CTOD Testing with H2S Hydrogen Pre-Charging: What Sour Service Projects Actually Require
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When a project specification calls for CTOD testing on weld materials destined for sour service, the standard air-environment fracture toughness test is not enough. Pipeline and pressure vessel engineers working under ISO 15156 or NACE MR0175 know this. The question that comes up repeatedly in procurement and QA discussions is: which Indian lab can actually run CTOD with hydrogen pre-charging in H2S, and do it to a documented, defensible standard?
TCR Engineering's Navi Mumbai laboratory now operates a formal test procedure for exactly this — Crack Tip Opening Displacement (CTOD) testing on weld metallic materials with hydrogen pre-charging in H2S saturated solution, executed to ISO 15653, ISO 12135, BS 8571, and NACE TM0177.
Why Standard CTOD Is Not Sufficient for Sour Service Welds
Fracture toughness data from specimens tested in air tells you how a material behaves in a clean, dry environment. That is useful, but it does not represent what happens in a wet H2S-containing system — which is the operating reality for pipelines, pressure vessels, and process equipment in oil, gas, and petrochemical plants.
In sour service conditions, hydrogen generated by the corrosion reaction at the metal surface diffuses into the steel. It concentrates at stress risers, grain boundaries, and weld heat-affected zones. The result is a measurable reduction in fracture toughness — sometimes dramatic, particularly in harder weld metals or heat-affected zones with residual stress.
CTOD testing after hydrogen pre-charging in H2S solution captures this degradation. The test deliberately loads the material in its most vulnerable state: after the diffusible hydrogen has had time to penetrate the specimen ligament.
Projects specifying materials to NACE MR0175 / ISO 15156, or requiring qualification under PDO, OQGN, or similar operator specifications, will often ask for this combined test as part of weld procedure qualification or materials acceptance.
What TCR's H2S CTOD Procedure Actually Involves
TCR's test procedure was developed for Larsen & Toubro and covers the full sequence from specimen preparation through to fracture surface reporting. Here is what the process looks like in practice.
Specimen preparation and fatigue pre-cracking
SENB (single edge notched bend) specimens are machined to B x 2B geometry. The notch is cut by EDM, and specimen orientation follows ISO 15653 requirements for weld centreline positioning. At least 75% of the crack front area must fall within the weld metal.
Fatigue pre-cracking is carried out in air at room temperature using a force-constant method, with stress ratio R between 0.1 and cycling between 10⁴ and 10⁶ cycles depending on specimen geometry. Total crack size (machined notch plus fatigue crack) must fall between 0.45W and 0.70W. Once pre-cracking is complete, the specimen does not go directly to CTOD loading — it goes to hydrogen charging first.
Hydrogen pre-charging in H2S solution
The charging solution is NACE TM0177 Solution A: distilled water with 5% sodium chloride and 0.5% glacial acetic acid by weight. The solution is de-aerated with nitrogen before H2S gas is introduced by con
Specimens are fully submerged for a minimum of 96 hours — four days — at 25 ± 3°C. H2S concentration is verified by iodometric titration at a minimum of 2300 ppm after saturation. Temperature and pH are recorded at the start, end, and at least every 24 hours through the soak period.
This is where the procedure gets operationally demanding. The time between removing the specimen from the H2S solution and starting the CTOD test cannot exceed 20 minutes. If that window is missed, the specimen goes back into solution for a minimum of another 24 hours. There is no shortcut here — the entire value of the test depends on retaining diffusible hydrogen within the specimen at the moment of loading.
CTOD loading and calculation
After removal and a quick rinse to clear residual acid and salt, the specimen goes immediately onto the three-point bend fixture. The COD gauge is attached, the load is applied at a controlled displacement rate not exceeding 3 MPa(m)^½ per second in the linear elastic region, and an autographic load versus displacement curve is generated.
The CTOD value (δ) is calculated per ISO 15653:2018 and ISO 12135:2021, combining elastic and plastic displacement components. The hydrogen charging affects the material's mechanical response, not the calculation methodology itself.
Post-test, specimens are broken open for nine-point crack front measurement and fracture surface examination. Evidence of hydrogen-related damage — HIC, secondary cracking, mixed fracture behaviour — is documented and reported alongside the δ values.
What This Test Reveals That Ordinary CTOD Does Not
A material that passes CTOD in air may fail at a significantly lower δ value after H2S pre-charging. The magnitude of reduction varies with steel grade, yield strength, weld process, and heat input. High-strength steels and hard HAZ microstructures are particularly sensitive.
Avinash Tambewagh, Technical Head at TCR Engineering, explains the practical significance: "The air CTOD gives you the baseline. The H2S pre-charged result tells you what actually happens when that weld joint is exposed to the operating environment. For sour service qualification, that second number is the one that matters to the project."
The comparison between charged and uncharged CTOD results — where both are available — provides a direct measure of hydrogen embrittlement susceptibility for the specific weld procedure being qualified. This is particularly relevant for EPC contractors qualifying new weld procedures for upstream oil and gas work, where operator-specific standards may require demonstration of retained toughness after sour exposure.
