Raindrops carry electric charge that pierces coatings, corroding metal

August 31, 2026 at 10:11 PM2 min

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Water drops landing on surfaces carry an electrical charge large enough to punch through insulating coatings, a study from the Max Planck Institute for Polymer Research found. The charge, built up as drops slide across surfaces, can reach thousands of volts and corrode metal beneath Teflon films after about 3,000 drops. The finding reveals an overlooked mechanism of rain-induced corrosion beyond dissolved salts and abrasion.

Key Facts

Water drops sliding across insulating surfaces can acquire charges up to 9,000 volts.
In experiments, 35-microliter drops with a pinch of salt slid 4 cm down a 50-degree incline, then fell 5 mm onto a copper plate coated with a 60-nanometer Teflon film.
After 3,000 drops, roughly an afternoon of moderate rain, the copper beneath the Teflon coating had corroded, with pits deeper than the coating thickness.
The study was led by Zhongyuan Ni, Rüdiger Berger, and Hans-Jürgen Butt at the Max Planck Institute for Polymer Research in Mainz, Germany.

Slide Electrification

When a water drop slides across an insulating surface like a leaf, painted wall, or plastic panel, it strips charge from that surface and leaves an opposing charge behind. Drops charged this way have been measured at up to 9,000 volts. The phenomenon, called slide electrification, has only been properly quantified in the past few years.

Experimental Setup

Researchers released 35-microliter water drops containing a pinch of salt onto a surface tilted at 50 degrees. The drops slid about four centimeters, picked up a charge, rolled off the edge, and fell five millimeters onto a copper plate coated with a 60-nanometer Teflon film. The tilted surfaces included a leaf from a Tradescantia spathacea plant, a PVC foam board, a sheet of transparent polystyrene, and a fluorinated coating on quartz. Charges picked up by the drops ranged from 0.2 nanocoulombs off the leaf to two nanocoulombs off the fluorinated quartz.

Coating Damage

After 3,000 drops, roughly equivalent to an afternoon of moderate rain, the copper plate beneath all four surfaces had corroded despite its Teflon coating. Atomic force microscopy of the impact zones found pits several nanometers deep, in places deeper than the entire thickness of the Teflon film. The damage ran clean through the coating and into the metal.