The continuous search for eco-friendly corrosion inhibitors due to differences in corrosive media remains an important point in corrosion control. The experimental studies on the corrosion inhibition of urea on mild steel in automotive gas oil (AGO) was conducted using gasometric techniques and scanning electron microscope (SEM). The theoretical approach on the density functional theory (DFT) on the urea molecule was carried out using Gaussian 09 software. The adsorption behavior of urea molecules on the surface of the mild steel was analyzed using Frumkin and Flory-Huggins adsorption isotherms models and Gibb’s free energy, respectively. The result of the experimental study shows a poor corrosion inhibitory effect of urea on mild steel in automobile gas oil (AGO) medium as the inhibition efficiency decreased from 69.30% in week 1 to 12% in week 11 at 200 ppm of inhibitor. The adsorption of urea on the mild steel surface obeys Frumkin’s adsorption isotherm model. Gibb’s free energy of adsorption of urea molecules onto mild steel surface revealed a physisorption mechanism. SEM results showed the non-inhibitive nature of urea on the studied mild steel. Quantum chemical parameters such as HOMO, LUMO, electron affinity, electronegativity, and the fraction of electrons transferred to the metal surface were calculated and interpreted to compare the experimental and theoretical results. The theoretical findings in the current investigation were not in agreement with the experimental result, thereby creating a need for further study using the electrochemical method.

The adsorption and inhibition performance of thiourea and lithium ion on aluminium corrosion in 3.5% NaCl were investigated using gravimetric measurement, scanning electron microscope (SEM) analysis and quantum chemical computational techniques respectively. Gravimetric analysis revealed that thiourea has a good inhibitory efficacy of 82% at 1 mM concentration of thiourea on the corrosion inhibition of aluminum under the conditions studied. Also, poor inhibitory effects were recorded with an increase in the concentration of inhibitor, and improvement in inhibition efficiency was observed with the addition of lithium ion. In addition, the effects of temperature (303–333K) on corrosion inhibition was investigated. The findings showed that the effectiveness of the inhibition rises with temperature. The adsorption of thiourea molecules onto an aluminium surface followed the Temkin adsorption isotherm, while the mixed inhibitor of thiourea and lithium ion followed the Langmuir adsorption isotherm model. SEM results confirmed that the inhibition mechanism is due to the formation of a protective thin film on the aluminium surfaces that prevents corrosion. Quantum chemical calculations based on the density functional theory (DFT) revealed that the presence of sulphur and nitrogen in the structure of thiourea molecules is responsible for the strong inhibitory performance due to possible adsorption with Al atoms on the metal surface. The computed experimental and theoretical parameters in this investigation are in good agreement.