The study, published in Discover Electrochemistry by Eric M. Garcia, Hosane A. Taroco, and Julio O. F. Melo of the Federal University of São João del-Rei, demonstrates that the raw aqueous extract of Eugenia dysenterica, the cagaita tree, acts as a highly efficient green corrosion inhibitor for AISI 1020 carbon steel in 10 weight percent hydrochloric acid. What makes the result remarkable is not just the number but the method. Many plant-based inhibitors described in the literature require organic solvents, elaborate purification, or chemical modification before they can protect a metal surface. The Brazilian team skipped all of that, using nothing more than ripe fruit pulp, deionized water, a knife mill, and standard laboratory equipment to produce an inhibitor that performed on par with far more processed alternatives.

The chemistry behind the protection lies in the fruit’s rich phenolic composition. Cagaita fruits are known to contain gallic, caffeic, vanillic, p-coumaric, syringic, ferulic, and salicylic acids, along with flavonoids such as epicatechin, quercetin, and rutin. Previous work by the same group had established that the aqueous extract is dominated by quercetin and epicatechin. These molecules carry exactly the structural features that corrosion scientists look for in an inhibitor: multiple hydroxyl groups, aromatic rings, and conjugated pi-electron systems that can donate electron density to vacant iron orbitals and form stable adsorbed films. In acidic media, protonation of these molecules shrinks their frontier orbital energy gap, with protonated epicatechin dropping from 4.61 to 0.66 electron volts, a change associated with enhanced electronic reactivity and stronger interaction with the metal surface.

To characterize the extract, the researchers combined several analytical techniques. Fourier-transform infrared spectroscopy of the pulp revealed broad hydroxyl stretching near 3325 inverse centimeters, carbonyl bands at 1726, aromatic carbon-carbon stretching at 1624, and glycosidic features around 817, all consistent with phenolic and polysaccharide constituents. Thermogravimetric analysis showed that fresh cagaita pulp is dominated by moisture, losing most of its mass below 140 degrees Celsius, with only a weak exothermic oxidation event near 450 to 520 degrees. Electrospray ionization mass spectrometry in negative mode detected the characteristic quercetin ion at mass-to-charge ratio 301, while tandem mass spectrometry of the ion at 289 produced fragments exclusively matching epicatechin. Ultraviolet-visible spectroscopy showed a strong absorption maximum near 275 nanometers, the signature of aromatic phenolic chromophores acting in concert.

The electrochemical evidence was compelling. Working with AISI 1020 carbon steel electrodes of one square centimeter exposed area, polished and stabilized for over two thousand seconds at open circuit, the team recorded potentiodynamic polarization curves at one millivolt per second in a standard three-electrode cell at 25 degrees Celsius. Adding just 0.3 grams per liter of the extract slashed the corrosion current density from 8.51 times ten to the minus four to 0.68 times ten to the minus four amperes per square centimeter, an inhibition efficiency of approximately 92 percent. Crucially, the corrosion potential shifted by less than 20 millivolts, and both the anodic Tafel slope, which fell from 123 to 99 millivolts per decade, and the cathodic slope, which dropped from 193 to 124, changed simultaneously. This pattern identifies the extract as a mixed-type inhibitor, suppressing both the anodic dissolution of iron and the cathodic evolution of hydrogen through adsorption-controlled surface blocking rather than selectively targeting one reaction.

Adsorption modeling added a deeper mechanistic layer. Surface coverage, calculated from the reduction in corrosion current, rose steadily with extract concentration and plateaued as the steel surface became saturated. Among the Langmuir, Freundlich, and Temkin isotherm models tested, the Freundlich equation fit best, with a regression coefficient of 0.9973 and the lowest chi-square value, pointing to non-ideal adsorption on a heterogeneous surface, exactly what one would expect from a multicomponent extract interacting with a corroding, chemically varied steel substrate. The Langmuir fit, though slightly weaker, yielded a high apparent affinity constant of 21.99 liters per gram and an estimated standard free energy of adsorption of about minus 31.6 kilojoules per mole at 298 kelvin, indicating spontaneous adsorption with contributions from both physical and chemical interactions.

Temperature experiments reinforced the picture. Arrhenius plots of the corrosion rate at 5, 25, and 45 degrees Celsius showed linear, thermally activated behavior in both media, but the inhibited solution corroded more slowly at every temperature. The apparent activation energy decreased slightly from 53.8 to 48.6 kilojoules per mole in the presence of the extract, suggesting that the adsorbed organic layer modifies the corrosion pathway itself rather than acting as a simple physical barrier. Inhibition efficiency declined modestly at higher temperatures, consistent with partial desorption of the protective film, yet the protection remained significant even as thermal agitation increased.

