Role of C═C Double Bond in Improving the Protective Film Properties of Schiff-Base Inhibitors on N80 Steel: An Integrated Experimental and Theoretical Study

In oil and gas operations, hydrochloric acid pickling causes severe corrosion of N80 steel. Corrosion inhibitors offer protection. However, the mechanism by which unsaturated bonds enhance performance remains unclear. This study clarifies the role of a carbon-carbon (C═C) double bond. Two structurally similar Schiff-base inhibitors were compared: cfar (with an extra C═C) and cfab. They were synthesized from 4-aminoantipyrine with cinnamaldehyde and phenylpropionaldehyde, respectively. Their inhibition performance on N80 steel in 1 M HCl was evaluated using electrochemical tests, weight-loss measurements, and surface analysis methods. The underlying mechanisms were investigated via density functional theory (DFT), molecular dynamics (MD), mean square displacement (MSD), reduced density gradient (RDG), and electron density difference (EDD) simulations. At 200 mg/L and 298 K, cfar achieved a higher inhibition efficiency (90.71%) than cfab (88.10%). The superior performance of cfar is attributed to the synergistic effect of its C═C bond. Specifically, the C═C bond (1) enhances molecular reactivity and electron donation to the iron surface (DFT, EDD); (2) promotes a more compact, parallel adsorption layer with lower porosity (MD, MSD); and (3) strengthens noncovalent interactions (RDG), leading to a smoother and more hydrophobic surface upon adsorption (X-ray photoelectron spectroscopy, scanning electron microscopy (SEM), contact angle). This work provides direct molecular-level evidence. Introducing a strategically placed C═C double bond significantly enhances the adsorption capacity and quality of the protective film of Schiff-base inhibitors. This offers a clear design principle for next-generation corrosion inhibitors.