Every steel structure operates in a different environment – a railing in a heated office corrodes differently than a viaduct at a seaside industrial facility. Choosing the right protection begins with a single question: how aggressive is the environment in which the steel is expected to survive for decades? The answer is organized by corrosivity classes – a standardized system that allows us to move from a general "it's wet and salty here" to a specific decision regarding zinc coating thickness. Below, we explain what these classes are, how to recognize the correct one, and how to match it with corrosion protection.
What are corrosivity classes?
Corrosivity classes (or corrosivity categories) are a standardized way of describing, how strongly a given environment damages unprotected metal. The scale includes five basic levels – from C1 (practically non-aggressive environment) to C5 (very aggressive) – and since 2017 also the CX category for extreme conditions. The classification is defined by ISO standards, and the reference point is not a subjective assessment, but measurable loss of mass or thickness of standard steel and zinc samples after one year of exposure.
Thanks to this, the "corrosion class" has become part of the common language of investors, designers, and contractors. One designation (e.g., C4) clearly defines how intensively the corrosion will proceed. corrosion and how durable the protection must be – and this directly translates into the choice of technology and the expected lifespan of the structure.
ISO 12944 and ISO 9223 standards – the basis for classification
The corrosivity classes are not conventional, but result from two related standards. ISO 9223 classifies the corrosiveness of atmospheres based on three factors: surface wetting time (humidity and temperature), sulfur dioxide (SO₂) contamination, and air salinity (chlorides). These parameters determine whether a given location is classified as C2 or C4.
On the other hand ISO 12944-2 – part of the standard dedicated to the corrosion protection of steel structures – transfers this knowledge to design. It defines atmospheric corrosivity categories based on the mass (or thickness) loss of standard samples after the first year of exposure, and additionally describes the typical environments assigned to each class.
In practice, class is determined in two ways: precisely – by exposing standard samples for a year and measuring their loss (according to ISO 9226), or approximately – comparing site conditions with the environmental descriptions in the standard. When the results for steel and zinc indicate different classes, the higher (more aggressive) class is adopted.
Corrosion classes C1–C5 – characteristics
The table below lists all classes along with an example of the annual material loss – it is a good starting point for an initial assessment.
| Class | Corrosiveness | Typical environment | Steel loss (µm/year) | Zinc loss (µm/year) |
| C1 | very small | heated interiors, clean air | ≤ 1.3 | ≤ 0.1 |
| C2 | small | unheated interiors, rural areas | > 1.3–25 | > 0.1–0.7 |
| C3 | mean | cities, moderate industry | > 25–50 | > 0.7–2.1 |
| C4 | big | industry, coastal zones | > 50–80 | > 2.1–4.2 |
| C5 | very large | heavy industry, maritime zones | > 80–200 | > 4.2–8.4 |
C1 – very low corrosivity (heated interiors, offices)
Environments with a clean, dry atmosphere, virtually free of condensation: office buildings, schools, hotels, shopping malls. Steel corrodes here at a negligible rate and often requires no special protection beyond aesthetic considerations.
C2 – low corrosivity (unheated interiors, rural areas)
Atmospheres with low levels of pollution and unheated interiors with periodic condensation – warehouses, sports halls, and outdoor rural areas away from industry. Corrosion is slow, and zinc coatings here achieve a very long service life.
C3 – medium corrosivity (cities, moderate industry)
Corrosion class C3 is one of the most common environments in Poland. It covers urban and industrial atmospheres with moderate SO₂ concentrations and coastal areas with low salinity. Typical objects include structures in urban areas, fences and roadside shelters, as well as production facilities with elevated humidity (breweries, dairies, food processing plants).
C4 – high corrosivity (industry, coastal zones)
Corrosion class C4 describes distinctly aggressive environments: chemical plants, swimming pools, shipyards, and coastal areas with moderate salinity. The combined effects of moisture and aggressive contaminants mean that choosing protection requires a conscious decision regarding coating thickness, and often a complex system.
