Question

What are the oxidation and reduction equations for when zinc is acting as a sacrificial anode for iron?

Answer 1

the oxidation of iron `Fe=Fe^(2+)+ 2 e`, E= -0,44 V, may be prevented if you supply electrons to this reaction.

To supply electrons you can use a metal that has a lower potential compared to iron and where the reaction is only little thermodinamically favourite, but not cinetically faster.
The Zinc is the best of these metals and is used in the pipes of drinking water . It could give `Zn=Zn^(2+)+ 2 e`, E= -0,76 V,.
So Zn is anode and it oxides itself while iron becomes catode and dosen't oxide itself also if a part of iron is not covered.
Also Mg can act as a sacrifical anode for iron but its oxidation is more faster and it must be replaced most frequently. Mg can be used on the ships