Corrosion is one of the most common enemies of steel. The process progresses over time and can seriously weaken structures, installations and connecting elements. To protect steel effectively against degradation, it is essential to select the right material and protective coatings – depending on the conditions in which the steel will be used.
This is precisely why the PN-EN ISO 12944-2:2018-02 standard was developed, introducing corrosivity categories for environments. It makes it easier to assess how intense the corrosive action is and which solutions will work best.
What is steel corrosivity?
The corrosivity of steel is simply its susceptibility to damage caused by chemical agents such as moisture, oxygen, industrial pollutants or salts. Over time and without adequate protection, it can lead to serious material loss – and, consequently, to failures or costly repairs.
That is why it is so important to take environmental conditions into account when designing steel structures, industrial buildings, tanks or balustrades.
ISO corrosivity categories for steel – what do the symbols C1–C5 and CX mean?
PN-EN ISO 12944-2 divides environments into seven corrosivity categories – from C1 (very low) to CX (extreme). These categories describe typical conditions inside and outside buildings in a temperate climate.
| Category | Environmental conditions | In practice |
|---|---|---|
| C1 very low corrosivity | Interior: heated, clean rooms, e.g. offices, schools, hotels. Exterior: not applicable (the outdoor environment is too aggressive). | Steel is exposed to minimal corrosion only. Basic protection is sufficient – e.g. a thin layer of anti-corrosion paint. |
| C2 low corrosivity | Interior: unheated warehouses and halls where condensation may occur. Exterior: rural areas with a low level of pollution. | Surface corrosion is possible, especially at points of damage. Light protective coatings and regular inspection are recommended. |
| C3 medium corrosivity | Interior: humid production plants, laundries, breweries. Exterior: urban and industrial areas with moderate pollution. | Corrosion may be clearly visible, especially in corners and at welds. Thicker protective coatings or the use of stainless steel with molybdenum are advisable. |
| C4 high corrosivity | Interior: chemical plants, swimming pools, repair shipyards. Exterior: industrial areas and coastal areas with moderate salinity. | The aggressive environment requires the use of highly resistant stainless steel (e.g. 1.4404, 1.4571) and regular surface maintenance. |
| C5 very high corrosivity | C5-I (industrial): heavily polluted industrial areas, high humidity. C5-M (marine): coastal areas, high salt content in the air. | It is necessary to use steels with increased resistance, e.g. duplex or super-austenitic, and advanced coating systems. |
| CX extreme corrosivity | Interior and exterior: tropical environments, offshore platforms, chemical plants with extreme conditions. | Only the most advanced materials (e.g. steel 1.4547, 1.4462) and multi-layer protective systems. Constant inspection and maintenance are required. |
How to choose the right material for a corrosivity category and why does it matter so much?
The choice of the right material and corrosion protection system should always be based on an analysis of the conditions in which the steel component will be used. The key factors are:
- place of installation (indoors or outdoors),
- type of environment (e.g. humid, marine, industrial),
- accessibility for cleaning and maintenance,
- expected service life (e.g. 15, 25 or 50 years),
- and the requirements set out in standards such as PN-EN ISO 12944, ISO 9223 or EN 10088.
The higher the corrosivity category of the environment, the more advanced the materials and protective systems that should be used – both chemically (e.g. stainless steel with molybdenum) and structurally (e.g. multi-layer protective coatings).
Understanding the corrosivity category is the foundation of the longevity of any steel structure. Choosing the wrong steel can lead to premature damage, costly repairs and, in extreme cases, a hazard to users. That is why it is worth taking a moment to analyse the environment in which a structure will be operated and choosing materials that can withstand the conditions in which they are to work for years.
