Twin-Ringed Molecule Shields Steel From Acid With 99% Efficiency

Mild steel is the workhorse of modern industry, prized for its strength, low cost and easy fabrication, yet it dissolves alarmingly fast when it meets acid. Petroleum refining, acid pickling, descaling and oil-well acidizing all expose steel components to aggressive acidic media, producing billions in economic losses each year alongside serious safety and environmental concerns. A research team led by Ankit Kumar of Hindu College, University of Delhi, together with collaborators in India, Nigeria and South Africa, now reports a strikingly effective answer: a newly synthesized bis-benzimidazole diamide compound that suppresses steel corrosion in sulfuric acid with efficiencies approaching 99 percent.

The molecule, named N1,N4-bis((1H-benzo[d]imidazol-2-yl)methyl)cyclohexane-1,4-dicarboxamide, or BCHDA, was assembled by coupling 2-aminomethylbenzimidazole with cyclohexane-1,4-dicarboxylic acid in pyridine, using triphenyl phosphite as a dehydrating agent. The reaction produced a white solid in 44 percent yield with a melting point of 310 degrees Celsius. Its structure was confirmed through elemental analysis, FT-IR and UV-visible spectroscopy, proton and carbon-13 NMR, high-resolution mass spectrometry and single-crystal X-ray diffraction. The proton NMR revealed a roughly 7:1 mixture of cis and trans isomers around the central cyclohexane ring, a detail the team tracked carefully through the characteristic resonances of the axial and equatorial hydrogens.

What makes BCHDA special is its architecture. Where most previously studied benzimidazole inhibitors carry a single fused aromatic ring, BCHDA carries two, linked through amide bridges to a cyclohexane core. Each benzimidazole unit is rich in electron-donating nitrogen atoms, and the amide groups add further oxygen and nitrogen donor sites. In principle, this gives the molecule multiple anchoring points for bonding to an iron surface, along with an extended conjugated pi-electron system capable of sharing electron density with vacant metal d-orbitals. The researchers set out to test whether this dual-ring design translates into superior protection.

The electrochemical evidence was unambiguous. Using potentiodynamic polarization in 0.5 molar sulfuric acid, the team found that adding BCHDA suppressed both the anodic dissolution of iron and the cathodic evolution of hydrogen gas. The corrosion potential shifted by less than 85 millivolts relative to the blank solution, the accepted signature of a mixed-type inhibitor acting on both halves of the corrosion reaction. At the highest concentration tested, 10 to the minus 3 molar, and at 298 kelvin, the corrosion current density fell so dramatically that the calculated inhibition efficiency reached 99.13 percent. Efficiency rose with concentration but declined with temperature, dropping to 92.80 percent at 328 kelvin at the same dose, a pattern consistent with partial desorption of the inhibitor film as thermal agitation increases.

Electrochemical impedance spectroscopy told a matching story. The Nyquist plots showed depressed capacitive semicircles whose diameters grew steadily with inhibitor concentration, reflecting a rising charge-transfer resistance at the metal-electrolyte interface. The data fitted well to a two-time-constant equivalent circuit, with one relaxation process assigned to the dielectric response of the adsorbed inhibitor film and the other to charge transfer across the double layer. From the impedance measurements the team derived a maximum inhibition efficiency of 94.32 percent at 10 to the minus 3 molar and 298 kelvin. The small discrepancy with the Tafel-derived figure arises because the two techniques probe different aspects of the corrosion process, one under finite overpotential and the other near open circuit, yet both showed the same concentration-dependent trend.

Adsorption analysis showed that the surface coverage data followed the Langmuir isotherm almost perfectly, indicating monolayer adsorption on energetically equivalent sites with negligible interaction between adsorbed molecules. The standard free energy of adsorption came out near minus 40 kilojoules per mole, right at the boundary between physical and chemical adsorption, while the adsorption enthalpy of minus 72.23 kilojoules per mole pointed clearly toward dominant chemisorption. The negative adsorption entropy indicated that the molecules become more ordered as they assemble into a protective layer at the metal-solution interface. Arrhenius analysis added further support, with activation energies for the inhibited systems exceeding those of the blank acid, confirming that the adsorbed film acts as a genuine kinetic barrier.

