Electrified Raindrops May Help Strip Protective Coatings From Metal

Rain may wear away metal-protective coatings in more than one way. Alongside abrasion and acidity, a new study suggests that electrically charged water droplets could contribute to the damage. The research, published August 26 in Nature, examined what happens when droplets slide across insulating surfaces before landing on coated metal. Water was run over materials including plant leaves, PVC foam boards and glass, then allowed to fall onto copper covered with Teflon. A small number of drops produced no visible effect. After about 3,000 electrified droplets, however, microscopy showed that the Teflon had been breached and the copper beneath it was pitted. Water placed directly onto the coated copper did not produce the same corrosion in the experiment. The researchers propose that droplets gain a small positive charge while moving across a surface. As a charged drop approaches the metal, the electrical potential difference between them can exceed 1,000 volts. The measured charges were only 0.2 to 2.0 nanocoulombs—less than the charge in a typical static-electricity zap—but the team found that even these small amounts could cause damage through tiny electrical discharges. The idea builds on triboelectric charging, the process in which contact and movement transfer electrons. Although static electricity has long been associated with rubbing solid objects together, researchers have increasingly found that sliding water droplets can also become charged. The findings do not suggest that rain poses an immediate threat to modern cars or ships. Hans-Jürgen Butt, a study co-author at Germany’s Max Planck Institute for Polymer Research, said their commercial coatings are generally thicker than the Teflon layer used in the experiment. The process could instead help explain how coatings on older structures eventually fail after prolonged exposure to the elements. The study may also broaden how scientists think about corrosion. Materials scientist Guangwen Zhou, who was not involved in the research, said the electrical condition of a droplet—not only its acidity, chemistry or mechanical impact—may affect how corrosion starts. Researchers said that insight could eventually inform the design of coatings intended to withstand charged water droplets.