Sacrificial corrosion of magnesium protects iron pipes

Steps:

• Identify the setup: Magnesium, more reactive than iron, is attached to form a galvanic cell where Mg acts as anode.
• Determine corrosion mechanism: Underground pipes corrode via oxidation of Fe to Fe²⁺ in moist soil; protection requires Mg to oxidize instead.
• Analyze choices: Select reaction showing Mg corroding preferentially in water, preventing Fe oxidation.
• Confirm protection: Mg's reaction supplies electrons to iron, making it cathode and halting its corrosion.

Why D is correct:

• Magnesium corrodes via Mg(s) + 2H₂O → Mg(OH)₂ + H₂, acting as sacrificial anode per galvanic series, where more active metal (Mg, E° = -2.37 V) protects less active iron (E° = -0.44 V).

Why the others are wrong:

• A: Shows iron reduction, which occurs at cathode but doesn't explain Mg's role in protection.
• B: Implies direct displacement, but underground corrosion involves water/oxygen, not just Fe²⁺(aq).
• C: Depicts Mg²⁺ reduction, opposite of Mg oxidation needed for sacrificial protection.

Final answer: D