Generally, we do not look at the gentle patter of raindrops on a surface with much concern, but according to a study by [Zhongyuan Ni] et al. in Nature, we should probably regard these droplets with a little scrutiny for their corrosion potential. What they found is that these drops can gather a significant electric potential as they gently slide down a surface, with over 1 kV measured. By first having droplets charge up on an insulating surface before hitting a target metal surface, they were able to induce significant corrosion.
Despite the target metal surface being coated with a protective layer, these charged droplets managed to gradually break down the coating, exposing the bare metal. In this example, a Teflon coating was used, with water droplets containing a small amount of dissolved sodium chloride to simulate natural raindrops.
It was postulated that this causes dielectric breakdown of the insulating protective coating, as the charged water drop acts as one electrode and the — often grounded — metal surface as another electrode. Subsequent investigations on the samples showed that this appears to be indeed the case.
A potential defense here would be to discharge any water before it can reach sensitive surfaces, but it’s not a straightforward problem to solve. As noted in the study’s conclusion, charged droplets can also be generated in clouds and waves, in addition to the insulating materials demonstrated in the study. It’s also a phenomenon that can cause issues anywhere charged droplets occur, such as in a wide range of industrial processes.
If you’re interested in the electrical generating properties of falling water, check out Lord Kelvin’s Generator! Hackaday’s own [Steven Dufresne] explored this phenomenon a few years back.