Explain the sacrificial anode cathodic protection method with a neat and well-labelled diagram.

Cathodic protection: the metal to be protected is forced to act as the cathode of an electrochemical cell, so it does not corrode. Corrosion is shifted to an auxiliary anode.

1. Sacrificial anode method: the structure is joined by an insulated wire to a more active metal (Mg, Zn or Al), which is higher in the galvanic series. The active metal becomes the anode and corrodes ("is sacrificed"), while the structure stays cathodic. Spent anodes are replaced from time to time.

      ground level
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   iron pipeline (cathode)        insulated Cu wire
   ===========================------------+
                                           |
                                      [ Mg block ]  sacrificial anode
                                      in backfill (gypsum + bentonite)
   Mg -> Mg2+ + 2e-  (anode corrodes)
   e- flow to pipe: O2 + 2H2O + 4e- -> 4OH- (on pipe)

Examples: Mg blocks on buried pipelines and underground tanks; Zn plates on ship hulls; Mg rods in domestic water heaters.

Principle: Mg (E° = −2.37 V) is more active than Fe (E° = −0.44 V), so Mg is oxidised in preference and electrons flow to the iron, keeping it cathodic.