TL;DR: Rain can arrive electrically charged and punch through protective coatings by dielectric breakdown, letting salty water corrode the metal underneath, according to a Max Planck-led study published in Nature on Aug. 26, 2026 and reported by Ars Technica on Aug. 31, 2026.

Rain does not just wet cars and buildings. It can zap them. A team led by Zhongyuan Ni, Rüdiger Berger and Hans-Jürgen Butt at the Max Planck Institute for Polymer Research in Mainz, Germany, found that water drops pick up enough charge sliding across everyday surfaces to electrically puncture thin insulating films and start corrosion, even on copper shielded by Teflon.

The result reframes a familiar maintenance headache. Paint on a garden fence, a car body, a ship hull or landmarks such as the Eiffel Tower and the Golden Gate Bridge has long been assumed to fail mainly by abrasion and by acids and salts in water, as summarized by the University of Bonn. The new work adds a third pathway: spontaneous charging followed by a microscopic discharge, described by the researchers as acting like a small flash of lightning.

Why sliding makes water electric

Friction without solids

The starting mechanism is called slide electrification or contact electrification. When a drop moves over an insulating solid, it takes one polarity of charge and leaves the opposite polarity on the solid, according to Nature.

That behavior has been documented on plant leaves, insect wings, building walls, window glass and plastics, according to Nature. Common outdoor encounters — a drop running off a leaf, a window, a painted wall or a plastic panel — are therefore enough to electrify the liquid before it strikes metal elsewhere.

The voltages are surprisingly large. Drops charged by sliding have been measured at up to 9,000 volts, according to Ars Technica, while the Nature abstract reports potentials that can exceed 1 kilovolt and reach several thousand volts depending on materials and insulating-layer thickness.

Study lead Hans-Jürgen Butt told Scientific American the finding also touches a basic physics puzzle: why water moves so slowly down smooth surfaces despite low friction. The group suspects hidden chemistry at the sliding interface. Solids are known to charge by rubbing, but liquids were not expected to behave the same way. As Butt put it to Scientific American:

“For a liquid, there is no force which could break a bond”

That expectation made the scale of liquid charging especially striking.

How the team tested rainy impacts

35 microliters, 3,000 hits, one copper target

The Mainz group built a two-stage rain simulation. First a drop picked up charge by sliding. Then it fell onto a coated metal target, according to Ars Technica and Nature.

The drops were 35 microliters, about the size of a large raindrop, and contained salt to mimic natural rain, according to Ars Technica. Nature specifies the liquid as water with 1 mM sodium chloride. The release interval in reference trials was 12 seconds from a height of 4 centimeters, according to Nature.

The target was copper protected by a 60-nanometer Teflon film. Nature details the fabrication: a 35-nanometer copper layer sputtered onto smooth quartz, then a 60-nanometer Teflon film dip-coated from a 1-weight-percent Teflon AF 1600 solution and annealed at 160 degrees Celsius under vacuum for 24 hours. Teflon was chosen because it is chemically robust, making it a stringent test of a non-chemical failure mode.

For charging, the team tilted the first surface at 50 degrees, let drops slide about four centimeters, roll off the edge and fall five millimeters onto the copper plate, according to Ars Technica. The reference samples were tilted by 10 degrees so drops ran off before the next impact, according to Nature. Surface checks used reflection-mode confocal microscopy and atomic force microscopy.

Leaves, hardware-store board and lab quartz

To keep the test close to real weather, three of the four charging surfaces were ordinary materials. One was a leaf from a Tradescantia spathacea plant growing in a researcher office. Another was PVC foam board from a hardware store. A third was transparent polystyrene sheet sold as window glazing, according to Ars Technica. Nature describes the set more generally as plant leaves, commercial walls, commercial glass and fluorinated quartz.

Only the fourth charging surface, a fluorinated coating on quartz, was primarily a laboratory creation rather than an everyday item, according to Ars Technica. The charge picked up ranged from 0.2 nanocoulombs after the leaf to two nanocoulombs after the fluorinated quartz, according to Ars Technica. At raindrop scale, one nanocoulomb corresponds to a few thousand volts.

After 3,000 charged impacts — roughly an afternoon of moderate rain, according to Ars Technica — the copper beneath the Teflon had corroded for all four charging surfaces. Neutral drops that fell directly onto the target without first sliding left the surface flat and smooth after the same 3,000 hits, with no notable change in morphology, according to Nature and Ars Technica. That control is central: same water, same salt, same number of hits, different electrical history.

