Synthesis, characterization and corrosion inhibition mechanism on Meldrum’s acid derivatives experimental and theoretical study

Four derivatives of Meldrum’s acid in this study were successfully synthesised: Vanillin Meldrum’s acid (VanMA), 4-nitrobenzaldehyde Meldrum’s acid (NitroMA), Benzaldehyde Meldrum’s acid (BenMA) and 4-formylbenzonitrile (FormylMA). In ATR-FTIR analysis, a distinct stretching band corresponding to ν(C=O) was detected between 1698 – 1790 cm-1, confirming the presence of the Meldrum’s acid. Specific functional groups are identified for each compound in which VanMA, showed OH peaks (3337 cm-1), NitroMA exhibited p-NO2 peak (852 cm-1) and FormylMA displayed a C≡N peak (2238 cm-1). 13C NMR identifies a peak between 162-164 ppm, corresponding to C=O. Single crystal X-ray crystallography confirmed the structure of VanMA and BenMA, with VanMA crystallizing in the triclinic system with P-1 space group and BenMA crystallizing in the monoclinic system with P1211 space group, which these space group define the arrangement of the atom in solid. The corrosion inhibition potential of these compounds in 1M hydrochloric acid (HCl) on mild steel was evaluated through weight loss (WL) analysis, electrochemical impedance spectroscopy (EIS) and potentiodynamic polarisation (PDP) and surface analysis. FormylMA demonstrated the highest inhibition efficiency, achieving 94.86% (WL), 94.11% (EIS) and 77.89% (PDP) at a concentration 0.1mM. PDP results indicated that all inhibitors acted as mixed-type inhibitors, predominantly suppressing the cathodic reaction. Scanning electron microscopy (SEM) revealed a smoother surface in the presence of inhibitors compared to the blank sample. Energy dispersive x-ray (EDX) mapping confirmed the adsorption of inhibitors, evidenced by the presence of oxygen and nitrogen elements on the mild steel. Atomic force microscopy (AFM) further supported this observation, with the FormylMA-treated surface exhibiting reduced roughness (102 nm) compared to the blank sample (356 nm). X-ray photoelectron spectroscopy (XPS) confirms Fe-O and C=O peaks, indicating interaction via oxygen in chemisorption. Adsorption studies suggested that all Meldrum’s acid derivatives followed the Langmuir isotherm, suggesting the formation of a monolayer of inhibitor molecules on mild steel. Gibbs free energy calculation suggested chemisorption for all inhibitors, while PDP showed mixed adsorption. WL analysis shows that VanMA and NitroMA preferred mixed adsorption while BenMA and FormylMA favoured chemisorption. Molecular Electrostatic Potential (MEP) analysis highlighted high electron density around the carbonyl oxygen in Meldrum’s acid, NO2 oxygen in NitroMA and CN nitrogen in FormylMA, suggesting these atoms are the primary sites for electron donation to mild steel. Mulliken analysis confirmed oxygen, nitrogen and aromatic ring as key adsorption sites. In 1M HCl, the substituents influenced protonation sites, with VanMA favour OH deprotonation, NitroMA and FormylMA favoured nitrogen protonation, and BenMA preferred carbonyl oxygen protonation which leads to physisorption. Meldrum’s’ acid derivatives have the potential to inhibit corrosion by adsorbing onto the mild steel with oxygen atoms adsorbing via chemisorption, protonated oxygen and nitrogen, and deprotonated oxygen via physisorption in acidic conditions. Overall, experimental analysis supported by DFT study confirmed that the corrosion inhibition mechanism of Meldrum’s acid derivatives involves mixed adsorption where both electrostatic physisorption and chemisorption, contribute to the formation of protective layer on mild steel surface.