New Synthesized Nicotinonitrile Derivatives as Effective Corrosion Inhibitors for Carbon Steel in Acidic Environment: Electrochemical, Surface Analysis, and Quantum Methods

2-((2-Aminophenyl)thio)-4,6-dimethylnicotinonitrile (APTDN) and 4,6-dimethyl-2-(phenylamino) nicotinonitrile (DPAN) as new organic derivatives were investigated as corrosion inhibitors for C-steel in 1 M HCl at various temperatures (25–45 °C) using chemical and electrochemical methods. The adsorption of inhibitors on the C-steel surface fits to Langmuir adsorption isotherm and polarization curves showed that APTDN and DPAN are mixed corrosion inhibitors. The topography maps of atomic force microscopy, X-ray photoelectron spectroscopy, and Attenuated total refraction infrared were performed for examination of C-steel surface. Moreover, quantum chemical calculations using Density functional theory were carried out to correlate between theoretical and experimental data.