Impediment of Iron Corrosion by N , N -Bis[2-hydroxynaphthylidene]amino]oxamide in 3.5% NaCl Solution

: Hydrazone [ N , N (cid:48) -bis[2-hydroxynaphthylidene]amino]oxamide] derived from the condensation of ethanedihydrazide with 2-hydroxynaphthalene-1-carbaldehyde was synthesized and assessed on the basis of elemental analysis (CHN) and spectral (IR, mass, 13 C/ 1 H NMR and UV-Vis) measurements. The inﬂuence of N , N -bis([2-hydroxynaphthylidene]amino)oxamide (HAO) in terms of the inhibition of iron corrosion in concentrated sodium chloride solution (3.5 wt.% NaCl) after various exposure periods was assessed. Numerous electrochemical and spectroscopic assessment techniques were performed. Cyclic potentiodynamic polarization experiments indicated that the presence of HAO and its increased concentration decreased the corrosion of iron in NaCl solution by decreasing the corrosion values, anodic and cathodic currents, and corrosion rate. The electrochemical impedance spectroscopy results showed that HAO molecules greatly increased the corrosion resistance. The chronoamperometric experiments performed at − 475 mV (Ag/AgCl) revealed that the HAO molecules decreased the absolute currents and reduced the probability of the occurrence of pitting corrosion. The effect of HAO on the inhibition of iron corrosion was also conﬁrmed through scanning electron microscopy micrographs and energy-dispersive X-ray proﬁle analyses, which proved that the surface of the iron sample exposed to chloride solution alone was pitted, while the presence of HAO molecules reduced the severity of the pitting corrosion. The results conﬁrmed that the presence of HAO molecules inhibits the corrosion of iron and this impact increased when the exposure time was increased to 48 h. ethandihydrazide and 2-hydroxynaphtha-lene-1-carbaldehyde in hydrazide–aldehyde molar ratio of 1:2. Elemental analysis (CHN), UV–Vis spectra, FT-IR spectroscopy, and 1 H and 13 C NMR spectra showed the identity of the inhibitor. Electrochemical experiments proved that iron corrodes in chloride solution via both uniform and pitting corrosion. Regarding the effects of the time of exposure at 1 and 48 h, it was found that increased time of exposure led to less uniform corrosion. The addition of a low concentration of HAO of 5 × 10 −5 M to the chloride solution was found to mitigate the corrosion of iron by decreasing its j Corr and K Corr as well as increasing its R P . Increasing the content of HAO to 1 × 10 −4 M and more to 5 × 10 −4 M greatly inhibited the corrosion of iron by decreasing all its corrosion reactions, which was reflected its decreased values of K Corr and increased values of R P . Furthermore, the extension of the exposure time to 48 h enhanced the effects of HAO in terms of the inhibition of iron corrosion. The results revealed showed iron suffered uniform and pitting corrosion in NaCl solution and that HAO molecules inhibit this corrosion, particularly at high concentrations and after long immersion periods.


Introduction
N,N -bis [2-hydroxynaphthylidene]amino]oxamide is an example of polyfunctional hydrazones, which are organic compounds that have fascinating biological properties, such as anti-inflammatory, analgesic, anticonvulsant, antitumor, and anti-HIV properties [1]. Additionally, they are significant in drug design and organocatalysis and have uses as inhibitors and antioxidants [2]. In analytical chemistry, some hydrazones have been employed as indicators [3] and analytical reagents for the extraction of copper(II), nickel(II), and cobalt(II) traces [4][5][6].
A literature survey revealed that the impediment of iron corrosion by N,N -bis [2hydroxynaphthylidene]amino]oxamide in a near-neutral concentrated chloride solution had not yet been achieved. Actually, the organic compounds containing heteroatoms such as nitrogen, oxygen, phosphor, and sulfur, as well as multiple bonds in their molecular structures, are mostly proficient inhibitors of metallic corrosion (M = copper, iron, aluminum, etc.) in NaCl brine and other media through eliminating the undesirable destructive influence of the corrosive media towards the metal surface [28][29][30][31][32]. The action of these inhibitors is clearly attributed to the physical or chemical adsorption of their molecules onto the surface of the metal [33][34][35][36]. In addition, the molecular weights and structures of the organic molecules are important parameters to be considered when choosing an organic corrosion inhibitor [37][38][39]. The aromatic N,N -bis [2-hydroxynaphthylidene]amino]oxamide was speculated to have excellent inhibition efficiency for iron due to its adequate molecular structure that contains adsorption centers (N, O) and π-bonds, which enable it to bind strongly with the iron sample's surface.
