Synthesis, Characterization and corrosion Inhibition Studies on Mn (II) and Co (II) Complexes Derived from 1-{(Z)-[(2-hydroxyphenyl) imino]methyl}naphthalen-2-ol in 1M HCl Solution

Schiff base derived from the reaction of 2-amino phenol and 2-hydroxy-1-naphthaldehyde and its Co (II), and Mn (II) complexes have been synthesized and characterized by solubility test, melting point/ decomposition temperatures, molar conductance, IR and magnetic susceptibility. The number of ligands coordinated to the metal ion was determined using Job’s method of continuous variation. Their molar conductance values indicate that all the complexes are non-electrolytes. Magnetic moment values of the complexes showed that both Mn (II) and Co (II) are paramagnetic. The spectroscopic data of metal complexes indicated that the metal ions are complexed with azomethine nitrogen and deprotonated oxygen atom. Corrosion inhibition of the schiff base and Mn (II) and Co (II) complexes were evaluated using the weight loss method in a 0.1MHCl solution for copper metal. The inhibition efficiency increased with increasing inhibitors concentration. The negative values of Gibb’s free energy of adsorption (ΔGads) confirmed the spontaneity and physical adsorption of the inhibition process which is inconsistent with Langmuir adsorption isotherm. for soft-cast steel in HCl acid solution using experimental and theoretical methods. The study revealed that the ligand and their metal complexes show good inhibition efficiency, with Ni complex showing significant inhibition efficiency at an optimum concentration of 25mg/L. (II) > Co (II) > Ligand. The adsorpt ion of studied inhibitors on the copper metal surface obeys Langmuir adsorption isotherm. The values of ΔG° obtained show that the adsorption of all inhibitors was spontaneous and physisorption. The surface morphology of copper was analyzed by the SEM method. The SEM images show that inhibitors adsorb to the copper metal surface.


Introduction 1
Deterioration of the material due to its environmental reaction is called corrosion which occurs in the presence of oxygen and humidity on the metal surfaces. Corrosion is a redox system in which anode oxidation and cathode reduction take place (Charles et al., 2020). The use of corrosion inhibitors is one of the most practical and cost-effective methods of corrosion safety available (Gupta et al., 2016). This is because of their cost-effectiveness and ease of implementation (Pandey et al., 2017). By adsorbing on the surface, they inhibit the metallic surface, creating a protective barrier between metal and electrolyte (Kannam et al., 2016). Singh et al., (2015) reported the synthesis of Mn(II), Co(II), Ni(II), Cu(II) and Zn(II) complexes with 2-amino-benzoic acid (phenylpyridin-2-yl-methylene)-hydrazide. The complexes were characterized by different physicochemical and spectral studies viz. molar conductance, magnetic susceptibility measurements, electronic, IR and NMR spectra. The molecular structures of the ligand Habph and its Mn(II), Ni(II), Cu(II), Zn(II) complexes were further confirmed by single-crystal X-ray diffraction technique. The results of electrochemical impedance spectroscopy (EIS) and adsorption behavior of the ligand and metal complexes showed appreciable corrosion inhibition efficiency for mild steel in a 1 M HCl medium. The metal complexes show a better inhibition effect than the ligand.

Literature Review
Anticorrosion properties in oilfield produced water of the N′-phenylbenzohydrazide and its metal complex derivatives by experimental studies and quantum chemical calculations as well as investigation of the relationship between experimental and theoretical study results through a correlation study was reported by Mahross et al., 2018. The molecular structures of N′phenylbenzohydrazide and its metal complexes (Cu, Mn, Co) were characterized using UV-Vis and FT-IR techniques. The electrochemical polarization measurements, electrochemical impedance spectroscopy (EIS) and scanning electron microscopy techniques were used experimentally in the discovery of corrosion inhibition properties of the metal complexes in oilfield produced water. The results showed that the metal complexes acted as anodic-type inhibitors. The experimental inhibition activities of studied metal complexes were found to be highly correlated with calculated some parameters, ET, ELUMO, EHOMO and EGap.  2·4H2O, CuPAB, on the corrosion behavior of carbon steel in 0.5 M sulfuric acid solutions by electrochemical impedance spectroscopy (EIS), and potentiodynamic polarization techniques. Theoretical fitting of different adsorption isotherms such as Langmuir, Flory-Huggins, and the kinetic-thermodynamic models has been tested. The adsorption isotherm parameters indicate that the adsorption behavior of complex on the metal surface is not ideal. The high binding constant of CuPAB showed a stronger interaction between the metal surface and the complex. The obtained data revealed that CuPAB complex has remarkable inhibiting effects on the corrosion of steel in 0.5 M H2SO4.

