Why Does Rain Eat Through Protective Coatings on Metal Structures? Researchers Propose a New Idea: Electrically Charged Droplets
Scientists long attributed rain damage to physical abrasion and chemical degradation from acidic substances. A new study offers a third idea, which could help researchers develop longer-lasting coatings and preserve important objects
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Throughout the year, painters slather a reddish-orange coating onto San Francisco’s Golden Gate Bridge. The structure’s protective layer needs constant upkeep, since the steel underneath could corrode if exposed to the salty air.
Harsh environmental conditions, including rainfall, chip away at the bridge’s paint. Scientists have long thought raindrops contribute to the wear and tear because they physically scratch the coating as they slide down it and have acidic components that can chemically degrade it. Now, new research suggests another culprit might help drive this destruction: static electricity.
It can play a role because water droplets can become electrically charged as they slide across smooth surfaces, such as plant leaves, insect wings, building walls and window glass. When these electrified drops fall onto coated metals, they generate micro-discharges capable of breaching protective coatings like Teflon and damaging the underlying metal, according to a study published on August 26 in the journal Nature.
People have known for centuries that rubbing two solids together can produce an electric charge in what’s called triboelectric charging. Rub a balloon against your hair, and the contact causes electrons from your hair to jump to the balloon, making the latter gain a negative charge and your hair gain a positive one.
Liquids were long assumed incapable of generating static charge this way since they lack the atomic-scale surface roughness of solids. “For a liquid, there is no force which could break a bond,” study co-author Hans-Jürgen Butt, a physicist at the Max Planck Institute for Polymer Research in Germany, tells Scientific American’s Megha Satyanarayana.
Fun fact: Triboelectric charging on Mars
Last year, researchers reported that they recorded audio of zaps of mini lightning on Mars, thanks to NASA’s Perseverance rover. They suspect the phenomenon is caused by triboelectric charging from dust swirling on the Red Planet during storms.
But in recent years, researchers have realized that sliding water droplets can pick up a charge. To investigate whether electric rain could affect coatings on metals, Butt and colleagues dropped water droplets onto sloped insulating materials, including plant leaves, PVC foam boards and glass, and allowed the liquid to run down and fall onto Teflon-coated copper.
Applying only a few dozen drops resulted in no visible wear. But when the team applied 3,000 electrified droplets, a powerful microscope revealed a surprising amount of damage. The water had eaten through the Teflon and left the underlying copper pitted, resembling scraped skin. By contrast, water dropped directly onto the coated copper caused no corrosion.
The researchers posit that as the droplets slide down a surface, they acquire a tiny positive charge. When a droplet approaches the coated metal, the electrical potential difference between the droplet and the underlying metal can exceed 1,000 volts.
The droplets carried a minuscule electrical charge between 0.2 and 2.0 nanocoulombs—less than a typical zap of static electricity from the doorknob—the team found. But “even with small amounts of charge, they can still cause damage,” study co-author Zhongyuan Ni, a physicist at the Max Planck Institute for Polymer Research, tells Chemical and Engineering News’ Annie Roth. “It was quite surprising.”
Since commercial coatings on cars and ships are typically thicker than the Teflon coating used in the experiment, these charged drops pose no immediate danger to modern vehicles, Butt tells Physics World’s Tim Wogan. However, the process could help explain why protective coatings on historical structures fail after long periods of time, leaving the material underneath exposed to the elements. Corrosion in the United States is estimated to cost the country more than $2 trillion per year.
“The finding is important because it suggests that the electrical state of a water droplet, not just its chemical composition, acidity or mechanical impact, can influence how corrosion begins,” Guangwen Zhou, a materials scientist at Binghamton University who wasn’t involved in the work, tells C&EN. “This perspective could open up new approaches to designing protective coatings and materials that are more resistant to corrosion in environments where charged water droplets are present.”
Preet Singh, a materials scientist at the Georgia Institute of Technology, agrees. “I think if you want to have reliable products, whether it’s pharmaceutical or petrochemical or even implants, we need to have very good control on corrosion,” Singh, who wasn’t involved in the study, tells Scientific American.