Why are magnesium blocks attached to iron water pipelines?
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Solution and Explanation
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.
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