"C5" and "CX" appear constantly in wind fastener specifications, and they are not arbitrary labels — they are corrosivity categories defined by ISO 12944 and ISO 9223. Getting the category right is the first step in selecting a coating and material, because it sets how aggressively the environment will attack unprotected steel.
§ 01 What the corrosion categories are
ISO 12944-2 (with ISO 9223) classifies atmospheric environments by their corrosivity — how fast they corrode standard steel and zinc reference samples. The scale runs from C1 (very low) to CX (extreme), each band defined by the measured first-year mass/thickness loss of reference metals. The category is an environmental property of the site, not of the coating.
Once you know the category, you can select a protection system rated to deliver the required durability (low / medium / high / very high) within that environment.
§ 02 The category scale
| Category | Corrosivity | Typical environment |
|---|---|---|
| C1 | Very low | Heated indoor, dry |
| C2 | Low | Rural, low pollution |
| C3 | Medium | Urban / industrial, some coastal |
| C4 | High | Industrial, coastal with moderate salt |
| C5 | Very high | Industrial high-humidity, coastal high salinity |
| CX | Extreme | Offshore, splash zone, harsh marine |
CX was introduced specifically to cover the extreme offshore and splash-zone conditions that exceed even C5 — directly relevant to offshore wind.
§ 03 Where wind turbine sites fall
As a general guide:
- Onshore, inland — typically C3–C4 for external structure; sheltered internal areas may be lower.
- Onshore coastal — C4–C5 depending on distance from the shoreline and prevailing salt-laden wind.
- Offshore — C5 to CX, with the splash and tidal zones being the most aggressive of all.
The jump in corrosivity is exactly why offshore turbines need a different fastener strategy from onshore — explored in offshore vs onshore fastener materials.
§ 04 Matching protection to category
| Category | Typical fastener protection |
|---|---|
| C3–C4 | Hot-dip galvanizing or zinc flake (8.8 / 10.9) |
| C5 | Zinc-flake (Geomet) high-build; A4-80 stainless for clamps |
| CX | Geomet high salt-spray spec; duplex / super duplex; A4 minimum |
For high-strength bolts, zinc-flake systems are usually preferred at C5/CX because they reach high salt-spray endurance without the hydrogen-embrittlement risk of acid processes — see Geomet / Dacromet coatings and the broader HDG vs zinc flake comparison. For stainless clamps and hardware, the category drives the move from 304 to 316/A4 or to duplex — see 304 vs 316 stainless.
§ 05 Specifying by site
To specify correctly: establish the site's corrosion category (from the project corrosion assessment), set the required durability/service life, and then call out a coating system and base material rated for that combination — including the salt-spray hours where relevant. In mixed-metal assemblies, also check galvanic compatibility, since a higher-category environment accelerates galvanic attack: see how to prevent galvanic corrosion.