Overview
EN ISO 8993:2018 - Anodizing of aluminium and its alloys - Rating system for the evaluation of pitting corrosion - Chart method - specifies a standardized chart-based rating system to define performance levels of anodic oxidation coatings on aluminium and its alloys after corrosion testing. The method applies to pitting corrosion caused by accelerated laboratory tests, exposure to corrosive environments, and practical service conditions, and covers only pits that penetrate the protective anodic coating.
Key topics and technical requirements
- Chart rating system (A–H): Visual reference charts (Figures A.1–A.8) are used to select the chart designation most closely matching the pitting observed. A = no visible corrosion; H = greatest degree covered by the standard.
- Rating numbers (RN) and percent area: Each chart designation maps to a rating number and percentage of the significant surface area affected by pits (Table 1). Examples:
- A (RN 10) = 0.00% pits
- B (RN 9.8) ≤ 0.02%
- C (RN 9.5) ≤ 0.05%
- D (RN 9.3) ≤ 0.07%
- E (RN 9) ≤ 0.10%
- F (RN 8) ≤ 0.25%
- G (RN 7) ≤ 0.5%
- H (RN 6) > 0.5%
- Specimen preparation: The standard specifies preparation and cleaning methods so pits can be discerned, including:
- Abrading with fine pumice slurry, rinsing and air-drying;
- Dipping in 30% nitric acid (prepared per clause) for 5–10 min, then rinse/dry;
- Dissolving the anodic coating in a hot phosphoric/chromic acid mix (useful for dark coatings) - note chromium(VI) hazards;
- Wiping with dilute hydrochloric acid to remove deposited copper, then rinse/dry.
- Area for evaluation: A specified flat test area is required (see clause 4.1) and a mask (50 × 100 mm) may be used to define the evaluated area.
- Reporting: Results are expressed as chart rating and/or rating number and/or percentage area. Test reports must reference ISO 8993:2018 and include product identification, preparation method, comparison method (visual or electronic), ratings, unusual features and test date.
Applications and users
This standard is used by:
- Anodizing manufacturers and surface-treatment labs for quality control and process validation.
- Corrosion engineers and test laboratories conducting accelerated or exposure tests.
- Product designers and specifiers in aerospace, automotive, marine, architecture and electronics who require defined acceptance criteria for anodized aluminium.
- Standards writers and procurement teams incorporating objective pitting acceptance into specifications.
Related standards
- ISO 8994 - Rating system for pitting corrosion - Grid method (alternative)
- ISO 2106 - Determination of mass per unit area of anodic coatings
Keywords: anodizing, aluminium, pitting corrosion, ISO 8993:2018, chart method, anodic oxidation coatings, corrosion testing, rating system.