Inhibition Effect of Benzohydrazide Derivatives on Corrosion Behaviour of Mild Steel in 1 MHCl

e inhibiting effect of N-benzylidenebenzohydrazide (BBH) and N-(3-phenylallylidene) benzohydrazide (PABH) on the corrosion inhibition of mild steel in 1M HCl solutions were determined by weight loss and electrochemical methods. ese inhibitors were adsorbed on the surface according to Langmuir adsorption isotherm. Both BBH and PABH have offered inhibition efficiencies upto 97%. e free energy of activation for the corrosion process has suggested the participation of these molecules in the corrosion process. e adsorbed inhibitor complex has offered barrier protection and prevented the corrosion.e dissolution of iron was also retarded thereby they inhibited corrosion. e surface morphology analysis con�rmed the presence of a �lm on the surface.


Introduction
Acidizing inhibitors are added to reduce base metal corrosion [1]. Most of the acid corrosion inhibitors are organic compounds such as those containing N, S, O, and aromatic rings. ey adsorb on the metal surface. In most of the studies, the formation of donor-acceptor surface complexes between free or electrons of an inhibitor and vacant d-orbital of metal atoms was proposed [2][3][4]. e adsorption of organic inhibitors at the metal/solution interface takes place through the replacement of water molecules by organic molecules according to the following process [5]: where Org (sol) and Org (ads) are organic molecules in the solution and adsorbed on the steel surface, respectively, is the number of water molecules replaced by the organic molecules. e Schiff base of cinnamaldehyde and panisidine gave 65% inhibition [6] in acid solutions at 55 ∘ C. Other Schiff base compounds have offered better inhibition than their amines and aldehydes [7][8][9][10][11][12]. Benzimidazole molecule and their derivatives have been exploited as corrosion inhibitors, as the molecule has two anchoring sites available for surface bonding which are the nitrogen atom with its lonely sp 2 electron pair and the aromatic rings [13][14][15][16]. Triazoles, substituted triazoles adsorb on steel surfaces and inhibit corrosion [17][18][19]. Substituted thioamides were found to offer inhibition by forming Fe-inhibitor complex on the surface [20]. e search for environmental friendly corrosion inhibitor has resulted in the development of triazole derivatives [21]. e inhibition action of hydrazone derivatives was performed via adsorption on the surface obeying Frumkin adsorption isotherm [22]. In this paper, inhibitive action of BBH and PABH compounds on mild steel corrosion in 1 M HCl was studied using weight loss study and electrochemical techniques. e effect of temperature on the corrosion behaviour of mild steel in 1 M HCl with optimum concentration of inhibitor was studied in the temperature ranging 303-383 K.

Experimental Details
Mild steel coupons composed of 0.760% C, 0.019% Mn, 0.050% Cr, 0.026% Si, 0.012% P, 0.023% Al, 0.135% Cu, 0.050% Ni, and remainder being iron were used. Mild steel, strips coated with lacquer with an exposed area of 1 cm 2 were used. BBH and PABH inhibitors were newly synthesized by condensation of benzohydrazide with appropriate aldehydes. ese compounds were characterized through their spectral data. eir purity was con�rmed by TLC. Table 1 presents the structural formulae of these compounds. A conventional three-electrode cell containing platinum foil as counter electrode, saturated calomel electrode (SCE) provided with the Luggin capillary as reference electrode, and a mild steel specimen as working electrode were used.
Experiments were performed using EG & G Electrochemical analyzer (model 6310) at 303 K. Electrochemical impedance spectroscopy measurements were carried out between 100 kHz-10 mHz frequency range at steady open circuit potential with an amplitude of 10 mV. e protective �lm formed on the surface of the mild steel specimens was investigated by SEM.

Weight Loss Studies.
Corrosion inhibition efficiencies (%) offered by N � -benzylidenebenzohydrazide at 303 K aer 3 hours immersion are given in Table 2. IE % was increased with increase of concentration BBH. Table 3 presents the % IE efficiency offered by N � -(3-phenylallylidene) benzohydrazide. 2.00 mM concentration of PABH has offered 97.43% IE. Corrosion inhibition studies were carried out for different temperatures aer immersing the steel specimens for an hour. At different temperature studies, BBH and PABH have offered good corrosion inhibition. However, the inhibition efficiencies were decreased with rise in temperatures (Table 4). High inhibition efficiency of these compounds were attributed to the presence of extensively delocalized electrons of the phenyl rings, planarity and the presence of lone pair of electrons on N atoms, which favored greater adsorption of BBH and PABH on the metal surface at 303 K [23].

