The standard explanation for why rain causes corrosion is that water carries dissolved salts and acids to a surface, constant drumming of raindrops abrades whatever protective coating is on it, and oxygen does the rest. Nearly all our tools to prevent this—paints, polymer films, or oxide layers—are built around this idea. But we’ve apparently been missing something important about the rain.
A new study led by Zhongyuan Ni, Rüdiger Berger, and Hans-Jürgen Butt at the Max Planck Institute for Polymer Research in Mainz, Germany, has shown that water drops routinely arrive at a surface carrying an electrical charge large enough to punch through an insulating coating. Not scratch it. Not slowly dissolve it. Electrically blow a hole in it, the way a spark jumps a gap.
Charged rain
The starting point of the study is a phenomenon called “slide electrification,” which has only been properly quantified in the past few years. When a water drop slides across an insulating surface like a leaf, a painted wall, a windowpane, or a plastic panel, it strips charge from that surface and leaves an opposing charge behind. The voltages involved are not trivial. Drops charged this way have been measured at up to 9,000 volts.
The question Ni, Berger, Butt, and their colleagues asked was what this charge does to the surface that the drop lands on next. To find out, the team released 35-microliter water drops, about the size of a large raindrop, containing a pinch of salt to mimic rainwater, onto a surface tilted at 50 degrees. The drops slid about four centimeters, picking up a charge, rolled off the edge, and then fell five millimeters onto a copper plate coated with a 60-nanometer film of Teflon, which is one of the most chemically resistant coatings on the market today.
The tilted surfaces were mostly chosen to mimic raindrops in the real world. One was a leaf from a Tradescantia spathacea plant growing in one of the researchers’ offices. Another was a PVC foam board from a hardware store. The third one was a sheet of transparent polystyrene sold as window glazing. Only the fourth one, the fluorinated coating on quartz, was more of a lab creation than something people usually see everywhere around them. The charges the drops picked up ranged from 0.2 nanocoulombs off the leaf to two nanocoulombs off the fluorinated quartz. A nanocoulomb in something the size of a raindrop works out to be a few thousand volts.