The corrosion classes C1 to C5 (and subcategories C5-M/C5-I) are defined in ISO 12944-2, a standard for protective coating systems against corrosion. These classes categorize environments based on their corrosivity to steel, guiding material selection and coating requirements. Here's a detailed breakdown:
Corrosion Class Overview
| Class | Corrosivity | Environment Examples | Key Pollutants/Conditions | Typical Applications |
|---|---|---|---|---|
| C1 | Very Low | Heated buildings (offices, schools, homes) with low humidity and pollution. | Clean air, minimal moisture, no aggressive agents. | Interior steel structures, non-industrial settings. |
| C2 | Low | Rural areas, low-pollution urban zones, or unheated buildings. | Low levels of sulfur dioxide (SO₂), occasional condensation. | Warehouses, sports halls, parking garages. |
| C3 | Medium | Urban/industrial areas, coastal regions (moderate salt exposure), high humidity. | Moderate SO₂, chlorides (salt), or pollution; frequent condensation. | Factories, chemical plants, bridges, coastal buildings. |
| C4 | High | Marine environments, industrial zones with heavy pollution, chemical plants. | High chlorides (salt), sulfur compounds, humidity, and frequent condensation. | Offshore platforms, ports, swimming pools, power stations. |
| C5-M | Very High (Marine) | Offshore areas, coastal regions with salt spray, tropical climates. | Aggressive chlorides, high humidity, UV exposure, and constant salt exposure. | Ship hulls, offshore structures, coastal piers. |
| C5-I | Very High (Industrial) | Chemical plants, mines, pulp mills with harsh pollutants. | Acidic gases, high humidity, chemical fumes, and extreme temperature fluctuations. | Chemical processing equipment, mining infrastructure. |
Key Details
- Corrosion Rate:
- The classes correlate with the expected steel corrosion rate (μm/year of thickness loss):
- C1: < 2.5 μm/year
- C2: 2.5–5 μm/year
- C3: 5–15 μm/year
- C4: 15–30 μm/year
- C5-M/C5-I: 30–60 μm/year
- Note: Actual rates may vary based on specific conditions.
- The classes correlate with the expected steel corrosion rate (μm/year of thickness loss):
- Coating Requirements:
- C1/C2: Basic coatings (e.g., epoxy primers) for low-risk environments.
- C3/C4: Multi-layer systems (e.g., zinc-rich primer + epoxy + polyurethane topcoat).
- C5-M/C5-I: High-performance coatings (e.g., glass flake, fluoropolymers) with thick dry film layers.
- Testing & Standards:
- Coatings for C3–C5 environments often undergo accelerated tests like salt spray (e.g., ASTM B117) or cyclic corrosion tests (e.g., Prohesion®) to simulate long-term exposure.
Why It Matters
- Material Selection: Helps engineers choose coatings, metals, or alloys (e.g., stainless steel for C4/C5) based on environmental severity.
- Cost vs. Performance: Higher classes require more durable (and expensive) coatings but extend asset lifespans.
- Compliance: Critical for industries like construction, marine, and chemical processing to meet regulatory and safety standards.
Example Application
- A C5-M coating system for an offshore oil rig would include:
- Abrasive blasting (Sa 2½ surface profile).
- Zinc-rich epoxy primer (anti-corrosive).
- Glass flake epoxy intermediate layer.
- Fluoropolymer topcoat (UV/chemical resistance).
This classification ensures structures withstand their operational environment, minimizing maintenance and failure risks.