Electrochemical Methods.
Polarization experiments were studied in 1 M HCl solution containing different concentrations of BBH and PABH (Figures 1 and 2). e linear segments of the anodic and cathodic polarization curves were extrapolated to the corrosion potentials to calculate corrosion currents. e Tafel slopes were determined from Tafel region of current-potential curves. e corrosion current densities were calculated by extrapolation of linear parts of these curves to corresponding corrosion potentials. e electrochemical parameters that is, corrosion potential ( corr ) anodic ( ) and cathodic ( ) Tafel slopes, corrosion current densities ( corr ) and inhibition efficiencies (IE%) determined from polarization curves are presented in Tables 5 and 6. In both the solutions, corrosion potentials became nobler. It can be seen from Tables 5 and 6 Tables 7 and 8 at open circuit potential in presence and absence of different concentrations of BBH and PABH (Figures 3 and 4). Single distorted semicircles were observed in Nyquist plots suggesting a simple -circuit. Increase the BBH and PABH concentrations decreased the double layer capacitance values (Tables 7 and 8). In inhibitor free solution the observed capacitance values 492 F cm −2 was due to the pseudocapacitance. is capacitance would have been due to the adsorption of corrosion products on the surface of steel. Higher charge transfer resistances indicate the decreases in corrosion current densities. e percentage of inhibition efficiencies increased with BBH and PABH concentrations. e values of double layer capacitance ( dl ) were decreased and the charge transfer resistance ( ct ) increased with inhibitor concentration. e appearance of depressed semicircle is due to high frequency dispersion [24] or due to surface heterogeneity or corrosion products covering the surface at random sites [25,26]. Both the electrochemical methods offered nearly same % IE values for both inhibitors.   molecules and the metal surfaces are obtained from various isotherms. Percentage inhibition efficiency was obtained from weight loss measurements as surface coverage ( ). According to the Langmuir adsorption isotherm, the surface coverage ( ) is related to inhibitor concentration ( ) by the following equation where ads is the equilibrium constant of the inhibitor adsorption process. Figure 6 presents the plot of versus for both the inhibitors. A straight line �t with a correlation of 0.999 suggests the adsorption obeys Langmuir adsorption isotherm. Using ads value, (Δ 0 ads ) was calculated: where is the gas constant and is the absolute temperature. e value of 55.5 is the concentration of water in solution. e values of Δ 0 ads were −32.12 and −34.56 kJ/mol for BBH and PABH, respectively. e negative value of Δ 0 ads suggests the chemisorptions of these inhibitors. is is due to electrostatic interactions between the charged molecules and the metal. e Δ 0 ads values more than −40 kJ/mole involve charge sharing or charge transfer from organic molecules to the metal surface to form a coordinate type of bond (chemisorption) [27,28]. e presence of various electrons donating groups in the inhibitor molecules and delocalized aromatic ring may also provide electrostatic adsorption. e free energy of activation for the corrosion process was 11.23 kJ/mole which was raised to 29.26 kJ/mole by 2.23 mM of BBH and to 28.35 kJ/mole by 2.00 mM of PABH. is increase in the free energy of activation for corrosion suggests that corrosion reaction is hindered. e adsorption of these molecules led to the formation of a physical barrier which reduced the metal reactivity in the electrochemical reactions of corrosion.

Conclusions
e results showed that, BBH and PABH compounds offer corrosion inhibition for mild steel in 1 M HCl solution. ese compounds spontaneously chemisorbed obeying Langmuir isotherm. e free energy of activation values for corrosion process was suggested that the compounds participated in the corrosion process. e adsorbed chloro-inhibitor complex has offered better barrier protection to steel and thereby reduced corrosion. e positively charged complex was prevented; the formation of protons and retarded hydrogen evolution reaction. Surface morphology observations revealed the formation of a protective �lm.