Explain the sacrificial anode cathodic protection method with a neat and well-labelled diagram. (asked in MU Dec 2025)

Answer: The metal to be protected is connected to a more active metal (Mg, Zn or Al), which becomes the anode and corrodes instead of it.

Principle: the structure is made the cathode of a galvanic cell. A more active metal (higher in the galvanic series), connected to it by an insulated wire, acts as the anode and is consumed ("sacrificed"); the structure does not corrode.

Diagram (described): a buried steel pipeline runs horizontally under the soil surface. A short distance away, a magnesium block is buried in the soil, surrounded by a backfill of gypsum and bentonite (to keep it moist and conducting). An insulated copper wire from the Mg block is joined to the pipeline. Arrows show electrons flowing through the wire from Mg to the pipe; labels: "Mg anode (corrodes)", "steel pipe (cathode, protected)", "soil (electrolyte)", "backfill".

Reactions: anode: \(\text{Mg}\to\text{Mg}^{2+}+2e^-\); at the steel cathode, oxygen reduction (\(\text{O}_2+2\text{H}_2\text{O}+4e^-\to4\text{OH}^-\)) takes place, so iron is not oxidised.

Applications: buried pipelines and cables, ship hulls, water tanks, offshore platforms. The anodes are replaced when consumed.