Charged Raindrops Found to Corrode Cars and Buildings

The findings, reported by C&EN, build on years of research into how water droplets from rain, dew, ocean waves, and melting snow become electrically charged. The process is similar to tribocharging, which causes static when objects like a balloon on hair are rubbed together. "Water drops spontaneously become electrically charged when moving on different surfaces, such as plant leaves, insect wings, building walls, window glass, and plastic. This process, known as contact or sliding electrification, is analogous to tribocharging between solids," said study coauthor Zhongyuan Ni, a PhD candidate at Max Planck. "Water drops getting charged is not a new phenomenon, but for the past 200 years, people have not paid attention to this."

In 2023, Ni and colleagues set out to determine whether electrically charged water droplets affect the surfaces they contact. They poured water over four common surfaces — a plant leaf, polyvinyl chloride foam board, polystyrene glass, and perfluoro octadecyltrichlorosilane (PFOTS), a water-repelling coating — and let it drip onto sheets of Teflon-coated copper tilted at 50 degrees. Initially, the team allowed only a few dozen drops to fall before examining the copper with electron microscopy, repeatedly finding no evidence of decay. But one day, Ni let the drops keep falling. "After thousands of drops, it looked like the drops were pinning onto the surface," he said. Microscopy revealed the destructive strength of the charged drops.

Repeating the experiment with thousands of drops, the researchers found that electrified water drops not only broke down the Teflon coating but also corroded the underlying copper, leaving the sheets looking like scraped-up skin. When the experiment was conducted with nonelectrified drops, neither the copper nor its Teflon coating showed any wear. The charge on each drop was only around 0.2 to 2.0 nanocoulombs — about one billionth of a standard unit — yet the impact was significant.

"The finding is important because it suggests that the electrical state of a water droplet, not just its chemical composition, acidity, or mechanical impact, can influence how corrosion begins," said Guangwen Zhou, a professor of mechanical engineering at Binghamton University who was not involved in the study. "This perspective could open up new approaches to designing protective coatings and materials that are more resistant to corrosion in environments where charged water droplets are present."