In view of the inhibitive impacts of organic compounds containing N and O heteroatoms, we were motivated to investigate the influence of small concentrations of a synthesized aromatic hydrazone named N,N -bis [2-hydroxynaphthylidene]amino]oxamide (HAO) on the corrosion of pure iron in freely aerated 3.5% NaCl solutions after exposure periods of 1 and 48 h. The study was achieved through various electrochemical techniques, namely cyclic potentiodynamic polarization, chronoamperometric current-time at fixed value of potential (−475 mV (Ag/AgCl)), and electrochemical impedance spectroscopy, after the iron was immersed in the different solutions for 1 and 48 h before measurement. The scanning electron microscopy and energy-dispersive X-ray analyses were employed to investigate the morphology and compositions of the surfaces after being exposed to the chloride solution without and with HAO molecules present. It is suggested that the presence of HAO molecules would hinder the corrosion of iron via their adsorption onto the external surface, which would diminish the corrosion rate and increment of the corrosion resistance.

Materials and Methods
Ethanedihydrazide (EH) and 2-hydroxynaphthalene-1-carbaldehyde were purchased from Sigma-Aldrich (Gillingham, Dorset, UK). HAO powder was dissolved in dimethylformamide (DMF) to prepare a stock solution of 0.01 M. The desired chloride solutions with or without the existence of the tested HAO were synthesized from the stock solutions by dilution. Sodium chloride (99% purity) was purchased from Merck. Other chemicals and solvents were of the highest purity and used without further purification. The elemental analysis (CHN), mass spectrum analysis using the direct inlet system, and FT-IR were carried out as reported [40]. 13 C-NMR (at 75.46 MHz) and 1 H-NMR (at 300 MHz) measurements in dimethylsulphoxide (DMSO-d 6 ) at room temperature (RT) were performed using a Varian mercury VX-300 NMR spectrometer. The chemical shifts are quoted in δ (ppm) with the aid of an internal reference (tetramethylsilane, TMS). The UV-visible absorptions were scanned on a Shimadzu (UV-1900) spectrophotometer between 200 and 1100 nm. A scanning electron microscope (SEM) with an attached unit for an energy-dispersive X-ray (EDX) analyzer was used to obtain the SEM micrographs and the EDX profiles. The SEM/EDX machine was a JEOL JSM-7400F microscope (Tokyo, Japan). The secondary electron and back-scattered electron detector was an Everhart-Thornley instrument working in automatic mode. The microscopy machine was operated at 15 kV.
The electrochemical measurements were performed in a traditional three-electrode electrochemical cell that accommodated 250 mL solution. The working electrode was a pure iron electrode (99.999% purity) with a surface area of 0.25 cm 2 . This iron rod was purchased from Goodfellow (located at Ermine Business Park, Huntingdon, UK) and used as the working electrode in this examination. A Pt sheet and silver-silver chloride (Ag/AgCl in saturated KCl solution) were the counter and reference electrodes, respectively. Electrochemical experiments were performed using a PGSTAT-302N Autolab Potentiostat. CPP curves were measured between −1200 and +100 mV at a scan rate of 0.00166 V/s. The PCT was measured at a value of −475 mV vs. Ag/AgCl for 1 h. EIS plots were obtained from the open-circuit potential values at a frequency scan range of 100,000 to 0.1 Hz with ±5 mV amplitude sinusoidal wave perturbation.

