Protective paint systems for steel structures are addressed within the scope of the ISO 12944 series. The most frequently cited concept in the series is the atmospheric corrosivity categories that classify the environment in which the structure will operate.
Category is not a paint brand
C3, C4, and C5 are not product or brand names but environmental classes. For the same category, different paint manufacturers may have different coat combinations and different total thickness recommendations. The final system is always determined according to the project specification and the manufacturer's approved system.
Categories and typical environments
Category selection is made based on the structure's geographical location, humidity, pollution, and chemical exposure. The same steel requires different coatings in an enclosed workshop versus a coastal area.
- C3 moderate corrosivity: Urban and light industrial atmospheres, moderate humidity and pollution.
- C4 high corrosivity: Industrial and coastal areas, high humidity and pollution.
- C5 very high corrosivity: Aggressive industrial and coastal atmospheres.
What changes as the category increases
As the category increases, the expectation for barrier performance rises. This generally means more coats, higher total dry film thickness, and tighter application control. On C4 and above, stripe coat application on critical details and the expectation of traceable records become more pronounced.
Along with the category, the target durability period is also defined. For the same category, short and long durability targets require different systems; therefore, not only the category but also the target duration must be specified at the quotation stage.
Five components that sustain the system
The coating system's expected lifespan does not depend on a single parameter. The following five components work together; if one is lacking, even the highest-performance paint system will not yield the desired result.
- Sa cleanliness degree compliant with the specification
- Surface profile required by the paint manufacturer
- Compatible resin system between coats
- Measured and accepted total dry film thickness
- Curing conditions in accordance with the technical data sheet
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