A novel phthalocyanine based discotic liquid crystal for efficient corrosion inhibition of mild steel

ABSTRACT Discotic liquid crystals have been extensively investigated for their applications in electro-optical devices. Very recently their potential as corrosion inhibitor has been realised. Effective methods for the prevention of corrosion of mild steel (MS) are still challenging research for industrial chemists. In the present work, MS samples in 1.0 M HCl medium were used to test the corrosion inhibition capacity of a metal-free phthalocyanine (Pc)-based discotic liquid crystal (DLC) using potentiodynamic and electrochemical impedance parameters. The phthalocyanine discotic liquid crystal (PcDLC) exhibited an excellent corrosion inhibition with a maximum inhibition efficiency of 89.61%. This superior ability was attributed to the strong adsorption of PcDLC on to the surface of mild steel as confirmed from the potentiodynamic polarisation data that fit into the cathodic inhibitor category. The protective layer of inhibitors generated on the MS surface was examined using scanning electron microscopy (SEM) and FTIR. The density function theory (DFT) approach was used to investigate the relationship between molecular structures and inhibitory efficacy, which indicated a strong interaction between the inhibitor and the surface iron atoms. GRAPHICAL ABSTRACT


Introduction
Mild steel (MS), a carbon deficit alloy of iron, is used extensively for several industrial structural applications.All industries including petroleum, chemical, power plants, and power generation plants have iron as their basic material due to its structural strength, ease of fabrication and lower cost [1,2].Due to the various applications of this material getting exposed to the acidic environment commonly used in industries for processes like pickling, descaling, and other industrial acid cleaning activities, the material tends to undergo corrosion easily in acidic media at varying pH of 3-5 [3].These steel alloys react readily and get converted from metallic to ionic state, causes huge economic loss.Coating of corrosion inhibitors over MS is an efficient method of corrosion control.Thus, it is essential to develop some excellent corrosioncontrol methods that react with metals and form protective films on the anodes and cathodes.Typically, these inhibitors operate in neutral or alkaline media where the counter reaction is an oxygen reduction process with an oxide-film or hydroxide covering the corrosion-prone metal surface.Corrosion inhibitors can be categorised based on their chemical structure, method of action, reactivity etc. Corrosion inhibitors can be organic, inorganic, plant extracts, etc., which are soluble in water and act as most effective and flexible way of preventing corrosion.Organic compounds comprising elements of group V B, VI B or electron rich functionalities such as amines, carbonyls and alcohols are often more effective corrosion inhibitors.Generally, the inhibition performance of different inhibitors follows the reverse order of their electronegativity; the inhibition performance follows the order O<N<S.Protection in this instance is provided by the electron transfer to the electropositive species followed by surface deposition resulting in lowering of over voltage of primary cathodic depolarisation process.Also, keto sulphone has recently been tested as a green-inhibitor of corrosion of MS in 1 M HCl [4].A new strategy for improving the corrosion-inhibition efficiency against steel using protein was developed [5].The use of various triazole or imidazole-based compounds is synthesised by a one-step process, has shown good inhibition property [5].Hybrid nanocomposites of polyaniline (PANI) and [Cu 2 Cl 4 (maminophenylbenzimidazole) 3 ] prepared using benzoyl peroxide as an oxidant are also explored for their corrosion inhibition ability [6].Also, corrosion inhibition property of quinazoline derivatives in acidic condition was reported [7,8].
In our present study, we showed the excellent inhibition properties of a novel phthalocyanine discotic liquid crystal (PcDLC) against MS.Phthalocyanine (Pc), a cyclic tetramer of fused indole moiety, is widely used in materials and bio-sciences.Phthalocyanines and metallophthalocyanines have received great attention not only as dyes and pigments but also as building blocks for the construction of various advanced functional materials for electronic and optoelectronics applications [30].In the current findings, our research aims to synthesise PcDLC by following literature and study its corrosion inhibition activity in 1 M HCl solution, using PDP and EIS techniques.The quantum chemistry technique is used to match the theoretical results with the experimental findings.

Materials
All chemicals and reagents used in the present investigation are labelled to be AR grade and used without further refinement.MS samples of composition C (0.051%), Mn (0.179%), S (0.023%), P (0.0005%) and Fe (99.42%) were procured from the local market and were sliced into 1 cm 2 area, washed, ultrasonicated in absolute ethyl alcohol and polished by the sandpapers of 200-1500 grids.The samples were then desiccated and stored under vacuum till future use.

Synthesis of PcDLC
Phthalocyanine discotic liquid crystal was synthesised using the standard protocol [31] as shown in Scheme 1.Following this method, we have prepared phthalocyanine containing eight alkoxy chains of eight carbon atoms with two branching methyl groups.In brief, hydrogenated citronellol (1) was brominated in presence of N-bromosuccinimide and triphenylphosphine.The catechol was alkylated with brominated citronellol (1) followed by bromination gives compound 4. The Rosenmund-Von Braun reaction of compound 4 gives dicyanide that was finally cyclised under reflux condition to yield metal-free phthalocyanine discotic liquid crystal.The material was well characterised using standard spectroscopic techniques and elemental analysis.The analytical data of newly prepared material were in agreement with previously reported data [31].

