what is the Outer casing of alkaline battery made of?
The cathode end is connected to the outer can of the battery (not the plastic casing but the metal directly under it), it's all one piece that is separated from the anode on the anode end. 

There is a metalized plastic film (PVC) over the can which has the battery markings printed on it. This is called the casing and no it is not conductive. If you scratch through the outer plastic casing then you will hit the cathode though.