Electrodeposited Benzothiazole Phthalocyanines for Corrosion Inhibition of Aluminium in Acidic Medium

1 ) and gallium(III)chloride ( 2 ) were successfully electrodeposited onto aluminium for corrosion retardation in 1.0 M hydrochloric acid solution. The aim of this study was to compare the corrosion resistance of electrodeposited metallated phthalocyanines. Scanning electron microscopy, X-ray diﬀraction, electrochemical impedance spectroscopy (EIS), and polarization conﬁrmed the aluminium corrosion inhibition potentials of complexes 1 and 2 . EIS and polarization techniques showed that complex 2 performed better than complex 1 , with values from EIS measurements of 82% for 1 and 86% for 2 in 1.0 M hydrochloric acid solution. The importance of electrodeposition in industries and a dearth of research on the use of electrodeposited metallated phthalocyanines necessitated this study, and results show that coatings formed by electrodeposition of 1 and 2 onto aluminium reduced its susceptibility to corrosion attack.


Introduction
e strong electron delocalization property of metallophthalocyanines (MPcs) allow them to be used in many applications such as in nonlinear optics, dye-sensitized solar cells (DSSC), and photocatalysis and as gas sensors [1][2][3][4]. Aggregation limits the technological applications of MPcs, but this setback can be improved on (or prevented) by modifying them structurally, such as by changing the position and type of substituents [5]. Bulky substituents such as benzothiazole employed in this work reduce aggregation.
Organic molecules with π-electron systems (such as MPcs) in combination with substituents containing heteroatoms such O, N, and S are potential corrosion inhibitors due to their ability to adsorb on metallic surface [6,7]. is work therefore combines phthalocyanines with benzothiazole for synergetic corrosion inhibition and reports for the first time on the electrodeposition of phthalocyanines prior to corrosion control, as opposed to adsorption during corrosion inhibition. Phthalocyanines have been self-assembled onto electrodes for corrosion inhibition [8]. Electrodeposition employed in this work provides a homogeneous and uniform deposition of surface coating [9], and simple MPc complexes can be employed. Electrodeposition is a simple technique for electrode film (or coating) formation. Different deposition techniques include potentiodynamic (cyclic voltammetry), potentiostatic (chronoamperometry), and galvanostatic (chronopotentiometry) techniques, which influence structural properties and roughness features of coatings formed [10][11][12]. Cyclic voltammetry deposition technique however allows the control of the electrodeposition process, by monitoring the number of scans, and hence was employed in this work. e MPc complexes employed are tetrakis[(benzo[d]thiazol-2ylphenoxy) phthalocyaninato] gallium (III) chloride (1) [12] and tetrakis [(benzo[d]thiazol-2-yl-thio) phthalocyaninato] gallium(III)chloride (2) [13].
is study reports for the first time the transition dipole moment (μ e ) and oscillator strength (f ) of complexes 1 and 2 (structure in Figure 1) from which corrosion inhibition abilities of 1 and 2 were determined. In the light of this, we investigated the corrosion inhibition properties (on aluminium) of electrodeposited complexes 1 and 2.

