The research, led by P. R. Rajimol of the CSIR-National Institute for Interdisciplinary Science and Technology in Trivandrum, began with marigold (Tagetes erecta) floral waste gathered from the Attukal Bhagavathy Temple in Kerala. Marigolds are rich in phytochemicals, including flavonoids such as quercetin and quercetagetin, carotenoids like lutein and zeaxanthin, and a variety of phenolic compounds. Rather than simply using a crude extract, the team pursued an activity-guided approach: they first identified which solvent extraction produced the most potent corrosion inhibitor, and then isolated the single molecule responsible for that activity. This distinction matters, because earlier studies of marigold as a corrosion inhibitor had stopped at the extract level without pinpointing or deploying the active compound in a controlled-release coating system.

The extraction itself was deliberately simple and green. Roughly 750 grams of dried petals were extracted at room temperature using two solvents of very different polarity: hexane, which pulls out nonpolar compounds such as carotenoids and steroids, and a 60:40 ethanol-water mixture, which captures polar species including flavonoids and phenolic acids. After concentration under reduced pressure and freeze-drying, the hexane route yielded about 26 grams of crude extract and the hydroethanolic route about 33 grams. When the two extracts were compared head-to-head in salt water, the hydroethanolic extract clearly outperformed its hexane counterpart, protecting mild steel more effectively in immersion tests lasting 14 days and in electrochemical measurements.

Those electrochemical tests provided the mechanistic clues. In electrochemical impedance spectroscopy, bare mild steel in 3.5 weight percent sodium chloride solution could be modelled with a simple circuit of solution resistance, double-layer capacitance, and charge transfer resistance. In the presence of the hydroethanolic extract, however, an additional film resistance and capacitance appeared in the fitted circuit, indicating that active molecules were adsorbing onto the metal surface and forming a protective layer. Potentiodynamic polarization told a complementary story: the hydroethanolic extract shifted the corrosion potential in the anodic direction and suppressed the corrosion current density, behaving as an anodic-type inhibitor, while the hexane extract acted predominantly on the cathodic reaction. Peak performance for both extracts occurred around 200 parts per million; at higher concentrations, the physically adsorbed layers apparently began to desorb or peel, slightly reducing efficiency.

With the hydroethanolic extract confirmed as the more active fraction, the researchers used column chromatography, eluting with mixtures of ethyl acetate and hexane, to isolate the marker compound. At 90 percent ethyl acetate, a pale-yellow solid emerged. High-resolution mass spectrometry assigned it the molecular formula C15H10O8, and proton and carbon nuclear magnetic resonance spectra confirmed its identity as quercetagetin, a hexahydroxyflavone. The molecule is, in electrochemical terms, almost ideally suited to corrosion inhibition: it carries a dense array of hydroxyl groups and a carbonyl group whose lone pairs can form coordinate bonds with metal ions, while its two aromatic rings, a double bond, and the carbonyl provide a delocalized pi-electron system that can also accept electrons back from the metal in retro-donation. In effect, quercetagetin and the metal surface behave as a Lewis acid-base pair, and this molecular handshake blocks the oxidation reactions that drive corrosion.

Immersion tests bore this out dramatically. Mild steel coupons soaked for 15 days in saline solutions at acidic, neutral, and alkaline pH developed heavy corrosion products, pits, and cracks when no inhibitor was present. In the presence of quercetagetin, the surfaces remained smooth and essentially damage-free, and elemental analysis showed far less oxide formation. The hydroxyl and carbonyl groups, by coordinating to the metal, simply deny corrosive chloride ions access to the surface.

The next challenge was longevity. A free inhibitor dissolved in a coating would be consumed quickly and leach away, so the team loaded quercetagetin, and the whole hydroethanolic extract, into mesoporous silica nanoparticles synthesized by a template method using cetyltrimethylammonium bromide and tetraethyl orthosilicate. These nanocontainers are extraordinary sponges: nitrogen adsorption measurements showed a surface area of 729.67 square meters per gram and a pore volume of 0.68 cubic centimeters per gram. After loading the inhibitor at a 2:1 inhibitor-to-container weight ratio, the surface area dropped to 88.80 square meters per gram and pore volume to 0.12 cubic centimeters per gram, clear evidence that the organic molecules had filled the pores. Infrared spectroscopy confirmed that loading occurred by physisorption, with no chemical bonds formed or broken, so the inhibitor retains its full activity. Transmission and scanning electron microscopy showed spherical, slightly polydisperse particles whose surfaces became smoother and denser after loading, and thermogravimetric analysis revealed a bonus: the nanocomposite-filled epoxy was substantially more thermally stable than pristine epoxy, with the temperatures of 5 percent and 10 percent weight loss rising by 77 and 68 degrees Celsius respectively.

