The research, published in Polymer Bulletin by Muhammad Taqi-uddeen Safian, Pandian Bothi Raja and Mohamad Nasir Mohamad Ibrahim, describes the synthesis and optimization of a lignin-acrylamide composite, abbreviated LAA, designed specifically to protect carbon steel in water-based mud. Lignin is the aromatic polymer that gives wood its rigidity, and it is generated in enormous quantities as a by-product of the pulp and paper industry and of palm oil processing. Rather than being burned for low-grade heat, this phenolic-rich material can be chemically upgraded. The Malaysian team started with soda lignin, a type of lignin produced by alkaline pulping, and polymerized it with acrylamide, a water-soluble monomer, to create a graft composite with far better solubility in the aqueous drilling fluid than raw lignin, which tends to aggregate and settle out of solution.

Solubility is not a cosmetic detail; it is the gatekeeper of performance. A corrosion inhibitor can only protect steel if it can reach the metal surface, adsorb onto it and form a protective film that blocks the electrochemical reactions of corrosion. Lignin’s aromatic rings and hydroxyl groups provide plenty of anchoring sites for adsorption, but the native polymer’s poor dispersibility in brines and muds has long limited its practical use. By grafting acrylamide chains onto the lignin backbone, the researchers created an amphiphilic molecule that dissolves readily in the mud while still presenting the aromatic and amide functional groups that bind to steel surfaces. The result is a composite that behaves like a designed corrosion inhibitor rather than a raw biomass extract.

Getting the synthesis right, however, is a multi-variable puzzle. The yield of the grafting reaction depends on temperature, reaction time, the ratio of acrylamide to lignin and the concentration of the catalyst that initiates polymerization. Testing every combination would be prohibitively expensive, so the team deployed two complementary statistical optimization strategies. The first was the Taguchi method, a design-of-experiments technique that uses specially constructed orthogonal arrays to screen many factors with a small number of runs. Taguchi analysis produced a clear hierarchy of influence: temperature mattered most, followed by reaction time, then the acrylamide-to-lignin ratio, with catalyst concentration playing the smallest role. The model predicted a maximum composite yield of 4.893 grams under its optimal conditions.

The second strategy, response surface methodology based on a central composite design, went a step further. Where Taguchi screening identifies which factors matter and roughly where the optimum lies, response surface methodology builds a mathematical surface describing how the yield responds to simultaneous changes in each variable, then refines the optimum through analysis of variance. The CCD-RSM model predicted a maximum yield of 4.653 grams, remarkably close to the Taguchi prediction. Both approaches converged on the same operating window: a reaction temperature near 90 degrees Celsius, a reaction time of about four hours, and an acrylamide-to-lignin ratio of approximately two. This kind of head-to-head comparison of the two optimization frameworks, coupled with validation under drilling-fluid conditions, is what the authors identify as the central novelty of the study.

The convergence matters because it gives engineers confidence that the optimum is real rather than an artifact of one statistical model. Temperature dominating the hierarchy makes chemical sense: graft polymerization of acrylamide onto lignin depends on generating reactive radical sites on the lignin backbone, and higher temperatures accelerate both initiator decomposition and chain growth. But push too far and side reactions, chain termination and lignin degradation begin to erode the yield. The four-hour reaction time and the two-to-one monomer ratio represent the balance point where the grafting reaction has run to near completion without the polymerization running away into homopolymer formation. In effect, the statistics mapped the energetic sweet spot of a complex free-radical grafting reaction.

With the optimized LAA in hand, the researchers moved from the synthesis bench to the drilling-fluid laboratory, and this is where the work distinguishes itself from much of the green-corrosion-inhibitor literature. Many candidate inhibitors are evaluated only in simple salt solutions at ambient temperature, conditions that bear little resemblance to a downhole environment. Safian and colleagues instead aged their water-based mud containing the LAA additive at 90 degrees Celsius for 16 hours under API-type conditions, the standardized testing protocol of the American Petroleum Institute that mimics the thermal and mechanical stress a mud experiences during actual circulation. Only after this aging did they assess corrosion protection using weight-loss testing on carbon steel coupons, the industry’s most direct measure of how much metal a fluid destroys over time.

