The overview about Corrosion of structural steel
The corrosion of structural steel is an electrochemical process that requires the simultaneous presence of moisture and oxygen.
In the absence of either, corrosion does not occur. Essentially, the iron in the steel is oxidised to produce rust, which occupies
approximately 6 times the volume of the original material consumed in the process. The general corrosion process is illustrated here.
Atmospheric corrosivity categories and examples of typical environments (BS EN ISO 12944-2)
| Corrosivity category and risk | Low-carbon steel Thickness loss (μm)a | Examples of typical environments in a temperate climate (informative only) | |
|---|---|---|---|
| Exterior | Interior | ||
| C1 very low | ≤ 1.3 | - | Heated buildings with clean atmospheres, e.g. offices, shops, schools, hotels |
| C2 low | > 1.3 to 25 | Atmospheres with low level of pollution Mostly rural areas | Unheated buildings where condensation may occur, e.g. depots, sports halls |
| C3 medium | > 25 to 50 | Urban and industrial atmospheres, moderate sulphur dioxide pollution Coastal area with low salinity | Production rooms with high humidity and some air pollution e.g. food-processing plants, laundries, breweries, dairies |
| C4 high | > 50 to 80 | Industrial areas and coastal areas with moderate salinity | Chemical plants, swimming pools, coastal, ship and boatyards |
| C5-I very high (industrial) | > 80 to 200 | Industrial areas with high humidity and aggressive atmosphere | Buildings or areas with almost permanent condensation and high pollution |
| C5-M very high (marine) | > 80 to 200 | Coastal and offshore areas with high salinity | Buildings or areas with almost permanent condensation and high pollution |
Notes:
1μm (1 micron) = 0.001mm
a The thickness loss values are after the first year of exposure. Losses may reduce over subsequent years.
The loss values used for the corrosivity categories are identical to those given in BS EN ISO 9223[2].
In coastal areas in hot, humid zones, the mass or thickness losses can exceed the limits of category C5-M. Special precautions must therefore be taken when selecting protective paint systems for structures in such areas.