Overview
EN ISO 12687:1998, titled "Metallic coatings - Porosity tests - Humid sulfur (flowers of sulfur) test", is a European adoption of the international ISO 12687:1996 standard. Developed and published by CEN, this standard specifies a testing method to detect discontinuities and porosity in metallic coatings such as gold, nickel, tin, palladium, and their alloys, especially when applied over silver, copper, or copper-alloy substrates. The method is designed to evaluate whether metallic coatings meet specific porosity acceptance levels critical to their performance.
This test exposes coated specimens to a controlled humid sulfur environment where sulfur vapors react with exposed substrate areas through pores or cracks, resulting in visible tarnish or corrosion marks. This practical and reliable porosity testing technique is vital for quality control in the manufacturing and inspection of metallic coating layers.
Key Topics
- Purpose: Detect porosity and discontinuities in metallic coatings by exposing samples to humid sulfur vapor (flowers of sulfur).
- Applicable Coatings: Single or combined metallic layers that do not significantly tarnish in reduced sulfur atmospheres (e.g., gold, nickel, tin, tin-lead, palladium coatings).
- Test Principle: Sulfur reacts with exposed copper, silver, or their alloys at pores or cracks to form sulfide or oxide corrosion products visible as black or brown spots.
- Test Conditions:
- Temperature: 50 °C ± 2 °C
- Relative Humidity: 85% to 90%
- Test duration typically 24 hours or as specified by user requirements.
- Preparation: Thorough cleaning of specimens to remove contaminants that may cause false indications.
- Equipment: Transparent test vessel, potassium nitrate solution to maintain humidity, powdered sulfur, temperature and humidity sensors, optical stereomicroscope for visual inspection.
- Evaluation: Visual examination at 10x magnification to identify tarnish spots indicating porosity; optional quantification of pore density.
- Limitations: This accelerated corrosion test mimics conditions not found in service and should be correlated with actual usage experience for performance prediction.
Applications
- Quality Control: Industrial manufacturers rely on this porosity test to ensure that decorative and protective metallic coatings meet user-specified porosity thresholds before product release.
- Coating Development: Researchers and developers use this method to evaluate new coating compositions and multilayer systems for integrity and durability.
- Corrosion Prevention: Identification of coating defects early in the production process helps minimize corrosion failure risks in end-use environments.
- Failure Analysis: In metallurgical laboratories, it helps diagnose corrosion-related problems by highlighting coating discontinuities.
- Standard Compliance: Ensures that metallic coatings conform to international and regional standards regarding porosity and surface protection.
Related Standards
- ISO 10308:1995 - Metallic coatings: Review of porosity test methods, providing general guidance on porosity detection techniques.
- ISO 2079:1981 - Surface treatment and metallic coatings - General classification of terms used in coating technology.
- ISO 2080:1981 - Electroplating and related processes: Vocabulary, specifications, and test methods.
- ISO 3696:1987 - Water for analytical laboratory use, specifying water purity critical for test reagent preparation.
- Additional corrosion test standards under ISO/TC 107, Subcommittee SC 7.
Keywords: EN ISO 12687, metallic coatings, porosity test, humid sulfur test, flowers of sulfur, coating discontinuities, corrosion testing, copper substrate, silver substrate, quality control, coating integrity, sulfur vapor corrosion.
