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

2.2 Carbon steel sheets

All experiments were done by carbon steel sheets with dimensions 1 × 1 × 0.2 cm approximately. Carbon steel composed mainly of iron with traces of some different elements (wt%): (0.06% Si, 0.001% Ti, 0.022% P, 0.08% C 0.010% S, 0.030% Al, 0.3% Mn).²⁰ Working electrode for electrochemical experiments were made of carbon steel piece (area of 0.5 cm²) attached by welding to a bar of copper and covered with glass. The auxiliary anode was a platinum layer (1 cm²), the saturated calomel electrode is the reference electrode (SCE).

2.3 Methods used for corrosion calculations

2.3.1 Weight loss (WL) method.Weight loss measurements were done using carbon steel sheets measuring 1 × 1 × 0.2 cm. These sheets were first abraded carefully with (400, 600, 800 and 1200 grit size) coarseness emery sheet, washed and dried with bidistilled water before being weighed and sunk into 1 M HCl glass beakers of 100 ml + gradual concentrations of (H4 & H5 & H6). Glass beakers were put in a water bath with an automatic thermostat to achieve experiments in different temperatures; all experiments allowed to air. Sheets were removed from beakers every 30 minutes for 3 hours, washed with distilled water, dried, and weighed exactly. Weight loss was estimated for each period, degree of surface coverage (θ) and inhibition efficiency (% IE) of (H4 & H5 & H6) was determined from eqn (1):

% IE = θ × 100 = [(W − W)/W^0] × 100	(1)

where W and W⁰ are the values of the weight loss in the presence and absence of the inhibitors, respectively.
2.3.2 Electrochemical measurements.
2.3.2.1 Potentiodynamic polarization (PDP) method.The PDP experiments were conducted in a three-electrode cell, a carbon steel sheet working electrode (WE) attached to a copper bar and covered with glass, a reference electrode (saturated calomel electrode) (SCE) and a counter electrode platinum plate (CE). In each run, 100 mL beakers of 1 M HCl were examined without and with the addition of gradual inhibitor concentrations. The study was carried out using Potentiostat/Galvanostat/ZRA (Gamry reference 3000), and the experiment was configured with electrochemical software provided by a computer. The working electrode were adjusted for at least 30 min to achieve the equilibrium before measurements. The polarization drawings were carried out by sweeping the electrode potential below 1 mV s⁻¹ sweep rate and under air environment in the range from −500 to +500 mV versus open circuit potential. The cathodic branch of the Tafel curve was extrapolated to the corrosion potential, E_corr, for the measurement of corrosion current densities. The tests were performed with gradual inhibitor concentrations at 25 °C. The level of surface coverage (θ) and percentage of inhibition efficiency (IE%) is measured from eqn (2):

% IE = θ × 100 = [Icorr − Icorr(inh)/Icorr] × 100	(2)

where I_corr(inh) and I_corr are the inhibited and uninhibited corrosion current density, respectively.
2.3.2.2 Electrochemical impedance spectroscopy (EIS) method.The experimental EIS were estimated after open circuit potential (OCP) measurement using the equivalent circuit. The EIS measurements was carried out between 1 Hz to 100 kHz frequency, the perturbation was conducted with signal amplitude 10 mV. Essential parameters deduced from Nyquist graph (that is produced from computer program-Gamry) are the impedance R_p and the double layer capacitance C_dl,²¹ Bode diagrams were also drawn. % IE was got from the eqn (3) and C_dl was got from eqn (4):

% IE = [1 − (R^0p/Rp)] × 100	(3)

where R⁰_p and R_p are the polarization resistances without and with the inhibitor, respectively,

Cdl = 1/(2πfmaxRp)	(4)

where f_max is the angular frequency; C_dl is the double layer capacitance.