Question

Why are magnesium blocks fixed to the iron pipelines carrying water?

Hint:

This same principle is used in galvanization, where iron is coated with zinc to prevent rust.

Answer 1

Concept:

• Corrosion (rusting) of iron is an electrochemical process where iron is oxidized.

• This can be prevented by a technique called Cathodic Protection.

• A more reactive metal is used to protect a less reactive metal.
Step 1: Compare reactivity
Magnesium is much more electropositive (reactive) than iron. In the electrochemical series, Magnesium has a much lower standard reduction potential than Iron.

Step 2: Identify the sacrificial anode
When Magnesium is connected to the Iron pipeline, it acts as the anode in the local electrochemical cell. Magnesium undergoes oxidation preferentially over Iron. \[ Mg(s) \rightarrow Mg^{2+}(aq) + 2e^{-} \]

Step 3: Effect on Iron
The electrons released by the Magnesium flow to the Iron pipeline, making it the cathode. Since reduction happens at the cathode, the Iron is protected from being oxidized (rusting).

Step 4: Conclusion
Magnesium is called a sacrificial anode because it is "sacrificed" (corroded) to save the iron pipeline. These blocks must be replaced periodically. The final answer is for sacrificial protection of iron against corrosion.