Stress corrosion slow strain rate tensile testing (SSRT) is an accelerated laboratory method used to evaluate the susceptibility of metallic materials to environmentally assisted cracking in aggressive corrosive media. By stretching a tensile specimen at a slow, controlled extension rate—typically on the order of 10⁻⁶ s⁻¹—while it is immersed in the service environment, the test reveals stress corrosion cracking (SCC) susceptibility within days rather than months. This technical guide covers the testing principles, specimen types, measured parameters, governing standards such as ASTM G129, ISO 7539-7, and GB/T 15970.7, and typical industrial applications.
Technical Overview
Stress corrosion cracking (SCC) is a delayed, brittle failure mechanism produced by the synergistic action of a sustained or slowly increasing tensile stress and a specific corrosive environment acting on a susceptible material. Because SCC often develops over months or years under static load, conventional constant-load or constant-deflection tests can be extremely time-consuming. Slow strain rate tensile testing, also known as constant extension rate testing (CERT), was developed to solve this problem by applying a continuously increasing strain to the specimen, which repeatedly ruptures the passive film and accelerates crack initiation and propagation through either anodic dissolution or hydrogen-assisted mechanisms.
In a typical SSRT test, a machined tensile specimen is loaded into a corrosion cell or high-pressure autoclave containing the aggressive medium and pulled to complete fracture at a nominal strain rate between 10⁻⁷ and 10⁻⁴ s⁻¹, with 10⁻⁶ s⁻¹ being the most widely used value. A companion test is performed in an inert reference environment (air, nitrogen, argon, or inert oil) at the same temperature and strain rate. The degree of susceptibility is judged by comparing the mechanical response—time to failure, elongation, reduction of area, and fracture energy—in the corrosive environment against the inert baseline, supplemented by fractographic and metallographic examination of the fracture surfaces.
- Accelerated screening: complete results within 1 to 7 days instead of weeks or months.
- Dynamic straining continuously exposes fresh metal, promoting crack initiation at otherwise dormant sites.
- Applicable over a wide range of temperatures and pressures, from ambient brines to high-temperature pressurized water in autoclaves.
- Compatible with electrochemical control, cathodic charging, and hydrogen ingress studies.
Testing Samples
Smooth Round Bar Tensile Specimens
The most common specimen is a cylindrical smooth tensile bar with a reduced gauge section, typically with a gauge diameter of 3 to 6 mm and a gauge length-to-diameter ratio of approximately 4:1. The gauge surface is ground and polished to a fine finish to remove machining marks that could act as artificial crack initiation sites. Smooth specimens provide a conservative overall assessment of SCC susceptibility because crack initiation must occur naturally on the gauge surface.
Notched Specimens
Circumferentially notched specimens concentrate stress and strain at the notch root, promoting crack initiation in materials or environments where smooth specimens may not fail by SCC. Notched geometries are useful for ranking highly corrosion-resistant alloys and for studying hydrogen embrittlement of high-strength materials.
Flat and Sheet Specimens
For plate, sheet, strip, and thin-wall products where round bars cannot be extracted, flat tensile specimens with rectangular gauge sections are used. Sub-sized flat specimens are also employed when material is limited, such as for weld qualification coupons or additively manufactured builds.
Welded Joint Specimens
Specimens can be machined transverse to a weld so that the gauge section spans the base metal, heat-affected zone, and weld metal. This allows the most susceptible region of the welded joint to be identified and the effectiveness of post-weld heat treatment to be verified.
Applicable material families include:
- Carbon and low-alloy steels, including pipeline steels and high-strength quenched-and-tempered grades.
- Austenitic, duplex, super duplex, and martensitic stainless steels.
- Nickel-based corrosion-resistant alloys such as Alloy 625, Alloy 825, and Alloy C-276.
- Aluminum alloys (2xxx, 5xxx, and 7xxx series), magnesium alloys, titanium alloys, and copper alloys such as brass in ammoniacal environments.
Testing Items
The following parameters are recorded and evaluated in a standard SSRT campaign:
- Time to failure in the corrosive environment and in the inert reference environment.
- Ultimate tensile strength and the full load–extension (stress–strain) curve.
- Uniform elongation and total elongation at fracture.
- Reduction of area (ROA) measured on the fractured halves.
- Fracture energy, calculated as the area under the load–extension curve.
- Susceptibility ratios: time-to-failure ratio, elongation ratio, reduction-of-area ratio, and fracture-energy ratio, each defined as the value in the corrosive environment divided by the value in the inert environment.
- Density and morphology of secondary cracks on the gauge surface and in cross sections.
- Average stress corrosion crack propagation rate, estimated from the deepest crack divided by the cracking time.
- Fracture mode classification by scanning electron microscopy: ductile dimple rupture, intergranular cracking (IGSCC), transgranular cracking (TGSCC), or mixed modes.
Testing Methods
Governing Standards
- ASTM G129 — Standard Guide for Slow Strain Rate Testing to Evaluate the Susceptibility of Metallic Materials to Environmentally Assisted Cracking.
- ISO 7539-7 — Corrosion of metals and alloys, stress corrosion testing, Part 7: Method for slow strain rate testing.
- GB/T 15970.7 — Chinese national standard for slow strain rate stress corrosion testing, technically aligned with ISO 7539-7.
