With reference to hull protection against corrosion, describe how sacrificial anodes and impressed current systems each operate?
Short answer
| Feature | Sacrificial Anodes | Impressed Current |
|---|---|---|
| Power source | Galvanic (self-powered) | External DC (rectifier) |
| Anode material | magnesium, zinc, aluminium | platinised titanium |
| Control | Passive | Active (reference electrodes + regulator) |
| Maintenance | Renew wasted anodes | Inspect anodes, reference cells, wiring |
| Risk | Poor bonding → no protection | Over-protection blisters paint; under-protection leaves steel unprotected |
- Sacrificial anodes: magnesium, zinc or aluminium blocks bolted to hull, more anodic than steel, corrode preferentially, protect shell plating, rudder, propeller boss, thruster tunnels, bilge strakes, sea inlets, sea-cocks, ballast tanks
- Sacrificial anodes check: heavy wastage = working, unwasted = poor bonding
- Impressed current system: DC current from rectifier to platinised titanium anodes, reference electrodes monitor hull-to-sea potential, regulator adjusts output
- Impressed current risks: too high current = alkali blisters paint, too low = unprotected steel
- Impressed current maintenance: inspect anodes, reference cells, dielectric backing shields, wiring in dry dock
- Governing instrument: MSC.47(66), Annex 2, Part A-1, Regulation 3-1; IMO Resolution A.798(19), Annex 1, §2.3
- Officer’s duty: monitor anodes and ICCP, report excessive wastage, poor bonding, abnormal readings, maintain protective potential
Full answer
The full answer explains each point step by step, with the regulations and figures behind it and the discussion from other candidates. It is in the MEO Class IV section of the Marine One forum, open to members enrolled for the MEO Class IV exam.