Corrosion – one of steel's greatest threats
Corrosion is one of the most common enemies of steel. This process develops over time and can significantly weaken structures, installations, or connecting elements. To effectively protect steel from degradation, it is essential to select the right material and protective coatings—depending on the environmental conditions in which the steel will be used.
This is where the PN-EN ISO 12944-2:2018-02 standard comes in. It introduces corrosion categories for different environments, making it easier to assess the level of exposure and determine the best protective solutions.
What is steel corrosivity?
Corrosivity refers to steel's vulnerability to damage caused by chemical factors such as moisture, oxygen, industrial pollutants, or salt. Over time, and without proper protection, corrosion can lead to serious material loss—and in turn, to structural failures or costly repairs.
That’s why environmental conditions are a key factor in designing steel constructions, industrial halls, tanks, or railings.
Corrosivity classes according to ISO – what do C1 to CX mean?
The PN-EN ISO 12944-2 standard defines seven corrosivity categories—from C1 (very low) to CX (extreme). Each class describes typical indoor and outdoor conditions in a temperate climate zone.
| Class | Environmental conditions | Practical implications |
|---|---|---|
| C1 Very low corrosivity |
Indoor: heated, clean spaces such as offices, schools, hotels. Outdoor: not applicable (too aggressive). |
Minimal corrosion risk. Basic protection is sufficient, e.g. a thin anti-corrosion paint layer. |
| C2 Low corrosivity |
Indoor: unheated warehouses, halls with possible condensation. Outdoor: rural areas with low pollution. |
Possible surface corrosion, especially in damaged areas. Light protective coatings and regular inspection recommended. |
| C3 Medium corrosivity |
Indoor: humid industrial spaces, laundries, breweries. Outdoor: urban and industrial areas with moderate pollution. |
Noticeable corrosion may occur, especially in welds and edges. Thicker coatings or stainless steel with molybdenum is advised. |
| C4 High corrosivity |
Indoor: chemical plants, swimming pools, ship repair yards. Outdoor: industrial and coastal zones with medium salinity. |
A highly aggressive environment requiring corrosion-resistant stainless steel (e.g. 1.4404, 1.4571) and routine surface maintenance. |
| C5 Very high corrosivity |
C5-I (Industrial): heavily polluted industrial zones with high humidity. C5-M (Marine): coastal areas with high salt content in the air. |
Requires highly resistant steel grades (e.g. duplex or superaustenitic) and advanced protective systems. |
| CX Extreme corrosivity |
Indoor and outdoor: tropical regions, offshore platforms, highly aggressive chemical environments. | Only the most advanced materials (e.g. 1.4547, 1.4462) and multilayer protective systems should be used. Ongoing inspection and maintenance required. |
How to choose the right material for each corrosivity class—and why it matters
Selecting the right material and corrosion protection system always starts with an analysis of the environmental conditions in which the steel element will be used. Key factors include:
- Installation location (indoor or outdoor),
- Type of environment (e.g. humid, marine, industrial),
- Ease of cleaning and maintenance,
- Expected service life (e.g. 15, 25, or 50 years),
- Relevant industry standards such as PN-EN ISO 12944, ISO 9223, or EN 10088.
The higher the corrosivity of the environment, the more advanced materials and protective systems are needed—both in terms of chemistry (e.g. stainless steel with molybdenum) and structure (e.g. multilayer protective coatings).
Understanding the corrosivity class is key to the durability of any steel structure. Choosing the wrong material may lead to early damage, expensive repairs, or even safety risks. That’s why it’s worth analyzing the working environment and investing in materials that match the conditions—right from the start.