Hull Preservation and Coating Systems on Ships

Maintaining the ship’s hull and applying the correct coating system are essential for:

• Preventing structural corrosion
• Reducing hydrodynamic resistance
• Improving fuel efficiency

(a) Methods for Preserving Ship’s Hull & Types of Antifouling Coatings

1. Methods of Hull Preservation

(i) Cathodic Protection

Cathodic protection prevents corrosion by making the hull act as a cathode.

• Sacrificial Anodes:
  • Made of zinc or aluminum
  • Fitted to areas such as the stern, rudder, and sea chests
  • These anodes corrode instead of the steel hull, thereby protecting it
• ICCP (Impressed Current Cathodic Protection):
  • Uses a DC power source with permanent anodes
  • Supplies a controlled current to counteract corrosive electrochemical reactions
  • More effective and adjustable compared to sacrificial anodes

  (ii) Protective Coating System

  A multi-layer coating system acts as a physical barrier between steel and seawater.

  • Primer / Anti-Corrosive (AC) Coats:
    • Usually epoxy-based
    • Provide the primary protection against corrosion by preventing contact with seawater
  • Intermediate / Tie Coats:
    • Ensure proper adhesion between layers
    • Act as a bonding layer between anti-corrosive and antifouling coats
  • Antifouling (AF) Coats:
    • Final outer layer
    • Contain biocides to prevent marine growth such as algae and barnacles

    2. Types of Antifouling (AF) Coatings

    • Controlled Depletion Polymer (CDP):
      • Traditional soluble matrix coating
      • Biocides leach out gradually
      • Coating layer remains but becomes ineffective (“exhausted”) over time
    • Self-Polishing Copolymer (SPC):
      • Reacts chemically with seawater
      • Outer layer dissolves gradually as the ship moves
      • Continuously exposes fresh biocide
      • Maintains a smooth hull surface
    • Foul Release Coatings:
      • Biocide-free, typically silicone-based
      • Create a very smooth and slippery surface
      • Prevent firm attachment of marine organisms
      • Any growth is easily washed away when the ship reaches sufficient speed

      (b) Banned Materials in Antifouling Coatings and Reasons

      Banned Substance:

      • Tributyltin (TBT) (banned under the IMO Antifouling Systems Convention)
      • Cybutryne (also known as Irgarol 1051) A biocide in anti-fouling paints to prevent the growth of algae and other marine organisms.

      Reasons for Ban:

      • Severe Environmental Toxicity:
        • Highly persistent in the marine environment
        • Does not degrade easily
      • Endocrine Disruption:
        • Causes “imposex” in marine organisms (e.g., female snails developing male characteristics)
        • Leads to reproductive failure and population decline
      • Bioaccumulation:
        • Enters the marine food chain
        • Accumulates in higher organisms, including fish consumed by humans

        (c) Difference Between Deck Coating and Superstructure Coating

        Although both coatings must resist corrosion and ultraviolet (UV) radiation, their functions and requirements differ.

        1. Deck Coating (Main / Weather Deck)

        • High Abrasion Resistance:
          • Subjected to heavy wear due to crew movement, dragging of wires, and equipment handling
          • Uses thick, hard-wearing modified epoxy coatings
        • Non-Slip Surface:
          • Essential for crew safety
          • Non-skid materials (e.g., sand or grit) are added to prevent slipping on wet or oily surfaces
        • Impact Resistance:
          • Must withstand mechanical impacts from tools and cargo operations

          2. Superstructure Coating

          • Aesthetic Appearance & Gloss Retention:
            • Represents the visible “face” of the ship
            • Typically uses polyurethane-based topcoats for a smooth, glossy finish
          • High UV Resistance:
            • Usually light-colored (often white)
            • Must resist chalking, fading, and yellowing due to constant sunlight exposure
          • Ease of Cleaning:
            • Smooth surface allows easy removal of soot, salt deposits, and dirt
            • Can be cleaned effectively with fresh water