Question

Why are magnesium blocks attached to iron water pipelines?

Hint:

Compare magnesium and iron in the reactivity series. The metal that loses electrons more readily becomes the sacrificial anode and keeps the other metal cathodic.

Answer 1

Step 1: Problem being solved.
Iron pipelines corrode (rust) when in contact with moist soil and oxygen because iron is oxidised: $Fe \rightarrow Fe^{2+} + 2e^-$.
Step 2: Role of magnesium.
Magnesium has a more negative standard electrode potential ($E^\circ_{Mg} = -2.37$ V) than iron ($E^\circ_{Fe} = -0.44$ V), making Mg much more easily oxidised. When Mg blocks are connected to the iron pipeline, they form a galvanic cell.
Step 3: Sacrificial anode mechanism.
Mg acts as the anode and is preferentially oxidised: $Mg \rightarrow Mg^{2+} + 2e^-$. Iron acts as the cathode and is protected from oxidation. The Mg blocks slowly corrode instead of the iron pipeline.
Step 4: Conclusion.
Magnesium blocks act as sacrificial anodes (cathodic protection), preferentially corroding to protect the iron pipeline from rusting.