Overview
EN ISO 21227-4:2008 (ISO 21227-4:2008) specifies a digital optical-imaging procedure for the evaluation of filiform corrosion on coated surfaces. The standard describes only the image-based evaluation method - the filiform corrosion itself must be produced according to other standards (ISO 4623-1, ISO 4623-2 or EN 3665). The method converts photographs of a scribe and adjacent damaged area into digital images, thresholds and analyses them to provide objective, quantitative measures of corrosion damage.
Key topics and requirements
- Scope: Evaluation of filiform corrosion using digital optical imaging; intended to improve objectivity and reproducibility versus pictorial/visual methods.
- Image acquisition & illumination
- Use bright-field or dark-field illumination depending on surface gloss; directional reflection may help for low-contrast or glossy specimens.
- After coating removal, diffuse reflection often gives good contrast.
- Resolution & calibration
- Minimum required resolution: 6 pixels per millimetre.
- Calibrate scale in both X and Y (e.g., object micrometer) to allow accurate measurements.
- Evaluation measurements
- Determine the scribe area (A = scribe width × evaluated scribe length).
- Measure total damaged area (including scribe) from binary/thresholded image.
- Compute the mean filiform width from the filament area divided by the evaluated scribe length (accounting for filaments on both sides).
- Record maximum filament lengths on each side of the scribe.
- For irregular corrosion, use the extended evaluation (Annex A) including the g‑value for irregularity.
- Procedure rules
- Prepare panels per ISO 4623-1/2 or EN 3665.
- Exclude unusually different damaged areas at scribe ends when specified.
- Rules for multiple/intersecting scribes are given (sum lengths/areas; exclude intersections).
- Reporting requirements
- Include coating ID, reference to ISO 21227-4, illumination and acquisition setup (resolution, optics, image size, grey/colour depth, gamma), image processing steps, scribe length evaluated, results, deviations, anomalies, date and tester name.
- Precision
- The standard notes that method precision data are currently not available.
Applications and users
This standard is intended for:
- Coating manufacturers for product development and quality assessment.
- Independent test laboratories performing standardized durability and corrosion testing.
- Automotive, aerospace and industrial quality-control teams assessing protective coatings.
- R&D teams comparing formulations or pretreatment processes for filiform resistance.
Keywords: EN ISO 21227-4:2008, ISO 21227-4, filiform corrosion, digital optical imaging, paints and varnishes, coated surfaces, image analysis, scribe evaluation.
Related standards
- ISO 21227-1 (general guidance)
- ISO 4623-1 (filiform corrosion - steel)
- ISO 4623-2 (filiform corrosion - aluminium)
- EN 3665 (aerospace filiform corrosion test)
