Adding electric heat element to gas water heater

Then pull the element and see if you notice any corrosion on the threads. It none then screw the element back in and check again in another 3 months.

Unfortunately the corrosion will occur on the aluminum side not the brass side. Aluminum will form the anode. I'd expect corrosion to occur at the contact zone between the brass and aluminum.

If the tank has a sacrificial anode, which it probably does not in a propane only heater, that may corrode instead of the aluminum.