Why Epoxy Coatings for Corrosion Protection?
Corrosion costs industry 3–5% of GDP annually. Epoxy coatings based on KER 828 are the most widely used and cost-effective method for protecting steel structures, pipelines and tanks, providing:
- Barrier protection against oxygen, water and chloride ions
- Adhesion to steel ≥ 8 MPa (at Sa 2.5)
- Chemical resistance to oils, fuels, dilute acids and alkalis
- Service life 10–25 years depending on environment
Corrosivity Categories (ISO 12944)
| Category | Environment | Examples | Recommended DFT |
|---|---|---|---|
| C2 | Low | Heated warehouses, offices | 80–120 µm |
| C3 | Medium | Urban atmosphere, production facilities | 120–160 µm |
| C4 | High | Chemical plants, coastal zones | 160–200 µm |
| C5 | Very High | Marine atmosphere, aggressive chemicals | 250–350 µm |
| Im1 | Water immersion | Submerged pipes, tank bottoms | 300–400 µm |
Surface Preparation — The Key to Durability
Up to 80% of premature coating failures are caused by inadequate surface preparation. Primary methods:
Abrasive Blast Cleaning
| Grade | Description | Application |
|---|---|---|
| Sa 1 | Light blast — loose rust removed | C2 only |
| Sa 2 | Thorough blast — ≥ 2/3 of contamination removed | C3 |
| Sa 2.5 | Near-white blast — grey-white appearance | C3–C5, Im1 |
| Sa 3 | White metal — bright metallic sheen | Im1, Im2, Im3 |
Roughness profile: Rz = 40–75 µm is optimal for epoxy coatings. Too smooth (<20 µm) reduces adhesion; too rough (>100 µm) requires extra primer to cover peaks.
Three-Layer Protective System
Layer 1: Zinc-Rich Epoxy Primer
Zinc acts as a sacrificial anode in the galvanic couple with iron, providing cathodic protection. Zinc content in dry film: ≥ 80% by weight. Typical DFT: 60–80 µm. Recoating interval at 20°C: 4–6 hours.
Layer 2: Epoxy Intermediate Coat
The main barrier layer. Formulation based on KER 828 + KH-816 + blanc fixe (50–80 phr). Typical DFT: 80–120 µm. Provides chemical resistance and mechanical strength. Recoating interval: 8–12 hours.
Layer 3: Topcoat
Polyurethane (recommended for UV resistance) or epoxy finish coat. DFT: 60–80 µm. Provides aesthetics, additional barrier and UV protection.
| Layer | Type | DFT, µm | Base |
|---|---|---|---|
| Primer | Epoxy zinc-rich | 60–80 | KER 828 + Zn dust |
| Intermediate | Epoxy | 80–120 | KER 828 + KH-816 + BaSO₄ |
| Topcoat | Polyurethane | 60–80 | Aliphatic PU |
| Total DFT | — | 200–280 | — |
Chemical Resistance of Epoxy Coatings
| Medium | Concentration | Resistance |
|---|---|---|
| H₂SO₄ (sulphuric acid) | up to 10% | Resistant |
| HCl (hydrochloric acid) | up to 10% | Resistant |
| NaOH (caustic soda) | up to 40% | Resistant |
| Fuels (petrol, diesel) | 100% | Resistant |
| Mineral oils | 100% | Resistant |
| Sea water | — | Resistant |
| HF (hydrofluoric acid) | any | Not resistant |
| Strong oxidisers (HNO₃ > 30%) | — | Not resistant |
Common Defects and Causes
Blistering
Cause: moisture trapped under the coating, insufficient primer drying, application at high humidity (> 85% RH). Solution: apply when RH < 80%, surface temperature ≥ 3°C above dew point.
Poor Adhesion (Delamination)
Cause: oil/grease contamination, insufficient surface cleanliness (Sa 1 instead of Sa 2.5), violated recoating window. Solution: strict surface preparation protocol and proper solvent degreasing.
Pinholes
Cause: excessive viscosity during application, insufficient mixing, cold surface. Solution: add TEGO AIREX 900 defoamer (0.2–0.3%), apply at 15–30°C surface temperature.