Direct imaging sealed the case. Scanning electron micrographs of polished steel showed a smooth surface marked only by polishing lines. After 24 hours in uninhibited sulfuric acid the surface was ravaged by cracks, pits and irregular corrosion products. In acid containing BCHDA the damage was dramatically reduced, with the 10 to the minus 3 molar sample retaining the smoothest morphology. Energy-dispersive X-ray analysis quantified the difference: iron content on the surface fell from 98.73 percent on polished steel to 56.35 percent after uninhibited acid exposure, but recovered to 71.52 percent in the presence of the inhibitor, while oxygen from oxide corrosion products dropped from nearly 40 percent to about 13 percent. Crucially, carbon and nitrogen signals appeared on the protected surfaces, direct fingerprints of adsorbed BCHDA molecules.

To understand why the molecule works so well, the team turned to density functional theory at the B3LYP/6-311+G(d,p) level, computing descriptors for both the neutral molecule and its protonated form, which predominates in sulfuric acid. The neutral species showed a HOMO energy of minus 6.09 electron-volts, a small frontier orbital gap of 1.74 electron-volts and a high electrophilicity index, all markers of a molecule well suited to donating and accepting electron density at a metal surface. Protonation widened the gap to 3.06 electron-volts and reduced the electron-donating capacity, yet the protonated form remained strongly surface-active. Natural bond orbital analysis revealed powerful intramolecular charge-transfer interactions, including a sigma-to-lone-pair stabilization of 43.04 kilocalories per mole in the neutral molecule and a pi-to-pi-star delocalization of 45.23 kilocalories per mole in the protonated species, confirming that the conjugated framework stays electronically robust under acidic conditions.

Molecular dynamics simulations on a four-layer iron (110) slab in aqueous sulfuric acid provided the final molecular-scale picture. The neutral BCHDA molecule bound to the surface with an interaction energy of minus 185 kilocalories per mole, lying nearly parallel to the metal to maximize contact between its pi-system and the iron atoms. The protonated form, though slightly weaker at minus 160 kilocalories per mole, still adsorbed spontaneously through a synergistic combination of electrostatic attraction to the sulfate-charged surface and residual donor-acceptor bonding through unprotonated nitrogen sites. Together, the experiments and simulations reveal a dual-mode mechanism: rapid electrostatic coverage by protonated molecules followed by durable chemisorption of neutral ones through nitrogen and oxygen lone pairs and aromatic pi-electrons. The authors argue that this integrated experimental-theoretical strategy offers a blueprint for rationally designing the next generation of corrosion inhibitors, positioning bis-benzimidazole derivatives as promising candidates for protecting steel in some of the harshest acidic environments industry can produce.

Subject of Research: Corrosion inhibition of mild steel in sulfuric acid by a bis-benzimidazole diamide derivative

Article Title: Experimental and computational studies on the corrosion inhibition of Bisbenzimidazole diamide derivative on mild steel in acidic media

Article References: Kumar, A., Jain, R., Kumar, S., Sanyukta, Shabnam, Vashisht, H., Baxi, S., Mahiya, K., Mathias, G. E., Anadebe, V. C., Olasunkanmi, L. O., & Ebenso, E. E. (2026). Experimental and computational studies on the corrosion inhibition of Bisbenzimidazole diamide derivative on mild steel in acidic media. Discover Electrochemistry, 3(1), Article 86. https://doi.org/10.1007/s44373-026-00175-6

Image Credits: AI Generated

DOI: 10.1007/s44373-026-00175-6

Keywords: corrosion inhibition, mild steel, benzimidazole, sulfuric acid, electrochemical impedance spectroscopy, potentiodynamic polarization, density functional theory, molecular dynamics, Langmuir isotherm, chemisorption, SEM-EDX, protective film