Chemicals and Equipments

The aldehydes were obtained from Merck Company. Manganese (2+) acetate, diethyl oxalate, hydrazine monohydrate were purchased from Sigma-Aldrich. Oxalicdihydrazide was prepared by reacting diethyloxalate (1 mol) with hydrazine hydrate (2 mol) [ ¹⁰ ], (Exp/Lit. m.p = 240/240 °C). Other employed chemicals and solvents were of highest purity and used without further purification. Elemental analyses (CHNM), spectral (UV-Vis., FT-IR, Mass) and thermal (TG) measurements were executed as mentioned elsewhere [ ²⁹ , ¹⁰ ]. The NMR spectra were recorded on a Varian mercury VX-300 NMR spectrometer. ¹H spectra were run at 300 MHz and ¹³C spectra were run at 75.46 MHz in dimethylsulphoxide (DMSO-d₆). Tetramethylsilane (TMS) was used as an internal reference and chemical shifts are quoted in δ (ppm). Magnetic moments (μ_eff) of the samples were measured at room temperature (RT) using Faraday's method. The ESR spectra of the complexes were recorded in powder form at room temperature using Bruker-EMX-(X-bands-9.7 GHZ) spectrometer with 100 KHZ frequency, microwave power 1.008 mW and modulation/amplitude of 4 GAUSS at national center for radiation research and technology, Egypt. The band gap energy (E_g) of product compounds was calculated according to Tauc's equations [ ³⁰ , ³¹ ]. Corrosion test was performed on freshly prepared sheet of aluminum of chemical compositions (wt %) Ni (0.006), Pb (0.01), Zn (0.003), Cu (0.002), Fe (0.27) and Al (99.71). The metal specimens were prepared, degreased and cleaned as previously described [ ¹⁰ ].

Synthesis of oxaloyldihydrazone ligands

The dihydrazone ligands; bis(2-hydroxynaphthaldehyde)oxaloyldihydrazone (L₁) and bis(2-methoxybenzaldehyde)oxaloyldihydrazone (L₂) were prepared using the same procedure described in Ref. 10 as follows. Aqueous methanolic solution of oxaloyldihydrazine was mixed with absolute methanolic solution of the selected aldehydes (maintaining the molar ratio at 1:2) in hot condition. The reaction mixture was heated for about 3 hours accompanied by stirring. The precipitate was separated out on concentrating the solution to half of its volume and cooling. Afterwards, the isolated compound was filtered, recrystallized from DMF-MeOH_aq mixed solvent, washed thoroughly by acetone and then dried in an electric oven at 80 °C for 2 h. The molecular structure of the resulting hydrazones was confirmed by elemental analyses, (IR, mass and ¹H/¹³C NMR) spectroscopy ( Table 1 , 2 ). The mass spectral data of the ligands (L₁, L₂) are displayed in Fig. 1 and the obtained M⁺ values are set out in Table 1 . Mass spectrum of the bis(2-hydroxy-1-naphthaldhyde)oxaloyldihydrazone (L₁) revealed ion peaks at m/e = 427 (20%, molecular peak, (C₁₆H₁₄N₄O₄)), 410 (25%, M⁺(C₂₄H₁₈N₄O₃)), 270 (21%, M⁺(C₁₃H₁₀N₄O₃)), 157 (21.2%, M⁺(C₁₁H₉O)), 257 (50%, M⁺(C₁₃H₁₀N₃O₃)), 242 (40%, M⁺(C₁₃H₉N₂O₃)), 175 (71%, M⁺(C₁₁H₉NO)), 141 (4%, M⁺(C₁₁H₉)), 128 (37%, M⁺(C₁₀H₈)), 79 (40%, M⁺(C₆H₈)), 64.95 (71%, M⁺(C₅H₅)) and 50 (100%, base peak, M⁺(C₄H₄). Mass spectrum of the bis(2-methoxybenzaldehyde)oxaloyldihydrazone (L₂) exhibited ion peaks at m/e = 356.45 (33%, molecular peak, (C₁₈H₁₈N₄O₄)), 247.35 (26%, M⁺(C₁₁H₁₁N₄O₃)), 217.2 (62%, M⁺(C₁₀H₉N₄O₂)), m/e = 50 (100%, base peak, M⁺(C₄H₄)), 234.2 (42%, M₊(C₁₀H₁₀N₄O₃)), 202 (25%, M⁺(C₁₀H₉N₂O₃)), 176.5 (60%, M⁺(C₉H₉N₂O₂)), 149 (92%, M⁺(C₈H₉N₂O)), 190.6 (46%, M⁺(C₉H₈N₃O₂)), 175.55 (78%, M⁺(C₉H₇N₂O₂)), 119.4 (53%, M⁺(C₇H₇N₂)), 104.2 (51%, M⁺(C₇H₆N)), 134.6 (82%, M⁺(C₈H₈NO)), 107.15 (52%, M⁺(C₇H₈O)), m/e = 90.6 (75%, M⁺(C₇H₆)), 77.05 (86%, M⁺(C₆H₆)) as well as 66.65 (38%) corresponds to M⁺(C₅H⁶). The spectral data assured the chemical formula of the ligands as well the correct molecular weights.

