ISO 23721:2022

Overview

ISO 23721:2022 - Corrosion of metals and alloys: Rating method by appearance of rust and stains of atmospheric corrosion for stainless steels - defines a qualitative, appearance-based procedure for assessing rust and stains on stainless steels exposed to atmosphere. The standard uses 10 photographic standards and a rating number (RN 0–9) to express the aesthetic degree of atmospheric corrosion, making it suitable for specimens and real structures where visual appearance matters.

Key topics and technical requirements

• Photographic standards: Ten reference photographs (RN 0 to RN 9) represent increasing cleanliness (RN 0 ≈ 100% rust/stains; RN 9 ≈ 0.009% total area). Specimen figures use a 100 mm × 150 mm area for comparison.
• Rating procedure:
  • Compare the specimen or structure visually to the photographic standards to assign an RN.
  • Use the middle part of the specimen photograph for rating - exclude the 5 mm perimeter and areas around markings.
  • Perform ratings by two or more people; use the most frequent RN or the average if no consensus.
• Expression of results: Report the RN class (e.g., “rating number RN 4”) as the assessment outcome.
• Test report requirements: Include reference to ISO 23721, material identification, surface type and size, orientation and exposure conditions, test duration, cleaning procedure before rating, RN result, anomalies observed, and date of examination.
• Quantitative relationship (Annex A): Provides image-analysis-derived average total area percentages corresponding to RN values (e.g., RN 1 ≈ 69%, RN 4 ≈ 22%, RN 7 ≈ 0.41%).

Practical applications and users

ISO 23721:2022 is focused on the aesthetic assessment of stainless steels rather than structural loss or pitting depth. Typical uses:

• Visual quality control for architectural stainless-steel façades, roofs, and exterior trim.
• Acceptance criteria for stainless-steel components in transportation, marine fittings, and consumer products where appearance is critical.
• Field evaluation of in-service stainless-steel parts (e.g., eaves, braid materials) after atmospheric exposure. Typical users:
• Materials engineers, corrosion specialists, and QA/QC inspectors.
• Architects, façade consultants, and building maintenance teams.
• Test laboratories performing atmospheric exposure evaluations.

Related standards

• ISO 9223 / ISO 9224 - atmospheric corrosivity classification and guiding values
• ISO 11463 - pitting corrosion evaluation
• ISO 10289 - rating of corrosion test specimens These standards complement ISO 23721:2022 for broader corrosion assessment, quantitative corrosion metrics, and environmental classification.

Keywords: ISO 23721:2022, stainless steel, atmospheric corrosion, rust rating, photographic standards, rating number RN, aesthetic appearance, corrosion assessment, test report.