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

Carbon steel is the workhorse of the petrochemical world. It is inexpensive, strong, and durable, which is why it carries oil and gas through pipelines, lines storage tanks, and forms the backbone of boilers, reactors, and heat exchangers. But wherever steel meets acid, trouble follows. In sulfuric acid, iron atoms at the metal surface lose electrons and dissolve as ferrous ions, while hydrogen ions consume those electrons at cathodic sites to evolve hydrogen gas. The result is gradual thinning, pitting, and eventual failure of equipment, with enormous economic and environmental costs. The standard defense has been organic inhibitor molecules rich in nitrogen, oxygen, or sulfur heteroatoms, which adsorb onto the metal and block the reactive sites. Concerns about the toxicity and environmental persistence of many synthetic inhibitors, however, have pushed corrosion scientists toward greener chemistry.

Chrysanthemum indicum seemed like a promising candidate. Phytochemical analyses of its flowers have revealed volatile compounds such as eucalyptol, alpha-pinene, and germacrene D, along with abundant flavonoids and glycosides including quercitrin, myricetin, and luteolin-7-glucoside. These molecules carry electron-rich oxygen and nitrogen atoms and aromatic pi-systems, exactly the structural features that allow inhibitors to anchor themselves to iron surfaces through donor-acceptor interactions. The Egyptian team collected flowers from the Daqahlia Governorate in June 2023, dried and powdered them, and extracted 200 grams of material in 800 milliliters of methanol for 48 hours before concentrating the crude extract under vacuum.

The researchers then tested the extract at concentrations from 200 to 400 parts per million in 0.5 molar sulfuric acid, using a battery of complementary techniques. Weight-loss measurements on polished steel coupons, performed according to the ASTM G31-72 standard, showed that mass loss fell steadily as extract concentration rose. At the optimum dose of 400 ppm and 25 degrees Celsius, the inhibition efficiency reached approximately 91.53 percent, the highest value recorded in the study. When the temperature was raised to 45 degrees Celsius at the same dose, efficiency dropped to 78.28 percent, a decline that carries important mechanistic information.

That temperature dependence, combined with Arrhenius analysis, told the team that the extract binds to steel largely through physical adsorption. The apparent activation energy of the corrosion process increased with inhibitor concentration, a signature of weak electrostatic interactions between adsorbed molecules and the charged metal surface that weaken as heat disrupts them. Thermodynamic parameters reinforced the picture: the adsorption was exothermic, with negative enthalpy values, while positive entropy changes reflected the displacement of adsorbed water molecules as inhibitor species attached to the surface. The adsorption data fit the Langmuir isotherm almost perfectly, with slopes near one and high correlation coefficients, indicating that the phytochemicals form a monolayer on the steel. Free energies of adsorption fell between the classic thresholds for physisorption and chemisorption, suggesting that both mechanisms contribute, with electrostatic attraction dominating and chemical coordination adding strength.

Electrochemical measurements told a consistent story. Potentiodynamic polarization curves showed that the extract suppresses both the anodic dissolution of iron and the cathodic hydrogen evolution reaction, with corrosion current density falling sharply as concentration increased. Because the corrosion potential shifted by only about 4 millivolts, far below the 85-millivolt threshold, the extract qualifies as a mixed-type inhibitor. The Tafel slopes barely changed with dose, meaning the adsorbed film simply reduces the active surface area rather than altering the fundamental corrosion mechanism. Electrochemical impedance spectroscopy added quantitative depth: charge-transfer resistance jumped from roughly 4.5 ohm-square centimeters in bare acid to about 31.1 ohm-square centimeters at 400 ppm, while double-layer capacitance plunged from about 425 to 108 microfarads per square centimeter, evidence that bulky organic molecules were displacing hydronium ions and thickening the interfacial layer.

Surface imaging provided the most visually striking confirmation. Scanning electron micrographs of steel immersed in uninhibited acid for 24 hours revealed severe roughness and widespread pitting, the classic scars of acidic attack. The surface treated with 400 ppm of the extract, by contrast, appeared smooth and largely free of pits. Energy-dispersive X-ray analysis detected nitrogen and sulfur signals on the protected surface, chemical fingerprints of adsorbed organic constituents, alongside a stronger iron signal indicating suppressed dissolution. Atomic force microscopy quantified the transformation: average roughness fell from 647 nanometers on corroded steel to just 84 nanometers on protected samples, a nearly eightfold smoothing that directly reflects the uniformity of the protective film.

Fourier-transform infrared spectroscopy then identified which functional groups do the anchoring. The free extract shows a broad band at 3237 per centimeter from O-H and N-H stretching; after adsorption this band shifts to 3214 per centimeter, indicating that hydroxyl and amine groups participate in binding through hydrogen bonding and coordination with iron. A carbonyl band at 1635 per centimeter shifted to 1653 per centimeter, consistent with oxygen lone pairs donating electron density into the empty d-orbitals of iron atoms, while C-O and C-N bands near 1032 per centimeter also changed position and intensity. Together, these shifts sketch a mixed-mode mechanism in which electron-rich donor atoms coordinate directly with iron while weaker van der Waals and hydrogen-bonding forces add coverage.

The practical significance is considerable. API 5L X70 steel is a high-strength grade used in long-distance oil and gas transmission pipelines, and acidizing operations, pickling, and industrial cleaning all expose such steels to corrosive acids. An inhibitor derived from an abundant, biodegradable flower, effective at just 400 parts per million, could reduce both the environmental footprint and the health risks associated with conventional formulations. The authors note that this is the first reported use of Chrysanthemum indicum extract for protecting this particular steel grade in sulfuric acid, and they acknowledge that direct comparisons with other plant-based inhibitors remain a task for future work.

The convergence of evidence is what makes the study compelling. Weight loss, polarization, impedance, microscopy, elemental analysis, and spectroscopy all point to the same conclusion: molecules in the chrysanthemum extract spontaneously assemble into an adherent organic film that starves the corrosion reactions of active sites. As industries worldwide face mounting pressure to replace hazardous chemicals with sustainable alternatives, the humble chrysanthemum, already valued for centuries in medicine and ornament, may find a new career guarding the steel arteries of the global energy economy.

Subject of Research: Plant-derived green corrosion inhibitors protecting carbon steel in acidic media

Article Title: Chrysanthemum indicum extract as a sustainable inhibitor for acid-induced corrosion of API 5L X70 steel

Article References: Chrysanthemum indicum extract as a sustainable inhibitor for acid-induced corrosion of API 5L X70 steel. (n.d.). https://doi.org/10.1007/s44373-026-00170-x

Image Credits: AI Generated

DOI: 10.1007/s44373-026-00170-x

Keywords: Chrysanthemum indicum, corrosion inhibition, green inhibitor, API 5L X70 steel, sulfuric acid, Langmuir isotherm, electrochemical impedance spectroscopy, potentiodynamic polarization, adsorption mechanism, plant extract, carbon steel, sustainable chemistry

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