Quick answer: In cathodic protection, a more reactive metal such as zinc or magnesium is connected to iron. Because it is higher in the reactivity series, it loses electrons (gets oxidised) instead of iron, so it corrodes and the iron is protected. The corroding metal is called a sacrificial anode. This is also why galvanised iron does not rust even when its zinc coating is scratched.
Steel pipelines buried in moist soil and steel ships sailing through salty seawater are constantly exposed to oxygen and water, the two things iron needs to rust. Without protection, they would corrode badly over time.
One clever solution is to attach blocks of zinc or magnesium to the steel ship hull, or bury them beside an underground pipe and connect them with a wire. Over time the zinc or magnesium blocks slowly corrode away, while the steel stays protected. This method is called cathodic protection using sacrificial anodes.
In CBSE Class 10 Science, Chapter 3 (Metals and Non-metals), prevention of corrosion, galvanisation and why galvanised iron does not rust even when scratched are frequently asked board exam questions.
What You Will Learn
- What cathodic protection is and how sacrificial anodes work
- Why zinc and magnesium corrode before iron (the reactivity series)
- Where sacrificial anodes are used: ship hulls and underground pipelines
- What galvanisation is and the two ways zinc protects iron
- Why galvanised iron does not rust even after the zinc coating is scratched
- Zinc coating vs tin plating: why a scratch on a tin-plated can makes iron rust faster
- Other methods of preventing rusting: painting, oiling, greasing and alloying
1. Rusting of Iron: A Quick Recap
Iron rusts when it is exposed to both oxygen () and water (). Rust is hydrated iron(III) oxide:
In this reaction, iron loses electrons (it is oxidised), and oxygen gains electrons (it is reduced).
Key idea: There are two ways to stop iron from rusting: (1) block oxygen and water from reaching the iron (barrier protection), or (2) connect a more reactive metal that gets oxidised instead of iron (sacrificial or cathodic protection).
2. How Sacrificial Anodes Work (Cathodic Protection)
Part of the reactivity series:
Magnesium and zinc are above iron in the reactivity series. This means they lose electrons more easily than iron.
| Part | Metal | What happens |
|---|---|---|
| Sacrificial anode | Zinc or magnesium (more reactive) | Is oxidised and slowly corrodes: |
| Connection | Metal wire or direct contact | Carries electrons from the zinc to the iron |
| Protected structure (cathode) | Iron pipe or steel hull | Receives electrons, so it does not rust |
How It Works
Step 1: A block of zinc or magnesium is connected to the iron structure, and both are in contact with moist soil or seawater.
Step 2: Zinc is more reactive than iron, so zinc is oxidised first and releases electrons:
Step 3: These electrons flow to the iron, which becomes the cathode.
Step 4: Because the iron keeps receiving electrons, it is not oxidised and does not rust. The zinc block is slowly used up instead, which is why it is called a sacrificial anode.
Step 5: When the block is used up, it is simply replaced with a new one, without having to dig up the pipeline or dry-dock the ship.
3. Galvanisation: Two Kinds of Protection
Galvanisation is a method of protecting iron and steel from rusting by coating them with a thin layer of zinc, usually by dipping them in molten zinc.
Galvanised iron is protected in two ways:
1. Barrier protection: The zinc layer keeps oxygen and moisture away from the iron. Zinc also reacts with air to form a thin protective layer of basic zinc carbonate on its surface.
2. Sacrificial protection: If the zinc coating is scratched and the iron underneath is exposed, the zinc still protects the iron, because zinc is oxidised first.
Common board question: Why does galvanised iron not rust even after the zinc coating is broken?
Answer: Zinc is more reactive than iron. So at the scratch, where both metals are exposed to moist air, zinc is oxidised in preference to iron. Zinc acts as a sacrificial metal, and the iron does not rust as long as zinc is present.
4. Zinc Coating vs Tin Plating
Food cans are made of iron coated with tin rather than zinc, because tin is less reactive than zinc and does not react with the acids in food. However, tin protects iron very differently from zinc.
| Feature | Zinc coating (galvanisation) | Tin plating |
|---|---|---|
| Reactivity compared to iron | Zinc is more reactive than iron | Tin is less reactive than iron |
| Coating undamaged | Protects iron as a barrier | Protects iron as a barrier |
| Coating scratched | Zinc corrodes first; iron stays protected | Iron corrodes first; iron rusts faster than usual |
Remember: When a tin-plated can is scratched, iron is more reactive than tin, so the iron is oxidised instead of the tin. The exposed iron rusts quickly, which is why a scratched or dented tin can rusts faster.
5. Other Methods of Preventing Rusting
| Method | How it protects | Examples |
|---|---|---|
| Painting | Forms a barrier against air and moisture | Car bodies, bridges, window grills |
| Oiling / greasing | Forms a water-repelling barrier | Bicycle chains, machine parts, tools |
| Galvanisation | Barrier + sacrificial protection by zinc | Iron pipes, roofing sheets |
| Sacrificial anodes | More reactive metal (Mg or Zn) corrodes instead of iron | Ship hulls, underground pipelines |
| Alloying (stainless steel) | Iron mixed with chromium and nickel forms a protective oxide layer | Cutlery, cookware, surgical instruments |
Exam Tip: If asked why stainless steel does not rust, write that it is an alloy of iron with chromium and nickel. The chromium forms a thin, protective layer of chromium oxide on the surface that keeps oxygen and moisture away from the iron.
Common Mistake: Thinking that a sacrificial anode must cover the whole surface like paint. A sacrificial anode is a separate block of zinc or magnesium that protects the steel because it is electrically connected to it.
Frequently Asked Questions
What is a sacrificial anode?
A sacrificial anode is a block of a more reactive metal, such as zinc or magnesium, connected to iron or steel. It corrodes instead of the iron, so the iron is protected.
What is cathodic protection?
Cathodic protection is a method of preventing corrosion by making the iron structure the cathode, using a more reactive metal that gets oxidised in place of iron.
Why does galvanised iron not rust even when scratched?
Because zinc is more reactive than iron, it is oxidised first at the scratch, so the exposed iron is protected.
Why are food cans coated with tin and not zinc?
Tin is less reactive than zinc and does not react with acids in food, so it is safer for storing food.
Why does a scratched tin-plated can rust faster?
Iron is more reactive than tin, so once the tin layer is scratched, the exposed iron is oxidised and rusts quickly.
Which metals are used as sacrificial anodes?
Zinc and magnesium are commonly used, because both are more reactive than iron.