The results were striking. The optimized lignin-acrylamide composite achieved a maximum corrosion inhibition efficiency of 73.5 percent at a dosage of just 500 parts per million. In other words, a half-gram of plant-derived polymer per liter of mud stripped nearly three-quarters of the corrosive attack on the steel. The mechanism is consistent with what is known about lignin-based inhibitors: the aromatic rings and polar functional groups adsorb onto the steel surface, creating a barrier layer that impedes both the anodic dissolution of iron and the cathodic reduction reactions that drive corrosion. The acrylamide grafts improve the integrity and coverage of this film in the hot, saline, solids-laden mud environment, where unmodified lignin would struggle to remain dispersed and adsorbed.

The implications extend beyond a single laboratory result. Drilling operations consume vast quantities of mud, and the corrosion inhibitors added to them eventually end up in cuttings waste and produced water. Regulators worldwide are tightening restrictions on heavy-metal-based and poorly biodegradable additives, and operators are actively seeking green chemistry that does not sacrifice performance. Lignin fits the brief almost perfectly: it is cheap, abundant, renewable and derived from waste streams that would otherwise be flared or landfilled. Previous studies have explored lignin and its derivatives as corrosion inhibitors in acidic pickling solutions, alkaline-chloride media and water distribution systems, and lignin-acrylamide copolymers have found uses as paper strength additives and flocculants. The Malaysian study extends this family of materials into one of the most demanding applications in industrial chemistry, a hot, aging drilling mud in contact with carbon steel.

There is, of course, still distance between a 16-hour laboratory aging test and months of continuous downhole service. Field muds encounter shear from the drill string, contamination from formation brines and hydrocarbons, and temperatures well above 90 degrees Celsius in deeper wells, all of which will stress the inhibitor film in ways a standardized test cannot fully capture. The economics of scaling the graft polymerization from grams to tons, and the interaction of LAA with the other mud additives such as viscosifiers and fluid-loss agents, will also need attention. But the study provides something rare in the bio-based inhibitor field: a statistically rigorous synthesis optimum, cross-validated by two independent design methods, and a performance figure earned under industry-standard aging conditions rather than idealized bench chemistry. As drilling companies confront both corroding infrastructure and tightening environmental rules, a polymer built from palm and pulp waste that shields steel at 500 parts per million is exactly the kind of dual-purpose solution the industry has been waiting for.

Subject of Research: Bio-based lignin-acrylamide corrosion inhibitors for water-based drilling mud

Article Title: Bio-based lignin-acrylamide corrosion inhibitor for water-based mud: Taguchi -RSM optimization, and API-aging validation

Article References: Bio-based lignin-acrylamide corrosion inhibitor for water-based mud: Taguchi -RSM optimization, and API-aging validation. (n.d.). https://doi.org/10.1007/s00289-026-06673-2

Image Credits: AI Generated

DOI: 10.1007/s00289-026-06673-2

Keywords: lignin, acrylamide, corrosion inhibitor, water-based mud, drilling fluid, Taguchi method, response surface methodology, carbon steel, oil palm waste, green chemistry, API aging, polymer bulletin

The research team, led by Mohammad Ebrahim Jafarian of the Islamic University of Shahreza together with M. Bagherzadeh and Meysam Karimi of the Nuclear Science and Technology Research Institute in Tehran, prepared their green inhibitor using nothing more elaborate than dried walnut leaves from a private garden in Isfahan Province and double-distilled water. Five grams of washed, dried leaves were heated in 100 milliliters of water at 80 to 90 degrees Celsius, then strained to produce a clear extract. This deliberately simple, traditional preparation matters: if a corrosion inhibitor requires exotic solvents or complex synthesis, much of its environmental and economic appeal evaporates. Walnut leaves are rich in phenolic compounds, including coumaric acid, quercetin derivatives, gallic and ellagic acids, tannins, and juglone, a chemical arsenal that the authors identify as the likely source of the extract’s protective power.

To test the extract, the researchers immersed polished carbon steel coupons for 72 hours in 3.5 percent NaCl solutions containing increasing concentrations of walnut leaf extract, from 5 to 30 percent by volume. The unprotected steel lost a substantial 1.0156 grams per square centimeter over the three days, while inhibitor-treated samples lost dramatically less. The standout result came at the lowest dose tested: a 5 percent extract concentration delivered the maximum inhibition efficiency of 98.53 percent. Counterintuitively, adding more extract did not improve protection. Efficiency plateaued and even declined at higher concentrations, a nonlinear behavior the authors attribute to molecular aggregation, partial coagulation of tannins in solution, and irregular, heterogeneous adsorption that leaves localized weak points vulnerable to chloride attack.

