Because a tin plate is dense and resistant to prevent further oxidation, if you take a noncorroding material, such as brass and scratch away some of the surface exposing new material, then the oxide layer closes up immediately and helps protect the material. If iron is a contact with a more corrosion resistant metal such as tin, copper or lead, the other material can act as a large cathode that significantly increases the rate of reduction of oxygen.
It is because the oxidation of iron perpetuates the reduction of oxygen, which can result in a dramatic increase in the rate at which iron is oxidated of iron. Corrosion is a process by which metals deteriorate through oxidation. Out of all the metals vulnerable to corrosion, iron is by far the most important. A common cause of damage to metal objects is by poor storage, where objects are piled on top of each other, causing scratching.