Results and Discussion
Materials and equipment items used in this work can be found in Supplementary Materials (Available here).
Aggregation of phthalocyanines in solution is known [15], and the factors which affect this property are complexed metal ions, concentration, nature of solvent, temperature, and the substituents [16]. Aggregation is evidenced by peak broadening which is absent for complexes 1 and 2 in the studied solvents at concentrations ≤1.0 × 10 −5 M, shown in Figures S1 and S2 (Supplementary Materials). In corroboration, Lambert-Beer law was obeyed in the 1.0 × 10 −6 M to 1.0 × 10 −5 M concentration range. is confirms that complexes 1 and 2 are not significantly aggregated within this concentration range.
Complexes 1 and 2 showed Q bands at 689 nm and 692 nm in DMSO (Table 1). Observation from this study reveals that Q band maximum for 2 is red shifted compared to that for 1 (Table 1). A plausible explanation is the fact that complex 2 contains sulphur bridges between the benzothiazole moieties and the phthalocyanine core. is differs from complex 1 which has phenoxy bridge between the benzothiazole moieties and the phthalocyanine. Similar observation was earlier reported by Li and coworkers [17]. Sulphur groups are known to red shift the Q band of phthalocyanines [18].
Dipole moment is the measure of polarity in a bond which is related to the distribution of electrons in a molecule; it is an index which can be used for the prediction of the magnitude/extent of a corrosion inhibition process. It is generally agreed that polar compounds possessing high dipole moments possess good adsorption properties on the metal surface leading to better corrosion inhibition efficiency [19]. Oscillator strength is related to dipole moment, and it is employed in this work to determine the possible corrosion inhibition efficiencies of complexes 1 and 2 in different solvents. Even though complexes 1 and 2 are electrodeposited, these parameters may give an idea of which complex can be a better corrosion inhibitor. Determination of oscillator strength (f s ), transition dipole moment (μ), and integrated absorption coefficient (IAC) was performed using expressions derived elsewhere [20]: where where f (s) is the oscillator strength (in solution) and the transition dipole moment is represented by μ km . Also, μ e and μ g are the dipole moments of the excited and ground states, respectively, q is the charge of the electron,v max (values in Table 1) represents the wavenumber of maximum absorption, N 0 is the Avogadro number, h is the Planck constant, c is the speed of light, ε 0 is the permittivity of free space, and m e is mass of electron. IAC was determined using area under the Q band using absorption data such that molar absorptivity (ε) was plotted against the wavenumber [20]. e Q band maxima varies with the nature of the solvent, and hence, different solvents are used. Table 2 presents the parameters for complexes 1 and 2 in the studied solvents, recorded in the form of oscillator strengths (f (s) ) and transition dipole moments (μ km ).
e ease at which complexes 1 and 2 are polarizable is implied by the positive values of transition dipole moment, derivable from equation (3), and showing that μ e > μ g . is in turn explains why they adsorb very well on metal surfaces resulting in good inhibition efficiencies [12,13]. Complex 2 has higher μ km and f (s) than complex 1 in all solvents, suggesting better corrosion inhibition for the former. However, this conclusion is tentative since complexes 1 and 2 are electrodeposited onto Al in this work and are not in solution. Figure 2(a) shows the cyclic voltammogram peaks of complex 1 on GCE as an example. All peaks in Figure 2(a) are due to ring-based processes since the central metals are inactive. Complexes 1 and 2 were electrodeposited onto the Al by scanning from 1.0 to -2.3 V vs Ag/AgCl in dichloromethane (DCM) containing TBABF 4 ( Figure 2(b)). On the Al electrode, steady state behaviour is observed on the anodic side. It is the shape of the electrode that determines whether it will reach a steady state or not [21]. e peak near −1.6 V increased with continuous cycling indicating that a film was formed on the electrode as a consequence of electrodeposition. Electrodeposition is judged by increase in current without shift in potential or formation of new peaks as observed in Figure 2(b) [22]. e steady state peaks only shift to more negative potentials with cycling ( Figure 2(b)). e colour of the phthalocyanine solution became less intense confirming electrodeposition of the Pc from the solution to the electrode. Figure 3 presents the thermal degradation information of benzothiazole phthalocyanines. ese TGA curves show that the studied Pcs are thermally stable (before electrodeposition) at moderately high temperatures (below 300°C). Detachments of the substituents probably occur at temperature ranges of 250-360°C and 230-370°C for 1 and 2, respectively. e Pcs seem to degrade rapidly from ∼400°C for 1 and ∼370°C for 2. Electrodeposition is not expected to change the nature of complexes 1 and 2.

Electrodeposition.
e decreased stability following electrodeposition could be a result of change in conformation during the electrodeposition process [23]. ermograms obtained after HCl exposure are similar with those recorded for electrodeposited complexes 1 and 2 before corrosion.

Surface Morphology-Scanning Electron Microscopy (SEM).
Scanning electron microscopy (SEM) was used for the study of surface morphology [24] (Figure 4). e smooth Al surface becomes badly battered after immersion in 1.0 M hydrochloric acid solution as shown in Figure 4(b). Figure 4(c) shows the SEM image of electrodeposited-2, before immersion in HCl. An amorphous surface with intrinsic plateletlike flakes covering the metal surface is observed. e electrodeposited-2 covering strongly adheres to the metal surface such that even after exposure to the corrosive medium, the metal is protected from acid attack as Figure 4(d) shows. is is judged by the similarities of the images of the electrodeposited-2 before and after immersion in acid, showing that the observed surface is that of the phthalocyanine and not the metal. e aluminium coated with electrodeposited-1 possesses flowerylike flakes (Figure 4(e)) which adhere strongly onto the metal, protecting it from the corrosive attack of 1.0 M hydrochloric acid as seen in Figure 4(f ). However, the image of electrodeposited-1 after being immersed in acid is different from that before, suggesting less protection compared to electrodeposited-2. e electrodeposited aluminium surfaces of Figures 4(c) and 4(e) became more dense and rough necessitated by the formation of films, which in turn protected the metal from exposure to aggressive environments. is observation is in agreement with earlier findings [25]. Amorphous and crystalline surfaces of electrodeposited benzothiazoles have been reported earlier [26].

XRD.
Literature sources [27][28][29][30] Figure 5). For the corroded Al, the peak at 39.3°is less intense and the one at 78.8°increases in intensity ( Figure 5). Following protection with the coatings (electrodeposited-1 and electrodeposited-2), the peak at 78.8°is low in intensity (relative intensity 18.1% and 28.4%, respectively) almost to the level of uncorroded Al (at 17.8% relative intensity). e peak near 2θ � 45.0°has increased with 100% relative intensities (Table 3) for 1 corr and 2 corr compared to uncoated aluminium surface (at 9.8% relative intensity); this increase corroborates with corrosion protection by electrodeposited-1 and electrodeposited-2. Seifzadeh et al. [31] associated increase in XRD peaks in the presence of corrosion inhibitors with reduced corrosion.
is probably suggests there was some corrosion of the metal before electrodeposition in this work.
ere is a drastic decrease of the peak at 2θ � 65.8°, and such changes have   International Journal of Electrochemistry 5 been attributed to the possible complex formation between the metal and the inhibitor molecule [32].

