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

The research team, led by Nafiz Ahmed Badhan and S M Yeasin Habib of Lamar University together with colleagues at Idaho National Laboratory and Clemson University, synthesized two alloys by vacuum arc melting: the four-element base alloy AlMoNbTi and its five-element counterpart AlCrMoNbTi, with chromium added in equimolar proportion. Both ingots were remelted at least five times to homogenize their chemistry and then subjected to hot isostatic pressing at 1200 degrees Celsius under 100 megapascals of pressure for four hours, a treatment designed to eliminate the casting porosity that plagues arc-melted refractory alloys. By removing such artifacts before testing, the authors ensured that the hardness values and corrosion currents they measured reflected the intrinsic character of each composition rather than flaws introduced during processing.

Microstructural analysis told the first part of the story. Backscattered electron imaging in the scanning electron microscope revealed that both alloys share a three-region architecture: a grey matrix, white island-like features, and black precipitates. Adding chromium enlarged the grey regions and increased the density of black, titanium-rich particles. Energy-dispersive X-ray spectroscopy mapping showed that aluminum dissolves relatively uniformly, while the white regions are enriched in aluminum, molybdenum, and niobium, the grey regions concentrate titanium and chromium, and the black particles are titanium-rich precipitates. Crucially, the alloy remains body-centered cubic with an ordered B2 superlattice, a structure long associated with the room-temperature brittleness of aluminum-containing refractory high-entropy alloys. The chromium addition did not dismantle this framework, but it did intensify elemental segregation within it, a change with profound consequences for how the material deforms and corrodes.

Nanoindentation, performed with a Hysitron TI 980 Triboindenter and a Berkovich tip at a maximum load of 20 millinewtons, captured the mechanical fingerprints of that segregation. The base AlMoNbTi alloy displayed hardness values ranging from 9.97 to 14.41 gigapascals, with a single, well-defined peak in the hardness distribution near 12.25 gigapascals. The chromium-containing alloy behaved very differently: its hardness distribution became bimodal, with one peak near 12.25 gigapascals and a second near 9.25 gigapascals, and its load-displacement curves scattered far more widely. Of 66 analyzed indents, roughly 42 percent landed on the softer phase. The overall average hardness of AlCrMoNbTi fell to 10.81 gigapascals, an 11.68 percent decrease relative to the base alloy, even as the reduced modulus rose modestly by about 3.1 percent to 200.57 gigapascals.

The authors trace this localized softening to a subtle disruption of crystallographic order. In the B2 structure of AlMoNbTi, aluminum and molybdenum preferentially occupy one sublattice while niobium and titanium occupy the other, and this long-range order strengthens the material by forcing dislocations to glide in paired super-dislocations across anti-phase boundaries. Drawing on prior work showing that chromium-enriched, titanium-depleted regions wet B2 domains with a more disordered A2-like phase, the team argues that chromium locally destabilizes the B2 superlattice and promotes a softer, chemically homogeneous A2 body-centered cubic phase. That loss of anti-phase-boundary strengthening, rather than the formation of hard Laves phases, which appear only in small volume fractions, best explains the bimodal hardness and the 11.68 percent softening. Notably, both alloys remain considerably harder than many other body-centered cubic high-entropy alloys reported in the literature.

The corrosion story is where chromium truly earns its reputation. Using electrochemical impedance spectroscopy in a 3.5 weight percent sodium chloride solution, the same brine concentration that approximates seawater, the researchers found that the chromium-containing alloy exhibited a 4.5 percent higher charge transfer resistance, meaning ion exchange at the metal-electrolyte interface slowed. More striking were the changes in the dielectric properties of the surface: effective double-layer capacitance dropped by 75.8 percent, and the phase-shift exponent moved 10.7 percent closer to the ideal capacitive value. Under the Helmholtz model, lower capacitance corresponds to a thicker protective layer, indicating that chromium promotes the growth of a denser, more ideal passive film on the alloy surface.