Fig. 1 Mass spectra of L₁ and L₂. 

Synthesis of solid complexes

Manganese (2+) complexes were prepared by literature method [ ¹⁰ ]. Dihydrazone (1 mmol) were dissolved first in 20 ml DMF and then 50 ml methanol was added. The obtained solution was added slowly to a 20 ml methanolic solution of Mn (2+) acetate in 1:2 molar ratio. The resulting mixture was refluxed for 4 hours and then reduced to 30 cm³ by gentle heating. The desired product obtained on cooling down to room temperature was filtered off, washed vigorously in beaker with successive portions of hot solvents (dimethylformamide, methanol and acetone, respectively) and dried in an electric oven at 80 °C for 8 hrs.

Solutions

The aggressive solutions, 1 M HCl, were prepared by dilution of analytical grade HCl with distilled water. Stock solutions of the inhibitors were prepared in DMF. All test solutions contained 10 ml (20 vol.%) of DMF to ensure solubility. The concentrations of the used inhibitors (L₁ and L₂) were 5×10⁻⁴, 1 ×10⁻⁴, 5 × 10⁻⁵ and 1×10⁻⁵ M.

Gravimetric measurements

A simple test for measuring corrosion is the gravimetric method (weight loss). The method involves exposing a clean weighed piece of the metal or alloy to the corrosive environment for a specified time followed by cleaning to remove corrosion products and weighing the piece to determine the loss of weight.

A previously weighed metal (aluminum) was completely immersed in 50 ml of the test solution (1M HCl) in an open beaker. The beaker was maintained at a temperature of 301 K. The aluminum coupons were suspended in the beaker with the help of glass hooks. The coupons were withdrawn from the test solution at 1 h interval progressively for 4 h, dipped into saturated ammonium acetate solution to terminate the corrosion reaction, washed by scrubbing with a light brush, rinsed in acetone and finally dried in an oven at 80 °C before reweighing. The difference in weight in grams was taken as the

total weight loss (W) [ ³² ].

Where: W_i and W_f are the initial and final weights for aluminum, respectively.

The corrosion rate, ρ (in gm.cm⁻².min⁻¹) for aluminum at different concentrations of the acid was determined using the following equation [ ³³ ]:

Where: W = weight loss to the nearest 0.0001gm, A = area of specimen to the nearest 0.01 cm² (2 × area of one face) and t = exposure time (minutes).

The inhibition efficiency (%η) for each inhibitor and the degree of surface coverage (θ) were calculated using the equations [ ³³ ]:

Where ρ₁ and ρ₂ are the corrosion rates in the presence and absence of inhibitor, respectively.

Stereo chemical structure of Mn²⁺-complexes

IR Spectra

The comparative IR spectral study of the dihydrazones and their manganese (2+) complexes reveals the coordination mode of the ligands during complex formation. The important frequencies with their possible assignments are listed in Table 1 .

