Basic information about the interaction between inhibitors and the metal surface can be provided by the adsorption isotherm. Organic inhibitor molecules slow corrosion by adsorption to the metal surface [43]. The adsorption process was observed as a simple exchange process in which an inhibitor molecule Inh(aq.) in the aqueous phase replaces “x” number of water molecules adsorbed on the metal surface [H2O(ads) ] as follows: 45],Inh(aq.) + x H2O(ads)<=> Inh(ads) + x H2O(aq.) (5) where x was called size ratio and is simply the number of molecules d water replaced by an inhibitory molecule [46]. The adsorption of the inhibitor on the metal surface depends on the structure of the inhibitor, the nature of the metal surface, the temperature and the electrochemical potential at the metal-solution interface. The linear relationship between the surface coverage value (θ) and Cinh will be calculated to obtain the isothermal parameters. A direct relationship between the inhibition efficiency of thiourea and the degree of surface coverage (θ) [Inhibition efficiency = 100xθ] can be assumed for the different thiourea concentrations. The degree of surface coverage (θ) for the different thiourea concentrations was evaluated from potentiodynamic polarization measurements in 1M H2SO4 solution at room temperature. The obtained data were tested graphically by fitting them to various adsorption isotherms, including the Freundlich, Temkin, Flory–Huggins, Bockris–Swinkles, Langmuir, and Frumkin isotherms. The correlation coefficient (R2) was used to determine the best-fit isotherm obtained for the Langmuir adsorption isotherm. According to this isotherm, θ is related to the inhibitor concentration by the following values...... middle of paper ...... values ​​of the standard Gibbs adsorption free energy (∆G0ads) in solution aqueous up to -20 KJ/mol are consistent with an electrostatic interaction between the charged molecule and the charged metal (physisorption), while those around -40 KJ/mol or more (more negative) are associated with chemisorptions resulting from sharing or transfer of electrons from organic molecules. to the metal surface to form a coordinate type bond [55]. While other researchers have suggested that the range of ∆G0ads of chemical adsorption processes for organic inhibitors in aqueous media is between -21 and -42 KJ/mol [56]. Therefore, for the present work, the value of ΔG0ads is higher than common physical adsorption values, but lower than common chemical adsorption values ​​[57], which probably means that both physical and chemical adsorption are taking place (c i.e. complete adsorption).).