Question

when Fe2+ions migrate from anode region they react with ........ ions. it is called rust

Answer 1

Rust occurs when oxygen (O2) reacts with (Fe2+) ions.

About 20 percent of air has of oxygen but rusting doesn't occur in dry air.  

It occurs in moist air and water.  

Rust needs three chemicals to form: iron, oxygen, and water.

Here oxygen gas is not only source of oxygen in air/water.  

Carbon dioxide (CO2) also contains oxygen where they both reacts to form weak carbonic acid.  

Carbonic acid is better electrolyte than water.  

When acid attacks the iron water breaks into H2O.  

Free oxygen with dissolved iron form iron oxide which can flow to other part of metal.  

Example of corrosion/rust.  

          Iron(Fe2+) + Water(H2O) + Oxygen(O2)  →  Hydrated iron(III) Oxide(Rust)