# ISO 9223

Source: https://coatmentor.com/standards/iso-9223/ · Updated 2026-10-14

Classifying the corrosivity of atmospheres (C1 to CX).

**Publisher**: ISO

**Area**: Systems, environments and degradation

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).

## Related

- **Concepts:** [Corrosivity categories (ISO 12944-2)](https://coatmentor.com/learn/corrosivity-categories/)
- **Standards:** [ISO 12944](https://coatmentor.com/standards/iso-12944/)

## About this page

**Also published openly:** [Materials (MDPI, open access), Table 1: ISO 9223 categories](https://pmc.ncbi.nlm.nih.gov/articles/PMC8467660/table/materials-14-05219-t001).

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](https://coatmentor.com/method/). CoatMentor is an independent study aid. It is not affiliated with ISO, AMPP, SSPC, NACE International, ASTM or FROSIO.