Inhibition of Mild Steel Corrosion by Nickel Complex of 1-(8-hydroxy quinolin-2yl-methyl) urea in Sodium Chloride solution

The Corrosion inhibition of a inhibitor namely Ni complex of 1-(8-hydroxy quinolin-2yl-methyl) urea (Ni-HUF) in controlling corrosion of mild steel immersed in aqueous solution containing 60 ppm Clhas been investigated using weight loss method. The corrosion inhibition efficiency offered by 50 ppm of Ni-HUF is 74 %. The corrosion inhibition was observed due to the formation of more stable and compact protective film on the metal surface. Fluorescence spectral analysis was used to detect the presence of iron-inhibitor complex. Polarization study and Electrochemical Impedance spectra confirm the formation of a protective film formed on the metal surface. Introduction Metal complexes are widely used as catalyst of chemical reactions, e.g. Oxidative dehydrogenation (ODH) of ethane and epoxidation of geraniol [1-3] and as stabilizer or precursor in sol-gel processes [4-6]. Very few works have been performed to study anticorrosive behavior of metal complexes. Harms et al.[7] proposed corrosion inhibition through precipitation of Fe(II) phosphate and Fe(III) phosphate in presence of Fe(III) acetylacetonate and Fe(II) acetylacetonate respectively. Palladium acetylacetonate is suggested as an effective corrosion inhibitor for water cooled nuclear reactor [8]. Interaction of transition metal complexes with mild steel is greatly affected by their standard electrode potentials, their reactivity and the nature of the ligand that could stabilize the metallic complexes. Reduction of Cu (II) and Co (II) species on mild steel surface is possible due to their noble standard electrode potential compared to Fe (II). However, it should be noted that negative charged ligands like nitro, thiocyanate, Oxalato, glucinato and acetylacetonate could stabilize the higher oxidation states [9]. Hence reduction of Cu (II) and Co (II) on the steel surface could be affected by the ligands surrounded them. It is reported that sodium, zinc and calcium salts of gluconic acid could provide an effective corrosion inhibition for the mild steel immersed in near neutral media [10-13]. The effectiveness of gluconates on the anodic metal dissolution reaction and the cathodic oxygen reduction reaction in neutral solution depends on the inhibitor concentration and the nature of cations introduced in the solution as a gluconate salt [14]. The aim of the present work is to evaluate corrosion inhibitive performance of nickel complex of 1-(8-hydroxy quinolin-2yl-methyl) urea to mild steel immersed in aqueous solution containing 60 ppm Cl-. The corrosion inhibition efficiency was evaluated using weight loss method and electrochemical impedance spectroscopy. The protective film formed on the metal surface characterized with the help of surface analytical techniques such as fluorescence and UVVisible spectroscopy. Materials and Methods Mild steel specimens; (0.026% S, 0.068% P, 0.39 % Mn, 0.11 % C and the rest iron ) of dimensions 1.0 cm ×4.0×0.2 cm were polished to mirrors finish and degreased with acetone and used for weight loss method. Weight loss method: Mild steel specimens triplicate were immersed in 100 ml beaker containing 100 ml of aqueous solution containing 60 ppm of Clcontaining various concentrations of the Ni complex of 1-(8-hydroxy quinolin-2yl-methyl) urea -inhibitors for one day. After one day immersion the specimens were taken out, washed in running water, dried and weighed using a Shimadzu balance, model AY62. The corrosion inhibition efficiency (IE) was calculated using the equation: IE = 100[1-(w2-w1)] % Where w1 is the corrosion rate in the absence of inhibitor and w2 is the corrosion rate in the presence of inhibitor. Potentiodynamic Polarization study: Polarization studies were carried out in a CHI electrochemical workstation with impedance model 643, Austin, USA. A three electrode cell assembly was used. The working electrode was mild steel. The exposed surface area was 1 cm2. A saturated calomel electrode (SCE) was used as the reference electrode and a rectangular platinum foil was used as the counter electrode. The results such as Tafel slopes, Icorr, Ecorr and LPR values were calculated. AC impedance spectra The instrument used for polarization study was also used for AC impedance spectra. The cell set up was the same as that was used for polarization measurements. The real part (Z’) and the imaginary part (Z’’) of the cell impedance were measured in ohms at various frequencies. AC impedance spectra were recorded with initials E (v) =0V, high frequency limit was 1×105 Hz, low frequency limit was 1 Hz, amplitude =0.005V and quiet time tq=2 s. The values of charge transfer resistance Rt and the double layer capacitance Cdl were calculated.