Standards Covered by TCR's H2S CTOD Procedure
The test procedure references and aligns with the following standards (latest editions):
ISO 15653 — fracture toughness of welds
ISO 12135 — unified quasistatic fracture toughness method
BS 8571 — SENT specimen testing (hydrogen pre-charging methodology annex)
NACE TM0177 — laboratory testing in H2S environments
ISO 15156 / NACE MR0175 — materials for H2S-containing environments
NACE TM0284 — hydrogen-induced cracking evaluation
For projects requiring PDO or OQGN specification compliance, this procedure sits within TCR's broader corrosion testing capability, which includes HIC and SSCC testing at a laboratory approved by PDO, ONGC, and EIL. The NABL-accredited laboratory at Navi Mumbai underpins all test reporting.
Safety and Infrastructure Requirements for H2S CTOD
This testing category is not operationally simple. H2S has an IDLH concentration of 100 ppm and a permissible exposure ceiling of 20 ppm. Running this work requires a dedicated fume hood or ventilated enclosure, continuous H2S gas monitoring worn by all personnel, minimum two-person attendance during any handling operation, and SCBA on standby. Spent solution must be chemically neutralised before disposal.
TCR's procedure mandates a completed and approved H2S-specific risk assessment before any work begins under this method. This infrastructure investment is what separates labs that can genuinely offer this test from those that nominally list it on a capabilities brochure.
Where This Capability Fits in a Sour Service Project
Most clients engaging TCR for this work are in one of three situations:
Weld procedure qualification. An EPC contractor or fabricator needs to qualify a WPS for sour service piping or pressure vessels. The project specification — or the end operator's standard — requires CTOD with H2S pre-charging as part of the PQR dataset. TCR handles the fracture toughness component alongside welder qualification and corrosion testing under one roof.
Materials acceptance testing. A pipe or fitting manufacturer needs third-party verification that their product meets the fracture toughness floor in the sour-charged condition before shipment to a Middle East or Indian operator.
Failure investigation support. A weld joint has cracked in service in a sour environment. Understanding whether the material had adequate pre-charged CTOD at qualification is part of the root cause picture. TCR's failure analysis team can integrate fracture toughness data with metallographic and fractographic findings.
For clients in the Middle East, TCR Arabia in Dammam supports testing coordination for Saudi Aramco and SABIC-related projects, with sample logistics into the Navi Mumbai laboratory where the CTOD facility is located.
Getting This Test Done
Sample requirements, specimen sizing, and soak duration requirements should be confirmed before submitting material to avoid rework. For weld metal testing, the weld joint should arrive with sufficient parent material to allow SENB specimens to be machined to the correct B x 2B geometry with proper notch positioning on the weld centreline.
Minimum specimen count is three per notch position. For projects requiring both air-environment and H2S-charged CTOD, that means a minimum of six specimens per position — and the two datasets together give you the full sour service toughness picture.
To discuss testing requirements or sample submission, contact the laboratory directly at sales@tcreng.com or call +91-22-67380900. You can also download the sample size requirements guide from the TCR website.
For a full picture of TCR's fracture toughness and corrosion testing services, visit the CTOD and fracture toughness page and the corrosion testing page.
CTOD testing with H2S hydrogen pre-charging is a specialist capability — one that very few Indian laboratories can execute to a documented, internationally referenced procedure. TCR Engineering now does exactly that.
Frequently Asked Questions
What is CTOD testing with H2S hydrogen pre-charging? It is a fracture toughness test (Crack Tip Opening Displacement) where specimens are immersed in an H2S-saturated acidic solution for a minimum of 96 hours before CTOD loading. The purpose is to simulate sour service conditions by introducing diffusible hydrogen into the specimen, then measuring how that affects fracture resistance.
Which standards govern CTOD testing in H2S environments? The primary standards are ISO 15653, ISO 12135, BS 8571, and NACE TM0177. Material qualification for sour service also references ISO 15156 / NACE MR0175. TCR's procedure aligns with all of these in their latest editions.
When is H2S pre-charged CTOD required? It is typically required for weld procedure qualification in sour service applications — pipelines, pressure vessels, and process equipment operating in H2S-containing environments. Project specifications referencing NACE MR0175, PDO standards, or OQGN specifications commonly call for this test.
How long does the H2S pre-charging soak take? The minimum immersion period is 96 hours (four days) in NACE TM0177 Solution A. For thicker specimens, longer soak times may be required to ensure hydrogen has fully diffused through the specimen ligament.
What is the critical time limit between H2S removal and CTOD loading? The specimen must be placed on the test fixture and loading commenced within 20 minutes of removal from the H2S solution. Exceeding this window results in loss of diffusible hydrogen, which invalidates the test. If the window is missed, the specimen must be re-immersed for at least another 24 hours.
Can TCR test both charged and uncharged specimens for comparison? Yes. TCR can run CTOD in air and CTOD after H2S pre-charging on the same material, providing a direct comparison that quantifies hydrogen embrittlement susceptibility for the specific weld procedure being qualified.
Does TCR hold the necessary accreditations for this testing? TCR Engineering's Navi Mumbai laboratory is NABL-accredited and ISO 17025 certified. The corrosion testing facility is approved by PDO, ONGC, and EIL for sour service testing under NACE standards.
