Overview
ISO 21153:2018 - Corrosion of metals and alloys: Measurement of environmentally assisted small crack growth rate specifies a standardized method to measure how small surface cracks grow in aqueous and atmospheric environments. The standard addresses measurement of change in crack size with time for environment-assisted cracking, covering both stress corrosion and corrosion fatigue. It also defines controlled methods to generate crack precursors (for example, accelerated pits) and guidance on specimen preparation, monitoring, data analysis and reporting.
Keywords: ISO 21153:2018, small crack growth, environmentally assisted cracking, corrosion fatigue, stress corrosion.
Key topics and technical requirements
- Principle: Measure crack-size evolution with high resolution near the initiation site; introduce a controlled crack precursor to localize initiation and enable reliable monitoring.
- Specimen design and preparation: Use specimens (e.g., flat tensile specimens) that confine cracking to the precursor area; surface preparation, degreasing and characterization of near-surface microstructure, hardness and residual stress are required because they strongly influence early crack growth.
- Crack precursor generation: Describes multiple methods:
- Accelerated corrosion pitting (coating methods, droplet/microcapillary cells, anodic current control)
- Mechanical notching
- FIB (focused ion beam) milling
- Laser ablation
- Electric discharge machining (EDM)
- Crack-size monitoring techniques:
- Optical methods for surface crack tracking
- Electrical resistance / potential drop techniques (Annex A) for sub-surface detection and high-resolution monitoring
- Calibration of measurement methods is required to estimate crack depth and growth rates accurately.
- Test procedure and data analysis: Guidance on precracking, environmental control, calculating small crack growth rate, estimating crack depth scenarios and defining the mechanical driving force.
- Reporting: Required test parameters, environmental conditions and data analysis methods to ensure reproducibility.
Practical applications and users
ISO 21153 is directly relevant for industries where early crack development affects component life and safety:
- Power generation (including nuclear)
- Oil & gas
- Aerospace
- Automotive
Practitioners who would use this standard include corrosion engineers, materials scientists, failure analysts, R&D teams developing alloys/coatings, and qualification laboratories performing life-prediction tests for components susceptible to pit-to-crack transitions and other near-surface crack initiation mechanisms.
Related standards
- ISO 7539-1 (Stress corrosion testing - General guidance)
- ISO 7539-6 (Pre-cracked specimen preparation for constant load/displacement)
- ISO 8044 (Basic corrosion terms and definitions)
ISO 21153:2018 provides a practical, standardized framework for measuring environmentally assisted small crack growth rates, enabling consistent data for life prediction, materials selection and corrosion mitigation strategies.