Overview
ISO 26146:2012 specifies a standardized metallographic examination method for metallic samples exposed to high-temperature corrosive environments. It defines how to classify, identify and measure the thickness of corrosion layers that form during exposure, and gives required measurement accuracy, sample preparation and reporting conventions. The standard is intended to support reliable evaluation of metal loss, corrosion scales, deposits and sub-surface attack after high-temperature testing or plant exposure.
Key topics and technical requirements
- Scope and layer definitions: Classification and definitions for surface deposit, outward/inward growing corrosion scale, internal corrosion, grain-boundary attack, de-alloyed zone, remaining coating, interdiffusion zone and unaffected alloy.
- Measurement accuracy: Uncertainty at 95% confidence must be ±5 µm or 5% of measured material loss, whichever is less. Measurement system precision shall be ±1 µm.
- Microscopy and instrumentation: Measurements conducted under an optical microscope with an X–Y moving stage or graticule/cross wires. Systems must be fully calibrated (orthogonality and traceable to certified length standards) at intervals ≤ 12 months and checked against secondary standards at the start and end of each measurement series.
- Test-piece geometries: Typical sample forms are rod, disc or block. Machine tolerances preferably better than ±0.05 mm.
- Specimen preparation: Cross-sections must retain corrosion products and deposits. Use protective coatings or thermoplastic resins; vacuum impregnation for porous scale; avoid aqueous lubricants when salts (e.g., alkali sulphates or halides) may dissolve-use non‑aqueous lubricants.
- Measurement procedure: Divide specimen into eight sectors to locate areas of minimum remaining metal. Measure designated positions (c and d) to obtain remaining sound metal thickness (trm) and calculate metal loss tml = (t0 − trm)/2. For statistical analysis obtain a minimum of 24 measurements, report mean and standard deviation, and optionally plot data on probability axes.
- Complementary techniques: SEM, XRD or other analyses are optional for further characterization.
Applications
ISO 26146 is used to:
- Quantify metal loss and evaluate protective coatings after high-temperature corrosion testing.
- Support failure analysis and root-cause investigations in power generation, petrochemical, aerospace, and industrial furnaces.
- Validate corrosion-resistant materials, coatings and high-temperature alloys.
- Provide consistent, traceable data for corrosion testing programs and comparative studies.
Who should use this standard
- Corrosion engineers and metallurgists
- Materials testing laboratories and quality assurance teams
- Failure analysts and R&D groups developing high-temperature alloys and coatings
Related standards
- ISO 3611 - Micrometers for external measurements (referenced)
- ISO 13385-1 - Callipers; design and metrological characteristics (referenced)
- Prepared by ISO/TC 156 (Corrosion of metals and alloys)
Keywords: ISO 26146, metallographic examination, high-temperature corrosion, corrosion layer measurement, metal loss, metallography, corrosion testing.