Isolation of N,N-bis([2-hydroxynaphthylidene]amino)oxamide
N,N -bis[2-hydroxynaphthylidene]amino]oxamide (HAO) was prepared using the same recipe published earlier [41][42][43]. By heating on a hot plate, ethanedihydrazide (0.02 mol) was allowed to dissolve in 80 mL distilled water, then 30 mL absolute ethanol was added. The subsequent hot ethanedihydrazide solution was mixed well with a hot ethanolic solution of 2-hydroxynaphthalene-1-carbaldehyde (0.04 mol). During the gradual addition of aldehyde, sedimentation of oxamide was noticed. The resulting mixture was refluxed for 3 h with constant stirring. The desired oxamide was collected by concentrating the solution to half of its volume and cooling. Next, the product was isolated by filtration and then washed vigorously with hot water and absolute ethanol. Finally, the solid compound was dried at 80 • C for 3 h inside an electric oven. The molecular formula of the HAO was assured by elemental analysis, which was conducted as follows: HAO oxamide/C 24 H 18 N 4 O 4 (Mol. Wt. = 426.13), 67.60% C, 4.25% H, 13.14% N. The % values of the elements were also experimentally obtained as follows: 68.30% C, 5.11% H, 12.84% N. Furthermore, the structural formula of oxamide was determined based on IR, 1 H/ 13 C NMR, and mass spectroscopy data, as will be shown later in this paper.

Chemistry and Characterization of the Inhibitor (HAO)
The N,N -bis[2-hydroxynaphthylidene]amino]oxamide inhibitor was isolated in a pure form. The physical properties of the compound and its corresponding hydrazide are shown in Table 1. The template synthesis of HAO derived from ethanedihydrazide and aldehyde is represented in Scheme 1. The chosen oxamide (HAO) was dissolved in DMF to prepare a stock solution. The low concentrations of HAO of 5 × 10 −5 , 10 −4 , and 5 × 10 −4 M were prepared by dilution and added to 3.5% NaCl solution to obtain the desired electrolyte. Indeed, the presented work is considered an extension of prior studies. The configuration of the chosen inhibitor was discussed in detail elsewhere [25][26][27]41]. The structural formula of the synthesized HAO inhibitor was validated on this basis.

Mass Spectra
The mass spectra of HAO ( Figure 2) affirmed its correct molecular weight, exhibiting a molecular ion peak at: m/z = 427 (20%, (C 16    3.1.3. 13 C NMR Spectra 13 C NMR: The spectrum of HAO is depicted in Figure 3. HAO showed resonances at 155.7 ppm for C=O; 149.7 ppm for C=N; and 158.3, 132.5, 128.9, 127.8, and 123.6 for Ar-C2, Ar-C4, Ar-C6, Ar-C5, and Ar-C1, respectively. It is suggested that the obtained spectral lines represent more carbon types in the represented structure due to the presence of some molecules in dienol or diketo forms.  3.1.3. 13 C NMR Spectra 13 C NMR: The spectrum of HAO is depicted in Figure 3. HAO showed resonances at 155.7 ppm for C=O; 149.7 ppm for C=N; and 158.3, 132.5, 128.9, 127.8, and 123.6 for Ar-C2, Ar-C4, Ar-C6, Ar-C5, and Ar-C1, respectively. It is suggested that the obtained spectral lines represent more carbon types in the represented structure due to the presence of some molecules in dienol or diketo forms.  3.1.3. 13 C NMR Spectra 13 C NMR: The spectrum of HAO is depicted in Figure 3. HAO showed resonances at 155.7 ppm for C=O; 149.7 ppm for C=N; and 158.3, 132.5, 128.9, 127.8, and 123.6 for Ar-C2, Ar-C4, Ar-C6, Ar-C5, and Ar-C1, respectively. It is suggested that the obtained spectral lines represent more carbon types in the represented structure due to the presence of some molecules in dienol or diketo forms.  addition indicated the right positions of these groups. Because deuterium is invisible in the 1 H NMR spectrum and the proton in -OH and -NH-groups is labile (acidic proton), doping the NMR sample with D 2 O easily exchanged out these protons for deuterium, resulting in the disappearance of labile proton peaks.

