[Mike Pearson]

Looks like the pump has not been sealed well to the engine or cover and water has bee getting between the pump housing and the gasket causing the corrosion issue. I run what ever water I can find from home or the track and drain my system every day at the track and never leave water in the system when the car is stored between races. I usually leave the radiator cap off when the radiator is empty. My water pump is at least 25 years old and still working. Maybe try coating the gasket with some better sealer when the engine is assembled as the corrosion seems to be in the gasket area and not in the are where the Impeller is located