Electrochemical measurements told a consistent story. In potentiodynamic polarization tests, the corrosion current density fell from 2.544 microamperes per square centimeter for the bare steel to 0.606 microamperes per square centimeter with 5 percent extract, and the corrosion rate dropped from 1.478 millimeters per year to 0.352 millimeters per year. Tafel analysis revealed that the inhibitor acts as a mixed-type inhibitor with predominantly anodic suppression, meaning it slows the oxidation of iron into solution more strongly than it slows the cathodic oxygen reduction reaction. The corrosion potential shifted toward more negative values and the anodic Tafel slope decreased, both signatures of a barrier layer interfering with metal dissolution. Statistical analysis confirmed that these electrochemical effects were significant, with triplicate measurements varying by less than 5 percent.

Electrochemical impedance spectroscopy added a second, independent line of evidence. The charge transfer resistance at the steel-electrolyte interface, a direct measure of how hard it is for corrosion reactions to proceed, jumped from 126 ohms per square centimeter for the blank solution to 926 ohms per square centimeter with 5 percent extract, corresponding to the 86.39 percent efficiency peak. The impedance data, fitted to a modified Randles circuit with a Warburg diffusion element, showed enlarged Nyquist semicircles and reduced double-layer capacitance, exactly what is expected when organic molecules displace water at the metal surface and thicken the electrical double layer. Analysis of surface coverage against concentration produced an excellent fit to the Langmuir adsorption isotherm, with a correlation coefficient of 0.989, indicating that the phytochemicals form an orderly monolayer on a largely homogeneous surface.

The molecular mechanism appears to combine physical and chemical adsorption. Polar functional groups such as hydroxyl and carboxyl moieties in the polyphenols and tannins can form hydrogen bonds with surface iron atoms and donate electron density into the vacant d-orbitals of iron, creating Fe-organic coordination complexes. Infrared spectroscopy of the treated surfaces revealed new carbonyl peaks between 1600 and 1700 wavenumbers characteristic of tannins, enhanced carbon-hydrogen stretching near 2900 wavenumbers from aliphatic compounds, and the coexistence of iron-oxygen and carbonyl signals, which the authors interpret as direct evidence of chemisorption alongside physical shielding. The diminished intensity of chloride-related peaks near 850 wavenumbers showed that the film was genuinely keeping the aggressive ions away from the metal.

Surface imaging sealed the case. Scanning electron micrographs of steel after three days in plain salt solution showed the pitted, blotchy texture of uniform chloride-induced corrosion, and energy-dispersive X-ray analysis detected chlorine, sodium, and oxygen on the surface, the fingerprints of iron chlorides and oxide corrosion products. On the inhibitor-treated sample, those chlorine and sodium signals vanished entirely, replaced by a carbon-rich organic film: the carbon weight percentage rose from 23.99 percent on the corroded surface to 42.80 percent on the protected one, a relative increase of 78.41 percent. The overlapping layers with distinct boundaries visible in the micrographs are the physical embodiment of the adsorbed protective barrier that the electrochemistry had already implied.

But the study also delivered a sobering caveat. When the researchers repeated the polarization and impedance experiments at elevated temperatures using the optimal 5 percent concentration, the protection collapsed. At 25 degrees Celsius the extract still delivered 76.2 percent efficiency from polarization data, but by 35 degrees Celsius and above the corrosion rate of the inhibited system exceeded that of the unprotected blank. At 70 degrees Celsius the inhibition efficiency plunged to a negative 255.1 percent, meaning the extract had become a corrosion accelerant, and the charge transfer resistance cratered from 926 ohms per square centimeter to a mere 0.893 ohms per square centimeter. The authors attribute this dramatic reversal to three converging factors: adsorption is exothermic, so heat drives inhibitor molecules off the surface; the phenolic compounds thermally degrade and oxidize, potentially forming soluble iron complexes or porous layers that actively speed up charge transfer; and higher temperatures inherently accelerate both reaction kinetics and chloride diffusion through any compromised film.

That temperature sensitivity points to physisorption as the dominant adsorption mode and defines the practical limits of the technology. Walnut leaf extract, on this evidence, is a strong candidate for near-ambient applications such as cooling water systems, marine and coastal infrastructure protection, and other chloride-rich environments that do not run hot, rather than for high-temperature industrial streams. Compared with other plant-based inhibitors reported in the literature, including rosemary extracts reaching 97 percent efficiency and Ficus tikoua leaf extract at roughly 96 percent in acidic media, walnut leaf extract’s 86.39 percent electrochemical efficiency is competitive, and its preparation as a simple aqueous extract from an abundant agricultural byproduct gives it a distinct edge in cost and sustainability.