Cyclic Polarization Data
The cyclic potentiodynamic polarization (CPP) measurements were carried out to report the corrosion parameters and to evaluate the corrosion and corrosion inhibition of iron in chloride solutions with and without HAO molecules present. CPP curves collected for iron electrodes in 3.5% NaCl solutions (a) without HAO and with (b) 5 × 10 −5 M HAO, (c) 1 × 10 −4 M HAO, and (d) 5 × 10 −4 M HAO present after 1 h immersion in the different solutions before measurement are depicted in Figure 6. The CPP experiments were also obtained under the same conditions after prolonging the time of immersion in all solutions to 48 h, the curves for which are shown in Figure 7. The corrosion current (j Corr ), cathodic Tafel (β c ) slope, anodic Tafel (β a ) slope, corrosion potential (open-circuit potential, E Corr ), polarization resistance (R P ), and corrosion rate (K Corr ), in addition to the percentage of the inhibition efficiency (IE%) that were obtained from the polarization curves, are shown in Table 2.
The continuous sweeping of the potential in the positive direction in the presence of the chloride ions led to the breakdown of the formed oxide film, which was indicated by the abrupt increase in the current in the forward anodic side. Reversing the scanning the potential in the backward direction seemed to result in a higher current as compared to the obtained current in the forward direction. This led to the appearance of a hysteresis loop; the wide size of the hysteresis loop indicates the occurrence of pitting corrosion with high severity.     The values of E Corr and j Corr were obtained from the extrapolation of the cathodic and anodic Tafel lines that located next to the linearized current regions [44][45][46][47][48][49]. The values of R P and K Corr were calculated from the following equations, respectively [50,51]; The percentage of inhibition efficiency (IE%) was also obtained as follows [4,38]; where k equals 3272 and is the constant that defines the unit for the rate of corrosion (mm/amp cm year), E W is the equivalent weight, d is the density, and A is the surface area. Additionally, i 0 Corr and i i Corr are the corrosion current densities for iron in the absence and presence of the HAO, respectively.
The CPP curve obtained for iron in NaCl solution without HAO added ( Figure 6, curve 1) showed a decrease in the cathodic current with a potential swept in the cathodic Crystals 2021, 11, 1263 9 of 18 branch due to the reduction reaction that took place on the surface of the iron sample. This reaction was the reduction of oxygen on the surface of iron, as follows [10,44,52]; The minimum current was recorded at the value of j Corr (the intersection of the extrapolation of the cathodic and anodic Tafel lines). The current then started to rise again in the anodic branch due to the dissolution of the iron sample as per the following equation [10,52]; The current then slightly slowed down, showing a passive region as a result of the reaction of the iron sample with oxygen from the solution to form iron hydroxide (Fe(OH) 2 ) and iron oxide (Fe 3 O 4 ), as follows [38]; The continuous sweeping of the potential in the positive direction in the presence of the chloride ions led to the breakdown of the formed oxide film, which was indicated by the abrupt increase in the current in the forward anodic side. Reversing the scanning the potential in the backward direction seemed to result in a higher current as compared to the obtained current in the forward direction. This led to the appearance of a hysteresis loop; the wide size of the hysteresis loop indicates the occurrence of pitting corrosion with high severity.
The addition of 5 × 10 −5 M HAO to the chloride solution after 1 h immersion ( Figure 6, curve 2) showed smaller cathodic and anodic currents as well as the smaller size of the hysteresis loop as compared to the one obtained for iron in chloride solution alone ( Figure 6, curve 1). The data were obtained from Table 1, where the j Corr and K Corr showed smaller values as well. This confirms that the addition of 5 × 10 −5 M HAO decreases the uniform and pitting corrosion. Increasing the concentration of HAO to 1 × 10 −4 M led to introducing lower values of j Corr and K Corr , and this effect was further enhanced when the concentration of HAO was increased to 5 × 10 −4 M. Perhaps this was due to the adsorption of HAO molecules onto the surface of the iron, leading to blocking of the flawed and pitted areas on the surface and suppressing its corrosion.
