Overview
ISO 18089:2015 specifies a laboratory procedure for determining the critical crevice temperature (CCT) of stainless steels under potentiostatic control. The method rapidly measures the lowest specimen surface temperature at which stable, propagating crevice corrosion occurs under defined electrochemical and environmental conditions. The CCT obtained is intended as a relative performance index (e.g., to compare stainless-steel grades), not as a prediction of in-service failure temperatures.
Key technical topics and requirements
- Test principle: Heat the test solution to raise the specimen surface temperature at a controlled ramp rate while maintaining a set anodic potential with a potentiostat. Monitor current response to detect onset of crevice corrosion.
- CCT definition: Temperature at which current density exceeds a specified value (recommended 10 μA·cm⁻²) for a sustained period (recommended 60 s) to indicate stable crevice propagation.
- Equipment:
- Potentiostat capable of controlling potential to within ±1 mV.
- High‑input-impedance electrode potential measurement (sensitivity ≈1.0 mV).
- Auxiliary electrode (high‑purity platinum) and a reference electrode (Ag/AgCl preferred) connected via a Luggin capillary.
- Temperature controller/heating bath, stirring mechanism, and temperature probe (solution temperature accuracy ±1 °C).
- Crevice formers or specimen holders as described in Annex B.
- Specimen and solution:
- Minimum specimen area guidance and reproducible surface finish (abrasion, cleaning, 24 h air ageing to form stable oxide).
- Test solutions commonly chloride‑containing; reagent‑grade chemicals and high‑purity water (see ISO 3696).
- Volume-to-area ratio and stir‑rate requirements to ensure repeatability (solution volume ≥100 ml per cm² specimen area).
- Procedure controls:
- Start temperature depends on expected CCT (e.g., ~2 ± 1 °C for CCT < 40 °C).
- Validation and calibration of reference electrodes (potential difference checks vs. validation electrodes).
- Data recording of open-circuit potential, applied potential, current vs. temperature, and test end conditions.
Practical applications and users
- Who uses it: Corrosion engineers, metallurgists, materials scientists, R&D laboratories, quality control teams, and standards organizations comparing stainless-steel corrosion resistance.
- Why use it: Fast, repeatable ranking of stainless-steel grades for susceptibility to crevice corrosion under controlled electrochemical conditions; supports material selection, alloy development, and comparative testing programs.
- Limitations: Not intended to predict service crevice-corrosion temperatures and not applicable for materials with CPT values below 20 °C when measured under equivalent conditions (see ISO 17864).
Related standards
- ISO 17864 - Determination of the critical pitting temperature (CPT) under potentiostatic control
- ISO 18070 - Crevice corrosion formers with disc springs for stainless steels
- ISO 3696 - Water for analytical laboratory use
- ISO 8044 - Corrosion terms and definitions
Keywords: ISO 18089:2015, critical crevice temperature, CCT, potentiostatic control, stainless steels, crevice corrosion testing, potentiostat, Ag/AgCl reference electrode, corrosion standard.