Hydrogen Induced Cracking (HIC) Susceptibility Testing: Standards, Methods, and Applications

Hydrogen Induced Cracking (HIC) susceptibility testing is a critical laboratory evaluation used to determine the resistance of carbon and low-alloy steels to cracking in wet hydrogen sulfide (sour) service environments. Performed in accordance with internationally recognized standards such as NACE TM0284 and ISO 15156 (NACE MR0175), this test quantifies crack propagation indicators including CLR, CTR, and CSR to support material qualification for pipelines, pressure vessels, and refinery equipment. This article provides a comprehensive technical overview of HIC testing principles, sample requirements, test procedures, acceptance criteria, and typical industrial applications.

Technical Overview

Hydrogen Induced Cracking (HIC), also known as stepwise cracking, is a degradation mechanism that affects carbon and low-alloy steels exposed to wet hydrogen sulfide (H2S) environments, commonly referred to as sour service. The electrochemical corrosion of steel in an H2S-saturated medium generates atomic hydrogen at the metal surface. Unlike molecular hydrogen, atomic hydrogen can diffuse into the steel matrix and migrate to internal discontinuities such as elongated manganese sulfide (MnS) inclusions, oxide clusters, and segregation zones. At these sites, hydrogen atoms recombine into molecular hydrogen, generating extremely high internal gas pressure that initiates planar cracks parallel to the rolling surface.

As cracking progresses, neighboring cracks may link together in a characteristic stair-step pattern across the wall thickness, significantly reducing load-bearing capacity without any externally applied stress. In more severe cases, hydrogen blistering may appear on the steel surface, and under tensile loading conditions, stress-oriented hydrogen-induced cracking (SOHIC) can develop, combining HIC damage with through-thickness crack growth.

HIC susceptibility testing reproduces these degradation conditions in an accelerated laboratory environment. Test specimens are fully immersed in an H2S-saturated solution for a defined exposure period, after which they are sectioned, polished, etched, and examined metallographically. The extent of cracking is quantified through three principal indicators: Crack Length Ratio (CLR), Crack Thickness Ratio (CTR), and Crack Sensitivity Ratio (CSR). The governing methodology is defined by NACE TM0284, with material selection requirements specified in NACE MR0175 / ISO 15156, and the equivalent Chinese national standard GB/T 8650. Accredited third-party laboratories routinely perform this testing to support material qualification for oil and gas operators, refineries, and engineering contractors worldwide.

Testing Samples

Plate and Strip Steel

Full-thickness plate and hot-rolled strip are the most commonly tested product forms. Specimens are typically cut to dimensions of 100 mm in length by 20 mm in width, with a thickness representing the full product wall section (up to a maximum of 30 mm for thick plate). Sampling is normally performed at locations specified by the applicable product specification or purchase order.

Line Pipe (Seamless and Welded)

Both seamless and welded line pipe (including ERW, SAWL, and SAWH) are tested for sour service qualification. Specimens are removed from the pipe body, and for welded products, additional specimens are taken to evaluate the weld seam, weld cap, and heat-affected zone (HAZ), where cracking susceptibility can differ from the base metal.

Pressure Vessel and Boiler Steels

Steels intended for pressure vessels, separators, heat exchangers, and boiler components exposed to sour process streams are qualified using specimens representing the plate rolling direction and, where applicable, the head or shell forging sections.

Forgings, Castings, and Weldments

Forged components, castings, and welded joints can also be evaluated. For weldments, cross-weld specimens allow simultaneous assessment of base metal, HAZ, and weld metal. For machined or ground specimens, testing may be performed on reduced-thickness coupons provided the surface preparation is documented.

Sample Preparation Requirements

• Specimens must be identified and stamped on surfaces away from the test faces to avoid artificial crack initiation sites.
• Surfaces should be free of scale, oil, grease, and protective coatings; light grinding or degreasing with acetone is typically applied.
• Standard practice requires three replicate specimens per test condition to improve statistical reliability.
• Specimen dimensions and orientation relative to the rolling direction must be recorded prior to exposure.

Testing Items

The core deliverables of an HIC susceptibility test include the following quantitative and qualitative results:

• Crack Length Ratio (CLR) — the sum of individual crack lengths measured on each polished section divided by the section width, expressed as a percentage.
• Crack Thickness Ratio (CTR) — the sum of individual crack thicknesses divided by the specimen thickness, expressed as a percentage.
• Crack Sensitivity Ratio (CSR) — the total crack area divided by the cross-sectional area of the specimen, expressed as a percentage; it is frequently calculated as (CLR × CTR) / 100.
• Hydrogen blister inspection — visual examination of all specimen surfaces for blister formation, with size and location recorded.
• Metallographic examination — microscopic evaluation of crack morphology (planar, stepwise, or straight-through cracking) and crack association with inclusions.
• Solution monitoring — measurement of solution pH before and after exposure, and verification of H2S concentration in the test solution and gas phase.
• Post-test material characterization — optional microstructure analysis, inclusion rating, and hardness checks to support failure interpretation.

Testing Methods

Standard Reference Documents

• NACE TM0284 — Standard Test Method: Evaluation of Pipeline and Pressure Vessel Steels for Resistance to Hydrogen Induced Cracking.
• NACE MR0175 / ISO 15156 (Parts 1–3) — Petroleum and natural gas industries: materials for use in H2S-containing environments in oil and gas production.
• GB/T 8650 — Chinese national standard for evaluating pipeline and pressure vessel steel resistance to hydrogen-induced cracking.
• EFC Publication No. 16 — European guidance on materials selection for sour service, often referenced alongside the above standards.