Introduction
Metal complexes are widely used as catalyst of chemical reactions, e.g. Oxidative dehydrogenation (ODH) of ethane and epoxidation of geraniol [1][2][3] and as stabilizer or precursor in sol-gel processes [4][5][6]. Very few works have been performed to study anticorrosive behavior of metal complexes. Harms et al.[7] proposed corrosion inhibition through precipitation of Fe(II) phosphate and Fe(III) phosphate in presence of Fe(III) acetylacetonate and Fe(II) acetylacetonate respectively. Palladium acetylacetonate is suggested as an effective corrosion inhibitor for water cooled nuclear reactor [8]. Interaction of transition metal complexes with mild steel is greatly affected by their standard electrode potentials, their reactivity and the nature of the ligand that could stabilize the metallic complexes. Reduction of Cu (II) and Co (II) species on mild steel surface is possible due to their noble standard electrode potential compared to Fe (II). However, it should be noted that negative charged ligands like nitro, thiocyanate, Oxalato, glucinato and acetylacetonate could stabilize the higher oxidation states [9]. Hence reduction of Cu (II) and Co (II) on the steel surface could be affected by the ligands surrounded them. It is reported that sodium, zinc and calcium salts of gluconic acid could provide an effective corrosion inhibition for the mild steel immersed in near neutral media [10][11][12][13]. The effectiveness of gluconates on the anodic metal dissolution reaction and the cathodic oxygen reduction reaction in neutral solution depends on the inhibitor concentration and the nature of cations introduced in the solution as a gluconate salt [14]. The aim of the present work is to evaluate corrosion inhibitive performance of nickel complex of 1-(8-hydroxy quinolin-2yl-methyl) urea to mild steel immersed in aqueous solution containing 60 ppm Cl -. The corrosion inhibition efficiency was evaluated using weight loss method and electrochemical impedance spectroscopy. The protective film formed on the metal surface characterized with the help of surface analytical techniques such as fluorescence and UV-Visible spectroscopy.

Materials and Methods
Mild steel specimens; (0.026% S, 0.068% P, 0.39 % Mn, 0.11 % C and the rest iron ) of dimensions 1.0 cm ×4.0×0.2 cm were polished to mirrors finish and degreased with acetone and used for weight loss method.

Weight loss method:
Mild steel specimens triplicate were immersed in 100 ml beaker containing 100 ml of aqueous solution containing 60 ppm of Clcontaining various concentrations of the Ni complex of 1-(8-hydroxy quinolin-2yl-methyl) urea -inhibitors for one day. After one day immersion the specimens were taken out, washed in running water, dried and weighed using a Shimadzu balance, model AY62.
The corrosion inhibition efficiency (IE) was calculated using the equation: Where w 1 is the corrosion rate in the absence of inhibitor and w 2 is the corrosion rate in the presence of inhibitor.

Potentiodynamic Polarization study:
Polarization studies were carried out in a CHI electrochemical workstation with impedance model 643, Austin, USA. A three electrode cell assembly was used. The working electrode was mild steel. The exposed surface area was 1 cm 2 . A saturated calomel electrode (SCE) was used as the reference electrode and a rectangular platinum foil was used as the counter electrode. The results such as Tafel slopes, I corr , E corr and LPR values were calculated.

AC impedance spectra
The instrument used for polarization study was also used for AC impedance spectra. The cell set up was the same as that was used for polarization measurements. The real part (Z') and the imaginary part (Z'') of the cell impedance were measured in ohms at various frequencies. AC impedance spectra were recorded with initials E (v) =0V, high frequency limit was 1×10 5 Hz, low frequency limit was 1 Hz, amplitude =0.005V and quiet time t q =2 s. The values of charge transfer resistance R t and the double layer capacitance C dl were calculated.
Where f max is maximum frequency.

Surface Characterization studies:
The mid steel specimens were immersed in various test solution for a period of one day. After one day the specimens were taken out and dried. The nature of the film formed on the surface of the metal specimen was analyzed by various surface analysis techniques.
Surface analysis by fluorescence spectroscopy: Fluorescence spectra of solutions and also the films formed on the metal surface were recorded using Jasco-F-6300 spectra fluorometer.
Surface analysis by UV-Visible spectroscopy: UV-Visible spectra were recorded in a Cary Eclipse Varian (Model U.3400) spectrophotometer.

