Oikodomisis

Marine Corrosion & Coatings: The Battle Against Chlorides

Within 5km of the coast, metal faces more than just humidity; it endures a constant chemical barrage. C4 and C5-M (Marine) corrosion categories demand systems that go beyond simple paint, focusing on salt management and electrochemical shielding.

1. Why the Sea "Consumes" Metal? (Pitting & Osmosis)

Chlorides (Cl-) have the unique ability to disrupt the passive layer of steel, causing pitting corrosion - localized deep craters that compromise structural integrity.

Analysis of pitting corrosion on coastal steel structures.

🧂 Osmotic Blistering

Trapped salt under the film acts as a "pump," drawing moisture through the coating (osmotic drive). This creates acidic blisters that corrode the metal silently until the coating system detaches.

☀️ TOW (Time of Wetness)

In coastal areas, the TOW is higher due to morning mist. A coating must withstand continuous wet/dry cycles without losing its inter-coat adhesion.

2. Technical Preparation: The Bresle Test

Measuring salt contamination using Bresle Patches before priming.

C5 preparation is not just about sanding. Removing invisible contaminants is the most critical step.

🧪 Chloride Measurement

Professional projects utilize Bresle Patches (ISO 8502-6). If salt concentration exceeds 20mg/m², failure is imminent. High-pressure fresh water washing is mandatory before any mechanical cleaning.

📐 Surface Profile

Abrasive blasting to Sa 2½ with a profile of 40-70μm (Medium G) is required to ensure the mechanical "anchor" for heavy-duty epoxy sistemas.

3. C5-M Systems: The Rule of Three Layers

In marine environments, protection is multi-layered.

Coating sequence: Zinc Rich → MIO Barrier → PU Topcoat.

1️⃣ Prime: Zinc-Rich Epoxy

Provides cathodic protection. Zinc prevents rust from spreading under the film ("undercutting") if the surface is scratched.

2️⃣ Intermediate: MIO (Micaceous Iron Oxide)

Contains specialized flakes that align parallel to the metal, creating a "shingle effect" that blocks the pathway of water and oxygen.

3️⃣ Finish: Aliphatic Polyurethane

Protects the epoxy layers from UV-induced chalking and provides the final aesthetic finish with high gloss retention.

4. Duplex Systems: The Gold Standard

The Duplex System (Hot-Dip Galvanizing + Paint) is the ultimate solution for marine durability.

The paint shield prevents the salt from prematurely consuming the zinc layer. In C5-M/CX environments, the total Dry Film Thickness (DFT) should exceed 240-320μm to offer a maintenance-free life of over 15 years.

Synergy of Hot-Dip Galvanizing and Paint (Duplex).

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