Free ligand: Prepared oxloyldihydrazones can be represented either in the trans- (staggered) or cis- (anti-cis- or syn-cis-) configuration as revealing in Fig. 2 [ ³⁴ ]. The structures of the ligands were authenticated on the basis: (1) the ν(C=N) was observed in the IR spectra of the two ligands at 1600-1618 cm⁻¹ indicating the interaction of dihydrazide with the aldehyde. (2) The bands at 3476, 3166 and (1705(m) +1660(v.s) cm⁻¹ are attributed to ν(OH)naphthoic, ν(NH) and ν(C=O) for L₁, respectively, meanwhile, L₂ which does not contain o-hydroxy group revealed ν(NH) at 3202 cm⁻¹ and ν(C=O) at 1653 cm⁻¹. (3) Both ν(C=O) and ν(NH) are observed concurrently in the IR spectra of the two ligands giving an important reflection to the existence of keto forms. (4) Appearance of ν(C=O) as two bands in case of L₁ (1705, 1660 cm⁻¹) and one band for L₂ (1653 cm⁻¹) refers to the mixture of [cis(syn/anti-cis-structure) + trans(staggered)-structure)] isomers for L₁ and trans (staggered) structure for L₂, respectively, ( Fig. 2 ). This suggestion is assumed in the light of the field effect criteria where the cis isomer is absorbed at higher frequency than trans one [ ¹⁰ ]. The band observed at 1705 cm⁻¹ characterized cis form while the other two bands located at 1660 and 1653 cm⁻¹ indicated the presence of trans-form. Otherwise, NMR spectroscopy provides satisfactory agreement with this view as shown later. (5) The band observed at 3227 cm⁻¹ (L₂) may concern ν(OH)_enolic in H-bond bonding with C=N. (6) The bands corresponding to ν(C-O) in the spectra of L₁ and L₂ were located at 1287 and 964 cm⁻¹, respectively [ ³⁵ , ³⁶ ]. (7) Also, each ligand contains bands in the regions 1400-1600 and one band at 700-800 cm⁻¹ assignable to ν(C=C) and δ(CH)_{out of plane} of the aromatic ring, respectively.

Fig. 2 Proposed structures of oxaloyldihydrazone ligands: Where L₁ (R=ph, X=H), and L₂ (R=H, X=CH₃). 

Mn²⁺-complexes: The IR spectra of the complexes (I, II) are depicted in Table 2 . Virtually, it is difficult to unambiguously assign the bands due to foundation of various groups e.g., C=O, C=N, N-C=O, NH, OH, ph ring, C-N and C-H which absorb in the overlapping regions. On examining the IR spectra of the complexes in comparison with those of the free ligands, the complexes structures are suggested based on the following evidences:

For I, the splitting of the azomethine band (1600, 1618 cm⁻¹) justifies the imparting of this group in coordination [ ³⁷ ] where two dissimilar azomethine groups are presented, Fig. 3 [ ³⁸ ]. One of them is coordinated with manganese while the other is free. This occurred due to the enolization of left or right half of the dihydrazone molecule assuming keto-enol skeleton. Existence of ν(OH)_naphthoic at 3342 cm⁻¹ endorses this suggestion. Interestingly, the obscure of ν(NH) with noticeable of ν(C=O) at 1655 cm⁻¹ upon chelation give a conclusive evidence to the coordination of metal ion with deprotonated NH group. Persistence of ν(C=O) as one band, 1655 cm⁻¹ instead of two bands in L₁ with slightly change in its location agrees with the presence of the two oxygen atoms related to (O=C-C-OH) part, in trans direction. Doubtless the positive shift observed for ν(C-O) vibration (1287→1299 cm⁻¹) provides a clue to deprotonation of both phenolic (OH) groups during their coordination [ ⁶ , ³⁶ , ³⁹ ] but remarking ν(OH)_naphthoic at 3186 cm⁻¹ points to the existence of naphthoic (OH) in H-bonding, Fig. 4 . This can be explained on the basis of the deprotonation of one naphthoic (OH) group while the other is in H-bonding with C=N group.

Fig. 3 ¹³C NMR spectra of L₁ and L₂ in DMSO- d ₆ at RT. 

Fig. 4 Suggested structures of Mn^II-oxaloyldihydrazone complexes. 

In case of II and comparing with the IR spectrum of L₂ ligand, The mode of chelation is supported by following facts: (1) The remaining of the ν(NH), ν(C=N) and ν(C-OMe) at nearly the same position with disappearance of ν(M-O_methoxy) excludes involvement of the above groups in bonding. The feature of the interesting band assigned to ν(C=O) is varied from strong to very strong relative to other neighbor bands indicating the coordination of the above group with manganese ion. Undoubtedly, the observation of C=O groups in the IR spectrum of II with obscure of ν(NH) band implies that the ligand prefers its keto form to coordinate with Mn²⁺ ion. DMF showed band at 1729 cm⁻¹ assigned to the ν(C=O) stretch and this band is not observed in the free ligand [ ⁴⁰ ].

