ISO 4628-3:2016

Overview

ISO 4628-3:2016 - "Paints and varnishes - Evaluation of degradation of coatings - Part 3: Assessment of degree of rusting" defines a visual method to assess rust formation on coated steel by comparison with standardized pictorial references. The standard assigns a degree of rusting (Ri) based on the combined area of rust broken through the coating and visible underrust, and provides calibration images for use with optical imaging systems. Assessments are carried out under controlled illumination (see ISO 13076).

Key technical topics and requirements

• Assessment method: Compare the test area to pictorial standards (Figures for Ri1 to Ri5) to determine the Ri class.
• Rusted area percentages (approx.):
  • Ri 0 = 0%
  • Ri 1 = 0.05%
  • Ri 2 = 0.5%
  • Ri 3 = 1%
  • Ri 4 = 8%
  • Ri 5 = 40–50% (visual impression note)
• Illumination: Perform visual assessments under appropriate lighting as specified in ISO 13076.
• Optical imaging: Annex A contains calibration images (Ri1–Ri5) to calibrate imaging systems for automated or image‑based assessment.
• Expression of results: Report the degree of rusting as an Ri class, and where relevant include area subdivisions (different Ri in different parts) and spot‑size ratings per ISO 4628‑1 (e.g., "rusting; degree of rusting Ri3 (S4)").
• Test report minimum content: identification of coating, reference to ISO 4628-3, surface type/size/location, result (Ri), illumination used, anomalies, and date.
• Correlations: Annex B provides mappings to other scales - European rust scale (CEPE) and ASTM D610 (approximate correlation).

Practical applications and users

ISO 4628-3 is used for consistent, repeatable evaluation of rust on coated metal surfaces in situations such as:

• Coatings manufacturers - product development and quality control.
• Third‑party testing laboratories - conformity testing and certification.
• Asset owners & maintenance teams - condition surveys and lifecycle planning for structures, bridges, pipelines, and offshore assets.
• Contractors and inspectors - acceptance criteria for coating application and repair work.
• Corrosion engineers and specifiers - performance comparisons and reporting.

Benefits include standardized reporting, interoperability with imaging systems, and the ability to relate results to other common rating systems (CEPE, ASTM).

Related standards

• ISO 4628‑1: General system for designation of defects and appearance change
• ISO 13076: Lighting and procedure for visual assessments of coatings
• ISO 8501‑1: Rust grades for uncoated steel surfaces
• ASTM D610: Test methods for evaluating degree of rusting on painted steel surfaces

Keywords: ISO 4628-3:2016, degree of rusting, pictorial standards, rusted area, coatings assessment, calibration images, ISO 13076, ASTM D610, ISO 4628 series.