Overview
ISO 9591:2004 - Corrosion of aluminium alloys - Determination of resistance to stress corrosion cracking (SCC) specifies a standardized laboratory method to evaluate the SCC susceptibility of cast and wrought aluminium alloys. The standard covers sampling, specimen types, loading procedures, test environments and result interpretation. It is intended to assess SCC as a function of chemical composition, manufacturing route and heat treatment, and to compare performance in chloride-containing (e.g., marine) atmospheres, provided the failure mechanism remains the same.
Key Topics and Requirements
- Scope: Applies to castings, semi-finished products, parts and weldments of aluminium alloys.
- Loading methods: Two primary methods are specified - constant total strain and constant load (slow strain rate tests are outside this standard but referenced in ISO 7539-7).
- Immersion regimes: Two immersion procedures are defined - alternate immersion (wet/dry cycles to simulate marine atmospheres) and continuous immersion (by agreement). Alternate immersion uses a 3.5% NaCl solution (mass fraction) with controlled pH (about 6.4–7.2).
- Specimens & orientations: Uses specimen geometries from ISO 7539 series (tension, C‑ring, U‑bend, bent-beam, pre-cracked, etc.). Orientation guidance recommends short‑transverse for thick products and long‑transverse for thin products; grain-orientation examination is recommended where structure is unclear.
- Surface preparation: Prescribed degreasing and etching/cleaning sequences (including NaOH and nitric/hydrofluoric acid options) to ensure reproducible surface condition.
- Test execution: Details on loading apparatus, specimen holders (electrical insulation, inert materials), immersion rates, and control of specimen temperature during cycles.
- Assessment criteria: Defines threshold stress (σ_SCC) - the maximum stress under which no failure occurs during the test period - and time to failure (τ_SCC) for constant-strain specimens. Minimum sample counts and stress-level planning are recommended (e.g., at least three adjacent specimens per stress level).
Applications and Users
ISO 9591:2004 is used for:
- Material selection and alloy/temper comparison for marine, offshore and chloride-exposed applications
- Quality assurance and production testing for aluminium components and weldments
- R&D into heat treatment, processing and alloy development to reduce SCC susceptibility
- Failure analysis and qualification testing for aerospace, marine, automotive and structural industries
Primary users: corrosion engineers, materials scientists, testing laboratories, OEMs, alloy producers and metallurgists.
Related Standards
- ISO 7539 series (Parts 1–7) - general SCC test guidance and specific specimen preparation/use (bent-beam, U‑bend, tension, C‑ring, pre‑cracked and slow strain rate tests).
Keywords: ISO 9591:2004, stress corrosion cracking, SCC testing, aluminium alloys, chloride environments, alternate immersion, constant load, specimen orientation, marine atmospheres.
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