Surface analysis provided the visual proof. After 24 hours of immersion in the uninhibited acid, scanning electron microscopy revealed a severely degraded steel surface, rough, porous, and scarred by heterogeneous attack. The sample protected by the extract looked markedly more intact, with a compact morphology and far fewer corrosion defects. Energy-dispersive X-ray spectroscopy showed reduced oxygen signals on the protected surface, indicating less formation of iron oxides and oxyhydroxides, while X-ray diffraction confirmed that crystalline corrosion products such as hematite, magnetite, and iron oxyhydroxides were strongly suppressed, leaving metallic iron as the dominant phase. Together, these observations matched the electrochemical data point for point.

The broader significance of the work extends beyond one fruit and one acid. The researchers harvested wild cagaita fruits on the university campus in Sete Lagoas, Minas Gerais, in compliance with Brazilian native vegetation laws, deposited a voucher specimen in a public herbarium, and processed the pulp with no chemical treatments or high-energy inputs. The extract dispersed readily in the aggressive acid without precipitation, and its inhibition efficiency compares favorably with plant-derived inhibitors reported for carbon steel in hydrochloric media, from henna and fruit peel extracts to essential oils of oregano and juniper. For industrial pickling, acid cleaning, and acid-treatment operations, where carbon steel degradation is a persistent and costly problem, a renewable, biodegradable, essentially free-byproduct inhibitor is an attractive proposition.

Challenges remain before cagaita extract reaches industrial tanks. The authors note that electrochemical impedance spectroscopy, which would quantify charge-transfer resistance and film properties, was not available for this study, and that long-term immersion tests and scale-up trials are needed. The thermodynamic parameters derived from a multicomponent extract are necessarily apparent values rather than exact molecular energies. Still, the central message stands: a simple water infusion of a Cerrado fruit, prepared with a grinder and deionized water, protected steel against one of the harshest corrosive environments in industry with 92 percent efficiency. It is a vivid demonstration that solutions to billion-dollar problems can sometimes be growing, quite literally, in the savanna outside the laboratory window.

Subject of Research: Green corrosion inhibition of carbon steel in hydrochloric acid using aqueous cagaita fruit extract

Article Title: Green corrosion inhibition of carbon steel in 10 wt% HCl solution using cagaita (Eugenia dysenterica) extract

Article References: Garcia, E. M., Taroco, H. A., & Melo, J. O. (2026). Green corrosion inhibition of carbon steel in 10 wt% HCl solution using cagaita (Eugenia dysenterica) extract. Discover Electrochemistry, 3(1), Article 57. https://doi.org/10.1007/s44373-026-00144-z

Image Credits: AI Generated

DOI: 10.1007/s44373-026-00144-z

Keywords: corrosion inhibition, green inhibitor, carbon steel, cagaita, Eugenia dysenterica, hydrochloric acid, phenolic compounds, epicatechin, quercetin, electrochemistry, adsorption isotherm, Brazilian Cerrado

The study, published in Discover Green Chemistry by M. H. N. Revon and N. Priyantha, tackles a persistent gap in corrosion science. Traditional inhibitors for stainless steel, including nitrite- and phosphate-based compounds, have come under scrutiny for environmental and health concerns, while metal oxide nanoparticles synthesized by conventional chemical routes typically rely on toxic reducing agents and stabilizers. Plant extracts have emerged as greener alternatives, but very few reports have explored fruit waste, and orange peel in particular, as a vehicle for making MgO nanoparticles aimed at protecting stainless steel surfaces in aggressive media. The mechanism by which such biosynthesized particles defend steel had also remained underexplored.

The synthesis itself is disarmingly simple. Fresh peels of Citrus reticulata were crushed and refluxed in deionized water at 90 degrees Celsius for an hour to produce an extract rich in flavonoids, polyphenols and organic acids. That extract was then added to a magnesium nitrate solution, and the pH was raised to 10 by dropwise addition of sodium carbonate. After four hours of stirring, centrifugation and calcination at 500 degrees Celsius, a pale powder of magnesium oxide nanoparticles emerged. Powder X-ray diffraction confirmed the crystalline periclase structure, with characteristic peaks at 36.9, 42.9 and 62.3 degrees matching the (111), (200) and (220) planes. Broad diffraction features between 18 and 23 degrees betrayed residual phytoconstituents clinging to the particle surfaces, a detail that would prove central to the inhibitor’s performance.

