Corrosion Protection Technology for CCS AH36 Shipbuilding Steel Plates in Marine Environments

1. Basic Properties and Corrosion Resistance of CCS AH36 Steel Plates

1.1 Standard Chemical Composition (GB/T 712-2022)

1.2 Characteristics of Corrosion Behaviour

In seawater, mainly non-uniform general corrosion occurs, with the corrosion rate initially increasing and then decreasing.

Corrosion products consist primarily of Fe oxides and hydroxides, which have a certain inhibitory effect on medium- to long-term corrosion.

The corrosion rate of domestically produced AH36 in conventional marine atmospheres and seawater splash zones is approximately 0.12–0.18 mm/year, representing a gap of 0.02–0.04 mm/year compared to similar products from Japan and South Korea.

Microbial corrosion (MIC) is prone to occur in environments containing sulphate-reducing bacteria (SRB), accelerating the localised corrosion process.

2. Corrosion Characteristics and Zoning in the Marine Environment

The marine environment can be divided vertically into five typical corrosion zones, with significant differences in corrosion rates and dominant factors between each zone:

3. Major Corrosion Types of CCS AH36 Steel Plates in Marine Environments

3.1 General corrosion

Comprehensive electrochemical corrosion, relatively easy to predict and control.

3.2 Pitting Corrosion

Localised corrosion caused by chloride ions damaging the passivation film; prone to causing perforation failure.

3.3 Crevice Corrosion

Concealed corrosion occurring in crevices such as welds and bolted joints.

3.4 Galvanic Corrosion

Occurs when in contact with dissimilar metals such as copper alloys and stainless steel.

3.5 Stress Corrosion Cracking (SCC)

Occurs under the combined action of tensile stress and chloride ions in seawater.

3.6 Corrosion Fatigue

A synergistic effect of cyclic stress and corrosion, commonly found in propeller shafts, decks and similar locations.

3.7 Microbial Corrosion (MIC)

Corrosion is accelerated by the metabolic products of marine microorganisms, such as sulphate-reducing bacteria.

4. Full Life Cycle Corrosion Protection System

The corrosion protection for CCS AH36 marine structures should employ a ‘triple protection system’: substrate protection + coating protection + cathodic protection. Through the synergistic action of these three elements, a design life of 15–20 years can be achieved.

4.1 Substrate Protection

Corrosion-resistant AH36 steel plates containing 0.02–0.05% copper should be selected, as these form a dense, stable rust layer on the surface.

For polar vessels, a modified AH36 grade with a nickel content of 0.30–0.50% may be selected to enhance both low-temperature toughness and corrosion resistance.

Strictly control the content of harmful impurities such as P and S in the steel plates to reduce pitting corrosion caused by intergranular corrosion and sulphide inclusions.

4.2 Coating Protection

Coatings are the primary means of corrosion protection; appropriate systems must be selected according to different corrosion zones:

4.2.1 General Surface Preparation Requirements

All surfaces to be painted must achieve Sa2.5 grade (ISO 8501-1), with a surface roughness of Rz 40–75 μm.

For critical areas such as below the waterline and ballast water tanks, Sa3 grade is recommended.

Primer application must take place within 4 hours of surface preparation to prevent secondary rusting.

4.2.2 Zone-Specific Coating Systems

a) Marine Atmosphere Zone (Superstructure, Decks)

System: Epoxy zinc-rich primer (80 μm, dry film zinc content ≥85%) + Epoxy micaceous iron oxide intermediate coat (120 μm) + Polysiloxane topcoat (40 μm)

Total dry film thickness: ≥240 μm

Design life: 15 years

b) Spray and waterline zones

System: Inorganic zinc silicate primer (80 μm) + glass flake epoxy intermediate coat (2 × 150 μm) + polyurethane topcoat (50 μm)

Total dry film thickness: ≥430 μm

Characteristics: Glass flakes provide excellent barrier protection against cyclic wetting and drying and mechanical abrasion.

c) Full seawater immersion zone (hull bottom)

System: Epoxy zinc-rich primer (80 μm) + Epoxy bitumen intermediate coat (2 × 120 μm) + Antifouling paint (2 × 60 μm)

Total dry film thickness: ≥440μm

Anti-fouling paint requirements: The use of organotin compounds is prohibited; copper-based or bio-based anti-fouling agents must be used.

d) Dedicated seawater ballast tanks

System: Pure epoxy primer (2×160μm)

Total dry film thickness: ≥320μm (CCS mandatory requirement)

Special requirements: Non-soap-forming paint, compatible with sacrificial anodes, resistant to wet-dry cycling, light colour for ease of inspection.

