Charged Water Drops May Damage Metal Beneath Protective Coatings

Rain may be doing more than washing over exposed metal: according to research reported by Phys.org, water droplets can acquire a small electrical charge and damage protective coatings placed over metal. The study, published in Nature, examined how initially neutral water drops changed after moving across several everyday surfaces. Researchers led by Rüdiger Berger and Hans-Jürgen Butt allowed drops to slide over a plant leaf, PVC foam board, polystyrene glass and PFOTS, a water-repelling coating. The drops then landed on copper covered with a Teflon coating. The researchers measured charges ranging from 0.2 to 2 nanocoulombs. After 3,000 drops had fallen onto the coated copper, the protective layer had broken down and the copper beneath it had corroded. In comparison, drops without an electrical charge produced no visible surface damage in the experiment. Water droplets were already known to become charged after sliding across surfaces such as leaves and window glass. However, the role of that charge in the corrosion of coated metals had not previously been considered in the way described by the researchers. Corrosion caused by rain has generally been associated with the physical impact of drops or with acidic pollution. The new findings suggest that spontaneously charged droplets may represent an additional corrosion mechanism, although the report describes further research as necessary to determine how the process can be suppressed. The potential implications involve metal objects and structures exposed outdoors, including cars, ships, buildings and cultural heritage sites. The researchers say the findings may also be relevant to developing materials and preservation methods that better withstand these conditions.