Fourier transform infrared spectroscopy painted a complementary picture. A band at 522 wavenumbers confirmed the Mg-O stretching vibration, while features at 1744 and 1219 wavenumbers corresponded to carbonyl and carbon-oxygen stretching from the adsorbed flavonoids and polyphenols. Particle size analysis revealed primary nanoparticles roughly 37.6 nanometers in diameter, although scanning electron microscopy showed that the particles tend to cluster into larger, irregular flaky aggregates upon drying. The sharp-edged, high-surface-area morphology is precisely what corrosion scientists look for in an inhibitor, since more surface area means more contact points for adhesion to the metal.

Before testing the inhibitor, the team documented just how hostile acidic chloride environments are to Grade 202 steel. Ring-shaped specimens, cleaned with deionized water and acetone and left unpolished to mimic industrial conditions, were immersed in hydrochloric acid solutions of varying strength and weighed at 24-hour intervals over at least a week. In 1.00 molar hydrochloric acid, the steel dissolved so completely within two days that its mass could no longer be tracked. Adding sodium chloride made matters dramatically worse: after seven days in 0.10 molar sodium chloride, mass loss reached about 37 percent, compared with roughly 20 percent without the salt, an increase of about 85 percent. In the harshest combination tested, 1.00 molar sodium chloride with 0.50 molar acid, the steel lost essentially all of its mass. Electron microscopy of exposed surfaces showed deep pits and rough, fractured corrosion products where the polished alloy had once been smooth.

Against this bleak backdrop, the magnesium oxide nanoparticles produced remarkable results. In 0.25 molar hydrochloric acid, a 0.05 molar dose of the particles cut the corrosion rate from 4.02 millimeters per year to 0.136, an inhibition efficiency of 96.6 percent. Raising the dose to 0.10 molar drove the corrosion rate down to a startling 0.00395 millimeters per year. Even in the more aggressive 0.50 molar acid, where the uninhibited steel corroded at 7.80 millimeters per year, the nanoparticles delivered efficiencies of 91.9 percent at 0.05 molar and higher at 0.10 molar. The team notes that both the particles themselves and the electron-donating phytochemicals from the extract contribute to this protection, since the flavonoids and polyphenols adsorb alongside the oxide and reinforce the barrier layer.

Electrochemical impedance spectroscopy provided mechanistic depth. In the three-electrode cell, with the steel as working electrode, a platinum counter electrode and a silver/silver chloride reference, the inhibited samples produced much larger semicircles in Nyquist plots, indicating substantially higher polarization resistance. Bode phase-angle plots showed the intermediate-frequency peak growing and shifting to lower frequencies in the presence of the nanoparticles, the signature of a more capacitive, resistive surface film that retards charge transfer, a film that remained stable throughout a week of testing. The solution resistance itself changed when nanoparticles were present, suggesting that the particles adsorb ions and alter ionic conductivity at the electrode surface. Double-layer capacitance dropped markedly, further evidence of dense adsorption of particles and phytochemicals onto the steel.

Tafel polarization analysis and open circuit potential measurements rounded out the electrochemical case. Adding the inhibitor shifted the corrosion potential toward more positive, more noble values while sharply lowering the corrosion current density, and because both the anodic metal dissolution branch and the cathodic hydrogen evolution branch were suppressed, the nanoparticles act as a mixed-type inhibitor. The positive shift in open circuit potential, which grew with increasing inhibitor concentration and stabilized over thousands of minutes, pointed to the formation and consolidation of a protective surface film rather than any transient effect.

The proposed mechanism hinges on synergy between the oxide core and its organic shell. The hydroxyl and carboxyl groups of the peel phytochemicals, which cap the particles during synthesis and remain adsorbed on their surfaces, promote adhesion of the nanoparticles to the steel through hydrogen bonding and electrostatic interactions. The resulting layer blocks the diffusion of hydronium and chloride ions toward the metal while the extract simultaneously improves dispersion stability, preventing the agglomeration that would otherwise reduce surface coverage. Optimal dosing reflects this balance: 0.10 molar suffices in 0.25 molar acid, whereas 0.50 molar acid demands 0.20 molar for full protection, since lower doses cover too little surface and excessive loadings risk particle clumping. Compared with previously reported inhibitors, the magnesium oxide system performs on par with gold nanoparticles at a fraction of the cost, and far outpaces zinc oxide, which dissolves in acid.

