New Phenomenon: Car Paint Pierced by Raindrops Leading to Rust
For a long time, it was believed that rainwater damages objects because its acidic and saline content dissolves paint and erodes metal surfaces. However, recent research has uncovered a far more powerful cause. Raindrops actually carry an electrical charge strong enough to instantaneously puncture protective coatings, acting like miniature lightning strikes every time it rains.
This finding implies that our current methods for protecting vehicles, buildings, and outdoor equipment may be inadequate, according to reports from Ars Technica and the journal Nature. Researchers from the Max Planck Institute for Polymer Research in Germany discovered that as water droplets slide down any surface—be it leaves, walls, windows, or car bodies—they absorb electrical charges from the surfaces they traverse. These seemingly ordinary droplets can carry electrical voltages of up to thousands of volts.
When these droplets strike another surface, the charge jumps, puncturing the coating much like a spark jumping between two poles. This means the damage to paint layers is not merely due to water slowly dissolving the coating. Instead, the phenomenon resembles microscopic lightning strikes penetrating the paint layer directly to the metal underneath. Once the paint layer is breached, salt and acid-laden water enter, accelerating rust and much more severe damage.
How can raindrops puncture paint? In the study, researchers dropped water droplets, sized like raindrops, down an inclined surface so the water would slide before hitting a copper plate coated with a nanometer-thick protective layer. After approximately 3,000 droplets—equivalent to several hours of moderate rain—the protective layer, which was designed to be acid and water-resistant, was punctured through to the metal.
Water that fell directly without sliding and carried no electrical charge did not damage the protective layer at all. This proves that the damage is caused by the electrical charge carried by the droplets, rather than the properties of the water itself. As the droplets approach the metal surface, the charge forms an increasingly strong electric field until it exceeds the resistance of the coating, resulting in an instantaneous electrical discharge that pierces the layer. Protective layers designed to withstand water and chemicals were not designed to withstand this electrical voltage.
The damage starting from these small holes expands over time. After about 50,000 droplets, the holes widen to one millimetre. As the holes grow larger, more water becomes trapped, further accelerating the degradation. This means that objects frequently exposed to rain do not just suffer slow damage, but rather damage that accelerates over time.
This discovery suggests that almost all current protective coatings used for cars, bridges, ships, and buildings are inadequate. Historically, protective materials have been chosen based on their resistance to water and chemicals, without considering their resistance to the electrical voltage carried by raindrops. Researchers noted that thicker protective layers can withstand these electrical discharges, as a thicker layer weakens the electric field. However, not all objects can support thick coatings; window glass, aircraft coatings, and outdoor electronics must remain thin and lightweight. Consequently, researchers suggest the design of new protective materials that are not only water and acid-resistant but also capable of resisting electrical voltage.