Charged Raindrops Can Electrically Punch Holes in Metal Coatings, Study Finds

When it rains, metal rusts. Science and engineering have long had a clear explanation for this phenomenon: acidic substances and salts dissolved in rainwater trigger chemical reactions, while the ceaseless impact of water droplets physically wears down the surface, allowing oxygen to reach it and drive oxidation forward. Modern anti-corrosion technologies—rust-preventive paints, polymer coatings, corrosion-resistant oxide layers—have all been designed around this premise of "chemical barriers" and "resistance to physical wear."

However, a paper published in the August 26, 2026 issue of the journal Nature (DOI: 10.1038/s41586-026-10941-6) reports the existence of a third, previously overlooked pathway behind rain-induced corrosion.

A research team led by Zhongyuan Ni, Rüdiger Berger, and Hans-Jürgen Butt of the Max Planck Institute for Polymer Research in Germany, working with researchers from the University of Bonn, South China University of Technology, the Massachusetts Institute of Technology (MIT), and Johannes Gutenberg University Mainz, discovered that water droplets sliding down an insulating surface accumulate electrostatic charge equivalent to several thousand volts. When these droplets land, that charge can trigger microscopic "discharge" (dielectric breakdown) events that puncture the insulating protective coating at the point of impact.

Before a raindrop is a liquid carrying chemical substances, it can act as a tiny, locally high-voltage packet of charge capable of electrically shooting through a metal's coating.

【Figure summary】Conceptual diagram showing a charged water droplet approaching a coated metal surface, causing dielectric breakdown that leads to corrosion.
Credit: Ni et al., Nature (2026). DOI: 10.1038/s41586-026-10941-6

The blind spot in "slide electrification": droplets that steal charge from insulators

Why would a water droplet carry such a high voltage? The physical phenomenon at the root of this is known as "slide electrification."

When a water droplet slides down an insulating surface—a plant leaf, a painted building exterior, a window pane, a plastic panel—charge separation occurs at the solid-liquid interface. The droplet strips charge from the surface and accumulates it within itself, leaving behind an opposite charge on the substrate. While this phenomenon has been qualitatively known for a long time, quantitative measurements of how much charge a droplet actually carries and what that charge does upon landing have only recently begun in earnest. The reason this electrical aspect had been entirely absent from corrosion research until now lies in both the limitations of measurement technology and the entrenched assumption that "water is merely a solvent."

To examine what effect this slide electrification has on the surface where droplets eventually land, the research team built a meticulous experimental setup.

In the experiments, they prepared water droplets of 35 $\mu \text{L}$ (microliters, a size corresponding to a large raindrop) containing a trace amount of salt to mimic real rainwater. These droplets were released from the top edge of a substrate tilted at 50 degrees, allowed to slide roughly 4 cm, and then dripped off the edge to fall 5 mm onto a target. The chosen target was a copper plate coated with a mere 60 nm (nanometer)-thick film of Teflon (PTFE). Teflon is exceptionally chemically stable, boasting among the highest chemical resistance of any commercially available coating material.

Four types of tilted substrates were chosen to represent everyday environments:

1. A fresh leaf of Tradescantia spathacea (oyster plant) grown in the lab
2. A rigid PVC (polyvinyl chloride) foam board purchased from a hardware store
3. A transparent polystyrene (PS) panel of the kind used for window glazing
4. Quartz glass coated with a fluorinated treatment (PFOTS-coated quartz) prepared in the lab

The amount of charge the droplets acquired while sliding 4 cm varied by up to a factor of 10 depending on the substrate material—from about 0.2 nC (nanocoulombs) on the leaf to about 2 nC on the fluorinated quartz. At first glance, a value measured in nanocoulombs seems minuscule. But when 2 nC of charge is concentrated in a volume as small as a raindrop, the resulting electrostatic potential can reach several thousand volts (up to roughly 9,000 V).

After subjecting the samples to 3,000 droplet impacts (a number of collisions roughly equivalent to a few hours of moderate rainfall in the real world), the research team examined the impact sites using atomic force microscopy (AFM). In every case involving the four sliding surfaces, they found corrosion pits tens of nanometers deep that had breached the Teflon coating and reached the underlying copper.

What proved critical in this experiment was a control test using "neutral, uncharged water droplets" that were dripped directly onto the target without undergoing the sliding process. Even when salt-water droplets of identical composition were struck against the target the same 3,000 times, the uncharged droplets left absolutely no marks on the Teflon surface, and the copper plate remained completely protected.

Further precise measurements using an electrometer revealed that charge transfer upon impact was nearly complete. A droplet arriving with 2.0 ± 0.021 nC of charge released 1.8 ± 0.022 nC into the underlying copper plate at the moment of impact, leaving only 0.016 ± 0.003 nC (less than 1% of the initial charge) in the droplet that rebounded. It was neither the droplet's chemical composition nor the impact force that caused the coating to break down—it was the charge the droplet carried.