The implications extend well beyond the laboratory. Stainless steel Grade 202 is normally confined to mild service because of its vulnerability to localized corrosion, but the authors suggest that phytochemically protected surfaces could survive moderate and even aggressive industrial conditions, from pickling baths to marine settings. The approach also exemplifies circular economy chemistry, converting a ubiquitous food-processing waste stream into a functional nanomaterial without toxic reagents. And the same phytochemical-capped magnesium oxide particles hold promise in drug delivery, agricultural fertilizers, active food packaging, ultraviolet protection in cosmetics and water purification. If the green synthesis scales as readily as it performed in acid, the humble orange peel may prove to be one of the most valuable byproducts in the fight against corrosion.

Subject of Research: Green synthesis of MgO nanoparticles using orange peel extract for corrosion inhibition of stainless steel Grade 202 in acidic media

Article Title: Phytochemically-aided synthesis of MgO nanoparticles facilitated by orange peel extract for corrosion inhibition of stainless-steel Grade 202 in acidic media

Article References: Phytochemically-aided synthesis of MgO nanoparticles facilitated by orange peel extract for corrosion inhibition of stainless-steel Grade 202 in acidic media. (n.d.). https://doi.org/10.1007/s44509-026-00009-z

Image Credits: AI Generated

DOI: 10.1007/s44509-026-00009-z

Keywords: corrosion inhibition, magnesium oxide nanoparticles, orange peel extract, stainless steel Grade 202, green synthesis, hydrochloric acid, electrochemical impedance spectroscopy, Tafel polarization, pitting corrosion, phytochemicals, open circuit potential, sustainable materials

Mild steel is the workhorse metal of pipelines, automobiles, construction, and machinery because it is cheap, ductile, and strong, but its Achilles heel is corrosion. One of the standard industrial remedies is acid pickling, in which hydrochloric, sulfuric, or phosphoric acid strips rust and mineral scale from steel surfaces before further processing. The problem is that once the scale is gone, the acid keeps attacking the freshly exposed metal. Synthetic organic inhibitors can slow this secondary attack, but many are expensive, poorly biodegradable, and environmentally hazardous, which has pushed corrosion scientists toward so-called green inhibitors derived from plant extracts rich in nitrogen, oxygen, sulfur, and phosphorus-bearing phytochemicals capable of bonding to metal surfaces.

Cosmos sulphureus caught the team’s attention because prior phytochemical surveys had already catalogued a dense inventory of flavonols, anthocyanins, chalcones, and related phenolics in its tissues, molecules loaded with hydroxyl, carbonyl, and aromatic groups that are ideal electron donors for coordination with iron. The researchers collected matured aerial parts of the plant around the university gardens, shade-dried and ground them, and extracted roughly 5.56 grams of crude material from 300 grams of powder using a Soxhlet apparatus with 95 percent ethanol, yielding about 1.85 percent by weight. Fourier transform infrared spectroscopy then confirmed the presence of broad hydroxyl stretching near 3307 inverse centimeters, methylene bands at 2925, carbonyl absorptions around 1624, and ester, ether, and phenolic C-O stretches between roughly 1047 and 1268 inverse centimeters, precisely the functional architecture needed for metal binding.

The electrochemical evidence came from a standard three-electrode cell in which mild steel coupons of API X70 grade pipeline steel served as the working electrode in one molar hydrochloric acid at 298 kelvin. Electrochemical impedance spectroscopy, which measures how the metal-solution interface resists charge flow, showed a well-defined semicircular capacitive loop for the blank acid, signaling rapid metal dissolution. As the extract concentration rose from 200 to 1000 parts per million, the charge transfer resistance climbed while the double-layer capacitance fell from 6.55 x 10^-4 to 2.11 x 10^-4 microfarads per square centimeter, a classic signature of organic molecules displacing water and crowding the surface with an insulating barrier. Inhibition efficiency from impedance measurements reached 88.77 percent at the highest concentration, up from 76.93 percent at the lowest, confirming a concentration-dependent protective effect.

