ISO 15720:2001

Overview - ISO 15720:2001 (Gel‑bulk electrography for porosity)

ISO 15720:2001 specifies a gel‑bulk electrography test method for detecting porosity in gold and palladium metallic coatings on metal substrates. The technique uses a semi‑solid gelatin electrolyte with indicators; the coated specimen is made the anode and an applied DC current drives substrate metal ions through pores. Coloured “decorations” form at pore sites and are examined (typically at 10× magnification) to determine porosity quantitatively in low‑density areas.

Key topics and technical requirements

• Scope and applicability
  • Designed principally for electrodeposited coatings used on electrical contacts:
    • Gold coatings containing ≥75% Au on Ag, Ni, Cu and their alloys.
    • Palladium coatings ≥95% Pd and Pd‑Ni alloys (≈75% Pd) on Ni, Cu and alloys.
  • Intended for measurement areas with pore densities up to about 25/cm² where individual pore sites are resolvable.
• Apparatus
  • Transparent test vessel, DC power supply (0–1 A, 0–10 V; constant‑current preferred), milliammeter/voltmeter, inert cathode (Pt/Au/Ti foil), timer and stereomicroscope (≈10×).
• Reagents and gel
  • Food‑grade gelatin (10% solution) plus specific chemical indicators (e.g., rubeanic acid, dimethylglyoxime, potassium ferrocyanide) chosen by substrate metal; indicators are light/heat sensitive and require controlled storage.
• Procedure highlights
  • Cleaning and handling protocols (ultrasonic cleaning, solvent rinses) to avoid contamination.
  • Preparation and solidification of gel around suspended samples; electrical attachment and application of calculated current.
  • Visual counting of coloured reaction sites through the gel; test is destructive and contaminates the surface.
• Safety & limitations
  • Chemical hazards (indicator reagents) and electrical safety considerations. The test produces corrosion products unlike in‑service corrosion, so electrical performance or service life cannot be inferred without correlation.

Practical applications and users

• Primary users
  • Plating and electroplating laboratories, quality control for electrical contact manufacturers, metallurgists and failure‑analysis teams.
• Use cases
  • Acceptance testing of noble‑metal platings on contacts and connectors.
  • Quantitative porosity reporting (pores per unit area or per contact) where low pore density allows accurate counting.
  • Inspection of complex geometries (e.g., pin‑socket), noting limitations in deep recesses.
• Why use ISO 15720
  • Provides a standardized, reproducible electrographic method to detect coating discontinuities that can lead to substrate corrosion, useful in qualifying coatings for electrical and decorative applications.

Related standards

• ISO 10308 (porosity review), ISO 14647, ISO 15721 (other porosity tests), ISO 12687, ISO 2064, ISO 2079, ISO 2080 - refer to these for alternative methods, terminology and thickness/measurement conventions.