipelines
- Material qualification for refineries, gas processing plants, and petrochemical units handling sour feeds
- Pressure vessels, separators, heat exchangers, and storage tanks in sour hydrocarbon service
- Offshore platforms, subsea flowlines, and risers designed to NACE MR0175 / ISO 15156
- Heater tubes, boiler components, and amine or sour water systems where hydrogen charging is anticipated
- Research and development programs evaluating inclusion control, calcium treatment, and low-sulfur steelmaking practices for HIC-resistant grades
- Failure analysis and remaining-life assessment of equipment removed from sour service
FAQ
What is the difference between HIC and sulfide stress cracking (SSC)?
HIC is driven by internally generated hydrogen pressure at inclusions and occurs without externally applied stress, producing stepwise internal cracks parallel to the plate surface. SSC is a hydrogen embrittlement mechanism that requires tensile stress (applied or residual) and typically propagates perpendicular to the stress direction, often affecting weldments and hard microstructures. HIC is evaluated by immersion exposure and post-test sectioning per NACE TM0284, while SSC is evaluated under sustained load using methods defined in NACE TM0177.
How long does an HIC area determination take?
The sour exposure itself is standardized at 96 hours. Including coupon machining, solution preparation, metallographic preparation of nine sections, microscopic measurement, calculation, and reporting, a typical turnaround is two to three weeks, subject to laboratory scheduling and any additional metallographic requirements.
What acceptance limits are typically applied?
Commonly specified average limits are CLR ≤ 15%, CTR ≤ 5%, and CSR ≤ 2%. Some end-user specifications also cap individual section maxima or require stricter values for critical service. The governing acceptance criteria always come from the purchase specification or applicable design code.
Which test solution should be used, Solution A or Solution B?
Solution A (acidified brine) represents a more aggressive, unbuffered environment and is the default for most pipeline qualifications. Solution B (buffered) is selected when the specification calls for tighter pH stability or when simulating environments where buffering is representative. The selection is a contractual decision based on the service environment being simulated.
Can welded pipe and clad materials be tested?
Yes. Coupon location and orientation are adjusted so that the weld seam, heat-affected zone, and adjacent base metal are all included in the evaluation sections. For clad products, the interface region is sectioned to detect cracking at or below the bond line.
Is HIC testing destructive?
Yes. The test consumes coupon material and the evaluation requires destructive sectioning, metallographic grinding, and etching. For production lot qualification, coupons are taken from the same heat, plate, or pipe as the delivered product so that results are representative of the material supplied.