Why keep the foil constantly connected to the HF ground ... especially when youre not using it to transmit.

The dissimilarities of wetted metals and their electro-potenial is a prime cause of the corrosion. A simple knife switch with large contact surface that affects full capacitance or full disconnect should go a long way to reducing 'corrosion'. A knife switch is constantly 'self cleaning' by the 'wiping' action of the 'knife'.

Ditto on use silver loaded (enhanced conductivity) 'greases' will help with the 'mechanical connections' ..... Dynaplate sells the silver enhanced conductive greases to use on their grounding plates.

Copper corrodes primarily through the formation of verdigris (copper acetate - green) or copper oxide (brown) .... I use Boeshield (a technical anti-corrosion wax) to lessen the surface corrosion on the foil. Straight lemon juice or citric acid quickly removes verdigris ... plus a wee bit of rubbing.