The coatings themselves were made by dispersing the loaded nanocontainers in a bisphenol A epoxy resin with a polyamine hardener, then dip-coating polished mild steel and aluminium-6061 panels and curing them at room temperature followed by a post-cure at 80 degrees Celsius. The average coating thickness was a modest 34 micrometers. Electrochemical testing in 3.5 percent sodium chloride revealed a clear optimum. For the whole extract, a 2 weight percent loading raised the charge transfer resistance of mild steel from 1,128 to 21,300 ohm square centimeters and cut the corrosion current density from 12.53 to 0.066 microamperes per square centimeter, an inhibition efficiency of 99.47 percent. The isolated quercetagetin did even better: at 3 weight percent, resistance climbed to 31,240 ohm square centimeters, corrosion current density fell to 0.0060 microamperes per square centimeter, and the corrosion potential shifted from minus 0.698 to minus 0.274 volts, yielding an inhibition efficiency of 99.95 percent relative to bare steel. Beyond that concentration, nanoparticles began to agglomerate, creating porous pathways that let electrolyte penetrate, so more was not better. The quercetagetin coating was also notably stable and non-hygroscopic, unlike the extract-loaded version at its highest loading. On aluminium-6061, the 3 percent quercetagetin coating raised resistance from 3,634 to 77,050 ohm square centimeters and delivered 99.67 percent inhibition by polarization measurements.

Perhaps the most remarkable results came from the coating’s responsive behavior. Because mesoporous silica releases its payload faster in acidic conditions, and because quercetagetin’s hydroxyl groups form more soluble anionic structures in alkaline media, the coating releases its inhibitor precisely where and when corrosion threatens, whether the environment is acidic, neutral, or basic. When the researchers scratched a coated aluminium panel with a razor blade and immersed it in salt water, scanning electron microscopy showed healing beginning within 3 hours and completing within 48 hours, with no corrosion products or leakage at the scratch site. Released quercetagetin fills the void, coordinates to the exposed metal, and hydrogen-bonds with the surrounding polymer, sealing the wound much as biological tissue repairs itself. The coating also proved self-sanitizing. In zone-of-inhibition tests, it produced a 3.0-centimeter inhibition zone against the gram-negative bacterium Escherichia coli and a 2.3-centimeter zone against the gram-positive Staphylococcus aureus, and quantitative testing under the AATCC-100 protocol showed 92 percent and 81 percent reductions in bacterial colonies respectively after one hour of contact. The stronger effect against gram-negative bacteria likely reflects differences in cell wall structure and their interaction with the flavonoid.

The significance of the work extends beyond the impressive numbers. Corrosion consumes an estimated 3.4 percent of global GDP each year, and the traditional inhibitors used to fight it, particularly hexavalent chromium compounds and heavy-metal complexes, are toxic, persistent, and carcinogenic, prompting tightening regulation under frameworks such as REACH. By converting temple flower waste, a documented source of water pollution, into a high-performance green inhibitor, the CSIR team addresses two environmental problems at once in a genuine waste-to-wealth strategy. The combination of passive barrier protection from the epoxy matrix and active, pH-triggered inhibition from the nanocontainers points toward a new generation of smart coatings for marine structures, healthcare surfaces, and industrial equipment, where a single layer of paint derived from discarded petals can resist rust, mend its own damage, and keep microbes at bay.

Subject of Research: Green multifunctional epoxy nanocomposite coatings from marigold floral waste for anticorrosive, antimicrobial, and self-healing metal protection

Article Title: Multifunctional epoxy nanocomposite coatings with anticorrosive, antimicrobial, and self-healing characteristics derived from floral biomass

Article References: Rajimol, P. R., Samuel, A. S., Ulaeto, S. B., & Rajan, T. P. D. (2026). Multifunctional epoxy nanocomposite coatings with anticorrosive, antimicrobial, and self-healing characteristics derived from floral biomass. Discover Electrochemistry, 3(1), Article 49. https://doi.org/10.1007/s44373-026-00122-5

Image Credits: AI Generated

DOI: 10.1007/s44373-026-00122-5

Keywords: corrosion inhibition, quercetagetin, marigold floral waste, mesoporous silica nanoparticles, epoxy nanocomposite, self-healing coating, antimicrobial coating, green chemistry, mild steel, aluminium alloy, electrochemical impedance spectroscopy, waste valorisation