Design life: 15 years

4.3 Cathodic protection

Cathodic protection is used in conjunction with coatings to protect steel plates where the coating is damaged. It is divided into two methods: the sacrificial anode method and the impressed current method:

4.3.1 Sacrificial anode method (Most commonly used)

Anode material: Al-Zn-In-Cd alloy (preferred for seawater environments), current efficiency ≥85%.

Protection current density: 110–150 mA/m²

Protection potential: –850 mV to –1100 mV (relative to a copper/copper sulphate reference electrode).

Design life: 5–15 years, aligned with the ship’s dry-docking cycle.

Installation principles: Even distribution, avoidance of shielding effects, and good insulation between the anodes and the hull.

4.3.2 Impressed Current Method

Suitable for large vessels and offshore platforms, with a design life of over 20 years.

System components: Potentiostat, auxiliary anodes, reference electrode, cables

Advantages: Adjustable output current, wide protection range, low maintenance costs

Precautions: Over-protection devices must be installed to prevent hydrogen embrittlement

4.4 Structural Design Optimisation

Avoid sharp angles and dead corners; use rounded transitions to reduce stress concentration and liquid accumulation.

Ensure unobstructed drainage to prevent seawater stagnation and the formation of localised corrosion environments.

Insulating washers should be installed where dissimilar metals come into contact to prevent galvanic corrosion.

Sufficient space should be provided for inspection and maintenance to facilitate future testing and repairs.

4.5 Other Auxiliary Measures

Add corrosion inhibitors in enclosed spaces such as ballast water tanks to suppress microbial corrosion.

Perform post-weld stress relief on welded joints to prevent stress corrosion cracking.

Employ surface strengthening techniques such as shot peening and roller burnishing to introduce residual compressive stress into the surface.

5. Key Considerations for Corrosion Protection in Specific Areas

5.1 Ballast Water Tanks

Ballast water tanks are among the areas of a ship most prone to corrosion. Due to frequent alternation between wet and dry conditions and the prevalence of an oxygen-deficient environment, they are susceptible to microbiological corrosion.

Strictly comply with the IMO’s ‘Performance Standards for Protective Coatings in Ballast Tanks’ (PSPC).

A full recoating must be carried out when the coating damage rate exceeds 20%.

Sacrificial anode protection must be provided, with anodes positioned to cover all corners.

Regular internal inspections and thickness measurements must be conducted to identify potential corrosion hazards in a timely manner.

5.2 Spray Zone

The splash zone is the area subject to the most severe corrosion, where traditional coatings struggle to provide long-term effective protection.

Use a thick-film glass flake epoxy coating with a total thickness of ≥500 μm.

A composite protection system comprising thermal spray aluminium-zinc alloy plus sealing treatment may be employed.

For critical structures such as offshore platform legs, the use of fibreglass wrapping or corrosion-resistant alloys may be considered.

6. Latest Corrosion Protection Technologies

6.1 Smart Self-Healing Coatings

Containing microcapsules or nanocontainers, these coatings automatically release corrosion inhibitors when damaged, thereby repairing microcracks.

6.2 Nano-Reinforced Coatings

The addition of nanomaterials such as graphene and titanium dioxide significantly enhances the coating’s barrier properties and mechanical strength.

6.3 MOF-Based Corrosion Inhibitors

Metal-organic framework materials serve as carriers for corrosion inhibitors, enabling controlled release and improving corrosion inhibition efficiency.

6.4 Intelligent Cathodic Protection Monitoring System

Real-time monitoring of protection potential and current, with automatic adjustment of output to optimise protection effectiveness.

7. Corrosion Prevention, Maintenance and Inspection Requirements

Regular external hull inspections shall be carried out, with a comprehensive dry-dock overhaul every 5 years

Internal inspections of ballast water tanks shall be carried out every 2.5 years, with comprehensive thickness measurements conducted every 5 years

Damaged areas of the coating shall be repaired promptly; surface preparation and coating thickness in the repaired areas shall not be less than the original design requirements

Sacrificial anodes shall be replaced promptly when consumption exceeds 80%

A comprehensive anti-corrosion record shall be established, documenting coating application, cathodic protection system operation and corrosion inspection data

Summary

Corrosion protection for CCS AH36 shipbuilding steel plates in marine environments is a systematic engineering process requiring comprehensive control from material selection and structural design through to construction quality and subsequent maintenance. The adoption of a triple protection system comprising ‘substrate protection + coating protection + cathodic protection’, combined with zone-specific design and regular maintenance, can effectively extend the service life of CCS AH36 structures and ensure the safe operation of ships and offshore engineering equipment.