Consider the following metals: Ag, Au, Mg, Ni, and Zn.

Which of these metals could be used as a sacrificial anode in the cathodic protection of an underground steel storage tank?

Steel is mostly iron, so use - 0.447 V as the standard reduction potential for steel.

Mg and Zn

Explanation:

In cathodic protection, the sacrificial anode corrodes instead of the cathode which it protects. The anode is usually higher than the cathode in the electrochemical series. This also means that the reduction potential of the sacrificial anode is more negative than that of the cathode. Consider the reduction potentials of the metals listed in the question:

Mg=-1.185V

Zn = - 0.7618V

Ag = + 0.7996V

Au = + 1.629V

Ni = - 0.251V

The reduction potential of the cathode stated in the question is - 0.447V hence only magnesium and zinc can function as sacrificial anode.