The broader significance of the work lies in its demonstration that corrosion protection does not have to come from a petrochemical plant. By systematically combining weight loss, polarization, impedance, adsorption isotherm analysis, and two independent surface characterization techniques, the study builds an unusually complete mechanistic picture of a green inhibitor in a realistic marine simulant, while honestly documenting the concentration and temperature windows where it works and where it fails. The authors note that future work will isolate and identify the specific active compounds within the extract using chromatographic techniques, a step that could allow even more efficient formulations. For now, the humble walnut leaf, usually discarded as garden waste, has earned a place in the growing catalog of nature-derived materials that could help industry fight rust without poisoning the planet in the process.

Subject of Research: Use of walnut leaf extract as a green corrosion inhibitor for carbon steel in sodium chloride solution

Article Title: Walnut leaf extract as a green corrosion inhibitor for carbon steel in NaCl solution across temperatures from ambient to 70 °C

Article References: Jafarian, M. E., Bagherzadeh, M., & Karimi, M. (2026). Walnut leaf extract as a green corrosion inhibitor for carbon steel in NaCl solution across temperatures from ambient to 70 °C. Discover Electrochemistry, 3(1), Article 43. https://doi.org/10.1007/s44373-026-00128-z

Image Credits: AI Generated

DOI: 10.1007/s44373-026-00128-z

Keywords: corrosion inhibition, walnut leaf extract, carbon steel, green inhibitor, electrochemical impedance spectroscopy, potentiodynamic polarization, Langmuir adsorption, phytochemicals, marine corrosion, sodium chloride solution, surface analysis, temperature dependence

Carbon steel is among the most widely used engineering materials in the world, serving in automotive components, machinery, construction structures, offshore platforms, and ship hulls. Yet it corrodes readily in the presence of chloride and sulfate ions, the chief aggressive species in coastal and industrial atmospheres. Chloride ions drive localized pitting corrosion, while sulfate species raise the conductivity of the aqueous film on the metal surface and participate in the formation of iron sulfates and oxides. Traditional protection strategies, including coatings, cathodic protection, and synthetic inhibitors, often rely on compounds containing toxic heavy metals such as chromium and lead, which environmental legislation like the United States Toxic Substances Control Act and the European Union Restriction of Hazardous Substances Directive increasingly restrict.

That regulatory pressure has pushed researchers toward greener alternatives, including plant extracts and expired pharmaceuticals. Expired drugs are attractive because their active molecules typically contain unsaturated bonds, aromatic rings, and electronegative heteroatoms such as oxygen, nitrogen, and sulfur, all of which can anchor to metal surfaces. Omeprazole, a benzimidazole-based proton pump inhibitor, is particularly well suited: its structure packs nitrogen and oxygen atoms into heterocyclic rings, a sulfoxide group, and a delocalized pi-electron system capable of donating electron density to iron atoms. Previous studies of expired omeprazole in strong acids such as hydrochloric, phosphoric, and sulfuric acid have reported inhibition efficiencies around 90 percent, and an earlier electrochemical noise study by the same group found efficiencies above 90 percent for AISI 1018 steel in the same chloride-sulfate electrolyte.

The new work goes further by asking how the protection evolves with time. The team prepared AISI 1018 steel electrodes, polished them with silicon carbide papers, and immersed them in a neutral solution of 0.1 M sodium sulfate plus 3 weight percent sodium chloride. The inhibitor was the powdered content of expired commercial 20 mg omeprazole capsules, dissolved in methanol and added at nominal concentrations of 25, 50, 75, and 100 ppm. Rather than relying on a single measurement, the researchers combined open-circuit potential, potentiodynamic polarization, linear polarization resistance, and electrochemical impedance spectroscopy, following each specimen over 72 hours and extending the best condition to 96 hours.

The electrochemical data told a clear story. Polarization curves showed that the inhibitor acts mainly as an anodic-type inhibitor, suppressing iron dissolution and producing passivation regions in the anodic branch, while leaving the cathodic oxygen-reduction kinetics largely unchanged. Linear polarization resistance measurements revealed that 75 ppm delivered the best sustained performance: polarization resistance climbed steadily from about 2,298 ohm-square centimeters at the start of immersion to 5,198 ohm-square centimeters after 72 hours, corresponding to an inhibition efficiency of roughly 70 percent. Impedance spectroscopy confirmed the trend, with charge-transfer resistance rising to a maximum and the inhibition efficiency reaching 66.54 percent at 72 hours for the 75 ppm condition.

