Corrosivity categories in the surface treatment of steel

Steel must be protected against corrosion for it to withstand the service life requirements set out for it. The combination of the structure or component to be surface treated, their pre-treatment and the paint film formed by the paint used for protection is called a paint system. The purpose of the system is to protect the steel surface from corrosion and give the structure the desired appearance. The structure or component must be protected against corrosion in such a way that it can later also be maintenanced and repainted.

The paint system may include several different paints, which supplement each other. On the other hand, some systems only use one paint, which is painted once or several times until the desired film thickness is achieved.

European standards dictate which anti-corrosion methods are selected to be used. There are paint systems for both wet painting and powder coating. When planning a paint system, the protection requirements set out by the location of the object to be painted primarily affect the selection. In other words, corrosivity categories are determined for the structure to be treated.

Climate corrosivity categories and immersion categories for submerged and underground structures  

When implementing corrosion protection, it is important to know what type of climate stress the component or structure to be protected is exposed to. It is not indifferent in terms of protection whether the structure or component is exposed to sunlight, wind, or maritime climate.

Climate corrosivity has been categorised according to its intensity into six different categories. These categories are described in the corrosion protection standard SFS-EN 12944-2.

Two different methods are used to determine climate corrosivity category, and they are based on either measurements or assessments. In the measurement of climate corrosivity category, a metal test piece is exposed to climate stress. The corrosion of the test piece is measured for one year, and the obtained results are compared with the benchmark values defined in standard SFS-EN 12944-2.

The second method is based on assessment. The assessment considers the duration that the surface has been wet and the environment’s impurities, such as the air’s chlorine salt and sulphur dioxide concentration fallouts. The area’s climate corrosivity category is determined based on these factors.

Three corrosivity categories have been defined for submerged and underground structures. Soil types and the differences of their variable are not categorised.

The durability of paint systems is not a warranty of the painting

The durability of the paint systems is categorised into four classes according to standard SFS-EN ISO 12944-1: low (2-5 years), moderate (5-15 years), high (15-25 years) and very high (more than 25 years).

However, the durability class does not refer to the warranty period. The purpose of the categorisation is to express the assumed duration of the painted structure before it requires any significant maintenance or repainting work. The durability classification works as a technical reference, which can be used to plan the maintenance and service of structures and components.