Raindrops Corrode Surfaces Electrically
A study finds raindrops carry an electrical charge strong enough to damage protective coatings like Teflon, challenging standard corrosion models.
A new study has found that raindrops can carry an electrical charge strong enough to punch holes in protective surface coatings. The research, led by Zhongyuan Ni, Rüdiger Berger, and Hans-Jürgen Butt at the Max Planck Institute for Polymer Research, challenges the standard explanation for rain-induced corrosion.
Conventional wisdom holds that corrosion from rain is caused by water carrying dissolved salts and acids, physical abrasion from drops, and oxygen. Nearly all protective tools, such as paints and polymer films, are designed around this idea. The new findings suggest an overlooked electrical mechanism is also at work.
The Mechanism of Slide Electrification
The study focuses on a phenomenon called "slide electrification." When a water drop slides across an insulating surface, it strips electrical charge from that surface. The drop leaves behind an opposing charge. Voltages involved are significant, with drops measured at up to 9,000 volts.
The researchers investigated what this charge does to the next surface a drop lands on. They released 35-microliter water drops, containing salt to mimic rainwater, onto a surface tilted at 50 degrees. The drops slid about four centimeters, picked up a charge, rolled off, and fell five millimeters onto a test plate.
Testing on Common Materials
To mimic real-world conditions, the team used several tilted surfaces. They tested materials commonly found around homes and in nature. The goal was to see how much charge drops would pick up from everyday surfaces.
| Surface Material | Source Description | Charge Picked Up (nanocoulombs) |
|---|---|---|
| Plant Leaf | Leaf from a Tradescantia spathacea plant | 0.2 |
| PVC Foam Board | Hardware store PVC foam board | Not specified in source |
| Polystyrene Sheet | Transparent polystyrene sold as window glazing | Not specified in source |
| Fluorinated Quartz | A fluorinated coating on quartz (lab creation) | 2.0 |
The charges ranged from 0.2 nanocoulombs off the leaf to two nanocoulombs off the fluorinated quartz. In a raindrop-sized volume, a nanocoulomb translates to a few thousand volts.
Impact on Protective Coatings
The target for the falling, charged drops was a copper plate coated with a 60-nanometer film of Teflon. Teflon is one of the most chemically resistant coatings available. The electrical charge from the drops was large enough to electrically blow a hole in this insulating layer.
Zhongyuan Ni, Rüdiger Berger, and Hans-Jürgen Butt's work shows the charge does not just scratch or slowly dissolve a coating. It can cause a spark-like discharge that punctures it. This creates a pathway for further corrosion to begin, undermining the protection offered by paints, polymers, or oxide layers.
The study indicates that slide electrification is a routine event. It happens on surfaces like leaves, painted walls, windowpanes, and plastic panels. This means the electrical corrosion mechanism could be widespread. The research was published by Ars Technica. It suggests future corrosion prevention strategies may need to account for this electrical effect. The work was completed at the Max Planck Institute for Polymer Research in Mainz, Germany.
