Standard · Systems, environments and degradation
ISO 9223
Classifying the corrosivity of atmospheres (C1 to CX).
ISO 9223 classes the corrosivity of an atmosphere from C1 (very low) to CX (extreme). The class comes either from the measured first-year corrosion of standard metal samples, or is estimated from environmental data. This data includes temperature, humidity, sulfur dioxide and chloride deposition.
The bands in numbers
Each class is a band of first-year loss. For carbon steel, in µm a year: C1 up to 1.3; C2 above 1.3 to 25; C3 above 25 to 50; C4 above 50 to 80; C5 above 80 to 200; CX above 200 to 700. Zinc has its own, much lower bands. The same bands are also given as mass loss in g/m² a year.
ISO 12944-2 uses the same classes for paint systems on steel, with typical examples of each environment. When steel and zinc give different classes, ISO 12944-2 takes the higher one.
Common mistakes
- Multiplying the first-year loss by the years of service. Corrosion does not run at a constant rate, so a first-year loss cannot simply be extrapolated.
- Reading a class as a coating life. The class describes the environment. The durability ranges of ISO 12944-1 are separate planning figures, not guarantees.
- Looking for immersion here. ISO 9223 covers atmospheres. The water and soil classes Im1 to Im4 are in ISO 12944-2.
3 practice items in the members area mention ISO 9223 (CIP 1: 2 · CIP 2: 1).
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Also published openly: Materials (MDPI, open access), Table 1: ISO 9223 categories.
This page describes the subject of ISO 9223 in our own words; it does not reproduce the standard. For the full text, buy the current edition from ISO. Checked on 2026-10-14; errors go in the corrections log. CoatMentor is an independent study aid. It is not affiliated with ISO, AMPP, SSPC, NACE International, ASTM or FROSIO.