Orange Peel Waste Yields Magnesium Oxide Nanoparticles That Shield Stainless Steel From Acid Attack

Corrosion costs the global economy hundreds of billions of dollars every year, and few materials illustrate the challenge better than stainless steel Grade 202, a chromium- and nickel-containing alloy found everywhere from cookware and cutlery to food-processing plants, automotive components and outdoor fittings. Although the alloy owes its everyday resilience to a thin, chromium-rich passive film, that film is far from invincible. In acidic, chloride-laden environments, hydronium ions dissolve the metal while chloride ions burrow into the oxide layer and trigger localized pitting and crevice attack. Now, researchers at the University of Peradeniya in Sri Lanka report a strikingly simple and sustainable answer: magnesium oxide nanoparticles grown with the help of nothing more exotic than orange peel extract, which protect the alloy with inhibition efficiencies approaching 96 percent in hydrochloric acid.

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