Overview
ISO 6957:1988 - "Copper alloys - Ammonia test for stress corrosion resistance" specifies a laboratory corrosion test that uses an ammoniacal atmosphere to reveal applied or residual stresses in copper alloys that can lead to stress corrosion cracking (SCC) in service or storage. The method is simple, reproducible and its severity is adjustable by changing the pH of the test solution. An informative annex provides guidance on selecting pH values for common ambient exposure conditions.
Key topics and technical requirements
- Test principle: 24‑hour exposure of cleaned specimens to an ammoniacal vapour atmosphere followed by visual/microscopic inspection for cracks (recommended magnification x10–x15).
- Test solution preparation: dissolve 107 g ± 0.1 g ammonium chloride (NH4Cl) in distilled/deionized water (prepare partially, adjust pH with sodium hydroxide 30–50% solution, then dilute to final volume and re-check pH).
- pH control: pH of the ammoniacal solution is specified in the product specification or by agreement; Annex A recommends typical values (around pH ~10.0–10.5) according to environment corrosivity.
- Exposure conditions: specimen must be placed so vapours reach all surfaces; temperature maintained between 20 °C and 30 °C (use 25 °C ±1 °C in case of dispute); exposure duration 24 h.
- Specimen details: minimum specimen length 150 mm for drawn products ≤75 mm diameter; number and exact geometry of specimens by agreement or product spec.
- Solution/volume requirements: prescribed minimum volumes of test solution relative to container volume and specimen surface area to ensure adequate vapour generation.
- Cleaning and inspection: degrease, clean in 5% sulfuric acid solution (optionally with hydrogen peroxide 20–50 ml/l for post-test cleaning), rinse, dry and inspect. Fine cracks may be opened gently to aid visual detection.
- Reporting: include sample identification, reference to ISO 6957, pH, exposure temperature, specimen count, results (cracks/no cracks), any deviations and test date.
Applications - who uses this standard
- Quality control and acceptance testing of copper alloys and finished copper products.
- Material selection and specification for components susceptible to SCC (e.g., fittings, tubing, assemblies).
- Corrosion engineers, metallurgists, test laboratories, manufacturers and purchasers specifying copper-alloy performance in ambient environments or storage.
- Evaluation of assemblies and partial assemblies where residual or applied stresses may be present.
Related standards and references
- ISO technical work by ISO/TC 26 (Copper and copper alloys).
- References cited in ISO 6957 include ASTM publications and BS 2871: Part 3:1972; Annex B lists bibliography for further background on ammonia testing and SCC of copper alloys.
Keywords: ISO 6957, copper alloys, ammonia test, stress corrosion resistance, stress corrosion cracking, ammoniacal atmosphere, pH control, corrosion testing.
