Overview
ISO/TR 15922:2011 is an ISO Technical Report that provides methods to evaluate the dark‑stain phenomenon observed on chromate conversion‑coated coiled or sheet products. It addresses appearance and corrosion‑related discoloration that can occur on zinc‑coated (hot‑dip and electroplated) steel after long‑term storage in elevated temperature and high relative humidity. The report is informative (a Technical Report) and specifies both qualitative assessment procedures and a quantitative colorimetric method to measure change in surface colour and luminosity.
Key topics and requirements
- Scope and purpose: Defines the dark‑ or black‑stain phenomenon affecting chromated zinc coatings and its influence on appearance and corrosion resistance.
- Specimen selection: Unstacked (outer‑surface) and stacked (inner‑surface under stress) plate specimens, typically 120 × 80 mm.
- Test environment and apparatus:
- Environmental test cabinet per IEC 60068‑2‑30 (damp heat, cyclic) and chamber performance per IEC 60068‑3‑6.
- Photoelectric absorptiometer/colorimeter with ~520 nm filter and 10 mm optical path.
- Apparatus to apply constant pressure to simulate stacking.
- Typical test conditions (unless otherwise specified): 60 ± 1 °C, 95 % ± 1 % RH, applied pressure about 147 N/m, and durations ranging from 24 h up to 1 000 h.
- Qualitative method: Visual assessment for appearance, defect distribution (black stain, white rust), time to first defect, and coating mass per unit area.
- Quantitative method: Colorimetric measurement following ASTM E1347 / ASTM D2244 principles using L*, a*, b* coordinates. Changes are reported as ΔL*, Δa*, Δb* (before vs. after exposure) and mean colour intensity differences.
- Reporting: Test parameters, number of samples, coating mass, results (individual and mean), any deviations, and dates.
Practical applications and users
ISO/TR 15922:2011 is useful for:
- Steel and zinc‑coating producers validating chromate conversion coatings for storage robustness.
- Quality and corrosion engineers assessing risk of dark stain and its impact on downstream parts.
- Product purchasers and specifiers requiring contractually defined test conditions and AQL sampling.
- Independent test laboratories performing accelerated damp‑heat and stacking simulations to quantify discoloration.
Practical benefits include reproducible evaluation of aesthetic and functional degradation, improved storage/packing guidelines, and data to support material acceptance decisions.
Related standards
Normative and referenced documents include:
- ISO 3613, ISO 3892, ISO 4520 (chromate conversion/coating mass methods)
- IEC 60068‑2‑30, IEC 60068‑3‑5/3‑6 (environmental chamber tests)
- ASTM E1347, ASTM D2244 (colorimetry and color difference calculations)
Keywords: ISO/TR 15922:2011, dark‑stain phenomenon, chromate conversion coatings, zinc‑coated steel, colorimetric method, damp heat testing.