The CPP curves obtained after 48 h (Figure 7) showed similar behavior but with currents lower than those obtained for the iron after 1 h exposure in all solutions. The data listed in Table 1 confirm that the values of j Corr and K Corr decreased for iron after 48 h immersion in all solutions as compared to the values obtained after only 1 h immersion, regardless of whether HAO molecules were present or not. For j Corr , a value of 3.2 µA/cm 2 was recorded for iron in chloride solution alone after 1 h immersion, which increased to 2.1 µA/cm 2 for iron in chloride solution alone after 48 h exposure. These values decreased in the presence of 5 × 10 −5 M HAO to 1.5 µA/cm 2 and 1.2 µA/cm 2 . It is also worth mentioning that lower values were recorded for iron in chloride solution in the presence of 5 × 10 −4 M HAO after 1 and 48 h, with values of 0.5 µA/cm 2 and 0.4 µA/cm 2 , respectively. The values of K Corr also decreased and R P values increased in the same order as those obtained for j Corr . This proved that the presence and increase of HAO molecules decrease the corrosion of iron and that this effect increases with prolonged exposure time from 1 to 48 h.

Chronoamperometric Current-time Results
In order to shed more light on the effect of HAO on the inhibition of iron corrosion and the possibility of the occurrence of pitting corrosion in the chloride solution at a constant potential, chronoamperometric current-time measurements were carried out. The chronoamperometric current with time measurements for iron in (1) 0.0 HAO, (2) 5 × 10 −5 HAO, (3) 1 × 10 −4 HAO, and (4) 5 × 10 −4 M HAO obtained at −475 mV (Ag/AgCl) were performed after 1 and 48 h immersion, the curves of which are shown in Figures 8 and 9, respectively. It can be seen from Figure 3 that the currents for iron in all solutions showed very low values, possibly due to the formation of an oxide film during the immersion of iron for 1 h before applying the anodic potential. The recorded current increased with increasing time of the experiment as a result of the dissolution of the oxide film and the continued increments in current due to the occurrence of pitting corrosion.     Figure 8 (curve 3), greatly decreased the obtained current values through the whole time of the experiment as compared to the current values collected for NaCl alone and NaCl + 5 × 10 −5 M HAO, confirming that the increase of HAO from 5 × 10 −5 M to 1 × 10 −4 M greatly reduced both the uniform and pitting corrosion for iron. This suggestion was further supported by the increase of HAO molecules to 5 × 10 −4 M, as shown in Figure 3 (curve 4), as the obtained current at this concentration was the lowest. Figure 9 shows the current-time curves for iron samples that were immersed in the chloride solutions in the absence and presence of HAO molecules for 48 h before fixing the constant potential for 1 h. It can be seen that the increase in the current with time continued and was the highest for iron in NaCl solution that had no HAO present, proving that NaCl solution had harsh effects on the surface. The presence of 5 × 10 −5 M HAO (Figure 9, curve 2) decreased the overall currents, and this effect increased with the increases of the HAO content to 1 × 10 −4 M and 5 × 10 −4 M. Extending the exposure time of iron in the different solutions reduced the corrosiveness of the chloride ions by decreasing the current obtained over time, particularly when HAO molecules were present. The change of current with time measurements confirmed the results of the CPP measurements.

