The mechanism by which raindrops corrode car bodies has been revealed: the key is that electric charge is released from the raindrops onto the car body.
Recent research has revealed the mechanism by which raindrops corrode car bodies. It has become clear that the 'electrical charge of raindrops,' which had not received much attention until now, is causing damage to the body.
Spontaneously charged water drops induce corrosion | Nature
Raindrops are tiny lightning bolts, and they're corroding cars, study finds - Ars Technica
https://arstechnica.com/science/2026/08/raindrops-are-tiny-lightning-bolts-and-theyre-corroding-cars-study-finds/
The commonly accepted explanation for why rain causes corrosion is that water carries dissolved salts and acids to the car's surface, the constant impact of raindrops wears down the protective coating, and oxygen then acts on it. Most corrosion-preventing measures, such as paints, polymer films, and oxide films, are based on this idea. However, recent research has revealed that a crucial element in the mechanism by which rain causes corrosion has been overlooked.
According to research led by John Yuen Nyi, Rüdiger Berger, and Hans-Jürgen Butt, who work at the Max Planck Institute for Polymer Research , a world-leading basic research institution in polymer science and soft materials located in Mainz, Germany, raindrops reach the car body surface with enough charge to penetrate the insulating coating. In other words, it has become clear that raindrops do not damage or slowly melt the car body surface, but rather electrically create holes that allow sparks to jump through the gaps.
When raindrops glide across insulating surfaces such as leaves, painted walls, windowpanes, and plastic panels, they absorb an electric charge from those surfaces. These charged water droplets can have a voltage of up to 9,000 volts. The research team investigated the effects of these charged water droplets when they land on the surface of an object. Charged droplets are formed not only in natural phenomena such as clouds, thunderstorms, ocean waves, fountains, and waterfalls, but also naturally in industrial processes such as electrostatic spraying and inkjet printing.
For the study, the research team prepared water that mimicked rainwater by adding a small amount of salt, created 35-microliter (about the size of a large raindrop) droplets, and dropped them onto the surface of an object tilted at a 50-degree angle. The droplets slid about 4 centimeters from the surface, and, carrying an electric charge, fell from the object's surface onto a copper plate coated with a 60-nanometer Teflon film 5 millimeters below. The Teflon film was reportedly the most chemically resistant coating available commercially.
The inclined surfaces were chosen to mimic real-world raindrops: the first was
In the study, water droplets were dropped 3,000 times under conditions equivalent to a moderate afternoon rain. It was confirmed that all four Teflon-coated copper plates were corroded. Examination of the areas where the water droplets landed using an atomic force microscope revealed indentations several nanometers deep in some places. This exceeds the thickness of the Teflon film, meaning the damage completely penetrated the coating and reached the metal (copper plate). On the other hand, water droplets that landed directly on the copper plate without sliding on the object surface beforehand (droplets without an electric charge) showed no damage whatsoever to the copper plate surface, even after being dropped 3,000 times.
Furthermore, the research team used a high-speed camera to film the falling of water droplets. They confirmed that when a neutral droplet approached the copper plate, its base remained smooth and round until contact was made. On the other hand, charged droplets exhibited completely different behavior, with their bases elongating into a sharp, angular shape called
The research team modeled the droplet as a conductive sphere floating on a conductive wall and calculated how the electric field strength changes with distance. For a droplet with a charge of 2 nanocoulombs, the electric field strength reached 60 kilovolts per millimeter, the dielectric breakdown threshold for Teflon, when the droplet was about 10 micrometers away from the surface. This is the electric field strength at which the insulator ceases to provide insulation. For a polystyrene coating that undergoes dielectric breakdown at 19 kilovolts per millimeter, the breakdown appears to occur at a distance of 50 micrometers.
When the coating reaches its dielectric breakdown threshold, it loses its function as an insulator, causing a minute dielectric breakdown that penetrates the coating. This is the same phenomenon that breaks down the insulation of capacitors and transformers. Furthermore, the research team's charge measurements confirmed that the charge transfer was almost perfect. When a droplet carrying 2 nanocoulombs of charge collided with a copper plate, 1.8 nanocoulombs of charge were released onto the copper plate, and the droplet itself bounced back carrying only 0.016 nanocoulombs (less than 1% of its original charge).
The amount of charge is proportional to the sliding distance of the droplet, and the water resistance of the coating is proportional to its thickness, so there are limits to how much raindrops can penetrate a coating. A 12-micrometer polystyrene film was penetrated, but a 130-micrometer film was not. However, since most automotive paints are only a few micrometers thick, the research team points out that charges on the nanocoulomb scale can essentially penetrate automotive coatings.
The research team discovered that when the coating is broken, the exposed metal is immersed in saltwater, creating a new potential difference on its surface. They point out that this allows electrochemical reactions that cause corrosion to act freely on the car body surface, which should otherwise be protected from corrosion.
Furthermore, the research team analyzed the corrosive deposits formed on copper plates whose coatings were damaged by water droplets using Raman spectroscopy and X-ray diffraction. As a result, components such as cuprous oxide and cupric chloride were detected. Elemental mapping of the damaged area also showed that oxygen and chlorine had flowed in, and fluorine and carbon had leached out from the Teflon. According to the research team, this is exactly what happens when the coating is damaged.
The research team also performed impedance measurements to investigate how quickly the protective performance of the coating deteriorates. After dropping charged droplets 10,000 times, it was revealed that the barrier performance of the Teflon film was worse than after immersion in salt water for four hours. Furthermore, once damage to the coating begins, its progression accelerates, and the damaged areas become more easily wetted, allowing water to remain for longer periods and expand. As a result, after dropping 50,000 charged droplets, the corroded area was found to be more than 1 millimeter wide.
In addition to these experiments, the research team created a plate made of two-thirds quartz and the remaining one-third copper, and coated the entire surface with a uniform layer of Teflon to create a smooth surface. They also found that when charged droplets slid across this plate, corrosion lines formed along the boundary between the embedded quartz and copper. This is because the charged droplets did not react with the surface they came into contact with, but rather with something close enough to supply the opposite charge.
The research team argues that charged droplets have a significant impact on composite materials in general. They point out that charged droplets, as they slide, will find wherever conductive or high dielectric constant components are embedded in an insulating matrix. Specifically, they suggest that bridges, ship hulls, painted steel, polymer casings for outdoor electronic equipment, and metal structures of historical buildings could all be affected by charged raindrops.
Apparently, increasing the thickness of protective layers such as Teflon is one solution to protect the car body from charged droplets. A 130-micrometer polystyrene film would be unaffected by charged droplets. However, increasing the thickness is not always a feasible option. Optical films need to maintain transparency, aircraft protective layers must be lightweight and flexible, and protective layers for outdoor electronics are designed to be thin. The research team argued that since most commercially available coatings are chosen based on chemical resistance, there is a need for 'coatings with a specified electric field resistance.'
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