Most metallic materials change their properties when they are exposed to outdoor weathering, i.e. due to corrosion. Corrosion occurs when metal building materials react with their environment. Providing an exact account of the environment’s impact as well as the resulting corrosion is not possible.

However, durable corrosion protection which is in accordance with regulations and standards is easy to manufacture.  The exposure to corrosion through environmental strain(s) is divided (by DIN EN ISO 12944-2) into categories depending on the environment and exposure to the elements.

For establishing the environmental conditions for atmospheric conditions (C), the following corrosiveness categories have be defined: 

Corrosiveness Category   Explanation
C2 low

Rural areas, unheated buildings, in which condensation may occur, e.g. warehouses or gymnasiums.

C3 moderate

Urban or industrial areas with moderate air pollution, coastal areas with low salinity, production premises with high humidity and some air pollution, e.g. food production plants, laundries, or breweries. 

C4 high

Industrial areas, coastal areas with moderate salinity, chemical plants, swimming pools.

C5-I very high

(Industry) Industrial areas with high humidity and aggressive atmospheres.

C5-M very high

(Sea) Costal and offshore areas with high salinity, buildings with virtually permanent condensation and/or high air pollution.