3. Classification of corrosion environments

Environments causing corrosion can be classified in different categories according to their corrosivity. The classification is based on standard ISO 9223:1992 Corrosion of metals and alloys -- Corrosivity of atmospheres - Classification. It categorizes environments on the basis of wet time, as well as sulphur dioxide and chloride contents.

Standard ISO 9224 Corrosion of metals and alloys -- Corrosivity of atmospheres -- Guiding values for the corrosivity categories gives the corrosion rates of steel, zinc, copper and aluminium in the first five years.

In the two following standards, the corrosivity categories are determined by the loads imposed by the atmosphere and through immersion:

EN ISO 12944-2

Paints and varnishes -- Corrosion protection of steel structures by protective paint systems -- Part 2: Classification of environments

ISO 14713

Protection against corrosion of iron and steel in structures -- Zinc and aluminium coatings - Guidelines

Table 1 gives examples of the environments in each corrosion category and the rates of corrosion for steel and zinc in the first year.

Table 1.Categories of environment

Corrosivity categoryEnvironment
(guiding examples)
C1
very low		 Indoor spaces with occasional condensation
Outdoor air: inland rural
C2
low		 Dry indoor spaces
C3
medium		 Indoor spaces with high moisture content, not much impurities
Outdoor air: inland urban, mildly saline
C4
high		 Indoor spaces: chemical industry, swimming pools, seaside docks.
Outdoor air: inland industrial plants, seaside urban areas
C5-I
very high		 Outdoor air: very humid industrial atmosphere
C5-M
very high		 Outdoor air: saline seaside atmosphere
Corrosivity categoryRate of corrosion in the first year
ISO 9223 and EN ISO 12944-2EN ISO 14713
Carbon steel
µm/year		Zinc
µm/year		Zinc
µm/year
C1
very low		 <= 1,3 <= 0,1 <= 0,1
C2
low		 1,3 - 25 0,1 - 0,7 0,1 - 0,7
C3
medium		 25 - 50 0,7 - 2,1 0,7 - 2
C4
high		 50 - 80 2,1 - 4,2 2 - 4
C5-I
very high		 80 - 200 4,2 - 8,4 4 - 8
C5-M
very high		 80 - 200 4,2 - 8,4 4 - 8

Table 2. Categories of immersed and buried structures.

Category of environmentEnvironment
(guiding examples)
lm1 Fresh water: river constructions, water power plants
lm2 Sea water in temperature zones: Structures in harbour areas
lm3 Soil: Buried tanks, steel piles, steel pipes
Category of environmentRate of corrosion in the first year
ISO 9223 and EN ISO 12944-2EN ISO 14713
Carbon steel
µm/year		Zinc
µm/year		Zinc
µm/year
lm1      
lm2     10 - 20
lm3      
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