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