CAUSES OF CORROSION IN SEA WATER PIPELINES AND PREVENTION

Causes of corrosion in sea water pipelines:

1. Galvanic (bimetallic) corrosion: Dissimilar metals in contact in sea water (e.g. steel pipe with bronze/ brass fittings, or copper-nickel with steel) cause the more anodic metal to corrode preferentially.
2. Pitting corrosion: Localised attack on the pipe wall, initiated by chloride ions, deposits, or biofouling, leading to perforation.
3. Erosion-corrosion (impingement): High-velocity sea water and turbulence (at bends, valves, reducers) erode the protective film and accelerate corrosion.
4. Crevice corrosion: Under deposits, gaskets, and at joints, where stagnant sea water creates a differential aeration cell.
5. Microbiologically influenced corrosion (MIC): Marine bacteria/ organisms create localised corrosion cells.
6. Dezincification (of brass/ bronze fittings): Selective removal of zinc, weakening the metal.
7. General/ uniform corrosion: On unprotected steel pipe, especially where the coating is damaged.
8. Stress corrosion cracking: Under tensile stress in a chloride environment.
9. Biofouling: Marine growth on the pipe surface creates localised corrosion and blocks flow.

Methods to prevent corrosion:

1. Material selection: Use corrosion-resistant materials (copper-nickel, bronze, or lined/ coated steel) appropriate for sea water service.
2. Cathodic protection: Fit sacrificial anodes (zinc/ aluminium) in the system, or use impressed-current cathodic protection.
3. Protective coatings/ linings: Apply epoxy, rubber, or other corrosion-resistant linings/ coatings to the pipe interior and exterior.
4. Velocity control: Design and operate within the recommended sea water velocity to avoid erosion-corrosion.
5. Filtration/ strainers: Fit sea water strainers to remove debris and reduce impingement and deposit formation.
6. Biofouling control: Use anti-fouling coatings, chlorination/ electrolytic anti-fouling, or periodic cleaning.
7. Corrosion inhibitors: Add inhibitors to the sea water/ cooling water where appropriate.
8. Regular cleaning and inspection: Clean the pipes, remove deposits, and inspect for pitting/ erosion; replace damaged sections.
9. Proper drainage and venting: Drain the system when idle to avoid stagnant sea water corrosion.
10. Sacrificial/ replaceable fittings: Use ferrules/ inserts at vulnerable points (bends, inlets) to protect against impingement.

Systems implemented for corrosion prevention:

• Sacrificial anode systems (zinc/ aluminium anodes) fitted in sea water boxes, coolers and pipelines.
• Impressed-current cathodic protection (ICCP) for the hull and sea water systems.
• Anti-fouling/ chlorination systems (electrolytic chlorination) to control biofouling.
• Corrosion-resistant linings (epoxy/ rubber) on sea water pipes and tanks.
• Material selection (Cu-Ni, bronze) for critical sea water components.
• Regular inspection, cleaning and maintenance programmes, including ultrasonic thickness measurement to monitor wastage.