Summary: C5-M marine air is high salt, high humidity and strong UV, plus biofouling. Long protection needs a system matched to the durability class, not one thick coat. Total DFT is often about 240-320 µm.
Mechanism
Typical system: zinc or epoxy primer for adhesion and cathodic/barrier action; epoxy MIO or glass-flake intermediate for barrier; aliphatic polyurethane or fluoropolymer topcoat for UV and colour retention.
Key data
- ISO 12944-2 lists C5-M as very-high corrosivity marine and industrial salt atmospheres.
- Long-life systems are often about 240-320 µm total (primer+intermediate+topcoat).
- Choose aliphatic PU / fluorotopcoat for UV; aromatic PU yellows and chalks outdoors.
- Splash / tidal zones need heavier systems, glass flake, cladding or cathodic protection.
Layer function
| Layer | Function |
|---|---|
| Zinc/epoxy primer | Adhesion + cathodic/barrier |
| PU/fluorotopcoat | UV, gloss and colour retention |
Defects & fixes
- Topcoat chalking: aromatic PU used outdoors — switch to aliphatic/fluoro.
- Early rust-through: total DFT or durability class too low — raise system grade.
- Edge rust: low edge film — stripe-coat edges and welds.
FAQ
Why is C5-M hard?
High salt and humidity accelerate electrochemical corrosion while UV degrades the topcoat; layers must handle both.
Can zinc primer be the topcoat?
No — zinc lacks UV resistance and needs sealing; topcoat is aliphatic PU/fluoro.
What about the splash zone?
Use heavier DFT, glass flake, or cladding/cathodic protection combinations.
Ref: ISO 12944-2/-5 environment classes; offshore corrosion protection guides (public).
Kexin New Materials (Guangdong) Co., Ltd. · 2026-09-18