Results and Discussion
The corrosion rates (CR) of mild steel immersed in aqueous solution containing 60 ppm Cland also inhibition efficiencies(IE) in the absence and presence of inhibitor Ni complex of 1-(8-hydroxy quinolin-2yl-methyl) urea obtained by weight loss method are given in Table 1. It is observed from

Analysis of Polarization Curves:
The polarization study has been used to investigate the formation of protective film on metal surface [15][16][17][18][19]. The polarization curves of mild steel immersed in aqueous solution containing 60 ppm of Clare shown in Figure 1. The corrosion parameters such as Corrosion potential (E corr ), Corrosion Current density (I corr), Tafel slopes (b c and b a ) and linear polarization curves (LPR) are given in Table 2.  When mild steel is immersed in aqueous solution containing 60 ppm of Cl -, the corrosion potential is -472 mV Vs SCE. The formulation consisting of 50 ppm of Ni-HUF shifts the corrosion potential to -494 mV Vs SCE. It shows that the corrosion potential is shifted to negative side. This suggests that the cathodic reaction is controlled predominantly.
The corrosion current density value and LPR value for aqueous solution containing 60 ppm of Clare 1.261 × 10 -3 A cm -2 and 25.74 ohm cm 2 respectively. For the formulation of 50 ppm of Ni-HUF the corrosion density value has decreased from 1.261 × 10 -3 A cm -2 to 4.857 × 10 -5 A cm -2 and the LPR value has increased from 25.74 ohm cm 2 to 455.2 ohm cm 2 . The fact that the LPR value increases with decrease in corrosion current density indicates the absorption of the inhibitor on the metal surface to block the active sites and inhibit corrosion and reduce the corrosion rate with the formation of a protective film on the metal surface.
AC impedance spectra AC impedance spectra [electrochemical impedance spectra] have been used to confirm the formation of protective film on the metal surface [20][21][22][23]. The AC impedance spectra of mild steel immersed in aqueous solution containing 60ppm of Clin the absence and presence of inhibitors are shown in Fig.2( Nyquist plots) and Fig.3 (Bode plots). The impedance parameters namely charge transfer resistance (R t ) double layer capacitance (C dl ) and impedance lg(z/ohm) are given in Table-3. If a protective film is formed on the metal surface, R t value increases and the C dl value decreases.  Table 3. The AC impedance spectra of mild steel immersed in aqueous solution containing 60ppm of Clin the absence and presence of Ni-HUF inhibitor system.
When mild steel is immersed in aqueous solution containing 60ppm of Cl -, R t value is 20.19 Ω cm 2 and C dl value is 5.235 × 10 -5 F cm -2 . When Ni-HUF are added to the aqueous solution containing 60ppm of Cl -R t value increases from 20.19 Ω cm 2 to 316.88 Ω cm 2 and the C dl value decreases from 5.235 × 10 -5 F cm -2 to 3.335 × 10 -6 F cm -2 . The impedance value increases from 0.973 to 2.098. This account for the high inhibition efficiency of Ni-HUF system and a protective film is formed on the metal surface. This is also supported by the fact that for the inhibitor system the phase angle increases from 47.88 to 63.73 o (Fig.3). The UV-Visible absorption a spectrum of an aqueous solution containing HUF is shown in figure 4. A peak appears at 356nm. When Fe 2+ solution is added to the solution the intensity of the UV-Visible spectra increases at 584nm. This peak is due to formation of Fe 2+ -HUF complex in solution [24,25].
200.00 300.00 400.00 500.00 600.00 Abs. Fluorescence spectra: The emission spectrum (λ ex : 380nm) of solution containing HUF-Fe 2+ solution is shown in Figure 5a. A peak appears at 400nm. This is due to HUF-Fe 2+ complex formed in solution.
The emission spectrum of the film formed on the metal surface after immersion in solution containing 50 ppm of Ni-HUF is shown in figure 5b. Hence it is concluded that the protective film consists of HUF-Fe 2+ complex. The number peak obtained is only one. Hence it is confirmed that the complex of somewhat highly symmetric in solution [26].

Conclusion:
The conclusion drawn from the results may be given as: the formulation consisting of 50 ppm of Ni-HUF has 74% inhibition efficiency. Polarization study suggests that cathodic reaction is controlled predominantly. AC impedance spectra reveal that a protective film is formed on the metal surface. Fluorescence and UV-Visible spectra show that the protective film consists of HTF-Fe 2+ complex formed on metal surface.