Q:

Passive cathodic protection

Trying to understand what is happening when we need protect peace of Fe by placing on it more active metal (for example Zn) like anode. According to my understanding some of free electrons are transferred from Zn to more positive part - Fe. Fe becomes more electrical negative and Zn - more electrical positive. Please, correct me if I'm wrong.
Now, we have our system in liquid with Na+ and Cl- ions. Since Fe now has negative charge it don't want to meet negative Cl-. Result - no reaction. Please, correct me if I'm wrong once again. 
What is happening with Zn+ with leak of electrons? It is attacked by Cl- ions and we have more active chemical reaction. I suppose after some time we need to add more Zn plates to surface since it corroded?

A:

The phenomenon you are referring is also termed sacrificial protection. The more reactive metal (Zinc) releases electrons more readily than Iron and prevents Iron(II) ions from forming. The Zinc ions then react with oxygen and water and corrodes. Hence your guess is right: the Zinc blocks need to be replaced after use, but it is far cheaper than other methods.
The electrolyte solution ($NaCl$) you are referring to only speeds up rusting of iron by increasing its conductivity, such that electrons can flow quickly and complete the redox reactions faster.
According to my high school textbook, rusting occurs due to the formation of hydroxide ions, i.e. $$O_2 + 2H_2O + 4e^- \rightarrow 4OH^-$$ which reacts with iron ions $$Fe^{3+} + 3OH^- \rightarrow Fe(OH)_3$$ which appears as hydrated iron(III) oxide (rust).

