ISO 12944 corrosion classes explained: choosing C3, C4 or C5

Corrosion is what finishes off outdoor steel, and an enclosure on an energy site sits in the weather for its whole life. ISO 12944 is the standard that turns "make it last" into something you can actually specify. It is the standard for protecting steel structures with paint systems, and the decision at the heart of it is the corrosivity category: how aggressive the environment is, written as a class from C1 to CX. Get that class right and the protection matches the site. Get it wrong in either direction and you have either paid for too much or set the unit up to rust early. Here is how the classes work and how to choose.

The corrosivity categories, C1 to CX

ISO 12944 sorts environments by how corrosive they are:

- C1, very low. Heated, dry indoor spaces. Not relevant to an outdoor enclosure. - C2, low. Rural areas with low pollution, or unheated indoor spaces with some condensation. - C3, medium. Urban and industrial atmospheres with moderate pollution, and coastal areas with low salinity. A lot of inland sites sit here. - C4, high. Industrial areas with significant pollution, and coastal areas with moderate salinity. - C5, very high. Heavily polluted industrial environments and coastal areas with high salinity. - CX, extreme. Offshore and high-salinity marine environments. The 2018 revision added this and folded the old C5-I and C5-M split into the C5 and CX classes.

Most fabricated enclosures on energy sites land in C3, C4 or C5. C3 for a typical inland site, C4 where there is real industrial exposure or moderate coastal salt, and C5 for an exposed or genuinely coastal location.

Durability is a guide, not a guarantee

ISO 12944 pairs the category with a durability range, the expected time before the coating needs its first major maintenance. The 2018 version sets these as low (up to 7 years), medium (7 to 15), high (15 to 25) and very high (more than 25). The important word is expected. Durability is a planning guide to when the paint will want attention, not a warranty on the unit, and it assumes the coating was specified and applied properly in the first place. A buyer should read it as "how long before this needs repainting", not "how long until it fails".

Choosing the category from assessed exposure

The project team should select the category from site evidence such as location, pollution, salinity, humidity and shelter. Over-specification adds cost. Under-specification can bring maintenance forward and increase remedial work, especially on remote or hard-to-access units. The coating system should follow the assessed category, required durability range and maintenance plan rather than carrying a category over from the previous project.

How we approach it

Garendon can help the project team specify and manage coating systems for C3, C4 and C5 environments where required. We agree the category, durability range, preparation, application and inspection requirements from the site information and project brief.

If you have an enclosure or a modular steel structure to specify and want to work out the right corrosion class for the site, we are happy to talk it through early.

Information needed before specifying a coating

• Whether the steel is indoors, outdoors, sheltered, buried, immersed or exposed to persistent condensation.
• The site's location, distance from the coast, salinity, industrial pollution and any chemical exposure.
• The required durability range and how the structure will be inspected, cleaned and maintained.
• The substrate, fabrication condition and required surface-preparation method.
• Any colour, gloss, topcoat, repair, film-thickness and inspection requirements.

The assessed environment can then be carried into the coating and inspection scope. For a typical outdoor application, see the project inputs for a BESS steel enclosure; for the material decision before coating, read steel versus GRP enclosures.