Overview
ISO 21227-3:2007 - Paints and varnishes - Evaluation of defects on coated surfaces using optical imaging - Part 3: Evaluation of delamination and corrosion around a scribe - specifies a digital optical imaging method to quantify delamination and corrosion adjacent to a scribe (artificial scratch). The damaged surface may be produced using standard corrosion tests (ISO 9227, ISO 11997-1, ISO 11997-2). The method converts images to greyscale or binary format to measure damaged area, mean and maximum widths, and shape of the affected zone.
Key topics and technical requirements
- Digital optical imaging: Capture of the scribed area using calibrated imaging equipment; image processing (thresholding) to separate damaged vs. undamaged regions.
- Illumination: Choose diffused reflection for stripped/coated corrosion and directional reflection for low-contrast delamination to clearly distinguish defect zones.
- Resolution: Minimum imaging resolution of 6 pixels per millimetre; deviations must be reported.
- Calibration: Scale calibration in X and Y (e.g., graph paper or pictorial standards) so pixel counts map to millimetres for accurate area/width measurements.
- Quantitative evaluation:
- Determine evaluated areas (scribe area, delaminated area, corroded area) and the evaluated scribe length.
- Compute mean widths (and optionally maximum widths) of delamination/corrosion perpendicular to the scribe.
- Map measured mean widths to the numerical rating (0–5) per ISO 4628-8 for comparability with visual grading.
- Precision: Interlaboratory data show average relative standard deviation ~9% for mean width by imaging (visual assessment showed higher variability).
- Test report requirements: Identification of coating, reference to ISO 21227-3, imaging/illumination details, resolution, processing steps, results, deviations and tester identity/date.
Applications and practical value
- Objective measurement of delamination, coating failure, and corrosion around scratches for product development, quality control, and failure analysis.
- Useful in automotive, marine, offshore, infrastructure, and industrial coatings R&D where reproducible corrosion resistance data are required.
- Supports correlation between accelerated corrosion tests (salt spray, cyclic corrosion) and quantified defect metrics for coating performance claims.
- Enables more reproducible, quantitative assessment compared with pictorial visual methods, assisting labs in interlaboratory comparisons and process control.
Who should use this standard
- Coatings laboratories, quality assurance engineers, materials scientists, corrosion testers, and third‑party test houses looking to implement standardized digital evaluation of scribe-induced corrosion and delamination.
Related standards
Keywords: ISO 21227-3, delamination, corrosion, scribe, digital optical imaging, coatings evaluation, ISO 4628-8, resolution, calibration, paint testing.
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