Zinc is used to protect corrosion of iron because 

Zinc is used to protect iron from corrosion because it is more active metal than iron and it has more tendency to lose electrons with respect to iron. In other words, zinc has a negative reduction potential higher than that of iron.
$$\begin{gathered} E_{Z{n}^{2+}/Zn}^o=-0.76V \\ {E}_{F{e}^{2+}/Fe}^o=-0.44V \end{gathered}$$