Final answer:
Among Ag, Au, Mg, Ni, and Zn, both magnesium (Mg) with a standard reduction potential of -2.37 V and zinc (Zn) with -0.7618 V can be used as sacrificial anodes for cathodic protection of steel, with Mg being the more effective choice due to its more negative potential.
Step-by-step explanation:
To determine which of the metals Ag, Au, Mg, Ni, and Zn could be used as a sacrificial anode for the cathodic protection of an underground steel storage tank, we need to consider their standard reduction potentials. A sacrificial anode must have a more negative standard reduction potential than steel to be effective in protecting it from corrosion. The standard reduction potential of steel is given as -0.447 V.
Magnesium (Mg) has a very negative standard reduction potential of -2.37 V, and zinc (Zn) has a standard reduction potential of -0.7618 V (according to the provided information and common electrochemical series). Among the given options, these two metals are more negative than steel and hence can act as sacrificial anodes.
Since Mg has a more negative standard reduction potential than Zn, it will be more effective as a sacrificial anode. However, the specific environment and conditions may favor the use of Zn in some cases.
Therefore, the correct answers would be:
c) Mg
d) Zn