Synthesis, performance and optimization studies of Meldrum's acid derivatives for corrosion inhibition: employing response surface methodology, surface analysis and computational study

Corrosion of mild steel in acidic environments is a serious industrial problem that causes material damage and high economic losses. This creates a strong need for effective, optimized, and environmentally friendly organic corrosion inhibitors. Meldrum’s acid derivatives are considered promising inhibitors because its highly reactive and easier to react with other derivatives thus they can strongly interact with mild steel surfaces through heteroatoms and π-electron systems. In this study, four Meldrum’s acid derivatives were successfully synthesized: 4-formylbenzonitrile Meldrum’s acid (FBMA), p-tolualdehyde Meldrum’s acid (TOMA), 2-thiophenecarboxyaldehyde Meldrum’s acid (THMA), and 4-pyridinecarboxyaldehyde Meldrum’s acid (PYMA). The synthesized compounds were characterized using Attenuated Total Reflectance–Fourier Transform Infrared (ATR-FTIR), 1H and 13C Nuclear Magnetic Resonance (NMR) spectroscopy, elemental analysis, and single-crystal X-ray crystallography. Corrosion inhibition performance was evaluated using the weight loss method, adsorption isotherms, and Gibbs free energy analysis in 1 M HCl solution. Among the four inhibitors, FBMA showed the highest inhibition efficiency of 81.48 % at an inhibitor concentration of 7 mM due to electron-withdrawing cyano (–C≡N) group, which increases the polarity of the molecule and enhances its interaction with the mild steel surface. Adsorption studies revealed that all inhibitors followed the Langmuir isotherm, indicating monolayer adsorption, while Gibbs free energy results showed that FBMA adsorbed mainly through chemisorption, whereas the other inhibitors exhibited mixed adsorption behavior. Surface analysis using scanning electron microscopy (SEM) showed a smoother mild steel surface in the presence of inhibitors compared to the blank solution. Energy-dispersive X-ray (EDX) mapping confirmed inhibitor adsorption through the detection of oxygen and nitrogen on the mild steel surface. Atomic force microscopy (AFM) further supported these findings, with the Meldrum’s acid derivatives-protected surface showing significantly lower roughness than the blank sample. X-ray photoelectron spectroscopy (XPS) confirmed Fe–O and C=O interactions, indicating chemisorption through oxygen atoms. Density functional theory (DFT) calculations showed that PYMA possessed the most favorable electronic properties, while THMA exhibited the weakest performance. Molecular electrostatic potential (MEP) and Mulliken charge analyses identified oxygen, nitrogen, and aromatic rings as the main adsorption sites. Response Surface Methodology (RSM) optimization revealed that FBMA achieved a maximum inhibition efficiency of 90.02% at an inhibitor concentration of 5.96 mM, concentration acid 0.49M, immersion time of 19.98 h, and inhibitor-to-acid ratio of 3.99: 3.01 mL. The inhibition mechanism involves both chemisorption and physisorption, forming a dense and protective film that effectively reduces corrosion. Overall, FBMA, TOMA, THMA, and PYMA inhibit mild steel corrosion by adsorbing via electron-rich oxygen, nitrogen, and sulphur atoms, as well as protonated sites. This combined adsorption forms a compact and stable protective layer, blocking active corrosion sites and significantly improving corrosion resistance, highlighting their potential as efficient, eco-friendly inhibitors for industrial acidic systems.