# Corrosivity categories (ISO 12944-2)

Source: https://coatmentor.com/learn/corrosivity-categories/ · Updated 2026-10-04

How a corrosivity category (C1 to CX, Im1 to Im4) is decided under ISO 12944-2, what it changes on site, and why durability is no guarantee.

ISO 12944-2 classes the environment a structure sits in, so a coating system can be chosen for it. For the whole standard, part by part, read the [ISO 12944 guide](https://coatmentor.com/blog/iso-12944-guide).

- **Atmospheric:** C1 (very low) to C5 (very high), and CX (extreme), based on the corrosion rate of reference steel and zinc samples.
- **Immersion and soil:** Im1 fresh water, Im2 sea or brackish water, Im3 soil, Im4 sea water with cathodic protection.

For the nine parts, the durability table and the offshore tests in one place, read the [ISO 12944 guide for inspectors](https://coatmentor.com/blog/iso-12944-guide). This page covers how a category is decided and what it changes on site.

## How a category is decided

The proper method is a measurement. Expose standard specimens of carbon steel and zinc for one year. Measure the thickness they lose, then read the category from the table. ISO 12944-2 uses the same loss values as [ISO 9223](https://coatmentor.com/standards/iso-9223/).

| Category | Steel loss, first year (µm) | Zinc loss, first year (µm) |
| --- | --- | --- |
| C1 | up to 1.3 | up to 0.1 |
| C2 | above 1.3 to 25 | above 0.1 to 0.7 |
| C3 | above 25 to 50 | above 0.7 to 2.1 |
| C4 | above 50 to 80 | above 2.1 to 4.2 |
| C5 | above 80 to 200 | above 4.2 to 8.4 |
| CX | above 200 to 700 | above 8.4 to 25 |

Three rules apply to the table. If steel and zinc give different categories, take the higher one. Never extrapolate a first-year loss from a shorter or a longer exposure. The same table also gives mass losses, and 400 g/m² of steel marks the same C3 and C4 boundary as 50 µm.

Often nobody can expose specimens. Then the category is estimated from typical environments. A heated, clean office is C1. An urban or industrial area with moderate sulfur dioxide, or a coast of low salinity, is C3. An industrial area with high humidity, or a coast of high salinity, is C5. Offshore areas of high salinity are CX. The standard warns that these examples can mislead. Only the measured loss gives the proper classification. ISO 9223 also allows an estimate from the yearly wet period, the sulfur dioxide level and the chloride deposition.

Water and soil are different. Corrosion there is usually local, so a category is hard to define. Im1 is fresh water, such as river installations and hydro plants. Im2 is sea or brackish water without cathodic protection. Im3 is soil, such as buried tanks and pipelines. Im4 is sea or brackish water with [cathodic protection](https://coatmentor.com/learn/cathodic-protection/), and it is new in the 2017 edition. Cathodic protection can also serve Im1 and Im3 with a coating system tested for it. The old C5-I and C5-M categories became C5 and CX.

## What a category changes for the inspector

The inspector does not choose the category. The specification does. The category and the durability range together select the paint system in ISO 12944-5, with its coats and nominal DFT. They also set other limits.

- Preparation grade P3 is required for high and very high durability in C4 and above, and in Im1 to Im4. See [ISO 8501](https://coatmentor.com/standards/iso-8501/).
- Offshore structures in CX and Im4 at high durability follow ISO 12944-9, which replaced ISO 20340. Blast to Sa 2½ at least with a medium (G) profile, except on metal-coated surfaces.
- Laboratory qualification depends on the category. Cyclic ageing tests apply to C5 at high and very high durability, and to C4 at very high durability.

On site, check that the system applied is the system specified for the category on the drawing. Report a clear mismatch with the real environment to the specifier. An example is a structure next to a chemical plant that the specification calls C3.

## Worked example

A bridge pier stands at an estuary. After one year the steel specimen lost 62 µm and the zinc specimen lost 1.9 µm.

Check: 62 µm is above 50 and not above 80, so steel gives C4. Also, 1.9 µm is above 0.7 and not above 2.1, so zinc gives C3. The categories differ, so take the higher one: C4. The owner then asks for high durability. At C4 with high durability, grade P3 applies.

## Durability ranges

ISO 12944-1 (2017) gives four durability ranges for the time to the first major maintenance. Low (L) is up to 7 years and medium (M) is 7 to 15 years. High (H) is 15 to 25 years and very high (VH) is more than 25 years. A durability range is a planning figure, not a guarantee. The standard gives an example for first major maintenance: about 10 % of the coating has reached Ri 3 under [ISO 4628-3](https://coatmentor.com/standards/iso-4628/).

## Exam traps

- Trap: A durability range is a planning figure. It is not a guarantee time.
- Trap: "Im4 is harsher sea water than Im2." Im2 and Im4 differ only by cathodic protection.
- Trap: When steel and zinc give different categories, take the higher one, not the lower one.
- Trap: "Scale a three-month loss up to one year." A first-year loss is never extrapolated.
- Trap: ISO 20340 is not the current offshore standard. ISO 12944-9 replaced it.
- Trap: "The typical environments decide the category." Only a measured loss gives the proper classification.

**Use it:** [ISO 12944 for coating inspectors](https://coatmentor.com/blog/iso-12944-guide)

**27** practice items in the members area mention this topic (CIP 1: 2 · CIP 2: 6 · FROSIO written: 19).

## Related

- **Standards:** [ISO 12944](https://coatmentor.com/standards/iso-12944/) · [ISO 9223](https://coatmentor.com/standards/iso-9223/) · [ASTM B117](https://coatmentor.com/standards/astm-b117/) · [NORSOK M-501](https://coatmentor.com/standards/norsok-m-501/) · [ISO 12944-9](https://coatmentor.com/standards/iso-12944-9/) · [NACE SP0198](https://coatmentor.com/standards/nace-sp0198/)
- **Defects:** [Rust-through (rusting)](https://coatmentor.com/defects/rust-through/)
- **Coatings and corrosion:** [Protective coating types](https://coatmentor.com/learn/coating-types/) · [Zinc-rich primers](https://coatmentor.com/learn/zinc-rich-primers/) · [Galvanic corrosion](https://coatmentor.com/learn/galvanic-corrosion/) · [Cathodic protection basics](https://coatmentor.com/learn/cathodic-protection/)

## About this page

**Sources:** ISO 12944-2:2017 · ISO 12944-1:2017 · ISO 9223:2012 · ISO 12944-3:2017 · ISO 12944-9:2018 · ISO 4628-3:2024.

**Also published openly:** [Hempel: ISO 12944 brochure](https://www.hempel.com/-/media/Hempel/Files/Certification/ISO/ISO-12944/ISO12944_brochure_EN_LR.pdf) · [Jotun: selecting the right system (ISO 12944 durability)](https://www.jotun.com/ww-en/industries/energy/power/about/selecting-the-right-system).

Written from the published standards and coating practice, in our own words, and checked on 2026-10-04. Found an error? It goes 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.