How the categories are defined
ISO 9223 classifies atmospheric corrosivity from the first-year corrosion loss of standard specimens of carbon steel, zinc, copper and aluminium exposed at the site, using the exposure method of ISO 9226. Where no exposure data exist, the category can be estimated from environmental data: temperature, relative humidity, sulfur dioxide deposition and airborne chloride deposition. ISO 12944-2 takes over the same scale for the design of protective paint systems and lists typical environments for each class.
| Category | Corrosivity | Carbon steel, first-year loss (ISO 9223) |
|---|---|---|
| C1 | Very low | 10 g/m² or less (1.3 µm or less) |
| C2 | Low | 10–200 g/m² (1.3–25 µm) |
| C3 | Medium | 200–400 g/m² (25–50 µm) |
| C4 | High | 400–650 g/m² (50–80 µm) |
| C5 | Very high | 650–1,500 g/m² (80–200 µm) |
| CX | Extreme | 1,500–5,500 g/m² (200–700 µm) |
A C4 classification — the "C4 corrosion class" of many specifications — covers industrial areas and coastal areas with moderate salinity; ISO 12944-2 cites chemical plants and swimming pools as interior examples. CX, added in the 2017 edition, covers offshore areas with high salinity, industrial areas with extreme humidity and an aggressive atmosphere, and tropical and subtropical atmospheres.
Immersion and soil: Im1 to Im4
Atmospheric categories stop at the waterline. Immersed and buried steel is classed separately: Im1 for fresh water, Im2 for sea or brackish water, Im3 for soil, and Im4 for sea or brackish water with cathodic protection. A CX environment is therefore not an immersion environment. Systems for CX and Im4 are qualified under ISO 12944-9, whose test regime includes cyclic ageing and, for Im4, cathodic disbondment.
Reading codes such as C4-VH
Specifications usually pair the category with a durability class from ISO 12944-1: L (low, up to 7 years), M (medium, 7 to 15 years), H (high, 15 to 25 years) and VH (very high, more than 25 years). C4-VH therefore means a high-corrosivity environment and a system designed to reach first major maintenance after more than 25 years. Durability is a design target for planning maintenance, not a warranty period.
Where classifications go wrong
One plant often contains several categories: cooling tower drift, stack fallout and splash zones can push one structure into C5 or CX while the next is C3. Classifying from a regional map rather than the local exposure undersizes the system, and quoting a category without the edition of the standard invites dispute — the legacy C5-I and C5-M labels no longer exist. The category drives the system and its dry film thickness, so an error here propagates everywhere.