Coating is one of the most common sources of disagreement on a steel structure order, usually because the specification names a paint system without naming the environment it has to survive. ISO 12944 ties those together through corrosion categories and durability ranges.
This guide explains how the categories work and what a workable specification contains. The values below are typical ranges for orientation; the final system has to be confirmed against the project specification and the coating manufacturer's data sheet.
Corrosion categories, in one table
| Category | Environment | Typical examples |
|---|---|---|
| C1 | Very low | Heated, dry interiors such as offices and clean production halls |
| C2 | Low | Unheated interiors with low humidity, rural atmosphere |
| C3 | Medium | Urban and industrial atmosphere, moderate humidity, inland production areas |
| C4 | High | Coastal areas with moderate salinity, chemical plants, areas with high humidity |
| C5 | Very high | Coastal and offshore areas with high salinity, aggressive industrial atmosphere |
| CX | Extreme | Offshore installations, tropical and subtropical aggressive environments |
The category is chosen from the environment the steel will live in, not from the country. A warehouse 200 metres from the sea on a humid coast is not the same exposure as one inland, even when both are in the same project package.
Durability ranges and what they mean for thickness
ISO 12944 also defines a durability range, which sets the expected time to the first major maintenance rather than a warranty period.
| Durability | Expected first major maintenance | Practical effect |
|---|---|---|
| Low (L) | Up to 7 years | Thinner systems, more frequent access for repainting |
| Medium (M) | 7 to 15 years | The most common commercial range |
| High (H) | 15 to 25 years | Thicker systems, better surface preparation, higher inspection load |
| Very high (VH) | More than 25 years | Typically galvanizing, a duplex system, or a heavy multi-coat system |
Painted, galvanized or duplex
| Option | Suits | Watch for |
|---|---|---|
| Zinc-rich primer plus epoxy and polyurethane topcoat | C3 and C4, appearance-sensitive structures | Correct blast grade, stripe coating on edges and welds, total DFT measurement per coat |
| Hot-dip galvanizing to ISO 1461 | C4 and C5, outdoor and coastal structures | Vent and drain holes in closed sections, distortion control, thread and connection-face masking |
| Duplex, galvanized then painted | C5 and CX, very long maintenance intervals | Surface preparation and a compatible paint system over the zinc, otherwise adhesion fails |
For galvanizing, the required coating mass or thickness depends on the steel thickness. Confirm the values against ISO 1461 for the actual member thickness rather than specifying one number for the whole package.
What a workable coating specification contains
- The corrosion category and durability range for the project
- The surface preparation grade before the first coat, for example Sa 2 and a half
- Each coat with its type and the dry film thickness the inspector should measure
- The total dry film thickness, with the measurement method and acceptance rule
- The colour, referenced to a standard such as RAL
- Requirements for connection faces, machined surfaces and threaded parts
- Whether repair coating is required after transport damage, and who inspects it
This guide describes general practice for steel structure projects. It is not a design, and it does not replace the specification or the engineer responsible for your project.