Walnut Leaves Turn Out to Be a Powerful Green Shield Against Steel Corrosion

Corrosion is one of the most expensive and stubborn problems in modern industry, quietly devouring pipelines, storage tanks, cooling systems, and marine structures made of carbon steel. Traditional defenses rely on organic inhibitors synthesized from petrochemicals, many of which are toxic, poorly biodegradable, and increasingly out of step with sustainability goals. Now a team of Iranian researchers has reported that an unlikely candidate, an ordinary aqueous extract of walnut leaves, can protect carbon steel in a salt solution that mimics seawater remarkably well, achieving inhibition efficiencies as high as 98.53 percent in long-term weight loss tests and 86.39 percent in electrochemical measurements. The study, published in Discover Electrochemistry, is the first to examine walnut leaf extract in a neutral 3.5 percent sodium chloride medium, a condition that closely reproduces the marine and coastal environments where corrosion does its worst damage.

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