// Corrosion Testing — H₂S Environments
S.S.C. Testing
Sulphide Stress Corrosion — Accredited NACE TM0177 & TM0316 Testing
SMT srl performs accredited SSC testing on steel components exposed to H₂S environments. Using Proof Ring and Four Point Bending methods, our dedicated sour-service laboratory delivers accredited results per NACE TM0177, NACE TM0316, and UNI EN ISO 7539-2.
// Understanding the Test
What is Sulphide Stress Corrosion?
Sulphide Stress Cracking (SSC) is a form of hydrogen-assisted cracking that occurs when a susceptible steel is simultaneously subjected to tensile stress and a wet H₂S environment. It is a specific and particularly dangerous form of stress corrosion cracking, because it can cause sudden brittle fracture at stress levels well below the material's nominal yield strength.
When steel corrodes in a solution containing hydrogen sulphide, atomic hydrogen is released at the metal surface. The presence of H₂S poisons the hydrogen recombination reaction, promoting absorption of atomic hydrogen into the steel rather than its release as H₂ gas. This absorbed hydrogen embrittles the steel microstructure and, combined with applied or residual stress, drives crack initiation and propagation.
The resulting cracks are typically perpendicular to the applied stress direction, distinguishing SSC from HIC, where cracks run parallel to the rolling plane with no need for external stress. SSC primarily affects carbon and low-alloy steels at ambient temperature; on stainless steels, duplex alloys, and CRAs the equivalent phenomenon is termed Stress Corrosion Cracking (SCC).
The SSC test reproduces the sour service environment in the laboratory, applying a controlled stress to the specimen and exposing it to H₂S-saturated solution for 720 hours. Post-test examination determines whether any cracking occurred.
SSC — Key Characteristics
Cracks: perpendicular to stress direction
Mechanism: hydrogen embrittlement under load
- Occurs at ambient or near-ambient temperature
- Stress can be applied or residual (e.g. welds)
- H₂S poisons hydrogen recombination at surface
- Atomic H diffuses to high-stress regions
- Embrittlement leads to brittle fracture
- Higher strength steels are more susceptible
- Hardness limits (max HRC 22) defined by NACE MR0175
- Distinct from HIC: no external stress needed for HIC
// Test Techniques
Testing Methods
The SSC phenomenon can be studied through two complementary loading configurations, each codified by NACE and EFC. Both use H₂S-saturated solutions and a 720-hour immersion period.
Four Point Bending (FPB)
Flat specimen — bending stress
The Four Point Bending method uses a flat, parallelepiped specimen loaded at four contact points. Stress is concentrated in the central span between the two inner rollers, producing a uniform bending moment in that region.
The load is applied by deforming the specimen against a jig to a calculated deflection corresponding to the target stress level. A digital displacement gauge or strain gauges can be used to set and verify the applied strain. The loaded specimen is then immersed in H₂S-saturated solution for 720 hours.
After the test, the stressed area is examined for cracks characteristic of SSC. The FPB method is particularly suited to assessing weld zones, HAZ regions, and flat product forms such as plates and strip.
- Specimen geometryFlat parallelepiped
- Loading mode4-point bending
- Stress controlDisplacement / strain gauge
- Duration720 hours
Proof Ring
Round tensile specimen — uniaxial stress
The Proof Ring method uses a round tensile specimen loaded under uniaxial stress by a calibrated proof ring jig. The jig applies and maintains a constant tensile load on the specimen throughout the test, independently of any corrosion-induced relaxation.
The specimen is immersed in a dedicated sealed test cell that maintains the H₂S-saturated solution at the required conditions. The uniaxial loading configuration is ideal when a specific, well-defined stress direction must be applied — such as longitudinal stress in pipe or uniaxial testing to a defined fraction of the material's yield strength.
This method is particularly appropriate for tubular products, forgings, and round bar, and is the preferred technique when stress magnitude must be precisely controlled and documented.