Materials
2-hydroxy-1-napthaldehyde was of analytical grade and procured from Sigma-Aldrich. Other chemicals and reagents were of analytical grade and used without further purification. The metal salts: cobalt (II) and manganese (II) chlorides were obtained from a chemical store, Bayero University, Kano.

Instruments
All weighing was carried out on electrical balance model H3OAR, melting point and decomposition temperatures were recorded using Gallenkamp melting point apparatus. Conductivity measurements were carried out using Jenway conductivity meter model 4010 in DMSO solvent. The infrared spectral analyses were recorded using SHIMADZU FTIR 8400S model. Magnetic susceptibility measurements were conducted and recorded using mk1 Sherwood magnetic susceptibility meter.

Preparation of Schiff base
0.01mol 2-aminophenol in 25cm 3 ethanol was slowly added to a solution of 0.01mole of 2 -hydroxy -1-naphthaldehyde in 25cm 3 ethanol. The mixture was refluxed for two hours. The Schiff base that formed was filtered and washed with ethanol and dried over phosphorus pentoxide (Aliyu and Sani 2011).

Preparation of Metal (II) Complexes
A general method has been used for the preparation of complexes using the reaction of metal salts and the corresponding Schiff-base in a molar ratio (1:2). An ethanolic solution of ligand (0.04 mol) and metal (II) chlorides (0.02 mol) both were mixed gently and refluxed for 4 h. The volume of the resulting solution was concentrated by evaporating the solvent. The reaction mixture was cooled to room temperature which solidified on cooling. The solid thus obtained was filtered, washed thoroughly with ethanol to apparent dryness and dried over phosphorus pentoxide (Aliyu and Sani, 2011).

Determination of Number of Coordinated Ligand
The ligand to the metal ratio in the complexes was determined using the continuous variations method (Job's method) (Agelici, 1971). 0.001M of the ligand and the metal (II) Chlorides in DMSO were prepared. The following ligand to metal salt ratio (ml); 1:15, 3:13, 5:11, 7:9, 9:7, 11:5, 13:3 and 15:1 were taken from ligand solution and each of the metal (II) chloride respectively. A total volume of 16ml was maintained (in the order) through the process and the mole fraction of the ligand was calculated in each mixture. The solution of the metal (II) chlorides were scan (as blank) to find the wavelength λmax for the particular metal ion before taking the absorbance values. The absorbance values were extrapolated against the mole fraction of the ligand and the number of coordinated ligands was determined using the relation below Where n is the number of coordinating ligand and Xi is the mole fraction at maximum absorbance.

Preparation of Coupons
The sample (copper), purchased from yan-gwangwan along K/marusa road, Katsina, was mechanically pressed and cut. The specimen's size was 1.0 × 1. 0 × and is used in all experiments with pre-treatments such as cleaning, polishing, and degreasing. Surface cleaning is done by immersing the specimen in ethanol; polishing is done by using various grades of emery paper and then finally degreasing by acetone. The specimen was stored in dessicator (Obot et al., 2011).

Preparation of Solution
Acidic solution containing 1.0 moldm -3 HCl was prepared by dissolving 8.60 cm 3 of concentrated HCl in distilled water and diluting the resulting solution to volume in 100 cm 3 volumetric flasks (Maibulangu et al., 2017). The concentration of the inhibitors (Schiff base, Mn (II) complex and Co (II) complex) used for the study were 0.02, 0.04, 0.06, 0.08 and 0.10 gL -1 .

Gravimetric Experiment
The weight loss experiments were performed using standard methods described by Gupta et al., 2016. The corrosion rate was calculated using the equation: = Where W is the mean value of weight loss of three parallel metal coupons, A is the total area of a coupon, and t is the immersion time (2hr). The percentage of inhibition efficiency (ɳ%) and surface coverage ( ) was evaluated corrosion rates using the following relations:

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Where CR and CR(i) are the corrosion rate (mgcm -2 h -1 ) of copper coupons in the absent and presence of the inhibitors, respectively.

Discussion
The Schiff base derived by the condensation of 2-amino phenol and 2-hydroxy-1-naphthaldehyde was orange in colour, stable and non-hygroscopic solid at room temperature. The corresponding product was obtained in good yield (82%). The Schiff base melts at 186 0 C reacts with Co and Mn (II) chlorides and formed complexes with a yield of 56-75%. The colors of Mn (II), and Co (II), complexes are cream and brown, respectively. The decomposition temperatures of these complexes were 240 o C for Mn (II) and 286 o C for Co (II). The values of decomposition temperatures indicate that the complexes are stable (Aliyu et al., 2013). These results are presented in Table 1.

