′-((2-methoxy-naphthalene-1-yl)methylene) pyrimidine-5-carbohydrazide (H5) and

2.3.7 Monte Carlo simulation.The interaction between the three investigated hydrazide derivatives and the carbon steel surface (CS) was studied using Monte Carlo simulations. To interpret the more adapted metallic surfaces for the simulations process, Monte Carlo simulation method was used Materials Studio 2017.
The adsorption energy (E_ads) of each inhibitor molecule was calculated as:²³

Eads = Ecomplex − (Einh + ECS)	(11)

where E_complex, E_inh, and E_CS represent the total energy of the optimized inhibitor –CS complex, the isolated inhibitor molecule, and the CS crystal, respectively.

3 Results and discussions

3.1 Weight loss (WL) tests

Weight loss tests were conducted for (H4 & H5 & H6) compounds and the corrosion rates of carbon steel were estimated. Fig. 1 illustrates a change of WL in the absence and presence of gradual concentrations of H4 at 25° for example. The curves for H2 & H3 are not shown. The (θ) and inhibition efficacy (% IE) were illustrated in Table 2. As shown from the table, the % IE raises with increasing hydrazide derivative concentrations which mean that more hydrazide derivative molecules were adsorbed on the surface which increases the surface coverage. For example, the ideal concentration needed to provide inhibition efficacy (% IE) of 93.1% was seen at 20 × 10⁻⁶ M for H4 inhibitor.

	Fig. 1 Weight loss vs. time curves for the corrosion of carbon steel in 1.0 M HCl with and without altered concentrations of compound (H4) at 25 °C.

Table 2 Weight loss tests for (H4 & H5 & H6) at 25 °C [corrosion rate (C.R.), surface coverage (θ) and inhibition efficacy (% IE)]