Overview
ISO 26146:2025 - "Corrosion of metals and alloys - Method for metallographic examination of samples after exposure to high‑temperature corrosive environments" - defines a standardized metallographic procedure to classify, identify and measure corrosion layers formed during high‑temperature exposure. It covers sample preparation, cross‑sectioning, mounting, grinding/polishing, microscopic examination and thickness measurement of corrosion scales and related features.
Key topics and technical requirements
- Scope: Examination of samples exposed to high‑temperature corrosive environments; classification, identification and thickness measurement of corrosion layers.
- Terminology: Defines terms such as interdiffusion zone, outward/inward growing scale, external/internal scale, de‑alloyed zone, metal loss and defects (voids/cracks in corrosion layers).
- Measurement accuracy: Uncertainty at 95% confidence of ±5 µm or 5% of measured material loss (whichever is lesser). Measurement systems should be calibrated and traceable; precision of the measurement system target is ±1 µm.
- Microscopy and imaging: Use of metallographic optical microscopes with X–Y stages (typical magnifications ×100, ×400, ×1 000); digital microscopes with automatic X–Y stages and image analysis software are supported for stitching, measurement and evaluation.
- Specimen types & preparation: Typical geometries - rod, disc, block. Detailed procedures for sectioning, mounting (e.g., epoxy with vacuum impregnation to avoid gaps), grinding/polishing sequences (P100 → P1200, optional 1 µm diamond paste, colloidal silica) to preserve edge retention of corrosion layers. Overplating and specimen supports are permitted to minimize artefacts.
- Classification & evaluation: Methods to classify corrosion layers (external vs internal, grain‑boundary corrosion, de‑alloyed zones), identify layer composition and measure thickness and metal loss. Procedures include assessment of defects in layers.
- Quality control: Calibration against length standards at intervals ≤ 12 months; checks against secondary standards at start/end of measurement series.
Practical applications
- Quality control and acceptance testing of high‑temperature coatings, protective alloys and components used in power generation, petrochemical, aerospace and materials research.
- Failure analysis and root‑cause investigations of high‑temperature corrosion damage.
- Comparative testing of coatings/deposits (salts, ashes, molten metals) and evaluation of interdiffusion or internal corrosion beneath scales.
- Research labs and R&D assessing new alloys or protective systems under simulated service atmospheres.
Who should use this standard
- Corrosion engineers, metallurgists and materials scientists
- Testing and inspection laboratories conducting metallography after high‑temperature exposures
- OEMs and manufacturers of high‑temperature components and coatings
- Failure analysts and standards bodies specifying test and reporting requirements
Related standards
- ISO 8044 - Corrosion of metals and alloys - Vocabulary
- ISO 3611 and ISO 13385‑1 - Dimensional measuring equipment (micrometers, calipers)
Keywords: ISO 26146:2025, metallographic examination, high‑temperature corrosion, corrosion layer thickness, corrosion of metals, digital microscope, image analysis.
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