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2026-11-12T08:47:23

ISO 12944 Corrosivity Categories C1–CX: Choosing Paint for Steel Gates & Canopies

ISO 12944腐蚀性等级C1–CX：铁艺围栏与雨棚钢结构如何选防腐漆

ISO 12944 corrosivity categories are the standard way to state how fast bare steel rusts in a given environment, from C1 (very low) to CX (extreme). Before choosing paint for a steel gate, canopy, staircase or roof frame, first identify the category of the site and then the durability target: low, medium, high or very high. Urban homes typically sit around C3, while buildings near the sea can fall into C4 or C5.

Key Facts

What ISO 12944 corrosivity categories are

ISO 12944 is the international standard for protecting steel structures against corrosion with paint systems. Part 2 classifies atmospheric environments into six categories, and the basis is very concrete: how many micrometres of low-carbon steel are lost after one year of exposure. In C1 the loss is no more than 1.3 µm, while in CX it can reach 200 to 700 µm per year. These loss values are the same ones used in ISO 9223.

Beyond the atmosphere, the standard also has categories for immersed or buried steel: Im1 for fresh water, Im2 for sea or brackish water without cathodic protection, Im3 for soil (for example buried tanks and steel piles), and Im4 for sea water with cathodic protection.

Typical environments for each category

Based on the ISO 12944-2 table as reproduced by SteelConstruction.info and Teknos:

  • C1 very low: interiors only, such as clean offices, shops, schools and hotels.
  • C2 low: exteriors with low pollution (mostly rural); unconditioned interiors such as depots where condensation can occur.
  • C3 medium: urban and industrial exteriors with moderate SO2 pollution, coasts with low salinity; humid production rooms such as laundries and food plants.
  • C4 high: industrial areas and coasts with moderate salinity; chemical plants and swimming pools indoors.
  • C5 very high: industrial areas with high humidity and aggressive atmosphere, coasts with high salinity.
  • CX extreme: high-salinity offshore areas, industrial areas with extreme humidity, and aggressive sub-tropical and tropical atmospheres.

One building can contain several categories. SteelConstruction.info gives the example of a hospital that is generally C1, while its kitchens and laundry can be C3.

Durability: L, M, H and VH

Once the category is known, the owner chooses a durability target, which is the expected life of the paint system until the first major maintenance painting. ISO 12944 divides it into low (up to 7 years), medium (7–15 years), high (15–25 years) and very high (over 25 years). Teknos stresses that these ranges are planning parameters, not guarantees. System codes on coating data sheets often combine both, for example C4-H means a system designed for a C4 environment with high durability.

Example protection systems in practice

SteelConstruction.info lists standard systems drawn from UK industry experience. For a C3 exterior, one option is an 80 µm zinc phosphate epoxy primer, a 100 µm epoxy MIO coat and a 60 µm polyurethane finish, with Sa 2½ blast cleaning and a coating life of about 20 years. Hot-dip galvanizing to ISO 1461 is also listed; for steel profiles over 6 mm, the minimum average galvanized thickness is 85 µm. For C5 the example systems are heavier, such as a 40 µm zinc-rich epoxy primer, 200 µm total epoxy MIO and a 60 µm polyurethane finish.

These figures are examples, not mandatory recipes. The system used should follow the paint maker's data sheet that states the target category and durability.

Tips for choosing steel paint for homes and projects

First, define the location: a fence in an inland city is not the same as a canopy on a beachfront house. Second, check whether the paint data sheet states an ISO 12944 category and the dry film thickness (DFT) per coat. Third, pay attention to surface preparation; even the best paint fails quickly if rust and oil are not removed. Fourth, plan maintenance according to the chosen durability. The homeX team can help source primers, steel paints and steel materials to match your project specification.

ISO 12944 corrosivity categories: related articles and products

FAQ

Which category applies to a house near the beach? ISO 12944-2 lists low-salinity coastal areas as C3, moderate as C4 and high as C5. Distance from the sea, wind direction and humidity decide the real category, so ask the paint supplier for an assessment on important projects.

Does high durability (H) mean a 15–25 year guarantee? No. Durability ranges are technical estimates until the first major maintenance, not guarantee periods.

Does galvanized steel still need paint? Hot-dip galvanizing is a protection system in itself. Painting on top can be for appearance or more aggressive environments, but the galvanized surface needs special preparation for paint to adhere.

For building and renovation material needs, reach the homeX team on WhatsApp +62 821 1818 998, by email at homex9998@gmail.com or through our contact page.

Sources: SteelConstruction.info: Standard corrosion protection systems for buildings (BS EN ISO 12944-2); Teknos: Corrosivity categories and durability ranges (ISO 12944:2018)