Microscopic dielectric breakdown triggered by a sharpening droplet

What happens at the interface just before a droplet touches the target? The research team used a high-speed camera to capture the final moment as a charged droplet approached the coated metal plate.

A neutral droplet maintains a smooth, spherical bottom surface right up until it touches the target. A charged droplet, however, behaved very differently. As it approached the metal plate, the bottom of the droplet rapidly deformed into a sharp cone shape. This is a phenomenon known as a "Taylor cone," which occurs when electrostatic force overcomes the surface tension of a liquid. It offers visual proof that the electric field between the droplet and the underlying metal had grown strong enough to physically stretch the water.

As the droplet approaches the target, the gap between the two narrows and the electric field strength rises exponentially. The team modeled the droplet as a conducting sphere with a radius of 2.0 mm and the coated metal as a conducting wall, using an approximate capacitance formula to calculate the relationship between distance and local electric field strength.

Here, $R$ is the droplet's radius, $h$ is the distance between the droplet and the coating surface, ${\epsilon }_0$ is the vacuum permittivity, and $\epsilon$ is the relative permittivity of the medium. Calculations based on this model showed that when a droplet carrying 2 nC of charge approaches to within about 10 $\mu \text{m}$ (micrometers) of the surface, the electric field strength reaches 60 kV/mm—the dielectric breakdown field of Teflon. Dielectric breakdown refers to the phenomenon in which an insulator, normally non-conductive, loses its insulating properties under excessive electric field and abruptly allows current to flow. This is exactly the same failure mechanism by which the insulating layers of capacitors or transformers rupture when their voltage rating is exceeded.

For polystyrene coatings, which have a comparatively low breakdown field of 19 kV/mm, the dielectric breakdown threshold is reached even while the droplet is still as far as 50 $\mu \text{m}$ from the surface. Notably, these calculated values represent a "lower-bound estimate," since they assume the droplet is a perfect sphere. In reality, the formation of a Taylor cone locally reduces the curvature radius at the droplet's tip, meaning the electric field concentrated at the tip is far stronger than the theoretical value.

This breakdown mechanism shows a clear dependence on coating thickness. Since electric field strength is proportional to applied voltage divided by coating thickness, making the coating thicker can suppress the field below the breakdown threshold.

When the research team repeated the experiment while varying the thickness of the polystyrene coating, a thin 12 $\mu \text{m}$ film was punctured by the charged droplets, while a 130 $\mu \text{m}$-thick coating survived 3,000 droplet impacts completely unscathed. Even under the same voltage, increasing the coating thickness by more than tenfold was enough to bring the internal electric field strength down to a safe range.

Similar breakdown and corrosion were also observed with polystyrene films ranging from 60 nm to 5 $\mu \text{m}$ thick on copper substrates, polystyrene films on gold substrates, a silicon dioxide (${\text{SiO}}_2$) insulating layer just a few nanometers thick on gold substrates, and even on aluminum substrates, where the discharge produced tiny blisters on the surface.

Coating degradation and a self-accelerating chain of corrosion

The moment dielectric breakdown opens a hole in the protective coating, the fate of the underlying metal changes entirely. Salt-laden water droplets make direct contact with the metal substrate through the tiny opening—and at that interface, a new potential difference created by the discharge already exists. The barrier built to block chemical corrosion is blown apart from the inside, and the electrochemical reactions it was designed to keep out are suddenly unleashed.

The research team analyzed the substances formed at the damaged sites using Raman spectroscopy and X-ray diffraction (XRD). Cuprous oxide (${\text{Cu}}_2\text{O}$) and basic copper chloride (${\text{Cu}}_2(\text{OH}{)}_3\text{Cl}$) were detected on the copper plate's surface. Basic copper chloride is the primary component of the green patina that forms on copper roofs exposed to wind and rain over many years. Elemental mapping via scanning electron microscopy combined with energy-dispersive X-ray spectroscopy (SEM-EDS) captured a sharp spike in oxygen and chlorine concentration at the center of the breakdown holes, along with fluorine and carbon—the constituents of Teflon—being pushed outward.

The polymer making up the coating itself also suffered chemical structural damage from the electrical stress. When damaged sites on the polystyrene film that had received charged droplets were examined under a laser microscope, rough patches emitting a green fluorescence not present in untreated polystyrene were found to have formed. Tracking with nanoscale infrared spectroscopy revealed that these fluorescent regions contained newly formed carbon-carbon double bonds ($\text{C}=\text{C}$) and carbon-oxygen double bonds ($\text{C}=\text{O}$)—chemical traces of polymer chains being severed and effectively "scorched" by high-voltage discharge sparks.

