Corrosion Resistance of CCS Grade A Shipbuilding Steel Plates in Marine Environments and Coating Solutions

1. Corrosion Resistance of CCSA Steel Plates in Marine Environments

1.1 Material Composition and Basic Corrosion Resistance

Chemical composition (mass fraction): C ≤ 0.21%, Si ≤ 0.50%, Mn ≥ 0.50%, P ≤ 0.035%, S ≤ 0.035%. This is a low-alloy steel; no corrosion-resistant elements such as Cu, Cr or Ni have been intentionally added.

Microstructure: Ferritic in the hot-rolled condition with a small amount of pearlite; no dual-phase strengthening or corrosion-resistant alloy layer.

Mechanical Properties: Yield strength ≥235 MPa, tensile strength 400–520 MPa, elongation ≥22%. No low-temperature impact requirements (suitable for use above 20°C); suitable for use in marine environments above 0°C.

1.2 Corrosion in Marine Environments

Fully submerged in seawater: Cl⁻ (approx. 3.5%) penetrates the passivation film, causing pitting and general corrosion at a rate of approximately 0.15–0.30 mm/a.

Waterline zone (alternating wet and dry): Subject to the combined effects of oxygen concentration gradients, salt spray and wave action, corrosion is most severe here, with rates reaching 0.3–0.5 mm/a.

Marine atmosphere zone: Salt spray, humidity and ultraviolet radiation cause electrochemical corrosion via a thin liquid film, with a rate of approximately 0.08–0.15 mm/year.

Ballast tanks: Due to alternating wet and dry conditions, a high-salinity environment and the action of sulphate-reducing bacteria (SRB), pitting and microbiological corrosion are likely to occur, with localised rates reaching over 0.5 mm/year.

1.3 Key influencing factors

Cl⁻ concentration: Destroys the passivation film and induces pitting corrosion.

Dissolved oxygen: Acts as a cathodic depolariser, accelerating corrosion.

Temperature: The corrosion rate approximately doubles for every 10°C increase.

Flow velocity/scouring: Destroys the corrosion product film, exacerbating localised corrosion.

Surface roughness/cleanliness: Sandblasting to Sa2.5 grade can significantly improve coating adhesion and corrosion resistance.

2. Marine Coating Solutions

2.1 Surface Preparation

Rust removal grade: Sa2.5, requiring that over 95% of the surface be free of visible scale and rust; areas subject to harsh conditions, such as ballast tanks, should achieve Sa3.

Surface roughness: 50–80 μm for epoxy primers; 80–120 μm for thick-film coatings (e.g. glass flake).

Cleanliness: Free of oil, salt and dust; painting must be completed within 4 hours to prevent re-rusting.

2.2 Zone-Specific Coating Systems

2.2.1 Hull Plating

Primer: Epoxy zinc-rich (Zn ≥ 85%), 80 μm (providing cathodic protection).

Intermediate coat: Epoxy micaceous iron oxide, 120 μm

Topcoat: Self-polishing anti-fouling paint, 80–100 μm, anti-fouling, service life ≥5 years.

Total film thickness: 280–300 μm, design life 10–15 years.

2.2.2 Waterline area

Primer: Inorganic zinc-rich, 60 μm.

Intermediate coat: Epoxy glass flake, 150 μm, high barrier, crack-resistant.

Topcoat: Fluorocarbon topcoat, 80 μm, salt spray resistant, UV resistant.

Total film thickness: 290 μm, with emphasis on preventing blistering and cracking.

2.2.3 Superstructure/Open deck

Primer: Epoxy zinc-rich, 60 μm

Intermediate coat: Epoxy micaceous iron oxide, 80 μm; glass flake option available for decks to enhance abrasion resistance.

Topcoat: Polysiloxane topcoat, 60–80 μm; additional 40 μm abrasion-resistant polyurethane topcoat for decks.

Features: UV resistance, salt spray resistance, easy to clean, resistance to mechanical wear.

2.2.4 Ballast Tanks

System: Solvent-free epoxy coating (100% solids, VOC=0), dry film thickness 320±20 μm.

Surface preparation: Sa2.5 grade, roughness 50–75 μm; single-coat application to minimise joints.

Performance requirements: Salt spray ≥2000 h, resistance to cathodic disbonding.

Complementary cathodic protection: Sacrificial anodes (Al/Zn alloy), protection current density 110–150 mA/m².

2.3 Core performance requirements for coating materials

Primer: Zinc content ≥85%, adhesion ≥5 MPa, salt spray ≥1500 h.

Intermediate coat: Mica iron/flake content ≥65%, chloride ion permeability ≤5×10⁻⁶ cm²/s, salt spray resistance ≥2000 h.

Topcoat: Weather resistance ≥2000 h (artificial ageing), salt spray resistance ≥3000 h, elongation at break ≥150% (waterline area ≥300%).

2.4 Key Points for Application and Acceptance

Environmental conditions: Temperature 5–35°C, humidity ≤85%, steel plate temperature ≥dewing point +3°C; avoid condensation.

Film thickness control: Multi-point measurement of dry film thickness; pass rate ≥90%, minimum value ≥80% of the design value.

Acceptance criteria: CCS on-site inspection, PSPC film thickness report, salt spray/adhesion test report.

3. Long-term Protection System (Coating + Cathodic Protection)

Underwater areas: Epoxy zinc-rich + epoxy micaceous iron oxide + antifouling paint + Al/Zn sacrificial anodes, with a protection potential of -0.85 to -1.10 V, effectively inhibiting corrosion in the cathodic zone and anodic dissolution.

Ballast Tanks: Solvent-free epoxy + sacrificial anodes, meeting IMO PSPC requirements, with a design life of over 15 years.

4. Summary and Recommendations

CCSA is a low-carbon steel of ordinary strength with poor resistance to seawater corrosion; its use in a bare state is strictly prohibited. In marine environments, a combined protection system comprising coating and cathodic protection must be employed.

Recommended strategy: Depending on the service area, priority should be given to a system comprising zinc-rich epoxy primer + epoxy micaceous iron oxide/glass flake intermediate coat + specialised topcoat.

Key control points: Strictly enforce rust removal to Sa2.5 grade or higher + appropriate surface roughness control, and ensure compatibility with sacrificial anode protection.

Expected service life: Under the aforementioned system, long-term reliable service of 10–15 years can be achieved.