- Specimen geometryRound tensile (dog-bone)
- Loading modeUniaxial tension
- Stress controlCalibrated proof ring
- Duration720 hours
// Normative Framework
Applicable Standards
// Sour Service Damage Mechanisms
SSC vs HIC — Key Differences
Both SSC and HIC are hydrogen damage mechanisms occurring in wet H₂S environments, but they differ fundamentally in their driving force, crack morphology, and applicable test methods. Understanding the distinction is essential for correct material qualification.
| Parameter | SSC (Sulphide Stress Cracking) | HIC (Hydrogen Induced Cracking) |
|---|---|---|
| Driving force | Tensile stress (applied or residual) + H₂S | Internal H₂ pressure at inclusions — no external stress required |
| Crack orientation | Perpendicular to the stress direction | Parallel to the rolling plane (stepwise through thickness) |
| Primary materials at risk | High-strength carbon & low-alloy steels; hardness > HRC 22 most susceptible | Carbon steels with high inclusion content; poor cleanliness |
| Test method | Specimen under load immersed in H₂S solution (720 h) | Unloaded specimen immersed in H₂S solution (96 h) |
| Key standards | NACE TM0177, NACE TM0316, ISO 7539-2 ACCREDIA | ANSI/NACE TM0284 ACCREDIA |
| Pass/fail criteria | Presence/absence of cracks after 720 h | CLR, CTR, CSR values vs specification limits |
| SMT capability | FPB + Proof Ring — both methods available | Solution A + Solution B — both available |
// Official Recognition
ACCREDIA
Accredited Laboratory
SMT srl holds ACCREDIA accreditation No. 00588, issued in accordance with UNI CEI EN ISO/IEC 17025:2018 — the international standard for the competence of testing and calibration laboratories.
The SSC tests — both NACE TM0177 Method A (Proof Ring) and NACE TM0316 / ASTM G39+TM0177 / UNI EN ISO 7539-2 (Four Point Bending) — are explicitly included in SMT's accreditation scope. Results are accepted by international operators, EPCs, and certification bodies under the ILAC mutual recognition framework.
Testing is performed in a purpose-built, safety-controlled laboratory dedicated to H₂S-environment testing, equipped with gas management systems and all containment infrastructure required for NACE-compliant sour service testing.
Accredited SSC Scope
// Industrial Relevance
Where SSC Testing Matters
SSC qualification is required for any steel component that will be subjected to stress in a wet H₂S environment. The following sectors and applications routinely specify NACE TM0177 and TM0316 testing:
Oil & Gas Wellheads
Wellhead components, christmas trees, and downhole tubulars operating in sour gas wells must demonstrate SSC resistance. NACE TM0177 is the baseline qualification requirement for materials specified under NACE MR0175 / ISO 15156.
OCTG & Linepipe
Oil Country Tubular Goods (casings, tubing, drill pipe) and high-strength linepipe grades intended for sour service must meet SSC qualification criteria. Testing per TM0177 is integral to API 5CT and API 5L sour service annexes.
Pressure Vessels & Reactors
Weld joints, HAZ regions, and hard spots in pressure vessels used in sour gas processing are primary SSC crack initiation sites. FPB testing per TM0316 or ISO 7539-2 is widely specified for weld procedure qualification in this sector.
Petroleum Refining
Refinery vessels, heat exchangers, and piping systems handling sour crudes or wet H₂S streams require confirmed SSC resistance. High-strength fasteners and valve bodies are often the most critical items requiring testing.
Valves, Fittings & Fasteners
High-strength bolting, valve bodies, and fittings used in sour service are particularly susceptible to SSC due to elevated hardness levels. NACE MR0175 imposes hardness limits and requires qualification by testing where necessary.
Offshore & Subsea
Structural components, risers, and pipeline systems in offshore sour environments must withstand combined corrosive and mechanical loading. FPB and Proof Ring testing are standard in offshore material qualification packages for operators and certification societies.