The team also examined other protective coatings beyond Teflon and saw the same pattern of damage from charged drops, according to Scientific American.

The micro-lightning strike

A Taylor cone before contact

High-speed video revealed why charge matters. A neutral drop kept a smooth, round underside until it touched the plate. A charged drop stretched downward into a sharp Taylor cone as it approached, a shape liquids assume when electrostatic force overwhelms surface tension, according to Ars Technica.

The cone signaled a rapidly intensifying electric field in the shrinking air gap. The researchers modeled the drop as a conducting sphere above a conducting wall and calculated field strength versus distance, according to Ars Technica. Scientific American reports the same interpretation: the positively charged water and negatively charged metal form a growing field as separation narrows.

The numbers explain the puncture. A drop carrying two nanocoulombs was calculated to reach 60 kilovolts per millimeter, the breakdown threshold of Teflon, while still about 10 micrometers above the surface, according to Ars Technica. A polystyrene coating, which fails at 19 kilovolts per millimeter, would break down about 50 micrometers out.

At that point the insulator briefly stops insulating. Charge rips through in dielectric breakdown, the same class of failure that destroys capacitors and transformer insulation, according to Ars Technica. Measured transfer was almost complete: a drop arriving with two nanocoulombs deposited 1.8 nanocoulombs into the copper and rebounded with 0.016 nanocoulombs, under one percent of its initial charge.

Thin versus thick films

Thickness sets the limit. Twelve-micrometer polystyrene films were punched through, while 130-micrometer films survived, according to Ars Technica. Since many paint coatings are only a few micrometers thick, the authors note that nanocoulomb-scale drop charges can break through most of them.

The result does not mean rain threatens people. Charged drops are far below hazardous levels for humans, and attempts to harvest rain electricity at large scale have not succeeded, Butt told Scientific American.

What happens after the coating breaks

Salt, oxygen and fresh metal meet

A pinhole is enough. Once the film is breached, the exposed metal sits inside a salty drop with a fresh electrical potential, allowing ordinary electrochemical corrosion to attack a spot that was supposed to stay sealed, according to Ars Technica.

Atomic force microscopy found pits several nanometers deep in impact zones — in places deeper than the full Teflon thickness — showing damage running through the coating into the metal, according to Ars Technica. Elemental maps showed oxygen and chlorine moving in while fluorine and carbon from Teflon moved out, the expected signature of coating loss and metal attack.

On copper, Raman spectroscopy and X-ray diffraction identified cuprous oxide and basic cupric chloride, the pale green compound familiar from weathered copper roofs, according to Ars Technica.

That chemistry matters because copper roofs, car trim, building cladding, ships and cultural-heritage metalwork often rely on thin polymer or oxide layers. Nature states the mechanism may contribute to degradation of cultural heritage sites, buildings, ships, cars and other metal components. The University of Bonn likewise lists cars, garden fences and ships among the coated objects exposed to rain, dew, ocean spray and melting snow.

Why coatings sometimes fail early

Outside experts see practical relevance. Preet Singh, a materials scientist at the Georgia Institute of Technology who was not involved, told Scientific American that infrastructure breakdown is a trillion-dollar problem and better corrosion control could save money and lives. His broader point, quoted by Scientific American, was:

“I think if you want to have reliable products... we need to have very good control on corrosion”

The phrasing reflects applications from pharmaceutical and petrochemical equipment to implants, where reliable corrosion control is essential.

For consumers, the near-term lesson is not to abandon wax, paint sealants or Teflon-like films. The study used an ultra-thin 60-nanometer Teflon layer to prove the physics; commercial automotive and architectural coatings are usually thicker and multilayered. But the work suggests formulators should test resistance to electrical puncture, not only abrasion and chemical attack, and it points toward protective materials designed to drain, block or withstand drop charge, as Nature concludes.

What remains to be tested

The experiments establish that spontaneously charged drops form naturally and can degrade coated copper in the laboratory. They do not yet quantify how much of real-world vehicle or building corrosion comes from this route versus mechanical wear, pollutants, road salt, UV aging and poor application, the factors emphasized by the University of Bonn summary of prior understanding.

Still, the controls are difficult to dismiss. Neutral salty drops did nothing after thousands of hits. The same drops, after a four-centimeter slide across a leaf, plastic board, window-like sheet or lab quartz, corroded coated copper every time. In that contrast, rain looks less like passive water and more like a carrier of tiny, corrosion-starting sparks.

Sources referenced