Overview
ISO 12687:1996 - Metallic coatings - Porosity tests - Humid sulfur (flowers of sulfur) test specifies a laboratory method to reveal discontinuities and porosity in metallic coatings that expose a silver, copper or copper-alloy substrate. Commonly called the humid sulfur or flowers of sulfur test, it detects pores, cracks and other coating breaches by producing brown/black sulfide or oxide corrosion products at exposed substrate sites.
Key topics and technical requirements
- Scope: Designed for single or multi-layer metallic coatings that do not significantly tarnish in reduced-sulfur atmospheres (examples: gold, nickel, tin, tin-lead, palladium and their alloys). Test shows whether the coating meets a user-specified acceptable porosity level.
- Principle: Elemental sulfur vapor in a humid atmosphere reacts with exposed copper/silver to form visually detectable corrosion products.
- Controlled conditions:
- Temperature: 50 °C ± 2 °C (oven + vessel).
- Relative humidity: ~85–90% RH (created with a saturated potassium nitrate solution).
- Sulfur source: powdered “flowers of sulfur” placed in the vessel above the humidity solution.
- Apparatus & reagents: glass/acrylic test vessel with vent, petri dish for sulfur, potassium nitrate saturated solution, dielectric hygrometer, temperature sensor, stereomicroscope (×10) with optional reticule.
- Specimen preparation: strict cleanliness (ultrasonic cleaning, solvent rinses, oil-free handling). Internal copper control panel must be included to confirm system operation (should darken in a few hours).
- Examination & evaluation:
- Visual inspection under ×10 magnification for brown/black tarnish or corrosion spots.
- Count only stable corrosion products (not removable by mild air-dusting).
- Pore sizing: longest diameter; corrosion products <0.05 mm are normally not counted. Suggested size groups: ≤0.12 mm, 0.12–0.4 mm, >0.4 mm.
- Caveat: Corrosion products formed in this accelerated test may differ from service environments - correlation with field performance is needed before using results for lifetime prediction.
Applications and users
- Quality control and acceptance testing for electroplated, electroless and hot-dip metallic finishes.
- Used by plating shops, surface-treatment laboratories, electronics and connector manufacturers, jewelry and precision-component producers, and corrosion test labs.
- Practical for verifying coating integrity where exposure of underlying copper/silver would impair function or appearance.
Related standards
Keywords: ISO 12687:1996, humid sulfur test, flowers of sulfur test, porosity test, metallic coatings, porosity testing, corrosion detection, tarnish, copper, silver, potassium nitrate, relative humidity.
