Synthesis, characterization, and corrosion protection potential of a highly effective and water-miscible surfactant for mild steel in sulfuric acid solution

In the current study, a tetracationic quaternary ammonium salt (TCQAC) was synthesized and characterized and its ability to suppress corrosion on mild steel (MS) in a 0.5 M H₂SO₄ solution was examined. Various chemical, electrochemical, and surface characterization techniques were utilized to study the inhibition efficiency of TCQAC. The TCQAC manifests 99.83% efficiency at 20 ppm concentration. Out of all the examined isotherm models, the Langmuir isotherm offered the best fit for the TCQAC adsorption on the MS surface. A very high negative value of ΔG_ads (−45.18 kJ mol⁻¹) suggests that the adsorption of TCQAC followed the chemisorption mechanism. Electrochemical studies indicate that TCQAC increases the linear and charge transfer resistances (LPR and R_ct, respectively). TCQAC slows down the anodic and cathodic Tafel reactions; however, it acts as an anodic-type inhibitor at 5, 10, and 20 ppm. The appearance of extra Cl and N signals in the energy dispersive x-ray (EDX) spectrum and an improvement in surface smoothness in the scanning electron microscope (SEM) image of the inhibited sample corroborated the adsorption method of corrosion inhibition. X-ray photoelectron spectroscopy (XPS) study indicates that TCQAC creates corrosion preventive layers by chemical adsorption. In Frontier molecular orbitals (FMOs), highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) were delocalized around the central part that comprises two benzyls, four allyls, and one hydrocarbon ((CH₂)₆) moieties and two quaternary nitrogen atoms. The outcomes of XPS and density functional theory (DFT) analyses indicate that the chemisorption of TCQAC occurs by dπ–pπ bonding with the surface iron atoms. The π-electrons of aryl and allyl moieties extensively participate in the bonding.