In one line
Cathodic protection prevents corrosion by forcing the whole metal structure to act as the cathode, using either a more active sacrificial metal or an external DC current.
Explained simply
Corrosion happens only at the anode. So if we make the steel structure the cathode of a bigger cell, it cannot corrode. Either we attach a "bodyguard" metal that corrodes instead (sacrificial anode), or we push electrons into the structure from a power supply (impressed current).
How it works
1. Sacrificial anode method
- A more active metal (Mg, Zn or Al and their alloys) is connected by an insulated wire to the structure to be protected.
- The active metal becomes the anode and corrodes: e.g., Mg → Mg2+ + 2e−.
- Electrons flow to the iron structure, which becomes the cathode and is protected.
- Consumed anodes are replaced periodically.
Uses: ship hulls (Mg or Zn blocks), buried pipelines, underground cables, water tanks, marine structures. Mg is commonly used in soils; Zn and Al in sea water.
2. Impressed current method
- A DC source (battery or rectifier) is connected: negative terminal to the structure, positive terminal to an insoluble (inert) anode such as graphite, high-silicon iron, platinised titanium or scrap iron.
- The anode is buried in a backfill (coke, gypsum, bentonite) to improve electrical contact with the soil.
- The impressed current opposes the corrosion current and converts the structure into a cathode.
Uses: long pipelines, large underground tanks, transmission line towers, marine piers.
Key formula
No formula. Principle: protected structure = cathode, so the anodic reaction Fe → Fe2+ + 2e− is suppressed.
| Sacrificial anode | Impressed current |
|---|---|
| No external power | Needs DC power |
| Anode is consumed, replaced periodically | Inert anode, long life |
| Low driving voltage; small structures | Adjustable voltage; large structures, high-resistivity soils |
| Low installation cost, no interference | Higher installation and running cost; may disturb nearby structures |
Quick example
Zinc blocks bolted to the steel hull and near the propeller of a ship corrode away over time while the hull stays intact.
Common mistakes
- Connecting the structure to the positive terminal (that would make it the anode and speed corrosion).
- Choosing copper as a sacrificial anode; it is more noble than iron.
- Confusing cathodic protection with cathodic coating (tinning).
How MU asks it
"Explain cathodic protection by sacrificial anode and impressed current methods with diagrams" (5–6 marks). For full marks: principle, two labelled diagrams (structure, wire, anode/backfill, DC source with polarity), reactions, applications, and a short comparison.