| Corrosion category | Possible reduction in the thickness of unalloyed steel | Common cases |
| C1 - Negligible | =≤ 1.3 µm | Heated buildings, offices, schools, stores, etc. |
| C2 - Low | => 1.3 – 25 µm | Low-pollution atmospheres and unheated structures prone to condensation, such as storage areas and sports halls. |
| C3 - Moderate | => 25 – 50 µm | Urban and industrial settings with limited SO₂ pollution, coastal regions with minimal salt content, and production facilities with elevated humidity levels like laundries and breweries. |
| C4 - Substantial | => 50 – 80 µm | Industrial environments with moderate salt exposure, including chemical plants, swimming pools, and coastal docks. |
| C5 - Very high | => 80 – 200 µm | High-humidity industrial zones, salt-laden coastal areas, and sites with persistent condensation and heavy pollution. |
| CX - Extreme | => 200 – 700 µm | Marine environments characterised by elevated salt concentrations, industrial zones with exceptionally high humidity and corrosive conditions, and subtropical or tropical climates. |