// Why Choose Us
A Partner You Can Count On
Dedicated H₂S Laboratory
SMT operates a purpose-built, safety-controlled laboratory for sour-service testing. The facility features gas management and containment systems designed for continuous, safe operation with H₂S, enabling both FPB and Proof Ring tests to run simultaneously for fast throughput.
Both Methods — One Laboratory
SMT performs both Proof Ring (NACE TM0177 Method A) and Four Point Bending (NACE TM0316, ASTM G39+TM0177, ISO 7539-2) in-house, all under ACCREDIA accreditation. Combined qualification programmes — including HIC testing — can be managed from a single order.
Job Manager — Dedicated Contact
Each project is assigned a dedicated Job Manager who coordinates specimen receipt, machining, test execution, and report delivery. The Job Manager acts as direct technical contact for the client throughout the entire process — before, during, and after the test.
40+ Years of Experience
Founded in 1979, SMT has decades of experience in sour-service corrosion testing. With over 20,000 test reports issued per year and 90+ accredited tests, the laboratory is a recognised reference in materials testing for the oil and gas sector across Europe and beyond.
// All Testing Services
Discover All Services
Mechanical Testing
Tensile, hardness, impact, bend, creep, and DWT testing on metallic materials.
Fracture & Fatigue
CTOD, K₂C, J₂C, and fatigue crack propagation testing per ASTM and ISO standards.
Metallographic Examinations
Macro and micrographic examinations, fractographic analysis, and surface measurements.
Failure Analysis
Root cause investigation of component failures through integrated testing and analysis.
Chemical Analysis
Compositional analysis to verify conformance with material specifications and standards.
Corrosion Testing
SSC, HIC, pitting, crevice, intergranular, CSCC — full corrosion testing programme.
Non-Destructive Testing
UT, MT, PT and other NDT methods for integrity assessment of components.
Mechanical Workshop
In-house machining of specimens to international standards with full dimensional control.
// Company Overview
About SMT srl
Founded in 1979, SMT srl is one of Italy's leading independent materials testing laboratories, located in Pozzo d'Adda near Milan, Lombardia. The laboratory holds ACCREDIA accreditation No. 00588 and issues over 20,000 accredited test reports per year.
The H₂S corrosion laboratory — a purpose-built, safety-controlled facility — is designed for continuous sour-service testing with both FPB and Proof Ring setups running in parallel, together with the full HIC programme per NACE TM0284.
Each project is managed by a Job Manager — a dedicated technical contact with deep expertise in sour-service material qualification who accompanies the client from the first inquiry to report delivery.
Discover SMT srl →
SMT srl — Company Presentation Video
// Frequently Asked Questions
SSC Testing FAQ
SSC is a form of hydrogen-assisted cracking caused by the simultaneous presence of tensile stress and a wet H₂S environment. Hydrogen sulphide poisons the surface recombination of atomic hydrogen, promoting its absorption into the steel lattice. The absorbed hydrogen embrittles the material, and combined with applied or residual stress, drives crack initiation and propagation perpendicular to the stress direction. It primarily affects carbon and low-alloy steels at ambient temperature, and is most severe in higher-strength grades.
SSC requires an applied or residual tensile stress to occur. Cracks propagate perpendicular to the stress axis. HIC requires no external stress — cracking is driven entirely by internal hydrogen pressure at inclusions and segregation bands, producing cracks parallel to the rolling plane. The test methods reflect these differences: SSC specimens are loaded during immersion (720 h), while HIC specimens are unloaded (96 h). Both tests can be performed by SMT under ACCREDIA accreditation.
Both methods apply stress to the specimen during immersion in H₂S-saturated solution, but through different loading configurations. The Proof Ring (NACE TM0177 Method A) uses a round tensile specimen under uniaxial constant tensile load — ideal for tubulars, forgings, and round bar where a precise, well-defined stress level must be applied. Four Point Bending (NACE TM0316, ISO 7539-2) uses a flat specimen loaded in bending — better suited to plates, strip, and weld qualification where the bending stress configuration better replicates service conditions.