Actually, the appearance of new M-O bands in both complexes ( Table 3 ) asserts the formation of metal-ligand bond [ ⁴¹ ]. The broad bands within the range 3300–3450 cm⁻¹ in the complex I are attributed to the presence of crystalline and coordinated water molecules. Besides, the two bands corresponding to ν_as and ν_s carboxylic modes of acetate group are shown at 1351 cm⁻¹ and 1437 cm⁻¹, respectively. The larger difference between the above two frequencies [ν_as(OAc) and ν_s(OAc)] is an indicative of monodentate nature of acetato group [ ⁴² ]. Also, the slight changes in the positions of ν(C=C)_ph upon chelation are due to metal-ligand interaction.

Table 3 Corrosion parameters for aluminum coupons in (1M HCl) solution in presence and absence of different concentrations of hydrazone ligands obtained from weight loss measurement at room temperature after 4 hrs immersion time. 

Inhibitor	Conc. (M)	Weight Loss (g)	Rate of corrosion mg cm-2 h-1	θ	Inhibition efficiency η %
L1	Blank 5 × 10-4 1 × 10-4 5 × 10-5 1 × 10-5	0.0210 0.0139 0.0153 0.0163 0.0178	0.0070 0.0046 0.0051 0.0054 0.0059	- 0.34 0.27 0.22 0.15	- 34 27 22 15
L2	Blank 5 × 10-4 1 × 10-4 5 × 10-5 1 × 10-5	0.0188 0.0067 0.0084 0.0099 0.0114	0.0063 0.0022 0.0029 0.0035 0.0038	– 0.64 0.54 0.44 0.39	– 64 54 44 39

¹H/¹³C NMR Spectra

The spectra of the two investigated ligands ( Table 1 ) contain signals at 6.5–8.5, 9.73, 8.95 12.8, 12.6, 12.3 and 3.9 ppm which are assigned to aromatic(L₁, L₂), azomethine (L₁), azomethine (L₂), -NH (L₁), naphthoic –OH (L₁), secondary -NH (L₂) and -OCH₃(L₂) protons [ ¹¹ ], respectively. The disappearance of signals due to –OH and –NH protons, on D₂O exchange confirms to the assignment of signals in the ligand spectrum. In the spectrum of the L₁. the signal due to the naphthoic –OH protons appeared as a broad singlet suggesting its capability to establish intermolecular H-bonding with DMSO solvent. A probable existence of interamolecular hydrogen bonding between naphthoic -OH and CH=N group was ruled out owing to its appearance at normal position. The observation of ν(NH) with nonexistence of enolic OH in the same spectra (L₁) implies the ligand is present in the keto form. In the spectrum of the L₂, noticeable of ν(NH) at 12.29 ppm accompanied with very weak intensity of ν(OH)_enolic at 11.92 ppm infers domination of diketo form compared with dienol or keto-enol form. Surely, NMR spectroscopy can determine the skeleton of the oxaloydihydrazone. As reported in literature [ ⁴¹ ], if the dihydrazone exists in the syn-cis-configuration or staggered configuration, the νOH, νNH and νCH=N, resonances, each should appear as a singlet. However, the appearance of these signals in the form of six signals (doublet of doublet) indicates anti-cis (chair) configuration. Based on this supposition together with the obtained data it can be concluded that the two ligands are in conformity with syn-cis- or staggered configuration. But the IR information clearly indicates that the staggered configuration is well defined.

The ¹³C NMR measurement was performed for the two ligands and the resulting data ( Fig. 3 , Table 1 ) correlate well with ¹H NMR spectra without discrepancies. The ¹³C NMR spectroscopy data for L₁ gave resonances at 155.7 ppm for C=O, 149.7 ppm for C=N and 158.3, 132.5, 128.9, 127.8, 123.6 for Ar-C2, Ar-C4, Ar-C6, Ar-C5, Ar-C1, respectively, confirming a staggered symmetry for L₁. The greater number of lines than the carbon types in the proposed configuration may be recognized from the existence of some ligand molecules in diketo and/or dienol forms. ¹³C NMR spectrum of L₂ was also in good conformance with ¹H NMR assuming a keto-keto form (staggered structure). The spectrum showed three distinct peaks assigned to C=O (156.2 ppm), C=N (148,7 ppm) and OCH₃ (55.7 ppm). The peaks centered at 158.1, 132.1, 125.8, 122.0, 120.9 are due to Ar-C2, Ar-C4, Ar-C6, Ar-C5, Ar-C1, respectively. The signals observed in-between 25-50 ppm are attributed to the exploited solvent (DMSO).

