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The standard explanation for why rain causes corrosion is that water carries dissolved salts and acids to a surface, constant drumming of raindrops abrades whatever protective coating is on it, and oxygen does the rest. Nearly all our tools to prevent this—paints, polymer films, or oxide layers—are built around this idea. But we’ve apparently been missing something important about the rain.
A new study led by Zhongyuan Ni, Rüdiger Berger, and Hans-Jürgen Butt at the Max Planck Institute for Polymer Research in Mainz, Germany, has shown that water drops routinely arrive at a surface carrying an electrical charge large enough to punch through an insulating coating. Not scratch it. Not slowly dissolve it. Electrically blow a hole in it, the way a spark jumps a gap.
The starting point of the study is a phenomenon called “slide electrification,” which has only been properly quantified in the past few years. When a water drop slides across an insulating surface like a leaf, a painted wall, a windowpane, or a plastic panel, it strips charge from that surface and leaves an opposing charge behind. The voltages involved are not trivial. Drops charged this way have been measured at up to 9,000 volts.
The question Ni, Berger, Butt, and their colleagues asked was what this charge does to the surface that the drop lands on next. To find out, the team released 35-microliter water drops, about the size of a large raindrop, containing a pinch of salt to mimic rainwater, onto a surface tilted at 50 degrees. The drops slid about four centimeters, picked up a charge, rolled off the edge, and then fell five millimeters onto a copper plate coated with a 60-nanometer film of Teflon, which is one of the most chemically resistant coatings on the market today.
arstechnica.com
A new study led by Zhongyuan Ni, Rüdiger Berger, and Hans-Jürgen Butt at the Max Planck Institute for Polymer Research in Mainz, Germany, has shown that water drops routinely arrive at a surface carrying an electrical charge large enough to punch through an insulating coating. Not scratch it. Not slowly dissolve it. Electrically blow a hole in it, the way a spark jumps a gap.
The starting point of the study is a phenomenon called “slide electrification,” which has only been properly quantified in the past few years. When a water drop slides across an insulating surface like a leaf, a painted wall, a windowpane, or a plastic panel, it strips charge from that surface and leaves an opposing charge behind. The voltages involved are not trivial. Drops charged this way have been measured at up to 9,000 volts.
The question Ni, Berger, Butt, and their colleagues asked was what this charge does to the surface that the drop lands on next. To find out, the team released 35-microliter water drops, about the size of a large raindrop, containing a pinch of salt to mimic rainwater, onto a surface tilted at 50 degrees. The drops slid about four centimeters, picked up a charge, rolled off the edge, and then fell five millimeters onto a copper plate coated with a 60-nanometer film of Teflon, which is one of the most chemically resistant coatings on the market today.
Raindrops are tiny lightning bolts, and they’re corroding cars, study finds
Finding reveals an overlooked way that water can damage surfaces.
arstechnica.com