Electrochemical Impedance Spectroscopy (EIS).
Uncoated aluminium and that coated via electrodeposition of complexes 1 and 2 when immersed in 1.0 M hydrochloric acid solution gave Nyquist plots and phase angle-log (frequency) diagrams, presented in Figure 6. It was observed that the uncoated metal exhibited capacitive and inductive curves at high-and low-frequency values, respectively. On the contrary, the coated metal with the MPcs gave electrochemical signals which showed capacitive curves, followed by a Warburg diffusional tail which, as Ozyilmaz et al. [33] opined, is related to diffusion of corrosive species through the coating and the oxide layers. Figure 6(a) shows larger capacitive curves for coated surfaces, an indication that coating the metal causes an increase in the impedance of the inhibited substrate. Adsorption onto the electrode surface by species such as H + ads perhaps due to the relaxation    process explains the inductive curve at low-frequency values for the uncoated metal [34]. e equivalent circuit for uncoated metal, shown in Figure 6(d), consists of the following elements: the solution resistance (R s ), charge transfer resistance (R ct ), and constant phase element (CPE), while the inductive resistance (R L ) and inductance (L) are introduced to simulate the inductive curve. Analyses of the impedance data for coated metal were performed using an equivalent circuit as shown in Figure 6(e) with the following elements: the solution resistance (R s ), double layer capacitance (C dl ), charge transfer resistance (R ct ), and Warburg impedance element (W).
e EI% values calculated using equation S1 in Supplementary Materials are higher for complex 2 than for complex 1 (Table 4) as predicted by transition dipole   International Journal of Electrochemistry moment (μ e ) and oscillator strength (f ) discussed above. e value of R ct is a measure of electron transfer across the exposed area of the metal surface, and it is inversely proportional to the rate of corrosion [35]. us, large R ct (and IE %) values are associated with slower corroding process [36]. e R ct values of the coated metal are about six times higher for electrodeposited-1 and seven times higher for electrodeposited-2 than the uncoated metal when immersed in the corrosive solution for 30 min.
is confirms that electrodeposited coating provides strong corrosion protections for the metal. Similar analogy was reported by Lu and coworkers [37]. e values of inhibition efficiency calculated from EIS are in good agreement with those determined from polarization curves in Figure 6.
Bode modulus and phase angle plots are presented in  Figure 7 shows slight difference in the shape and slope of anodic/cathodic branches, and there is a shift in potential when the aluminium surface is coated with the Pcs and immersed in hydrochloric acid solution.
Classification of an inhibitor as predominantly anodic or cathodic is determined by the difference in corrosion potential (E corr ) of more than 85 mV when compared to the corrosion potential of the blank [40]. e indication that electrodeposited-1 and electrodeposited-2 possess mixed-type character is based on the fact that the differences in the corrosion potential values are all less than 85 mV compared to the blank (Table 5), which is indicative of possible retardation of anodic and cathodic aluminium corrosion processes [13]. Electrodeposited-1 and electrodeposited-2 exhibit predominant cathodic characters because values of cathodic Tafel slopes (β c ) are more positive than in their absence, as shown in Table 5. Values of corrosion current density (j corr ) decreased for aluminium electrodeposited-1 and electrodeposited-2 surfaces when compared to the uncoated metal surface (Table 5).
is observed decrease plausibly is due to corrosion protection of the metal from attack of Cl − , indicating that the electrodeposited phthalocyanines possess good corrosion inhibition properties. Similar trend was reported for electropolymerized polyaniline and its copolymer on carbon steel [41]. e inhibition efficiency (IE%) values were calculated using the corrosion current densities without coating and with coating (equation S2 in Supplementary Materials). e values of these parameters are shown in Table 5. IE% is larger for 2 as was the case for the data from EIS.

Conclusions
Gallium(III) chloride phthalocyanines complexes (1) and (2) were successfully electrodeposited on aluminium metal surface by cyclic voltammetry and used as anticorrosive coatings. e coated aluminium surfaces exhibited good corrosion resistance in comparison with uncoated Al substrate, as demonstrated by results from EIS and potentiodynamic polarization techniques in 1.0 M hydrochloric acid solution such that complex 2 was found to show higher inhibition efficiency than complex 1. is work has developed an efficient strategy for aluminium corrosion protection with good inhibition efficiency.

Data Availability
e data used to support the findings of this study are available from the corresponding author upon request. Disclosure e funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

Conflicts of Interest
e authors declare no conflicts of interest. Acknowledgments is work was supported by the Department of Science and Technology (DST) and National Research Foundation (NRF), South Africa, through the DST/NRF South African Research Chairs Initiative for Professor of Medicinal Chemistry and Nanotechnology (UID 62620) as well as the Rhodes University/DST Centre for Nanotechnology Innovation, South Africa.

Supplementary Materials
Electrodeposited benzothiazole phthalocyanines for corrosion inhibition of aluminium in acidic medium. Figure