I am doing extensive work on the front of my engine, specifically the water pump, thermostat and housings etc. I noticed on the outside of the necks of the aluminum components, there is corrosion. On the inside of the components, there is generally no corrosion. The neck corrosion is not on the pressure side (fluid) of the clamp but rather on the air side (no pressure) of the clamp.
I use Zerex G05 coolant but don't know what was used previously. Why is there corrosion where there is no fluid?
Is there a compound that can be used to prevent this?