Overview
ISO 20728:2018 - Corrosion of metal and alloys: Determination of resistance of magnesium alloys to stress corrosion cracking (SCC) - specifies a laboratory method to assess how magnesium alloys used in structural applications resist SCC. The standard covers sampling, specimen types, loading procedures (constant load, constant strain, slow strain rate), test environments (alternate or continuous immersion, chloride/sulphate solutions), and interpretation and reporting of results. It is intended to identify relative SCC susceptibility as a function of chemical composition, manufacturing route, and heat treatment, but it does not by itself qualify a material for service.
Key topics and technical requirements
- Scope and applicability: Applies to cast and wrought magnesium alloys in castings, semi‑finished products, parts and weldments for structural uses (e.g., automotive housings, steering components, electronic housings, power tools, medical equipment).
- Specimen types: Tension specimens, C‑ring, bend, bent beam, U‑bend, and optionally notched or pre‑cracked specimens to assess thresholds or crack growth.
- Sampling & orientation: Defines orientations (L‑T, L‑S, T‑L, T‑S, S‑L, S‑T) and recommends short transverse (S) for thick products, long transverse (T) for thin products; grain direction and texture considerations are required.
- Surface preparation: Controls on surface finish (e.g., Ra requirements) and machining practices to minimize residual stresses and metallurgical changes.
- Loading methods: Allows constant load, constant strain or slow strain rate testing (commonly strain rates in the order of 10^-7 to 10^-5 s^-1).
- Environments & immersion: Supports alternate immersion and continuous immersion in chloride or sulphate containing solutions; stagnant conditions are preferred to avoid removal of corrosion products.
- Test apparatus & materials: Requirements for inert construction materials, insulated specimen holders, and avoidance of contaminating coatings.
- Results & reporting: Procedures for assessment, expression of results and test reporting; intended to compare relative performance but not to certify in-service suitability.
Applications
ISO 20728:2018 is used to:
- Evaluate SCC susceptibility during alloy development and heat‑treatment optimization.
- Compare candidate magnesium alloys or tempers for structural components.
- Assess manufacturing effects (casting, extrusion, welding) on SCC performance.
- Support material selection for parts exposed to chloride/sulphate environments (automotive housings, brackets, intake manifold blades, electronic device enclosures, medical device components, power tools).
Who should use this standard
- Corrosion engineers, materials scientists and metallurgists
- OEMs and suppliers of magnesium structural components
- Test laboratories performing SCC resistance testing
- Quality assurance and R&D teams evaluating alloy/process changes
Related standards
- ISO 7539 series (stress corrosion testing procedures and specimen preparation)
- ISO 8044 (corrosion terms and definitions)
Keywords: ISO 20728:2018, magnesium alloys, stress corrosion cracking, SCC testing, corrosion testing, alternate immersion, slow strain rate, chloride environments.