EIS Data
In order to confirm the data obtained from the CPP and current-time experiment, EIS measurements were carried out. It has been reported [53][54][55][56][57][58] that the EIS method can be successfully employed to understand corrosion and corrosion protection for metals and alloys in the surrounding environments. Figure 10 presents the collected Nyquist plots for the iron samples that were immersed for 1 h in 3.5% NaCl solutions without (a) and with (b) 5 × 10 −5 M HAO, (c) 1 × 10 −4 M HAO, and (d) 5 × 10 −4 M HAO. The Nyquist plots were also obtained after 48 h immersion in the different solutions, as shown in Figure 11. All impedance data were best fitted to the equivalent circuit model that is depicted in Figure 12. The values of the impedance parameters of the circuit were obtained and are listed in Table 3. These parameters were R S (solution resistance), Q (constant phase elements, CPEs), and R P (polarization resistance). The values of the inhibition efficiency (IE/%), which were obtained for the HAO inhibitor, are also presented in Table 3 and were obtained as per the following relation [56][57][58];

Scanning Electron Microscopy (SEM) and Energy-Dispersive X-ray (EDX) Analyses
The SEM image and EDX spectra obtained from the surface of iron after its immersion for 48 h in 3.5% NaCl solution (before applying a potential amount of −475 mV (Ag/AgCl) for 60 min) are shown in Figure 13. The SEM micrograph shows numerous pits spreading over the surface of the iron sample. This confirms that the increase in the current with time that is shown in Figure 8 (curve 1) was due to the occurrence of pitting corrosion. The EDX profile analysis indicated that the atomic percentages (at%) for the elements found on the iron samples were 72.54% Fe, 13.53% O, 12.46% C, 0.78% Na, and 0.68% Cl. The at% values for Fe and O were the highest, revealing that the layer formed on the surface during the current-time experiment was mostly iron oxide, as represented by Reactions (6) and (7). The low at% values for both Na and Cl resulted from the deposition of a thin layer of NaCl salt on the surface. Here, R Un P and R In P are the values of the polarization resistance without and with HAO, respectively. The Nyquist plots for the iron samples in the different solutions after 1 h as well as after 48 h show only one semicircle, the diameter of which increased in the presence of HAO and with the increase of its concentration. The smallest diameter for the semicircle was obtained for iron after 1 and 48 h immersion in 3.5% NaCl solution in the absence of HAO molecules. Table 3 also shows that the R S and R P values were the lowest while the Y Q were values the highest for iron in chloride solution alone. The presence of the HAO compound increased the corrosion resistance of iron, as indicated by the wide diameter of the Nyquist plot's semicircle. This was confirmed by the R S and R P values, which increased with the increase in HAO content within the chloride test solution. The values of Y Q and the exponent n values varied between 0.83 and 0.89, revealing that Q can be considered as a double layer capacitor with some porosity.
Increasing the immersion period of time from 1 to 48 h before measurement ( Figure 11) resulted in almost similar Nyquist plots, whereby the presence of HAO molecules increased the diameter of the drawn semicircle. This effect increased with the increases in HAO from 5 × 10 −5 M to 1 × 10 −4 M and further to 5 × 10 −4 M. Looking at the digits listed in Table 3, the values of R S , R P , and IE% greatly increase with the increase in HAO content and further with the increase in immersion time to 48 h as compared to the shorter exposure period of 1 h. The values of Y Q also decreased with the content of HAO and with the increase in immersion time. The EIS data, thus, were in good agreement with the data collected via polarization, confirming the fact that the increase in HAO content and extended immersion time increased the corrosion resistance of the iron in the chloride solution.

Scanning Electron Microscopy (SEM) and Energy-Dispersive X-ray (EDX) Analyses
The SEM image and EDX spectra obtained from the surface of iron after its immersion for 48 h in 3.5% NaCl solution (before applying a potential amount of −475 mV (Ag/AgCl) for 60 min) are shown in Figure 13. The SEM micrograph shows numerous pits spreading over the surface of the iron sample. This confirms that the increase in the current with time that is shown in Figure 8 (curve 1) was due to the occurrence of pitting corrosion. The EDX profile analysis indicated that the atomic percentages (at%) for the elements found on the iron samples were 72.54% Fe, 13.53% O, 12.46% C, 0.78% Na, and 0.68% Cl. The at% values for Fe and O were the highest, revealing that the layer formed on the surface during the current-time experiment was mostly iron oxide, as represented by Reactions (6) and (7). The low at% values for both Na and Cl resulted from the