ISO 8993:2018

Overview

ISO 8993:2018 - "Anodizing of aluminium and its alloys - Rating system for the evaluation of pitting corrosion - Chart method" defines a standardized chart-based rating system for assessing pitting corrosion of anodic oxidation coatings on aluminium and its alloys. The standard is intended for use after corrosion tests (accelerated laboratory tests, exposure to corrosive environments, and in-service evaluations) and applies only to pits that penetrate the protective anodic coating.

Key topics and requirements

• Chart rating system: Uses standardized visual charts (designations A to H) to classify the extent and severity of pitting corrosion. A = no visible corrosion; H = greatest degree covered by the standard.
• Rating numbers (RN): Each chart designation is associated with a rating number (e.g., A = RN 10; B–H have progressively lower RNs). The standard provides conversion to percentage area of the evaluated surface covered by corrosion pits (see Table 1 in the document).
• Specimen preparation: Requires a defined significant surface area (the document specifies a flat test specimen area of at least 5 000 mm) and prescribes cleaning options to reveal pits:
  • abrasion with fine pumice slurry, rinsing and drying;
  • nitric acid dip (specified dilution and time);
  • dissolution of the anodic coating in hot phosphoric/chromic acid mixtures (note: Chromium(VI) is toxic - handle with care and environmental precautions);
  • wiping with dilute hydrochloric acid to remove deposits.
• Evaluation procedure: Compare the cleaned surface (usually a masked area) visually or electronically to the rating charts (Figures A.1–A.8). Disregard edge effects. Select the chart that most closely resembles the corroded specimen.
• Reporting and results: Express results as chart rating and/or RN and/or percentage area. Test reports must reference ISO 8993:2018 and include product identification, preparation method, comparison method (visual or electronic plus apparatus if used), rating, unusual observations, and test date.

Applications and users

ISO 8993:2018 is practical for:

• anodizers and surface-treatment shops for quality control and process validation;
• corrosion engineers and materials laboratories performing accelerated corrosion and salt-spray tests;
• OEMs and specifiers in aerospace, automotive, architectural and electronics industries who require consistent evaluation of anodized aluminium performance;
• standards writers and procurement teams who include acceptance criteria in product and test specifications.

Keywords: ISO 8993:2018, anodizing, aluminium, pitting corrosion, rating system, chart method, anodic oxidation coatings, corrosion testing, rating number, percentage area.

Related standards

• ISO 8994 - Rating system for pitting corrosion - Grid method
• ISO 7583 - Anodizing of aluminium - Terms and definitions
• ISO 2106 - Determination of mass per unit area of anodic oxidation coatings (gravimetric method)