Cathode region in corrosion The anode region is on the iron where the following reaction occurs: 
$$\ce{Fe(s) -&gt; Fe^2+(aq) + 2e-}$$

and at the cathode region, the following reaction occurs
$$\ce{O2(g) + 4H+(aq) + 4e- -&gt; 2H2O(l)}$$

However, the textbook does not explicitly state what the cathode region is. Can someone provide an answer?
I would assume that the cathode region is a source of $\ce{O2(g)}$, which is probably the air in contact with the iron. Therefore the reacting should occur at the interface iron/air.