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

3.3 (EIS) measurements

The corrosion of carbon steel in 1 M HCl medium with and without various concentrations of (H4 & H5 & H6) was researched by EIS technique at 25 °C after 30 min. of immersion. Fig. 7 shows circuit model used to carry out EIS experiment and Fig. 8 shows the Nyquist plot for carbon steel in 1 M HCl medium in the absence and presence of various amounts of (H4 & H5 & H6). The way that impedance charts have as semi-round appearance clarifies that the oxidation of carbon steel in 1 M HCl is faced by a charge transfer impedance process.³⁵ The semi-circle is observed in EIS measurements and the diameter of semicircle is increased by increasing (H4, H5 and H6) concentrations. This is attributed to the occurrence of low charge transfer in the solution.³⁶ The charge transfer resistance R_ct at active carbon steel/electrolyte interface is increased by increasing (H4, H5 and H6) concentrations confirmed by the increase in radius of the semicircle. The increase in resistance value predicted the decrease in electronic conduction which results into low-kinetic reaction.³⁷ Fig. 9 shows The Bode plot for carbon steel in 1 M HCl at gradual concentrations of (H4) at 25 °C which shows the same behavior. The circuit appeared in Fig. 7 was applied to calculate (R_P) from eqn (18):

RP = (Rct × RL)/(Rct + RL)	(18)

where (L), is the inductance, R_L is the inductive resistance.³⁸

	Fig. 7 Circuit used to investigate EIS.

	Fig. 8 The Nyquist plots for corrosion of C-steel in 1.0 M HCl in the absence and presence of different concentrations of compound (H4) at 25 °C.

	Fig. 9 The Bode plots for C-steel in 1 M HCl at different concentrations of inhibitor (H4) at 25 °C.

EIS information from Table 7 illustrates that, the R_P values raised (due to increase in the thickness of double layer) and the C_dl values decreased with the increase of (H4 & H5 & H6) amounts, which is most probably due to the decrease in local dielectric constant and/or increase in the thickness of the electric double layer. This suggests that these derivatives act through adsorption of inhibitor molecules on the metal/acid interface.³⁸ This is because of the continuous substitution of (H4 & H5 & H6) molecules by H₂O molecules adsorbed on the carbon steel surface and decreasing the degree of the dissolution reaction. The large R_P values are referred to a small corrosion procedure.³⁹ C_dl is lowered with increasing concentration because of the decrease of the dielectric constant or may be due to the enlargement double layer,⁴⁰ which assure that the adsorption of (H4 & H5 & H6) mitigates carbon steel corrosion for a large extent.
Table 7 Electrochemical kinetic parameters obtained by EIS technique for in 1 M HCl without and with various concentrations of investigated compounds at 25 °C