Problems of different metals in a cavity resonator: Galvanic series
A problem building a cavity resonator is the potential of electrolytic corrosion between different metals, such as copper and aluminium. This can be overcome by keeping the cavities dry and using a sacrificial anode, such as zinc, already part of the design.
The Galvanic series for metals in sea water is shown below.
From the table mixing copper and aluminium, is potentially not a good idea. However...
A cavity usually resides in the same room as the repeaters that is usually dry, hence no electrolyte to enable galvanic corrosion. Putting the cavity outside is a different story.
Another sneaky trick is to use a sacrificial anode to protect the other metals. Zinc, in the form of galvanised steel can do this. An integral part of my cavity design is the use of galvanised threaded rod to hold everything in place. A secondary function is the galvanic protection of the device with the sacrificial zinc, if there does happen to be an electrolyte, such as water on the cavity.