Quantitative evaluation via electrochemical impedance spectroscopy (EIS) laid bare just how severe this breakdown was. After 10,000 charged droplets had struck the Teflon coating, its electrical resistance (barrier performance) had dropped substantially lower than that of an identical coated plate continuously immersed in salt water of the same concentration for four hours. That four-hour immersion time corresponds to the cumulative time the 10,000 droplets spent in contact with the target surface. In other words, intermittent impacts from charged droplets degrade the coating far faster than simply soaking it in salt water.

More troublingly still, once this corrosion process begins, it takes on a "self-accelerating" character. Once a coating is electrically punctured and exposes hydrophilic corrosion products, that spot becomes easier for water droplets to wet and hold moisture longer. The longer water lingers there, the more readily it can absorb the charge from the next incoming droplet, expanding the site of electrochemical reaction. By the time the team had completed an experiment involving 50,000 droplet impacts, corrosion spots that had initially been nanometer-scale had grown to a macroscopically visible size exceeding 1 mm in diameter.

To examine behavior in composite materials mixing conductors and insulators, the research team prepared a flat plate that was two-thirds quartz and one-third copper, coated seamlessly across its entire surface with a uniform Teflon film.

When charged droplets were made to slide across this plate, corrosion concentrated intensively along the embedded "boundary line between quartz and copper." While a droplet remains over the non-conductive quartz, no opposite charge is drawn from the substrate below, keeping the electric field weak. But the instant the droplet reaches the copper region, the free electrons within the copper instantaneously rearrange themselves in response to the droplet's charge, concentrating a strong image charge just beneath the coating. This subjects the Teflon coating sandwiched between the droplet and the copper to an abrupt, intense electric field, causing a chain of dielectric breakdown events along the boundary.

Challenges remaining in extrapolating to real-world environments

The paper's co-authors point out that this dielectric breakdown caused by charged water droplets may be contributing to the deterioration of a wide range of outdoor infrastructure—bridges, ship hulls, painted steel structures, resin enclosures of outdoor electronics, and metal fixtures on historic buildings. Droplet charging is not limited to rain; it also arises naturally in turbulent clouds, thunderstorms, breaking waves, and near fountains and waterfalls, and it is ubiquitous in industrial processes such as electrostatic painting and industrial inkjet printing.

However, one must be cautious about immediately treating this finding as "a complete overturning of conventional corrosion theory" or as "the primary cause of outdoor infrastructure degradation." This study was conducted in a highly controlled laboratory environment, and the extent to which natural rainfall actually degrades structures via this mechanism remains, at this stage, an untested hypothesis.

Outside experts not involved in the study have likewise acknowledged the academic value of the finding while urging a measured view of its real-world applicability. Professor Guangwen Zhou of the State University of New York at Binghamton told Chemical & Engineering News that while he recognized the significance of the discovery, further research is needed to determine how frequently similar breakdown events actually occur under the complex meteorological conditions of the real world. Professor Preet Singh of Georgia Institute of Technology told Scientific American that, given corrosion costs the world trillions of dollars annually, identifying a previously unrecognized degradation pathway is extremely valuable—but he also emphasized the importance of quantitatively assessing its impact in field conditions.

As the science outlet ScienceAlert has reported, the charge distribution of natural raindrops is far from uniform, varying dramatically with droplet size, fall speed, atmospheric humidity, and wind conditions around a given structure. The 35 $\mu \text{L}$ droplets used in the experiment fall on the larger end of natural raindrops, and not every raindrop cleanly slides 4 cm across an insulating surface before jumping onto a metal part.

As a countermeasure, thickening coatings to 130 $\mu \text{m}$ or more has been proposed as one solution. But thick coatings cannot be adopted for every application. Optical coatings on cameras and sensors must remain thin to preserve transparency; protective films for aircraft and drones must meet strict weight limits and flexibility requirements; and the insulating barriers of increasingly miniaturized outdoor IoT devices are often constrained by design to nano- or micrometer-scale thin layers. Conventional anti-corrosion coatings have traditionally been selected based on chemical resistance—that is, what chemicals they can block—but going forward, material design will also need to account for dielectric strength, or how much localized electric field a coating can withstand.

Ultraviolet-induced polymer photodegradation, abrasion from wind-blown sand, chemical erosion from acid rain, and now electrostatic dielectric breakdown from water droplets—outdoor materials face a harsh environment where all these factors intertwine in complex ways. Now that laboratory experiments have demonstrated that charged water droplets can electrically puncture metal, the next stage of research—bridging theory and field conditions—is set to begin: measuring how much this mechanism actually contributes in natural environments, and designing new protective coatings capable of withstanding electrical breakdown.