C5 – very high corrosivity (heavy industry, marine zones)
The most demanding of the basic classes: areas with almost constant condensation and high pollution, as well as coastal and marine zones with high salinity – heavy industrial installations, coastal port structures, and facilities in the immediate vicinity of the sea. Thin coatings wear out quickly here, and reinforced protection is often necessary.
Zinc coating thickness and corrosion class
This is where we get to the heart of design. Thickness zinc coating in hot-dip galvanizing it is not arbitrary - it depends primarily on the thickness of the steel, and its minimum is specified in the standard ISO 1461.
| Thickness of the steel element | Local thickness (min.) | Average thickness (min.) |
| < 1.5 mm | 35 µm | 45 µm |
| ≥ 1.5–3 mm | 45 µm | 55 µm |
| > 3–6 mm | 55 µm | 70 µm |
| > 6 mm | 70 µm | 85 µm |
The second half of the equation is the zinc consumption rate in a given class – the higher the class, the more intensely the coating "works." Combining the thickness with the annual zinc loss yields an estimated durability, which is time to first maintenance. The values below (for a typical coating of approximately 85 µm on steel thicker than 6 mm) are based on ISO 14713 data.
| Class | Annual zinc loss | Estimated coating durability 85 µm |
| C1 | ≤ 0.1 µm | over 100 years |
| C2 | 0.1–0.7 µm | about 100 years and more |
| C3 | 0.7–2.1 µm | approx. 40–100+ years |
| C4 | 2.1–4.2 µm | approx. 20–40 years old |
| C5 | 4.2–8.4 µm | approx. 10–20 years |
In conditions of corrosivity class C3, the coating thickness at the standard level of 85 µm provides several dozen years of protection without interference – hot-dip galvanizing works flawlessly here. For corrosion class C4, the coating thickness must be higher or maintenance must be more frequent; the same 85 µm will last for about 20–40 years.
How to choose anti-corrosion protection for your corrosivity class?
A practical decision-making process for the designer and investor can be reduced to a few steps:
- Determine the corrosivity class – based on location, proximity to industry, distance from the sea and humidity (ISO 9223 / ISO 12944-2).
- Determine required durability – i.e. the expected time until the first maintenance (e.g. 15, 25 or over 50 years).
- Check maintenance access – structures that are difficult to access or non-replaceable require a greater reserve of durability.
- Choose the technology – zinc coating thickness or complex system.
For classes C1–C3 Hot-dip galvanizing alone provides very long-term protection and is usually fully sufficient. C4 still works well, especially when the structure is made of galvanized steel with greater thickness or reinforced coatings were used. In the most difficult class C5 – with constant humidity and high salinity – it is worth considering duplex system, i.e., hot-dip galvanizing with a paint coating. Both barriers work synergistically: the paint protects the zinc from rapid wear, and the zinc cathodically protects the steel, even where the paint is damaged.
If you are faced with the decision to protect a steel structure and want to be sure that the coating meets its operating conditions – contact Strumet. We'll help you choose a solution that's appropriate to your corrosion class and expected durability.
FAQ – most frequently asked questions about corrosion classes
What is the difference between corrosivity classes C1–C5?
They differ in environmental aggressiveness, measured by annual steel and zinc loss. C1 represents virtually non-aggressive conditions (heated interiors), while C5 represents very aggressive environments (heavy industry, marine zones), where metal corrodes many times faster.
How do I check which corrosivity class my environment belongs to?
The most accurate method is to expose standard samples for a year and measure their deterioration (ISO 9226). As a guide, it is sufficient to compare the conditions at the investment site (humidity, pollution, proximity to the sea and industry) with the environmental descriptions in the ISO 12944-2 standard.
Which corrosivity class applies to coastal and industrial zones?
Most often, C4 (moderate salinity, industry, coastal zones) or C5 (high salinity, heavy industry, immediate proximity to the sea). Classification into a specific category is primarily determined by the level of chlorides and pollutants in the air.
Is hot dip galvanizing sufficient for class C5?
Yes, although in the harshest C5 conditions (constant moisture, very high salinity), the durability of the zinc coating itself may be limited. In such cases, thicker coatings or a duplex system (zinc + paint) are recommended, which significantly extends the time until the first maintenance.
