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: Concept
This is an application of cathodic protection (sacrificial anode method) to prevent corrosion.

Step 2: Analysis
Iron pipelines in contact with water and electrolytes (soil moisture) are prone to corrosion (rusting). Magnesium is more electropositive (more reactive) than iron, having a more negative standard electrode potential ($E^\circ_{Mg} = -2.37$ V vs $E^\circ_{Fe} = -0.44$ V).

Step 3: Conclusion
When magnesium blocks are attached, Mg acts as the sacrificial anode and preferentially oxidizes ($Mg \rightarrow Mg^{2+} + 2e^-$). The iron pipeline becomes the cathode and is protected from oxidation. The Mg blocks corrode slowly instead of the iron. This is called the sacrificial anode method of corrosion prevention.

Final Answer: Magnesium (more electropositive) acts as a sacrificial anode, preferentially corroding instead of iron. This protects the iron pipeline -- a method called cathodic protection.

Answer 2

Concept:

• When two connected metals are exposed to an electrolyte, the more reactive metal becomes the anode and loses electrons.
• Supplying electrons to iron prevents iron atoms from undergoing oxidation.

Step 1: Compare magnesium and iron in the reactivity series.
Magnesium is more reactive than iron, so it has a stronger tendency to lose electrons.

Step 2: Form the protective electrochemical pair.
When the magnesium block is electrically connected to the iron pipe in moist soil or water, magnesium becomes the anode.
$Mg\rightarrow Mg^{2+}+2e^-$

Step 3: Explain the protection of iron.
The released electrons flow to the iron pipeline and keep it cathodic. Iron therefore does not lose electrons readily and rust formation is suppressed.

Step 4: State what must be maintained.
The magnesium slowly gets consumed in place of the pipe and must be replaced when necessary. This protection is called the sacrificial-anode method.

Final Answer: Magnesium blocks act as sacrificial anodes and corrode instead of the iron pipeline.