Test Environments and Solutions

NACE TM0284 defines two standard test solutions, with the selection agreed between the purchaser and the testing laboratory based on the intended service environment:

• Solution A — synthetic seawater (5.0 wt% sodium chloride and 0.5 wt% glacial acetic acid in distilled or deionized water), saturated with H2S, with an initial pH of approximately 2.6 to 2.8. This is the default environment for most pipeline and pressure vessel qualifications.
• Solution B — a buffered solution (saturated H2S in a boric acid and sodium acetate buffer) with an initial pH of 3.5 to 4.0, typically selected for service environments where carbon dioxide coexists with H2S.

Exposure Procedure

• Specimens are placed in the test vessel, completely immersed in the deaerated solution, with the vessel purged with inert gas (typically nitrogen) to remove dissolved oxygen.
• H2S gas is bubbled through the solution at a controlled rate for a minimum initial purge, after which a positive H2S blanket of at least 0.1 vol% in the gas phase is maintained throughout the test.
• The exposure duration is 96 hours at a temperature of 25 °C ± 3 °C.
• Solution pH is measured at the start and end of the test; H2S concentration is verified to confirm environmental validity.
• All operations involving H2S are conducted in fume hoods or sealed systems with continuous toxic gas monitoring, in compliance with laboratory safety protocols.

Post-Exposure Evaluation

After the 96-hour exposure, specimens are removed, visually inspected for blisters, sectioned at predefined planes perpendicular to the rolling direction, mounted, ground, polished, and lightly etched. Each polished section is examined under optical microscopy, and crack dimensions are measured along a standardized reference grid. The CLR, CTR, and CSR are calculated for each section and specimen, and the maximum values are reported.

Typical Acceptance Criteria

Acceptance limits are specified by the purchaser, end user, or applicable standard. The widely applied criteria for sour service qualification, consistent with NACE MR0175 / ISO 15156 and common pipeline purchase specifications, are:

• CLR ≤ 15%
• CTR ≤ 5%
• CSR ≤ 1.5%
• No visible hydrogen blistering on the specimen surfaces (in most specifications)

Applications

HIC susceptibility testing is essential wherever steel equipment contacts wet H2S environments. Principal applications include:

• Oil and gas production — qualification of line pipe, tubing, casings, and flowlines for reservoirs containing H2S (NACE MR0175 / ISO 15156 compliance).
• Refining and petrochemical plants — material selection for sour water systems, amine units, hydroprocessing units, and sour crude processing equipment.
• Midstream transportation — certification of transmission pipelines handling sour natural gas and sour crude oil.
• Offshore and subsea facilities — qualification of platform structural components, risers, and subsea hardware exposed to produced fluids.
• Storage and pressure vessels — evaluation of plate used in tanks and vessels containing H2S-bearing media.
• Research and failure analysis — mill development of HIC-resistant steel grades, evaluation of heat treatment effects, and root-cause investigation of in-service cracking incidents.
• Quality control and certification — routine lot release testing required by purchase specifications and periodic audits at accredited third-party laboratories.

FAQ

What is the difference between HIC and SSC testing?

HIC testing (NACE TM0284) evaluates cracking caused by hydrogen generated by corrosion in wet H2S without any externally applied stress, whereas Sulfide Stress Cracking (SSC) testing (NACE TM0177) evaluates cracking under sustained tensile stress in a sulfide environment. HIC is driven by internal hydrogen pressure at inclusions, while SSC is a stress-driven cracking mechanism affecting harder microstructures. Many sour service qualifications require both tests.

What are CLR, CTR, and CSR?

CLR (Crack Length Ratio) measures the total crack length relative to the specimen width, CTR (Crack Thickness Ratio) measures the total crack depth relative to specimen thickness, and CSR (Crack Sensitivity Ratio) represents the total crack area relative to the cross-sectional area. These three parameters are the standard metrics used to grade HIC resistance.

How long does a standard HIC test take?

The standard exposure duration per NACE TM0284 is 96 hours. Including sample preparation, sectioning, metallographic analysis, and reporting, a complete test campaign is typically completed within five to ten working days.

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

Solution A (acidified synthetic seawater, pH ≈ 2.7) is the default and most widely specified environment. Solution B (buffered, pH 3.5–4.0) is selected when the service environment contains significant CO2 or when the purchase specification calls for it. The choice should be agreed between the purchaser and the accredited third-party laboratory.

Can welded pipes be tested for HIC resistance?

Yes. Welded line pipe is commonly tested with specimens taken from both the base metal and the weld region, allowing separate assessment of the weld seam, cap, root, and HAZ in accordance with NACE TM0284 and line pipe specifications.

Do HIC tests require applied stress?

No. The standard HIC test is performed on freely immersed, unstressed coupons. Where combined stress and hydrogen effects must be evaluated, supplementary methods such as SOHIC testing or SSC testing per NACE TM0177 are recommended.

What steel characteristics influence HIC resistance?

Key factors include sulfur content, inclusion morphology (especially elongated MnS), calcium treatment for inclusion shape control, centerline segregation, microstructure uniformity, and hardness. Low-sulfur, fully killed, inclusion-shape-controlled steels with homogeneous microstructures generally exhibit superior HIC resistance.