In industrial valve procurement, terms such as “C4 paint” or “C5 coating” are common specifications. These grades originate from the ISO 12944-2 standard, which defines the corrosion protection requirements for valves in different atmospheric environments.
The standard classifies atmospheric corrosion environments into six grades, from low to high: C1 to CX.
| Grade | Corrosivity | Typical Environment | Minimum Total DFT (μm) |
| C1 | Very low | Clean heated interiors: offices, hotels, virtually no condensation | Not specified |
| C2 | Low | Rural areas; unheated warehouses, occasional condensation | 80–160 |
| C3 | Medium | Urban areas, light industrial zones; low-salinity coastal areas; food plants | 120–200 |
| C4 | High | Industrial areas, chemical plants, swimming pools, shipyard interiors | 160–280 |
| C5 | Very high | High-humidity heavy-pollution industrial areas / high-salinity coastal areas | 240–320 |
| CX | Extreme | Offshore platforms, tropical high-salinity mist; governed by ISO 12944-9 | 380+ |
For valves, selecting a coating grade requires considering both environmental corrosivity and expected durability. The standard divides durability into four categories: low (≤7 years), medium (7–15 years), high (15–25 years), and very high (>25 years). The harsher the environment, the more complex the coating system required, and the greater the total dry film thickness.
It is important to note that “C5 paint” is not a specific product, but a corrosion grade requirement. When a specification states “C5, 250µm, Sa 2½,” it actually requires a complete coating system: surface preparation grade, number of coats, coating types, total dry film thickness, and inspection requirements. For valves, surface preparation typically must reach Sa 2½, and total dry film thickness ranges from approximately 160µm to over 320µm depending on the grade.
A valve’s corrosion protection depends half on the paint and half on the application. Only by integrating coating grade, surface preparation, application conditions, and inspection standards into a unified specification can valves reliably serve their intended environments over the long term.
