There are some very basic principles you should know BY HEART before you start solving problems on corrosion.
1. Overall corrosion process can be divided at least into two reaction(at least one oxidation and at least one reduction). So find and write down at least one oxidation and one corrosion reaction before you start to solve.
2. Oxidation is anodic reaction and reduction is cathodic reaction by definition.
3. Overall oxidation and overall reduction both occur SIMULTANEOUSLY and at the SAME RATE (the consequence of charge conservation).
4. Whether or not oxidation and reduction are separated or occur at the same point of the surface does not affect the 3rd principle.
5. During corrosion more than one oxidation and one reduction reaction may occur (see the 1st principle)
6. Anodic and cathodic reaction occuring during corrosion are mutually dependent(see the 3rd principle). So it is possible to reduce corrosion by reducing the rate of either reaction.
...to be continued...