Overview
ISO 9224:2012 - "Corrosion of metals and alloys - Corrosivity of atmospheres - Guiding values for the corrosivity categories" provides guiding values of corrosion attack for metals and alloys exposed to natural outdoor atmospheres for exposures greater than one year. It is intended to be used in conjunction with ISO 9223 (classification and estimation of atmospheric corrosivity) and ISO 8044 (terms). The standard gives engineering-relevant guidance to predict long‑term corrosion attack (up to ~20 years) from first‑year corrosion measurements and environmental corrosivity classifications.
Key topics and technical requirements
- Guiding corrosion values: Defines corrosion rates, mass loss or penetration that represent each corrosivity category for standard structural metals (steel, zinc, copper alloys, aluminium alloys).
- Time‑dependence model: Uses a power‑law relation (total attack D related to first‑year corrosion rate r and a time exponent b) to predict extended exposure attack. The exponent b is typically < 1 and reflects decreasing rate with time.
- B1 / B2 exponents: Provides generalized time‑exponent values (B1 = average, B2 = conservative upper bound including two standard deviations) for calculations and simplified tables for time up to 100 years.
- Annexes:
- Annex A: examples of maximum corrosion attack for six standardized corrosivity categories (up to 20 years).
- Annex B: presumed average initial and steady‑state corrosion rates per corrosivity interval.
- Annex C: calculation procedure to adjust corrosion predictions for steel composition (weathering steels and alloy effects).
- Long‑term guidance: Notes Equation (1) is validated up to ~20 years; offers an alternative conservative approach for t > 20 years where corrosion may become linear with time.
- Limitations: Does not address localized phenomena (pitting, crevice, galvanic corrosion, environmental cracking, corrosion product wedging). Zinc is especially sensitive to SO2; in highly aggressive atmospheres a linear (b = 1.0) assumption may be prudent.
Practical applications
- Predicting long‑term corrosion attack and useful life of metallic structures and coatings exposed to outdoor atmospheres.
- Performing engineering calculations for material selection, thickness loss estimates and structural durability.
- Determining whether protective measures (coatings, material upgrades) are needed and informing specification of corrosion protection (see ISO 11303).
- Asset management, lifecycle cost analysis, and maintenance planning for civil infrastructure, building exteriors, bridges, and marine/industrial installations.
Who uses ISO 9224:2012
- Corrosion engineers and materials scientists
- Structural and civil engineers
- Coating and corrosion protection specifiers
- Asset managers, maintenance planners and risk assessors
- Standards writers and regulatory bodies concerned with atmospheric corrosion
Related standards
- ISO 9223 - Corrosivity of atmospheres: classification, determination and estimation
- ISO 8044 - Basic terms and definitions for corrosion of metals and alloys
- ISO 11303 - Selection of corrosion protection methods (referenced for protective measures)
- ISO 14713‑1 - Guidance on zinc coatings for corrosion protection (relevant background for zinc materials)
Keywords: ISO 9224:2012, corrosivity categories, atmospheric corrosion, corrosion rates, guiding values, corrosion attack, steels, zinc, copper, aluminium.
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