Rain may cause corrosion in a way scientists previously overlooked. In addition to carrying dissolved salts and acids to exposed surfaces, raindrops can generate powerful electrical charges as they move across leaves, paint, glass, and plastic. That charge may puncture insulating protective coatings and accelerate corrosion.
Most corrosion-prevention methods—including paints, polymer films, and oxide layers—are designed to block water, oxygen, and corrosive chemicals. However, a new study led by Zhongyuan Ni, Rüdiger Berger, Hans-Jürgen Butt, and colleagues at the Max Planck Institute for Polymer Research in Mainz, Germany, suggests that electrically charged rainwater can damage these barriers in a different way.
How charged raindrops may cause corrosion
The research focuses on a phenomenon known as sliding electrification, or “sliding charging.” Scientists have only quantified this effect in detail during the past few years. When a water droplet moves across an insulating material—such as a leaf, painted wall, window, or plastic panel—it removes electrical charge from the surface and leaves behind a charge of the opposite polarity.
The resulting voltages can be surprisingly high. Previous experiments have found that water droplets charged through this process can reach as much as 9,000 volts. Although a raindrop contains very little energy, its high voltage can still create a localized electrical discharge capable of breaking through an insulating film—much like a spark jumping across a small gap.
Ni, Berger, Butt, and their colleagues wanted to determine how this electrical charge affects the surface where a droplet eventually lands. The researchers released 35-microliter water droplets—roughly the size of large raindrops—onto a surface tilted at 50 degrees. They added a small amount of salt to the water to better simulate natural rain.
Each droplet traveled approximately four centimeters, became electrically charged, rolled off the edge, and then fell five millimeters onto a copper plate covered with a 60-nanometer layer of Teflon. Teflon is widely recognized as one of the most chemically resistant insulating coatings.
To replicate real-world conditions, the team tested several sloped surfaces. These included leaves from Tradescantia spathacea, a plant grown in one of the researchers’ offices; PVC foam board; and a clear polystyrene sheet commonly sold as window glazing. The researchers also tested a fluorinated quartz surface produced in the laboratory.
The amount of charge transferred to the droplets varied by material. Drops collected approximately 0.2 nanocoulombs after sliding across the leaf and up to 2 nanocoulombs after moving across fluorinated quartz. For a droplet the size of a raindrop, those charges can produce voltages of several thousand volts.
These findings suggest that rain-related corrosion is not driven solely by water, oxygen, salts, acids, and mechanical wear. The electrical charge created when droplets slide across a surface may also help create tiny breaches in protective coatings, allowing corrosive substances to reach the underlying material.
Source: arstechnica.com