Optical absorption and magnetic investigation

The electronic spectral bands for the oxaloylhydrazones and their manganese complexes in DMF solvent have been given in Table 2 . The electronic spectra of the investigated ligands shows peaks at 350-392, 338-375, 306-340, 292-315 and 280-303 nm due to (n-π^*, C=N), (n-π^*, C=O), (π-π^*, C=N), (π-π^*, C=O) and (π-π^*, aromatic ring), respectively. The proposed geometry of the isolated complexes was deduced from the electronic spectra in DMF and magnetic measurements. The optical absorption spectrum of I revealed two absorptions at 460 and 425 nm assigned to ⁶S→⁴G and ⁴P transitions, respectively. The observation of these bands suggests a tetrahedral environment around the Mn²⁺ ion [ ⁴³ ]. The electronic spectra of II showed two weak bands at 430 and 380 nm assignable to ⁶A_1g→⁴T_1g(G) and ⁴T_2g(G) transitions, respectively, considering an octahedral configuration around the Mn²⁺ ions [ ⁴⁴ ]. Other bands attributed to octahedral (⁶A_1g→⁴E_g, ⁴A_1g, ⁴T_2g(D) and ⁴E_g(D)) and tetrahedral (⁶S→⁴D and ⁴F) structures of Mn²⁺-complexes are overlapped with strong ligand bands [ ⁴⁴ ].

The room temperature magnetic moments (μ_eff., Table 1 ) of manganese complexes under study supported the proposed stereochemistry derived from the electronic data [ ⁴⁴ ]. The μ_eff values of compounds I and II, measured at 4 and 6 amperes (A), are close to spin only value and refer to high spin complexes. In other words, the results point to the presence of 5 unpaired electrons in both complexes confirming a paramagnetic nature and illustrating that Mn²⁺ ions were not oxidized by dihydrazone ligands.

Thermogravimetric analysis

The thermogram (TGA) data of complex I exhibited five steps of decomposition. The first step (50-150 °C) corresponds to the loss of 1/2 H₂O-crystalline (found/calculated % = 2.0/1.7), while the other four stages are related to the gradual decomposition of the complex with formation of speculated metal oxides at the final step. TG of II gave four stages of decomposition. The first one (140-280 °C) is attributed to the loss of 2OAc⁻ groups (found/calculated% = 18.0/17.5) while the other three stages (300-380, 400-550, 600-800 °C) concerned to the gradual decomposition of the complex with formation of MnO at the final stage (found/calculated % =11.1/10.5).

On the basis of the various physico-chemical and spectral studies presented above and their discussion, the complexes may be suggested to have structures shown in Fig. 4

Electron paramagnetic resonance (EPR)

The complexes I and II have been analyzed by EPR spectroscopy at room temperature and their spectra are shown in ( Fig. 5 ). The ground state of both octahedral and tetrahedral d⁵ ion (spin free) is ⁶A. The energy levels are at ±5/2, ±3/2, ±ν and a single isotropic resonance at g = 2 is easily attained. Accordingly, the EPR spectra of complexes I and II exhibited one broad signals with two “g” values (g_//, g_┴), Table 1 . The absence of hyperfine splitting for these complexes ruled out any axial distortion. The average g values were calculated using the formula:

Fig. 5 The plots of (αhν)² vs. hν of Mn(II)-oxaloyldihydrazone complexes. 

Where, the calculated values are inserted also in Table 2 .

Electron paramagnetic resonance (EPR)

The average g values (g_av) of I and II complexes are nearly 1.9 close to free electron g value indicating the absence of spin-orbit coupling. The g_// is a moderately function for covalent character. The complexes I and II exhibited g_// at 1.930 and 1.998, respectively which are less than 2.3, implying covalent nature of manganese-ligand bond in the present complexes, where, g_//>2.3 is character of ionic metal-ligand bond [ ⁴⁵ , ⁴⁶ ]. Fairly high values of g are in conformity with the oxygen and nitrogen coordination in these compounds [ ¹⁸ ].