Potentiodynamic polarization, which sweeps the electrode potential and extrapolates the anodic and cathodic Tafel branches, delivered the headline number. The corrosion current density collapsed from 3.83 x 10^-4 to 2.77 x 10^-5 amperes per square centimeter in the presence of 1000 ppm of the extract, corresponding to 92.77 percent inhibition. Because the corrosion potential shifted by less than 85 millivolts relative to the blank, the extract qualifies as a mixed-type inhibitor, damping both the anodic dissolution of iron and the cathodic hydrogen evolution. Polarization resistance values rose steadily with inhibitor concentration, and when the temperature was raised to 308 kelvin, efficiency fell to 69.45 percent, indicating that the adsorbed film loosens with heat but remains appreciably stable. The team deliberately capped testing at 308 kelvin to avoid thermal degradation of fragile flavonoid glycosides such as rutin.

Scanning electron microscopy provided the visual proof. Polished steel coupons emerged smooth, coupons bathed in plain hydrochloric acid for eighteen hours came out rough and pitted, and coupons immersed in acid containing the extract retained a comparatively even surface. Together with the sharp drop in double-layer capacitance, which reflects the replacement of high-permittivity water at the interface by a lower-permittivity organic layer, the micrographs support the formation of a continuous protective film that physically bars aggressive chloride ions from reaching the metal.

To understand which of the weed’s countless phytochemicals actually do the work, the team turned to density functional theory calculations on five representative molecules reported in the literature for Cosmos sulphureus: cosmonidin, cosmonidin 4′-O-glucoside, chlorogenic acid, quercetin, and rutin. Geometry optimizations at the B3LYP/6-311G(d,p) level yielded frontier molecular orbital energies, electronegativities, hardness, softness, dipole moments, and the fraction of electrons transferred. High HOMO energies in cosmonidin, chlorogenic acid, and rutin point to facile electron donation, while high softness in cosmonidin 4′-O-glucoside and quercetin indicates molecules that deform easily to bond with the metal. All selected molecules showed a fraction of electrons transferred below 3.6, the threshold beyond which electron-donating ability ceases to correlate with inhibition efficiency, meaning each can feed electron density into iron’s vacant d-orbitals through coordinate bonds.

Local reactivity descriptors sharpened the picture further. Fukui indices and dual descriptors computed with UCA-FUKUI software identified the specific carbon and oxygen atoms most willing to surrender electrons, including particular positions in caffeic acid, chlorogenic acid, cosmonidin, quercetin, and rutin. Mulliken charge analysis flagged electron-rich oxygen atoms in the phenolic and carboxylic groups as the primary donor centers. Monte Carlo simulations in the Adsorption Locator module then placed each molecule over a cleaved Fe(110) surface surrounded by roughly 200 explicit water molecules, and the resulting adsorption energies for chlorogenic acid, cosmonidin, and rutin were consistent with chemisorption, the stronger and more durable of the two adsorption modes. Side views of the most stable configurations show the large planar molecules lying flat against the iron lattice, acting as steric barriers against the corrosive medium.

The proposed mechanism weaves these threads together. Chloride ions first displace water molecules and form an anionic underlayer on the steel, which electrostatically attracts protonated heteroatoms of the phytochemicals. Simultaneously, lone pairs on oxygen and the pi-electron clouds of aromatic rings donate into iron’s empty d-orbitals, anchoring a dense chemisorbed film that suppresses both anodic iron dissolution and cathodic hydrogen liberation. The authors conclude that Cosmos sulphureus extract is a competitive, natural, and sustainable inhibitor suitable for room-temperature acid pickling at around 1000 ppm, and they suggest that deeper experimental and theoretical work could push the efficiency further and pin down the molecular mechanism with even greater precision.

Subject of Research: Green corrosion inhibition of mild steel in hydrochloric acid using Cosmos sulphureus plant extract studied by electrochemistry and computational simulation

Article Title: Electrochemical and computational study of a weed based inhibitor for mild steel corrosion in an acidic medium

Article References: Kotupalli, M. R., & Pulapa, V. K. R. (2026). Electrochemical and computational study of a weed based inhibitor for mild steel corrosion in an acidic medium. Discover Electrochemistry, 3(1), Article 81. https://doi.org/10.1007/s44373-026-00168-5

Image Credits: AI Generated

DOI: 10.1007/s44373-026-00168-5

Keywords: Cosmos sulphureus, green corrosion inhibitor, mild steel, hydrochloric acid, acid pickling, electrochemical impedance spectroscopy, potentiodynamic polarization, density functional theory, Monte Carlo simulation, Fukui indices, plant extract, chemisorption