Solubility Test
The result of the solubility test of Schiff base ligand and its Mn (II) and Co (II) complexes in water and common organic solvents, presented in table 2 revealed that the Schiff base and its Mn (II), and Co (II), complexes were all soluble in methanol, ethanol, DMSO and DMF. However, they were insoluble in distilled water and tetrachloromethane. Mn (II), and Co (II), complexes found to be slightly soluble in chloroform and n-hexane. The Schiff base was also found to be soluble in chloroform and slightly soluble in n-hexane. This is in agreement with results obtained by Aliyu et al., 2013, Aliyu and Sani, 2011.

IR Spectra
The IR values of the Schiff base, Mn (II) complex and Co (II) are presented in Table 3 and their spectra in figure 1-3. The Schiff base exhibits the following bands in its IR spectrum, 3119 cm −1 due to ѵ (-OH) of aromatic, and 1618cm -1 is characteristic of the azomethine group. Other bands appeared in the spectra of the complexes in the region range (3396-3401cm -1 ) which may be attributed to Ѵ(H2O) stretching vibration.

Molar Conductivity and Magnetic Susceptibility
The molar conductivity values for the complexes in DMSO were in the electrolytic range of 7.07-7.53 μs. It is clear from the conductivity data that the complexes present seem to be non-electrolytes. Also, the molar conductance values indicate that there is an absence of anions outside or inside the coordination sphere (Moamen et al., 2013). The observed magnetic moment values for Mn (II) and Co (II) complex are 2.11 BM and 3.21 BM, respectively, suggesting a high spin complex with octahedral geometry.

Weight Loss Experiment
The values of percentage inhibition efficiency (ɳ), corrosion rate (CR), and surface coverage (ϴ) obtained by weight loss method at various concentration of all compounds at 313 K are summarized in table 6. From the results, in 1M HCl medium, the Schiff base and its Mn (II) and Co (II) showed significant corrosion inhibition for copper. The inhibition efficiency seems to have improved with increased inhibitor concentration, and this is due to the copper surface interaction of the inhibitor molecule, allowing it to be adsorbed (Aouniti et al., 2016;Nassar et al., 2015).. The inhibitor's ability to prevent the copper's corrosion ability is due to its propensity to form bonds using the lone pair of electrons present in the O atom and the π-electrons present in both the azomethine moiety and the benzene ring (Nassar et al., 2015).

Adsorption Isotherms
Adsorption isotherms provide data on the interaction between inhibitors and metal surfaces (Hussaini et al., 2020). The surface coverage values at 313 K for different concentrations have been used to describe the best isotherms that assess the mechanism of adsorption. Attempts were made to fit (ϴ) values to different isotherms, like Langmuir, Frumkin, Freundlich and Temkin. The calculated values of ΔG 0 ads are given in Table 7. The value of ΔG 0 ads for the ligand, Mn (II) complex and Co (II) complex are -9.421KJmol -1 , -10.445KJmol -1 and -10.379KJmol -1 respectively, indicating that the adsorption of the inhibitors is physical adsorption (Verma et al., 2015).

Surface Morphological Examination
The surface analysis was carried out by using Scanning Electron Microscope (SEM) for the copper surface before immersing in acidic solution, after immersing in acidic solution in the absence of inhibitors, and after immersing in acidic solution in the presence of inhibitors. The results obtained were presented as scanning electron micrographs in Figures 4a, 4b, 4c, 4d, and 4e. The SEM image of polished copper metal (blank) surface was obtained and shown in Figure 4a. The micrographs of the copper metal surface showed an aggressive attack after immersion without inhibitor in 1M HCl solution was shown in Figure 4b. In the absence of an inhibitor, the surface appeared to be strongly damaged because of some cracks and pits as a result of exposure to the corrosive solution. In the presence of the inhibitor, the SEM micrograph shows the formation of a thick film copper metal surface and reduction of surface roughness compared to the micrograph of copper metal without inhibitor, and the result is presented in figure 4c, 4d and 4e. Therefore, it is understood that the inhibitor molecules adsorbed on the copper metal surface, forming an efficient protective layer. This formed layer protects the copper surface against the corrosion process in 1M HCl.

Conclusion
The present work describes the synthesis and characterization of Co (II) and Mn (II) complexes of 1-{(Z)-[(2hydroxyphenyl)imino]methyl}naphthalen-2-ol. The ability of this Schiff base and its metal (II) complexes to inhibit the corrosion of copper in 1M HCl solution using the weight loss method was then investigated. The results showed that corrosion rate increases with increasing inhibitors concentration. Also, corrosion rate was found to be in the order: Ni (II) > Co (II) > Ligand. The adsorption of studied inhibitors on the copper metal surface obeys Langmuir adsorption isotherm. The values of ΔG° obtained show that the adsorption of all inhibitors was spontaneous and physisorption. The surface morphology of copper was analyzed by the SEM method. The SEM images show that inhibitors adsorb to the copper metal surface.
Funding: This research received no external funding