- NACE TM0198 — Slow strain rate test method for screening corrosion-resistant alloys for stress corrosion cracking in sour oilfield service.
Apparatus and Environment Control
Testing is performed on a dedicated constant extension rate machine or a servo-electric/servo-hydraulic load frame capable of stable, low crosshead speeds. The specimen is enclosed in a glass corrosion cell for ambient-temperature work or in a pressurized autoclave for elevated-temperature service up to approximately 350 °C. Environmental parameters such as chloride concentration, pH, dissolved oxygen, and H₂S partial pressure are controlled and monitored throughout the test, and tests can be run at the free corrosion potential or under potentiostatic or galvanostatic control. Common test media include 3.5% sodium chloride solution, synthetic seawater, acidified sour brines, caustic solutions, and high-purity demineralized water for nuclear applications.
Typical Test Procedure
- Machining of specimens, followed by surface grinding, polishing, degreasing, and dimensional inspection.
- Conditioning of the test solution: deaeration with nitrogen or argon, saturation with H₂S where required, and pH adjustment to the target value.
- Mounting the specimen in the cell, filling the environment, and allowing the open-circuit potential to stabilize before loading.
- Applying the selected strain rate (commonly 1 × 10⁻⁶ s⁻¹) and pulling the specimen to complete fracture while continuously recording load and extension.
- Running a duplicate test in an inert reference environment at the same temperature and strain rate.
- Post-test measurement of reduction of area, metallographic cross-sectioning for secondary cracks, and fractographic examination by SEM.
- Calculation of susceptibility ratios and issuance of the test report with photographs and fractographs.
Data Evaluation and Acceptance
A susceptibility ratio close to 1.0 indicates that the environment has little effect on the mechanical response and that the material is resistant to SCC under the tested conditions. Significantly reduced ratios, together with intergranular or transgranular brittle facets and secondary gauge cracking, indicate susceptibility. Acceptance thresholds—for example, a minimum reduction-of-area ratio on the order of 0.8 with no significant secondary cracking—are typically defined by the purchaser, the material specification, or the qualification protocol referenced by standards such as ISO 15156 / NACE MR0175 for sour service. Because SSRT applies sustained dynamic straining, it is regarded as an aggressive screening method, and borderline results are often confirmed by constant-load tests of longer duration in accredited third-party laboratories.
Applications
SSRT is used across many industries in which environmentally assisted cracking governs equipment integrity:
- Oil and gas: qualification of corrosion-resistant alloys for tubing, casing, flowlines, and wellhead equipment exposed to wet H₂S, CO₂, and chlorides; screening under ISO 15156 / NACE MR0175 frameworks.
- Nuclear power: evaluation of austenitic stainless steels, Alloy 600/690, and weld metals in simulated PWR and BWR primary water at elevated temperature and pressure.
- Pipelines: assessment of near-neutral-pH and high-pH SCC susceptibility of line pipe steels in carbonate–bicarbonate and dilute groundwaters.
- Chemical and petrochemical processing: pressure vessels, heat exchangers, and piping in chloride, caustic, and polythionic acid environments.
- Aerospace: qualification of high-strength aluminum alloys, landing gear steels, and fasteners, including hydrogen embrittlement evaluation under cathodic charging.
- Marine and offshore: ranking of alloys in seawater and under cathodic protection, and evaluation of weldments and clad overlays.
- Research and development: comparison of heat treatments, inhibitors, surface finishes, and additively manufactured materials.
FAQ
What is the difference between SSRT and constant-load SCC testing?
Constant-load tests, such as those in NACE TM0177, hold a fixed stress or load until failure or a defined run-out time, which can take 30 days or longer. SSRT instead applies a continuously increasing strain at a slow rate, forcing failure within a few days and making it a much faster screening tool, although it is more aggressive and less representative of purely static service stresses.
What strain rate should be selected?
A nominal rate of 1 × 10⁻⁶ s⁻¹ is the most common choice and is suitable for many systems. Because SCC susceptibility is rate-dependent, a preliminary test series spanning 10⁻⁷ to 10⁻⁴ s⁻¹ is recommended for new material–environment combinations to identify the rate that maximizes cracking response.
How long does a single test take?
At a strain rate of 1 × 10⁻⁶ s⁻¹, most ductile alloys fracture within roughly 1 to 3 days; very high strain rates shorten this to hours, while the lowest rates may extend the test to about a week. Inert reference tests require the same duration.
Why is a reference test in an inert environment required?
The inert-environment test establishes the baseline mechanical behavior of the same material at the same temperature and strain rate. Only by normalizing the corrosive-environment results to this baseline can environmental degradation be separated from ordinary metallurgical variability.
Can SSRT be performed at high temperature and high pressure?
Yes. Autoclave-based SSRT systems routinely operate at temperatures up to about 350 °C and elevated pressures, enabling simulation of geothermal, nuclear, and deep oilfield conditions, including controlled H₂S and CO₂ partial pressures.
Is SSRT a qualification test by itself?
SSRT is primarily a screening and ranking method because the imposed dynamic strain is more severe than most service conditions. Critical applications typically combine SSRT results with constant-load or fracture-mechanics-based tests, following the acceptance criteria specified in the applicable design code or purchase specification.