Optical band gape energy

The energy band gap of the ligands and their manganese complexes has been calculated with the help of absorption spectra. To calculate the optical band gap energy from absorption spectra, Tauc's relation is used [ ³⁰ , ³¹ ]:.

Where hν is the photon energy, h is Planck's constant, α is the absorption coefficient, E_g is the optical energy gap, A is the constant, m is equal to 1/2 and 2 for direct and indirect transitions, respectively. The absorption coefficient (α) was calculated from to the relation:

Where d is the width of the cell and T is the measured transmittance.

We plot a graph between (νhα)² versus hν, the extrapolation of the straight line to (νhα)² = 0 axis gives optical band gap. Figure 6 shows Tauc's plot for all the samples. Band gap for ligands and Mn²⁺-complexes in DMF solvent lies in the ranges 3.06-3.42 eV and 2.38-3.39 eV, respectively and are mentioned in Table 1 . Careful inspection of Table 1 shows that the optical band gap values of the two ligands (E_g) are minimized upon complexation. This reduction in band gap is attributed to the mobilization of the electrons toward the metal [ ⁴⁷ ]. It is suggested that the manganese raises mobilization of the ligand electrons by accepting them in its shell thus the width of the localized levels in the resulting complex is expanded and in turn, the band gap is smaller. This result has many applications in optics, electronics and energy conversion devices [ ⁴⁸ ]. In fact small band gap facilitates electronic transitions between the HOMO-LUMO energy levels and makes the molecule more electro-conductive [ ⁴⁹ ]. From the observed values of optical properties, it is seen that the synthesized compounds lie in the same range of highly efficient photovoltaic materials [ ⁴⁷ , ⁵⁰ ] and can be used as semiconductors.

Fig. 6 The efficiency for aluminum coupons in 1M HCl solution in the presence and absence of different concentrations of oxaloyldihydrazone ligands at room temperature. 

Weight loss studies

The inhibitive actions of ligands on the dissolution of aluminum in 1 M HCl are investigated ( Table 3 ). Careful inspection of these results showed that, by increasing the inhibitor concentration, both (θ) and (η%) are increased while the weight loss is reduced. So, the dissolution of aluminum in the presence of the investigated inhibitor can be interpreted on the basis of interface inhibition mode where the adsorbed species mechanically screen the coated part of the metal surface from the action of the corrosive medium. This adsorption is related to some discriminated features of the investigated oxaloyldihydrazones like the functional groups (OH, NH, C=O and C=N), quite large molecular size, high molecular weight, aromaticity, electron density at the donor atoms (N:, :O:) and also the π-orbital character of the azomethine, carbonyl and phenyl groups. Adsorption and desorption of inhibitor molecules continuously occur at the metal surface and an equilibrium exists between the two processes at a particular temperature. The inhibition achieved by dihydrazone compounds decreases in the sequence L₂ < L₁ and this behavior can be discussed on the basis that the methoxy group (in L₂) is more electron donating group than naphthoic hydroxyl (in L₂). Positive mesomeric effect (+M effect) and negative inductive effect (–I effect) of -OCH₃ lead to push the electron density toward the donor atoms of O=C-NH-N=CH part via the phenyl ring as a result of delocalization and subsequently enhance the bond strength between the molecule and the corroding aluminum surface. Under these circumstances, the inhibition efficiency of (L₂) should be higher than that of L₁. Furthers, existence of two naphthyl rings in the structure of L₁ may cause crowding that encourages incomplete coating besides they can withdraw the electron pair of ortho-OH group toward themselves under the influence of resonance phenomenon. The iinhibition efficiency values of the dihydrazone are set out in Table 3 whereas the maximum protection efficiency percentages of L₁ is 34% while that for L₂ is 64%. The obtained data boosts the application of L₂ as plausible corrosion pickling inhibitor. It is evident that the inhibition efficiency values is not extremely high owing to the lower ligand concentrations used.

Kinetic treatment

Spontaneous corrosion reaction is a heterogeneous one, composed of anodic and cathodic reactions with the same rate. On this basis the kinetic analysis of the data is considered necessary. In the present work, the initial weight of aluminum coupon at time t is designated as W_i, the weight loss is ΔW and the weight change at time t is (W_i - ΔW) or W_f. When _logW_f was plotted versus time, a linear variation was remarked ( Fig. 7 ) considering a first order reaction [ ⁵¹ ]:

Fig. 7 Variation of log W_f with time (hour) for aluminum coupons in HCl solution containing oxaloyldihydrazone ligands (L₁ and L₂). 

