Exploring the Substitution Effect on Heterocyclic Compounds Performance as Corrosion Inhibitors for C‐Steel Under Aggressive Circumstances: A Detailed Experimental Study and Theoretical Calculations

ABSTRACT The behavior of organic inhibitors, particularly heterocyclic compounds, is significantly impacted by the replacement. Our aim is to determine the most important effects of (‐PhCl) and (‐PhCH₃) on the deportment of (PP‐PhCl) and (PP‐PhCH 3 ) during the corrosion process abstraction of C‐steel surface protection. The findings highlight the excellent performance of both inhibitors, with the highest values for PP‐PhCl and PP‐PhCH₃ reaching 95.1% and 92.3%, respectively, at the model concentration of 10 −3 M. A mixed‐type inhibitory behavior is demonstrated by the PP‐PhCl and PP‐PhCH₃ polarization curves from PDP. In the instance of PP‐PhCl, an energy effect mechanism with a beneficial anodic reaction blocking procedure is used. However, a geometric blocking mechanism is exerted by the PP‐PhCH₃. Due to the (‐PhCH₃) effect, the higher concentration showed a greater impact on the inhibitory effectiveness of PP‐PhCH₃ than PP‐PhCl. After overcoming the temperature effect, the pyran–pyrazole inhibitors continue to protect the C‐steel surface, despite a slight drop in inhibition efficiency as the temperature rises. This is explained by the fact that, particularly at low concentrations, the main cathodic region reactions are controlled rather than both reactions. According to the EIS study, the pyran–pyrazole inhibitors use the adsorption process to create a protective layer on the C‐steel surface by increasing concentration, which causes a drop in Cdl and a rise in Rp. The PP‐PhCl and PP‐PhCH₃ interact by physisorption predominance, as determined by the activation parameters and the Langmuir adsorption isotherm. This describes how sensitive the protective layer is to changes in temperature. The MEB surface characterization results approved the PP‐PhCl and PP‐PhCH₃ layers that were created. EDX analysis confirms their composition. Additionally, the theoretical calculation shows that PP‐PhCl is mostly found in the examined solution in its bi‐protonated state, whereas PP‐PhCH₃ is found in its protonated form. In addition, the DFT demonstrates that bi‐protonated PP‐PhCl is a stronger surface protector for C‐steel than protonated PP‐PhCH₃. According to FMO's theory and Fukui indices, the primary active sites that provide this protection are NH₂, N‐pyrazole heteroatoms, and O‐pyran heteroatoms. In actuality, Monte Carlo simulation (MC) has demonstrated that the molecule's orientation face to the C‐steel surface is caused by (‐PhCl) and (‐PhCH₃).