Which metal can be used as a sacrificial anode to protect an underground iron pipeline from corrosion?

A sacrificial anode must be more easily oxidised than iron, i.e. have a more negative reduction potential than $E^{\circ }(\mathrm{Fe}{}^{2+}/\mathrm{Fe})=-0.44\mathrm{V}$. Magnesium ($E^{\circ }=-2.37\mathrm{V}$) satisfies this and corrodes in place of iron, which becomes the cathode. Copper, tin and nickel have less negative potentials and would make iron the anode, speeding up its corrosion.