ISO 15721:2001

Overview

ISO 15721:2001 specifies a laboratory porosity test for gold and palladium metallic coatings using sulfurous acid / sulfur dioxide (SO2) vapour. The method is intended to reveal pores, discontinuities or defects in precious-metal platings (typically ≥95% Au or Pd) on substrates such as copper, nickel and their alloys - commonly used in electrical contacts. The test produces discrete corrosion products at pore sites that are counted microscopically to give quantitative or comparative porosity data.

Key topics and requirements

• Test principle: concentrated sulfurous acid emits SO2 vapour that reacts at coating defects; corrosion products delineate pore sites.
• Applicability: coatings with ≥95% gold or palladium; suitable for electrodeposits and clad metals on contact substrates.
• Apparatus & reagents
  • Glass/acrylic airtight test chamber (e.g., 9–10 L desiccator). Ratio of chamber volume (cm³) to generating-solution surface area (cm²) shall not exceed 25 ± 1.
  • Specimen holders made of inert materials; maintain minimum clearances to walls and solution surface.
  • Stereomicroscope (×10) with oblique illumination for pore counting.
  • Analytical-grade sulfurous acid (H2SO3) as SO2 source.
• Safety: SO2 is toxic and corrosive - tests must be performed in a fume cupboard with appropriate PPE and disposal procedures.
• Procedure highlights
  • Strict cleaning and handling procedures to avoid contamination.
  • Test temperature 23 °C ± 3 °C and relative humidity ≤ 60%.
  • Exposure durations: typically 2 h for gold ≥1.25 µm, 90 min for thinner gold, 60 min for palladium.
  • Post-test stabilization and oven development (125 °C ± 5 °C for ~15 min) to enhance visibility of corrosion products.
• Examination & counting
  • Count protruding corrosion products at ×10 magnification with oblique light; minimum count size normally 0.05 mm.
  • Quantitative use generally valid for pore densities up to ~100/cm²; above that the method is qualitative/comparative.
• Limitations: Destructive test (contaminates and undercuts coatings); not automatically predictive of electrical performance unless correlated with service data. Report precision shows