Notably, the results were not uniformly positive. Several concentration-time combinations produced negative inhibition efficiencies, meaning the inhibitor-containing solutions actually corroded faster than the uninhibited blank. At 50 ppm, for example, the polarization curves showed an apparent acceleration of corrosion. The authors retained these values deliberately, arguing that they reflect genuine electrochemical behavior rather than artifacts, and that the transition from negative to positive efficiency with immersion time demonstrates how strongly inhibition depends on both dose and exposure duration. At 100 ppm, the team suggests that excess inhibitor may cause electrostatic repulsion between adsorbed molecules, preventing efficient packing and leaving active sites exposed.

Time also proved to be a double-edged sword. When the optimal 75 ppm condition was followed to 96 hours, the impedance response collapsed, with charge-transfer resistance falling back toward blank levels. The phase-angle maxima in the Bode plots, which had risen steadily to 69.15 degrees at 72 hours, dropped to 62.47 degrees, and the fitted parameters indicated an increase in the electrochemically active area. The researchers interpret this as partial desorption, defect formation, or electrolyte penetration through the protective layer, marking the point at which the film begins to deteriorate.

Surface analysis and quantum chemistry filled in the molecular picture. Scanning electron microscopy showed far less deterioration on steel exposed to 75 and 100 ppm than on the blank, and energy-dispersive spectroscopy recorded higher relative iron content with lower oxygen and chlorine on the protected surfaces. Adsorption modeling showed that the Frumkin isotherm best described the data, with a positive interaction parameter of 2.52 indicating attractive lateral interactions between neighboring adsorbed molecules, and a negative adsorption free energy of -24.4 kJ per mole confirming that adsorption is thermodynamically favorable. Density functional theory calculations using the B3LYP functional with a water continuum model identified specific atoms as the most reactive adsorption centers: one oxygen atom, one nitrogen atom, and the sulfur atom of the sulfoxide group showed the highest dual-descriptor values and local electrophilicity, pinpointing them as the sites that donate electron density into vacant iron d-orbitals.

The proposed mechanism is therefore a sequence: omeprazole molecules displace water at the steel-solution interface, anchor through their oxygen, nitrogen, and sulfur atoms and aromatic pi-electrons, and progressively occupy electrochemically active sites, choking off charge transfer. At the optimum concentration, attractive lateral interactions help organize a compact monolayer whose stability peaks at 72 hours before slow degradation sets in. The authors caution that the material tested was a commercial formulation containing excipients rather than analytical-grade omeprazole, and that spectroscopy alone cannot prove molecular bonding to the surface.

The broader implications are twofold. Scientifically, the study demonstrates that inhibition efficiency numbers cannot be compared across techniques or time points without care, since electrochemical noise, impedance, and polarization probe different aspects of the corrosion process. Practically, repurposing expired pharmaceuticals as corrosion inhibitors could divert pharmaceutical waste from disposal streams, though the researchers stress that toxicity, biodegradation, leaching, and life-cycle impacts must be assessed before any environmental benefit can be claimed. For now, the image of a discarded heartburn capsule quietly defending a steel plate against salt and sulfate attack is a vivid reminder that valuable chemistry can outlive its expiration date.

Subject of Research: Expired omeprazole as a time-dependent corrosion inhibitor for AISI 1018 carbon steel in chloride-sulfate solution

Article Title: Expired omeprazole as a time-dependent corrosion inhibitor for AISI 1018 steel: adsorption, interfacial evolution, and film stability

Article References: Noriega, O. A. G., Nicolás, A. F., Pérez, C. A. G., Cárdenas, M. Y. D., Larios, A. K. G., Valis, A. S. M., Chavarín, J. U., & Campos, E. C. M. (2026). Expired omeprazole as a time-dependent corrosion inhibitor for AISI 1018 steel: adsorption, interfacial evolution, and film stability. Results in Chemistry, 31, Article 103904. https://doi.org/10.1016/j.rechem.2026.103904

Image Credits: AI Generated

DOI: 10.1016/j.rechem.2026.103904

Keywords: corrosion inhibition, expired omeprazole, AISI 1018 steel, electrochemical impedance spectroscopy, adsorption isotherm, Frumkin model, density functional theory, chloride-sulfate electrolyte, green corrosion inhibitors, pharmaceutical waste repurposing, anodic passivation, protective film stability

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

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