Overview
EN ISO 10309:2016 - Metallic coatings - Porosity tests - Ferroxyl test specifies a standardized wet-chemical method for revealing pores or other discontinuities in metallic coatings that are cathodic to iron and steel and not visibly affected by ferricyanide and chloride ions during the test period. The method (originally ISO 10309:1994, adopted by CEN in 2016) is particularly useful for thick, hard chromium coatings used for wear resistance.
Key topics and requirements
- Scope and principle
- Detects coating discontinuities by forming corrosion cells at exposed base metal sites; base metal ions react on a treated test paper to produce blue ferricyanide markings.
- Reagents and purity
- Use analytical-grade chemicals and distilled/deionized water (conductivity ≤ 20 μS/cm; see ISO 3696).
- Typical reagent recipes in the standard:
- Sodium chloride solution: 50 g NaCl + 1 g non‑ionic wetting agent per litre; optionally gelled with 50 g gelatin or agar per litre.
- Ferricyanide development solution: 10 g potassium hexacyanoferrate(III) per litre; pH checked to 6 ± 0.2.
- Test paper and contamination control
- Use iron‑free “wet‑strength” filter paper; verify absence of iron contamination by a control immersion test.
- Test procedure
- Clean and degrease specimen surface.
- Wet test paper with NaCl (or NaCl‑gel) and press firmly to make full contact with the coating.
- After contact (typical application period described), remove and immerse paper in the potassium ferricyanide solution to develop blue spots.
- Evaluation and reporting
- Count blue markings without magnification; express porosity as average pores/cm² or as maximum count using a 10 mm × 10 mm counting template.
- Test report must include reference to EN ISO 10309:2016, area tested, relevant coating/product standard, and results.
- Notes
- Pass/fail criteria are not specified in the standard - they depend on coating/product specifications and thickness.
- Retesting requires thorough rinsing and drying to avoid smudging.
Applications and users
- Practical for quality control and acceptance testing of metallic platings, especially hard chromium deposits used in:
- Automotive and aerospace components
- Hydraulic cylinders, molds and tooling
- Industrial wear-resistant parts
- Typical users:
- Plating shops, coating manufacturers, inspection labs, metallurgists, surface engineers, and procurement/QA teams specifying coating acceptance.
Related standards
- ISO 3696 - Water for analytical laboratory use (referenced for water purity)
- ISO/TC 107 (Metallic and other inorganic coatings) - technical committee responsible for related standards
This standard is essential when reliable, low‑cost detection of coating porosity is required for corrosion-prone substrates and wear-critical chromium coatings. Keywords: metallic coatings, porosity tests, ferroxyl test, EN ISO 10309:2016, chromium coatings, porosity detection.
