A drop of water moving down a surface might act like the belt of a miniature Van de Graaf generator: accumulating or dropping electrons at one end and vice versa at the other end of the drop as it rolls down the surface. This rolling polarized "belt" would be broken into at least 2 pieces each with a net accumulation of opposite charges as they fall and hit the metal surface below. The metal below is struck first with a droplet with net + charge and a moment later one with a net - charge. Lots of very low-level electrical activity!

Van de Graaf generator:

https://en.wikipedia.org/wiki/Van_de_Graaff_generator#Descri...

This means that you could theoretically produce electricity directly from rainfall with no moving parts, other than water. But the low "amperage" and coordination of current renders this an unlikely method. But it would, over time, be very destructive to a surface.