KEY POINTS
- ISO 12944 classifies environments from C1 (very low) to C5 (very high) and CX (extreme). C5 covers aggressive industrial and coastal atmospheres.
- A C5 result depends on the full system: substrate preparation, pretreatment chemistry, coating system, film thickness and testing.
- Ying Group C5 systems are available as a standalone line or as an upgrade package for an existing vertical line, designed for 1,000+ hours of neutral salt spray (ISO 9227). The hours your project needs depend on its durability range.
What "C5" actually means
ISO 12944 is the international standard for protecting steel structures with protective paint systems. Strictly, it covers paints that dry at ambient temperature and excludes powder coatings. Its categories and durability ranges are still the common language for specifying powder systems: the QUALISTEELCOAT label certifies powder coatings on steel against them, up to C5 with high durability. Part 2 defines the atmospheric corrosivity categories:
| Category | Corrosivity | Typical examples |
|---|---|---|
| C1 | Very low | Heated buildings with clean air (interior only) |
| C2 | Low | Rural areas with low pollution |
| C3 | Medium | Urban and light industrial; coastal with low salinity |
| C4 | High | Industrial areas; coastal with moderate salinity |
| C5 | Very high | Industrial areas with high humidity and aggressive atmosphere; coastal areas with high salinity |
| CX | Extreme | Offshore and extreme industrial environments |
The standard also defines durability ranges: how long the coating system should last before its first major maintenance. Low is up to 7 years, medium 7–15 years, high 15–25 years and very high more than 25 years. A "C5" request is only complete when it also states the required durability.
Five things to confirm before quoting
1. Substrate
Steel, galvanised steel and aluminium need different preparation. Heavy steel structures often need blasting before chemical pretreatment. Aluminium relies on etching and conversion coating. ISO 12944 itself applies to steel. For architectural aluminium on the coast, the usual reference is the QUALICOAT Seaside label, which requires deeper etching (at least 2 g/m²) during pretreatment.
2. Pretreatment chemistry and stages
C5 performance needs an intensified, well-controlled pretreatment, typically a longer multi-stage tunnel with chrome or chrome-free conversion configured to the substrate and certification requirement.
3. Coating system
One coat or two? Many C5 specifications use a primer plus topcoat system. The powder supplier's system data sheet defines which combinations are tested for which category and durability.
4. Film thickness
Film build must be controlled across the whole part, including edges and corners where corrosion usually starts. The target dry film thickness comes from the tested coating system, not from the line. C5 powder systems are commonly two-coat (primer plus topcoat) and specified well above the 60 µm minimum of a standard single architectural coat. Ying Group C5 lines are configured, booths and guns included, to hold the thickness your coating system requires.
5. Testing method
Agree how performance will be verified, for example neutral salt spray per ISO 9227, condensation testing, adhesion tests and a film thickness measurement plan. For reference, ISO 12944-6 sets 1,440 hours of neutral salt spray and 720 hours of condensation (ISO 6270-1) for C5 with high durability, and less for lower durability ranges. Ying Group C5 systems are designed for 1,000+ hours of salt spray. The hours you can claim depend on the powder system you choose.
Standalone line or upgrade package?
If you already run a vertical powder coating line, a C5 upgrade may be the faster route. It typically means pretreatment intensification, film build control and process monitoring added to your existing line. A new C5 corrosion coating system makes sense when you are building capacity for infrastructure steel, marine or coastal projects from scratch.
Typical C5 applications
- Coastal façades, windows and balcony systems (on aluminium, see QUALICOAT Seaside)
- Infrastructure steel: bridges, lighting poles, rails and barriers
- Gates, railings and fencing near the sea
- Industrial plants with chemical or high-humidity atmospheres
What to send us
Send the substrate, part dimensions and weights, the exposure environment and required durability, any coating system already approved by your client, the testing requirements, and your target output. With these, our engineers can confirm the pretreatment, film build and cure design. For the full checklist, see what to send for a quote.
Frequently asked questions
What is C5 corrosion protection?
C5 is the 'very high' atmospheric corrosivity category in ISO 12944, covering aggressive industrial environments and coastal areas with high salinity. A C5 coating system is designed and tested to protect steel in those conditions for a stated durability period.
Can powder coating meet ISO 12944 C5?
Yes, in practice. ISO 12944 formally excludes powder coatings, but powder systems on steel are tested and certified against its C5 category, for example under the QUALISTEELCOAT label (up to C5 with high durability). The full system has to be right: surface preparation and pretreatment, a powder system tested for C5, controlled film thickness including edges, and correct curing.
How many hours of salt spray does C5 require?
It depends on the durability range. ISO 12944-6 sets 1,440 hours of neutral salt spray (ISO 9227) plus 720 hours of condensation for C5 with high durability, and less for lower durability ranges. Ying Group C5 systems are designed for 1,000+ hours of neutral salt spray. Confirm the exact requirement in your project specification.
Can an existing powder coating line be upgraded to C5?
Often yes. Ying Group offers C5 as an upgrade package for existing vertical powder coating lines, covering pretreatment intensification and film build control.