Overview
ISO 7539-7:2005, "Corrosion of metals and alloys - Stress corrosion testing - Part 7: Method for slow strain rate testing," specifies procedures for conducting slow strain rate testing (SSRT) to assess a metal or alloy’s susceptibility to stress corrosion cracking (SCC), including hydrogen‑induced failure. The method is adaptable to many product forms (plate, rod, wire, sheet, tube), welded components and composites, and can use plain or notched specimens. A principal advantage of ISO 7539-7 is the rapid assessment of a metal/environment combination under controlled strain and environmental conditions.
Key topics and requirements
- Test principle: apply a controlled, relatively slow strain to specimens while exposed to a specified corrosive or inert environment to detect SCC initiation or growth.
- Specimens: plain or notched specimens; common forms referenced in ISO 7539-4 and ISO 7539-6. Grip design and exclusion of grips from the corrosive environment are recommended to avoid galvanic or crevice effects.
- Strain rates: commonly used nominal strain rates range from about 10^-3 to 10^-7 s^-1. An initial rate ~10^-6 s^-1 is often recommended, with follow-up tests at ~10^-5 and ~10^-7 s^-1; very low rates (e.g. 10^-8 s^-1) may be used where appropriate.
- Environmental control: specify electrode potential, temperature, solution composition, pH, dissolved gases, flow rate and pressure; gas purity is critical for gaseous environments. Tests may be run at open circuit or under potentiostatic/galvanostatic control.
- Controls and comparisons: run tests in an inert environment and include unstrained specimens to provide baseline data. Calibration of crosshead-to-gauge strain is advised to ensure meaningful strain-rate control.
- Assessment metrics: time to failure, plastic strain to failure, ductility measures (elongation, reduction of area), maximum load, area under stress–elongation curve, percentage SCC on fracture surface, and average crack growth velocity from crack depth/time.
Applications and users
- Practical uses: ranking alloy/environment susceptibility, screening materials for service, evaluating weld heat‑affected zones, assessing hydrogen embrittlement and informing failure analysis.
- Typical users: corrosion engineers, materials and metallurgical engineers, test laboratories, quality/control teams, R&D groups, manufacturers of metals/alloys and welded products, and regulatory bodies needing standardized SCC test data.
Related standards
- ISO 7539 series (stress corrosion testing): Part 1 (general guidance), Parts 2–6, 8–9 (specimen preparation and specific test methods). Refer to ISO 7539-1, -4 and -6 for specimen design, preparation and complementary procedures.
Keywords: ISO 7539-7:2005, slow strain rate testing, stress corrosion testing, stress corrosion cracking, hydrogen-induced failure, metals and alloys, strain rate, SCC testing.
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