Where: W_i = the initial weight before immersion, K = slope (the rate constant), and t is time.

From the rate constant values obtained from the graphs, regarding the adsorption reaction on manganese surface proceeds as first order, the half-life values (t_ν) of the metal in the test solutions were calculated using the equation [ ⁵¹ ]:

Adsorption isotherms and Gibb's free energy

The adsorption behavior of oxaloydihydrazones on the surface of aluminum was studied by fitting data calculated for the degree of surface coverage to different adsorption isotherms such as Freundlich, Temkin, Langmuir, Frumkin, and Flory-Huggins. The obtained correlation coefficients (R²) were used to determine the best fits [ ⁵² ]. The Gibb's free energy of adsorption ΔG_ads at room temperature, which can characterize the interaction of adsorbed molecules and aluminum surface, for all choice isotherms was calculated using the formula [ ⁵³ ]:

Δ G_ads = RT In 55.5K_ads , Where: T is the temperature ( in K )

Freundlich isotherm

Freundlich isotherm can be expressed according to the equation [ ⁵⁴ ],

Where C is the inhibitor bulk concentration in mol/L, K_ads.(mol⁻¹L) is the equilibrium constant of adsorption and n is Freundlich heterogeneity factor.

Plots of lnθ versus lnC of all ligands ( Fig. 8 ) were linear with acceptable correlation coefficient which illustrate the applicability of Freundlich isotherm. The thermodynamic parameters obtained for the Freundlich adsorption isotherm were recorded in Table 4 . This demonstrates that values of the adsorption parameter n are greater than unity implying favored adsorption process [ ⁵⁵ ].

Fig. 8 Plots of Freundlich lnθ vs. lnC (A), Langmuir Log(C/θ) vs. LogC (B), and Frumkin LogC[θ/(1-θ)] vs. θ (C) adsorption isotherms for aluminum coupons in HCl solution containing oxaloyldihydrazone ligands (L₁ and L₂). 

Langmuir adsorption

The assumption establishing Langmuir adsorption isotherm is given by the equation [ ⁵⁶ ],

Where: C is the concentration of the inhibitor in the bulk electrolyte, θ is the degree of surface coverage of the inhibitor and K is the equilibrium constant of adsorption of the inhibitor. Plotting of logC/θ against logC at 301 K ( Figure 8 ) for the two inhibitors gave straight lines with high correlation coefficients (very close to unity) indicating that Langmuir adsorption isotherm is applicable to the adsorption of oxaloylhydrazones on the surface of aluminum. Values of Langmuir parameters deduced from the isotherms are recorded in Table 4 . The application of Langmuir isotherm to the adsorption of oxaloylhydrazones on surface of aluminum indicated that there is no interaction between the adsorbate and adsorbent. The equilibrium constant (K) of the adsorption process can be calculated from the reciprocal of anti logarithm of the intercept.

Frumkin adsorption

Frumkin adsorption isotherm can be represented by the equation [ ⁵⁷ ],

Where: K is the adsorption-desorption constant and α is the lateral interaction term describing the interaction in adsorbed layer.

Plots of logC[θ/(1-θ)] versus θ shown in Fig. 8 were linear with acceptable correlation coefficients suggesting that the adsorption of oxaloylhydrazones on the aluminum obeys Frumkin isotherm. The values for Frumkin adsorption parameters are listed in Table 4 . It is of interest to note that, the positive α values reflect the attractive behavior of the inhibitor on the surface of aluminum [ ⁵⁸ ].

Adsorption consideration

The inhibition effect of inhibitor compound is ascribed to adsorption of the molecule on metal surface. This adsorption may be physical or chemical depending on the adsorption strength. For a physical adsorption mechanism, the value of the free energy (ΔGads,) of adsorption should be less negative than −40 kJ/mol required for chemical adsorption. The negative values of ΔG_ads set out in Table 4 indicate that the adsorption of oxaloylhydrazones on aluminum surface is spontaneous and in the same time it characterizes their strong interaction with the metal surface [ ⁵⁹ ]. Moreover, the magnitudes of ΔG_ads are in good agreement with physical adsorption mechanism [ ⁵⁹ ]. Accordingly, the prohibition mechanism is due to the formation of a protective film on the metal surface where the additive covers both the anodic and cathodic sites.