The type of corrosion cell that causes iron or steel to corrode
(and create rust) when a droplet of water is in contact with the
surface is Concentration cell.
The corrosion of iron or steel is an electrochemical process.
When droplet of water comes in contact with the surface , the
surface tends to become voltaic cell and starts to oxidise.
Concentration cell involving oxygen can initiate pitting through
generation of cathodic and anodic areas. Differential aeration
(oxygen concentration cell) and ion concentration (salt
concentration) cells create dissimilar polarities (anodic and
cathodic areas). The above question is an example for this .
