Sacrificial protection
From these observations it becomes clear that iron can be protected from corrosion by connecting to it a metal above iron in the activity series. The iron then acts as the cathode and is protected.
Sacrificial protection
Making iron the cathode of an electrochemical cell is called cathodic protection, or sacrificial protection.
Cathodic protection is used in these situations:
Galvanising
Coating iron articles with zinc: buckets, barbed wire, roofing sheets, GI pipes.
Blocks on ship hulls
Fixing blocks of magnesium and zinc to the bottom of ships that sail the sea. The magnesium and zinc blocks must be replaced from time to time.
Why it is called sacrificial
The protecting metal is given up so that the iron survives. Zinc on a bucket, or a magnesium block on a hull, is the anode of the cell: it is oxidised and slowly wears away, while the iron, as the cathode, only has oxygen and water reduced on its surface. That is why the blocks on a ship have to be renewed. Galvanising has a second advantage over tin plating. Zinc is above iron, so even where the coating is scratched through, the zinc around the scratch keeps corroding in place of the iron. Compare the scratched tin can on the protective-coatings page, where the iron was the one eaten away.
Galvanising is not tin plating
Both are coatings, but only zinc protects iron sacrificially. Tin protects only as a barrier, and a break in it makes rusting worse.
A more active metal is sacrificed
Joining iron to zinc or magnesium makes the iron the cathode. The zinc or magnesium corrodes instead — galvanising, and blocks on ship hulls.