Potentiodynamic polarization tests reinforced the picture. The corrosion potential shifted positively from minus 403 to minus 356 millivolts versus the saturated silver-silver-chloride reference electrode, and the corrosion current plummeted by 44.2 percent, from 52 to 29 nanoamperes per square centimeter. Pitting potentials exceeded 1 volt versus the reference in both alloys, evidence of excellent resistance to passive film breakdown, though the chromium-bearing alloy showed a distinct secondary passivation region at potentials above 1.7 volts relative to its corrosion potential. This secondary passivation, the authors explain, is the signature of chromium’s celebrated repassivation ability: when the protective chromium oxide film breaks down at high anodic potentials, dissolved trivalent chromium ions hydrolyze inside incipient pits to form a chromium hydroxide barrier that stifles the pit and allows a new chromium-rich passive layer to reform.

Surface imaging after the polarization experiments made the improvement visible to the eye. The base AlMoNbTi alloy corroded in clustered, non-uniform patches, consistent with preferential attack along galvanically coupled, aluminum-rich pathways in the ordered sublattice network. The chromium-containing alloy, by contrast, showed a far more random and even distribution of pits, suggesting that chromium’s disruption of the ordered structure created a chemically more homogeneous surface with fewer weak points. Quantitative image analysis with ImageJ revealed that the average pitted area, as a percentage of the surface, fell from 20.02 percent to 12.28 percent, a reduction of approximately 38.66 percent attributable to chromium addition.

The authors ground these observations in thermodynamics and strengthening theory. Chromium raises the alloy’s valence electron concentration from 4.5 to 4.8, still comfortably within the body-centered cubic regime, and its smallest atomic radius in the five-element group increases lattice distortion and the atomic size mismatch parameter, which helps explain the heightened segregation. Calculations of solid-solution strengthening show that chromium itself contributes the largest single increment, roughly 1112 megapascals, more than aluminum at 705 megapascals, and that the total solid-solution strengthening of the disordered A2 phase reaches about 2060 megapascals. Combined with an estimated 68 megapascals from Orowan-type precipitation strengthening by the titanium-rich particles, the calculated hardness of the soft phase, about 7 gigapascals, lands reasonably close to the measured 9.15 gigapascals, with the residual gap attributed to grain and phase boundary strengthening and impurity effects.

The broader significance of the study lies in its demonstration that alloying additions in high-entropy systems cannot be judged by a single metric. Chromium simultaneously strengthens the passive film, enables self-healing repassivation, redistributes and suppresses pitting, and yet softens the load-bearing matrix by eroding B2 order. For engineers contemplating refractory high-entropy alloys for marine, chemical, or high-temperature service, the message is that composition must be tuned against the full property envelope. The research, funded by the U.S. National Science Foundation under award number 2138674, provides both a rigorous experimental baseline and a mechanistic framework for that tuning, showing that even within a family of famously complex metals, a single element can rewire the balance between durability and strength.

Subject of Research: Chromium alloying effects on the mechanical and corrosion properties of the AlMoNbTi high-entropy alloy

Article Title: Effects of Cr addition on the mechanical and corrosion properties of the AlMoNbTi high-entropy alloy

Article References: Badhan, N. A., Habib, S. M. Y., Fan, Z., Fan, X., Zhang, X., & Sun, C. (2026). Effects of Cr addition on the mechanical and corrosion properties of the AlMoNbTi high-entropy alloy. Journal of Materials Science: Metallurgy, 1(1), Article 18. https://doi.org/10.1007/s44492-026-00018-w

Image Credits: AI Generated

DOI: 10.1007/s44492-026-00018-w

Keywords: high-entropy alloy, AlMoNbTi, chromium addition, nanoindentation, corrosion resistance, electrochemical impedance spectroscopy, potentiodynamic polarization, pitting corrosion, B2 ordering, passive film, refractory alloy, segregation