deposition of a thin layer of NaCl salt on the surface. Figure 13. SEM image and EDX profile spectrum after 1 h with a potential of −475 mV (Ag/AgCl) for the surface of the iron rod that was immersed for 48 h in 3.5% NaCl solution before measurement. Figure 14 shows the SEM micrograph and EDX profile taken from the surface of the iron sample that was left in 3.5% NaCl + 1 × 10 −4 M HAO for 48 h before setting the potential to −475 mV (Ag/AgCl) for 1 h. It can be seen from the SEM image that the sample shows no pitting but does show the formation of a surface layer. This layer was most probably due to the surface of iron being in contact with the electrolyte. In order to confirm the composition of the layers formed on the surface, the EDX analysis was carried out. The at% percentages for the majority of the surfaces were 55.88% Fe, 32.48% O, 7.60% C, 2.61%Cl, and 1.43% Na. It is clear that the highest at% values were obtained for Fe and O. As compared to the surface in chloride solution alone, the at% of O was too high but the at% for Fe was too low. This meant that the iron had a thick protective layer that covered its surface and prevented its corrosion thanks to the presence of HAO molecules within the chloride solution. The presence of Cl and Na at these low percentages was due to the adsorption of a thin layer of NaCl salt on the surface of the iron sample. The spectroscopic investigations confirmed the electrochemical measurements showing that the presence of HAO molecules and increases in their concentration increase the corrosion resistance for iron in 3.5% NaCl solution.   Figure 14 shows the SEM micrograph and EDX profile taken from the surface of the iron sample that was left in 3.5% NaCl + 1 × 10 −4 M HAO for 48 h before setting the potential to −475 mV (Ag/AgCl) for 1 h. It can be seen from the SEM image that the sample shows no pitting but does show the formation of a surface layer. This layer was most probably due to the surface of iron being in contact with the electrolyte. In order to confirm the composition of the layers formed on the surface, the EDX analysis was carried out. The at% percentages for the majority of the surfaces were 55.88% Fe, 32.48% O, 7.60% C, 2.61%Cl, and 1.43% Na. It is clear that the highest at% values were obtained for Fe and O. As compared to the surface in chloride solution alone, the at% of O was too high but the at% for Fe was too low. This meant that the iron had a thick protective layer that covered its surface and prevented its corrosion thanks to the presence of HAO molecules within the chloride solution. The presence of Cl and Na at these low percentages was due to the adsorption of a thin layer of NaCl salt on the surface of the iron sample. The spectroscopic investigations confirmed the electrochemical measurements showing that the presence of HAO molecules and increases in their concentration increase the corrosion resistance for iron in 3.5% NaCl solution.
Crystals 2021, 11, 1263 14 of 18 Figure 13. SEM image and EDX profile spectrum after 1 h with a potential of −475 mV (Ag/AgCl) for the surface of the iron rod that was immersed for 48 h in 3.5% NaCl solution before measurement. Figure 14 shows the SEM micrograph and EDX profile taken from the surface of the iron sample that was left in 3.5% NaCl + 1 × 10 −4 M HAO for 48 h before setting the potential to −475 mV (Ag/AgCl) for 1 h. It can be seen from the SEM image that the sample shows no pitting but does show the formation of a surface layer. This layer was most probably due to the surface of iron being in contact with the electrolyte. In order to confirm the composition of the layers formed on the surface, the EDX analysis was carried out. The at% percentages for the majority of the surfaces were 55.88% Fe, 32.48% O, 7.60% C, 2.61%Cl, and 1.43% Na. It is clear that the highest at% values were obtained for Fe and O. As compared to the surface in chloride solution alone, the at% of O was too high but the at% for Fe was too low. This meant that the iron had a thick protective layer that covered its surface and prevented its corrosion thanks to the presence of HAO molecules within the chloride solution. The presence of Cl and Na at these low percentages was due to the adsorption of a thin layer of NaCl salt on the surface of the iron sample. The spectroscopic investigations confirmed the electrochemical measurements showing that the presence of HAO molecules and increases in their concentration increase the corrosion resistance for iron in 3.5% NaCl solution. Figure 14. SEM image and EDX profile spectrum after 1 h with a potential of −475 mV (Ag/AgCl) for the surface of an iron sample that was immersed for 48 h in 3.5% NaCl solution containing 1 × 10 −4 M HAO before measurement. Figure 14. SEM image and EDX profile spectrum after 1 h with a potential of −475 mV (Ag/AgCl) for the surface of an iron sample that was immersed for 48 h in 3.5% NaCl solution containing 1 × 10 −4 M HAO before measurement.
