Charged Water Droplets Sliding Off Surfaces May Quietly Corrode Metal

Water droplets can become electrically charged simply by sliding along a surface. When they land on materials with the right electrical properties, including metals, that charge can drive corrosion — even through a protective coating, researchers report in a study published August 26 in the journal Nature. The effect could be widespread, since water may also pick up charge in clouds, thunderstorms and waterfalls.

Corrosion already has a large economic toll: this broad category of damage, which includes rust, cost the U.S. automotive industry alone more than $30 billion in 1999, and global costs are estimated in the trillions. Earlier work attributed such damage mainly to the physical impact of water droplets and to small amounts of corrosive compounds like acid that they might contain.

Physicist Zhongyuan Ni of the Max Planck Institute for Polymer Research in Mainz, Germany, led the study. In early experiments he found no damage even after hundreds of drops; it was only when an experiment ran longer than planned that he noticed signs of damage, prompting further investigation. The team found that thousands of drops, not hundreds, were needed to cause observable damage.

The researchers released water drops, charged by sliding along surfaces such as leaves, onto Teflon-coated copper. Every sample hit by these charged, sliding drops showed damage under a microscope, while a sample hit by drops released directly showed none. A different microscopy technique revealed the charged drops had penetrated the Teflon and corroded the copper beneath; similar damage appeared on polystyrene glass-coated copper and gold.

Stefan Weber, a physicist at the University of Stuttgart in Germany who was not involved in the study, said the work could pave the way for better anticorrosion coatings. Ni said the effects of individual charged drops are small and should not cause alarm, but they can add up over years for monuments, machinery and infrastructure exposed to the elements.