It has been common practise to silver plate waveguide and cavity interiors.
The stated objective is to reduce transmission losses.
The resistivity of copper: 1.7 ohm-meter, silver 1.6 ohm-meter
(Your references may vary)
I assert that it has been found that this silver plate is not always
more effective than copper - and the reason may be associated with the
resistivities of the respective surface corrosion films on silver and copper.
A correspondent noted that in Navy practise, microwave components were
stripped and replated, which seems to support my assertion.
A second possibility: some metals electrodeposit electrolytically pure
metal: some metal alloys electrodeposit alloys.
I would like some comfirmation of this latter suggestion, and particularly
whether silver used for electroplating electrodes is at approximately the
'Sterling' level ( >92.5%), higher, or lower in purity.