A rainy day typically washes your car clean, but it also slowly erodes at the surface. The explanation once seemed clear: rain corrodes metal because it carries salts, acids, and pollutants, but new research identifies a hidden culprit behind a water drop’s damaging effects.
Water drops can become electrically charged when sliding onto a surface. That tiny spark is powerful enough to break down protective coating and lead to corrosion of the metal, according to a recent study published in Nature. The findings highlight an overlooked reason for surface damage caused by water and could potentially change current tools for preserving metal objects like cars, bridges, and ships.
Rain, rain, go away
As a drop of water slides across a surface of a plant leaf, building walls, or glass windows, it strips the surface of its charge and leaves an opposing charge behind. The process is similar to tribocharging, which happens when we rub certain objects against each other like a balloon on our hair.
While this is not a new phenomenon, the role that it plays in water-caused corrosion has been largely overlooked. A team of researchers from the Max Planck Institute for Polymer Research set out to determine how electrically charged water drops affect the surfaces that they come in contact with.
The team poured 35-microliter water drops (around the size of a raindrop) over four common surfaces—a plant leaf, polyvinyl chloride foam board, polystyrene glass, and a commonly used water-repelling coating called perfluoro octadecyltrichlorosilane (PFOTS). They then let the drops fall onto Teflon-coated copper (a strong chemically resistant coating) positioned at a 50-degree angle.
The drops acquired a tiny electrical charge between 0.2 and 2 nanocoulombs. After 3,000 drops, the equivalent of a single rainy day, the Teflon coating broke down, leading to the corrosion of the metal underneath. When the experiment was conducted with drops that had no electrical charge, neither the copper nor its Teflon coating suffered any damage.
Take cover
The findings revealed that even small amounts of charge can break through tough, heat-resistant, and water-resistant coatings. This suggests that water’s electrical state, and not just its acidity or chemical composition, can cause corrosion.
The results of the study could help inform the design of protective coatings that are more resistant to electrically charged water drops. The data suggests that a thicker protective layer could do the trick, although more research is needed on designing a coating specifically geared toward protecting metal against tiny sparks.
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