The theory of hard and soft acids and bases (HSAB) can also explain the theory of inhibition. According to Ralph G. Pearson's principle [59,60], metals (M) are classified into three classes, namely hard acids, soft acids, and intermediate (borderline) acids. Bases are also classified into hard, soft, and intermediate (borderline) bases. Hard metals, i.e., those with a high charge and small size (charge/size = high), tend to bind with hard bases containing F − and: O donor atoms. Based on this supposition along with the obtained data, it can be concluded that iron ions (Fe 2+ ), which are classified as hard acids, will bind strongly with the inhibitor containing O donor atoms in its structure (Scheme 1). Moreover, the persistence of the: N donor in the HAO structure may also help its binding with iron ions, as it is associated with intermediate bases. The theory of hard and soft acids and bases (HSAB) can also explain the theory of inhibition. According to Ralph G. Pearson's principle [59,60], metals (M) are classified into three classes, namely hard acids, soft acids, and intermediate (borderline) acids. Bases are also classified into hard, soft, and intermediate (borderline) bases. Hard metals, i.e., those with a high charge and small size (charge/size = high), tend to bind with hard bases containing F − and :O donor atoms. Based on this supposition along with the obtained data, it can be concluded that iron ions (Fe 2+ ), which are classified as hard acids, will bind strongly with the inhibitor containing O donor atoms in its structure (Scheme 1). Moreover, the persistence of the :N donor in the HAO structure may also help its binding with iron ions, as it is associated with intermediate bases.

Conclusions
The N,N′-bis [2-hydroxynaphthylidene]amino]oxamide inhibitor (HAO) can be prepared via the usual condensation reaction between ethandihydrazide and 2hydroxynaphtha-lene-1-carbaldehyde in hydrazide-aldehyde molar ratio of 1:2. Elemental analysis (CHN), UV-Vis spectra, FT-IR spectroscopy, and 1 H and 13 C NMR spectra showed the identity of the inhibitor. Electrochemical experiments proved that iron corrodes in chloride solution via both uniform and pitting corrosion. Regarding the effects of the time of exposure at 1 and 48 h, it was found that increased time of exposure led to less uniform corrosion. The addition of a low concentration of HAO of 5 × 10 −5 M to the chloride solution was found to mitigate the corrosion of iron by decreasing its jCorr and KCorr as well as increasing its RP. Increasing the content of HAO to 1 × 10 −4 M and more to 5 × 10 −4 M greatly inhibited the corrosion of iron by decreasing all its corrosion reactions, which was reflected its decreased values of KCorr and increased values of RP. Furthermore, the extension of the exposure time to 48 h enhanced the effects of HAO in terms of the inhibition of iron corrosion. The results revealed showed iron suffered uniform and pitting corrosion in NaCl solution and that HAO molecules inhibit this corrosion, particularly at high concentrations and after long immersion periods.

Conclusions
The N,N -bis[2-hydroxynaphthylidene]amino]oxamide inhibitor (HAO) can be prepared via the usual condensation reaction between ethandihydrazide and 2-hydroxynaphthalene-1-carbaldehyde in hydrazide-aldehyde molar ratio of 1:2. Elemental analysis (CHN), UV-Vis spectra, FT-IR spectroscopy, and 1 H and 13 C NMR spectra showed the identity of the inhibitor. Electrochemical experiments proved that iron corrodes in chloride solution via both uniform and pitting corrosion. Regarding the effects of the time of exposure at 1 and 48 h, it was found that increased time of exposure led to less uniform corrosion. The addition of a low concentration of HAO of 5 × 10 −5 M to the chloride solution was found to mitigate the corrosion of iron by decreasing its j Corr and K Corr as well as increasing its R P . Increasing the content of HAO to 1 × 10 −4 M and more to 5 × 10 −4 M greatly inhibited the corrosion of iron by decreasing all its corrosion reactions, which was reflected its decreased values of K Corr and increased values of R P . Furthermore, the extension of the exposure time to 48 h enhanced the effects of HAO in terms of the inhibition of iron corrosion. The results revealed showed iron suffered uniform and pitting corrosion in NaCl solution and that HAO molecules inhibit this corrosion, particularly at high concentrations and after long immersion periods.

Data Availability Statement:
The data presented in this study are available within the manuscript.