Both Proof Ring and FPB methods require a minimum immersion time of 720 hours (30 days) in H₂S-saturated solution per NACE TM0177 and TM0316. Added to this are specimen machining time before the test and post-test examination and report preparation. From receipt of raw material, a typical total turnaround is 6–8 weeks for a complete accredited test report. Contact us for current scheduling availability.
The primary criterion for NACE TM0177 and TM0316 is the presence or absence of cracking at the end of the 720-hour test period. The stressed area is examined visually and, where required, by optical microscopy or metallographic cross-section. A specimen that passes shows no crack initiation or propagation attributable to SSC. The specific stress level applied (typically a percentage of the material's actual yield strength or SMYS) is defined by the applicable material standard or project specification.
The test solutions contain sodium chloride, acetic acid, and/or sodium acetate in proportions that vary depending on the applicable standard. All solutions are saturated with hydrogen sulphide (H₂S) gas during the test. NACE TM0177 specifies Solution A (5% NaCl + 0.5% CH₃COOH, saturated with H₂S at atmospheric pressure) as the default, with alternative solutions defined for specific applications. The composition and preparation of the solution are controlled parameters subject to ACCREDIA accreditation at SMT.
Yes. SMT's in-house mechanical workshop machines both Proof Ring (round tensile) and FPB (flat parallelepiped) specimens directly from material supplied by the client — pipe sections, plate, bar, or forging. Specimen dimensions comply with the applicable standard, and orientation relative to the product rolling or forging direction is documented. Full dimensional records are maintained as part of the accredited test dossier.
NACE MR0175 / ISO 15156 is the governing standard for material selection in H₂S-containing environments in oil and gas production. It defines the conditions under which SSC can occur (H₂S partial pressure thresholds, pH ranges), sets hardness limits for carbon and low-alloy steels (typically max HRC 22 / HV 250), and specifies when SSC testing per TM0177 or TM0316 is required or can be used to qualify materials above standard hardness limits. SMT's accredited test reports are directly applicable to MR0175 qualification packages.
Yes. SMT performs both SSC testing (TM0177 and TM0316) and HIC testing (TM0284) under the same ACCREDIA accreditation, in the same dedicated H₂S laboratory. Combined sour service qualification programmes — which typically require both test types — can be handled from a single order managed by one Job Manager, reducing coordination overhead and facilitating a single combined test report or coordinated report set.
Yes. SMT holds ACCREDIA accreditation No. 00588 under UNI CEI EN ISO/IEC 17025:2018. ACCREDIA is a signatory to the EA multilateral agreement (MLA) and the ILAC mutual recognition arrangement (MRA), meaning accredited test reports issued by SMT are recognised across Europe and internationally. This is essential for qualification projects involving international oil and gas operators, EPCs, and certification bodies such as Bureau Veritas, DNV, Lloyd's Register, and TÜV.
EFC Publication No. 16 (European Federation of Corrosion) provides guidelines on materials requirements for carbon and low-alloy steels in H₂S-containing environments in oil and gas production. It references and supplements the NACE TM0177 and TM0284 methodologies, defining Method A (uniaxial constant load, equivalent to TM0177 Method A) and Method B (bent beam / FPB). EFC 16 is widely cited in European project specifications alongside or in lieu of NACE standards, and the test methods it defines are covered within SMT's accredited scope.
SSC susceptibility increases strongly with steel hardness. Higher hardness correlates with higher strength and a microstructure more prone to hydrogen embrittlement. NACE MR0175 / ISO 15156 sets maximum hardness limits for carbon and low-alloy steels in sour service (typically HRC 22 / HV 250 for base metal). Hardness testing is therefore always performed alongside or as a prerequisite to SSC testing — a specimen that exceeds the hardness limit is non-conforming regardless of the SSC test outcome. SMT performs both hardness verification and SSC testing as part of a complete sour-service qualification package.
// Request a Quote
Request an SSC Test Quote
Send us your requirements — material, product form, method (FPB or Proof Ring), applicable standard, and quantity — and we will provide a customised quotation.