Corrosion behaviour of new oxo-pyrimidine derivatives on mild steel in acidic media: Experimental, surface characterization, theoretical, and Monte Carlo studies

In this work, the effects of new compounds, namely, 1-amino-5-(4-methylbenzoyl)-4-(4-methylphenyl) pyrimidin-2 (1H)-thione (AMMP), and 1-(5-(4-Methoxybenzoyl)-4-(4-methoxyphenyl) 2-oxopyrimidin-1 (2H)-yl)-3-phenylthiourea (MMOPH) has been successfully investigated as a corrosion inhibitor for mild steel in a 1 M HCl solution. This investigation has been done by electrochemical techniques (potentiodynamic polarization, and electrochemical impedance spectroscopy), surface characterization (scanning electron microscopy with energy dispersive x-ray spectroscopy, and atomic force microscopy), and theoretical calculations (density function theory and Monte Carlo simulation). The electrochemical results showed that both compounds act as mixed-type inhibitors. However, MMOPH is more efficient than AMMP (95.9% compared with 84.1% at 5 × 10−4 M and an immersion time of 1 h). Additionally, the effect of immersion time on inhibitor efficiency was studied. The current density was reduced with the presence of inhibitors from 517.93 to 56.18 and 9.96 μA.cm−2 at 5 × 10−4 M and an immersion time of 1 h for AMMP and MMOPH, respectively. In both substances, the Langmuir isotherm system showed the best fit, with physisorption and chemisorption being the types of adsorption. The results of surface characterization indicated that both compounds can be adsorbed on mild steel surfaces to minimize corrosion. The obtained Monte Carlo simulation results suggest that the inhibitors are adsorbed vertically and the formation of a protective layer on the metal surface. The density function theory calculations for inhibitors found the protonated state is more reactive than the neutral state and agree with experimental results and follow the order MMOPH ˃ AMMP. The results showed that both compounds can be used as new corrosion inhibitors for mild steel in aggressive environments.