Overview
ISO 21227-4:2008 - Paints and varnishes - Evaluation of defects on coated surfaces using optical imaging - Part 4: Evaluation of filiform corrosion describes a standardized, image-based procedure to quantify filiform corrosion on coated substrates using digital optical imaging. The document specifies the evaluation procedure only; production of filiform corrosion on test panels is carried out in accordance with ISO 4623-1, ISO 4623-2 or EN 3665.
Key topics and requirements
- Purpose: Provide an objective, reproducible method to measure filiform corrosion parameters from digital images rather than relying on subjective visual assessment.
- Imaging principle: Convert original colour or grey-scale images to binary form (thresholding) to detect damaged area and filaments, then calculate metrics such as damaged area, mean filament width and maximum filament length.
- Illumination: Must allow clear discrimination between damaged and undamaged areas. Bright-field or dark-field illumination can be used:
- Glossy specimens: directional illumination recommended.
- Matt specimens: dark-field illumination recommended.
- When coating remains in place, directional reflection may help; after coating stripping, diffuse reflection is often sufficient.
- Resolution: Minimum image resolution of 6 pixels per millimetre is required; different resolutions can affect measured damaged area.
- Calibration: Scale must be calibrated in both X and Y directions (e.g., object micrometer) so digital measurements are accurate.
- Evaluation metrics:
- Damaged area including scribe
- Scribe area and scribe length evaluated
- Mean filament width (calculated from measured areas and scribe length)
- Maximum filament lengths on each side of the scribe
- Optional extended metrics (Annex A): g-value (irregularity measure), most frequent filament length
- Reporting: Test report must include identification of coating, reference to ISO 21227-4, illumination and acquisition setup, image processing details, scribe length, assessment results, deviations and tester identity.
- Precision: Method precision data is not provided in the standard.
Applications and users
ISO 21227-4 is practical for:
- Paint and coating manufacturers validating corrosion resistance and comparing formulations
- Quality control laboratories performing reproducible, quantitative assessments
- Research and development teams studying coating performance and failure modes
- Aerospace, automotive and industrial coating testers who need standardized, image-based corrosion metrics
Using digital optical imaging for filiform corrosion supports faster, more objective comparisons across batches, coatings and test conditions.
Related standards
- ISO 4623-1 (filiform corrosion on steel substrates)
- ISO 4623-2 (filiform corrosion on aluminium substrates)
- EN 3665 (aerospace filiform corrosion test on aluminium alloys)
- ISO 21227-1 (general guidance on optical imaging evaluation)
- ISO 4628-10 (assessment of filament length distribution)
Keywords: ISO 21227-4, filiform corrosion, digital optical imaging, paints and varnishes, image analysis, corrosion evaluation, scribe, g-value, calibration, illumination, resolution.
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