Predicting the interaction between organic layer and metal substrate through DFTB and electrochemical approach for excellent corrosion protection

Abstract

Despite the growing interest in dealing with how to control the corrosion behavior of steel alloys via organic corrosion inhibitors (CIs), the adsorption mechanism of CIs has remained less understood with respect to the physical–chemical interactions as well as the self-assembly of organic coatings, which might be the primary sources for excellent electrochemical resistance. For this purpose, a new carbocyclic compound, namely 4-Hydroxy-3-(2-methoxybenzoyl)-2,6-bis(4-methoxylphenyl)-4-(2-methoxyphenyl)cyclohexane-1,1dicarbonitrile (PTC), was synthesized as a sustainable-green corrosion inhibitor for mild steel (MS) in HCl solution in a bid to understand the organic-metal mechanism, the adsorption behavior, and the correlation between charge transfer phenomena and corrosion properties of PTC compound. NMR (1H and 13C) spectroscopy, electrochemical techniques and surface analysis were used to characterize and evaluate the inhibitive performance of PTC compound on the metal substrate. The electrochemical results revealed that the PTC inhibitor exhibited high corrosion resistance with inhibition efficiency reaches 92.86 % at 10−3 M due to the significant growth of organic layer sealing the micro-defects present in corroded metal. Furthermore, thePTC inhibitor showed good performance at all temperatures (303–333 K) studied and maintainedprotective ability at the maximum temperature. Then, the protective layer assembled by the adsorption of PTC inhibitor presents robust protection and reliable corrosion stability. Multi-level theoretical calculations based on density functional theory (DFT), density functional based tight-binding (DFTB) and molecular dynamic (MD) simulations were performed to explore the corrosion protection mechanism activated by the presence of PTC inhibitor. As a result, the PTC-surface interactions are mainly dominated by the formation of strong covalent bonds such as N–Fe and O–Fe in the parallel adsorption geometries, in which the formation of the organic layer is consistent with improved charge transfer behavior. Thus, the PTC molecule was preferentially adsorbed through polar functional groups and exhibits high adsorption energy (–6.43 eV), resulting from self-assembly triggered by an organic-metal interaction. This was confirmed experimentally by the results of electrochemical assessments, which showed that reliable and excellent barrier properties were provided for more than 72 h owing to the significant role of functional groups in the π–π interactions of adsorbed PTC molecule. Finally, the computational perspectives provide a profound explanation for the interfacial mechanism of the PTC molecule and show a good correlation with the experimental observations.