Mesomorphic properties of PcDLC
The mesomorphic properties of PcDLC was observed under polarised optical microscopy (POM).The POM observation of metal-free phthalocyanine shows typical columnar texture upon cooling from isotropic liquid as shown in Figure 1.The Figure (a) shows early growth of columnar texture and fully grown texture is shown in Figure b.The thermal behaviour and mesophase structure of the newly prepared material was in accordance with the previously reported data [31].It exhibits a hexagonal columnar mesophase at room temperature and goes to the isotropic phase at about 300 ºC.The phase transition temperature and DSC data was mentioned in the ESI Table X and Figure S1 respectively.

Preparation of sample medium and mild steel (MS)
For the preparation of 1 M HCl, 37% HCl was used, followed by an addition of estimated amounts of PcDLC inhibitor to prepare concentrations 100, 200, 300, 400 ppm, which were coated by drop casting the PcDLC over the fine surface of MS.The specimens used for investigation are mild steel [6].Abrasion of the MS specimens was done with SiC emery sheets of 200-1600 grade, cleaning them in double distilled water, then cleaning them with tissue paper, then dipping them in ethyl alcohol for 5 seconds, drying them fully, and storing them in desiccators until needed.One cm 2 of exposed surface area was used for electrochemical investigation, and rest of the surface was masked using epoxy resin.

Electrochemical measurements (EIS and PDP)
CHI660E electrochemical workstation was used to perform EIS measurements [32].The experimental set-up constituted of three-electrode system containing platinum, Ag/AgCl and MS as auxiliary, reference and working electrodes respectively.Various concentrations of the PcDLC were electrochemically tested at  30°C after immersing it for 30 minutes in the test solution.In EIS investigations, the frequency range was from 10.0 KHz to 0.1 kHz, with an amplitude of 0.005 V.The inhibition efficacy η (%) was determined by utilising the Equation (1) given below.
Here, (R p ) a and (R p ) p being polarisation resistances in without and with PcDLC inhibitor correspondingly.
The potentiodynamic polarisation studies were done with a fixed scan rate of 0.4 mV/s across a potential range of −200 to +200 mV [33].Extrapolation the anodic/cathodic curves of Tafel plots to potential axis would yield corrosion current (i corr ) and potential (E corr ) respectively.Using the Equation ( 2), corrosion inhibition efficacy η (%) has been evaluated.
Here, i (corr)p and i (corr)a corrosion current (μAcm −2 ) values with and without PcDLC inhibitor respectively.

Contact angle measurements
A contact angle metre Goniometer Tech inc. was used to measure water contact angles (WCA) on the MS surface, using the sessile drop method and FAMAS (inter Face Measurement and Analysis System) software.On MS surfaces, 2 µL water droplets were placed.A contact angle measurement was performed before and after coating.The measurement was repeated ten times.

Scanning electron microscopy (SEM)
The SEM experiment aimed to observe morphological changes occurring over the surface of MS pre-and posttreatment with PcDLC corrosion inhibitor at ambient temperature.22-24 hours, MS samples were submerged in 1 M HCl solution containing 400 ppm of PcDLC inhibitor and after being air-dried, SEM micrographs were recorded.

Computational studies and modelling
Simulations were done with the ORCA 4.1.2software under the DFT framework [34,35].The selected methodology combined the PBE exchange-correlation functional with the def2-SVP [36] electron basis set [32], using DFT-D3 procedure and Becke-Johnson damping function for dispersion corrections.
The geometry optimisation had no structural constraints.The resulting optimised structure was utilised for single-point calculations to obtain the densities of the anionic and cationic forms of the molecule.These were used to calculate the Fukui functions; the Fukui dual descriptor was calculated with the following formula: where f D is the Fukui dual descriptor, f þ is the electrophilic Fukui function and f À is the nucleophilic Fukui functions.The Fukui functions were calculated by the numerical approximations: Here ρ Nþ1 stands for the electron density of the anion, ρ N is the electron density of the neutral molecular system and ρ NÀ 1 represents the electron density on the cation.The images were rendered on the Chem craft molecular visualiser software [37,38].

Electrochemical impedance spectral studies
Electrochemical impedance spectral analysis was used to evaluate the inhibitory efficacy of PcDLC in 1 M HCl medium over a frequency regime of 100 KHz to 0.1 Hz. Figure 2(a) depicts Nyquist plots of MS with and without PcDLC (100-400 ppm concentration) inhibitor using 1 M HCl medium.The observed Nyquist plots show semicircle nature of graph suggesting the charge transfer process.The plots being non-ideal and has depressed semicircles.Due to the inhomogeneity and impurities of the metal surface, the perfect semicircle deviates slightly from the perfect circle.We also see the diameters of the graph grow as the concentration of PcDLC inhibitor increases, indicating electron movement from MS to electrolyte (HCl) becoming futile.
Variation of impedance parameters against the concentration of PcDLC inhibitor has been presented in Table 1.A feasible increase in the charge transfer resistance from 1.923 (Blank) to 18.52 (400 ppm) indicates the adsorptive development of protective film of the PcDLC inhibitor over the surface of MS.An 89.61% efficiency was achieved with 400 ppm solution of PcDLC inhibitor.Development of double layer at the inter-junction is ascribed to the adsorption of PcDLC over MS surface, thus acting as a barrier between metal surface and corrosion media.It is also confirmed from the double layer capacitance.Equivalent circuit representing the electrochemical corrosion process at MS was computed with EIS data Z Simp Win software and is as shown in Figure 2(a) (inset), where Rs, Rp, Cdl represent solution resistance, polarisation resistance and double layer capacitance respectively.The Bode's plot and Bode phase plots are given in Figure 2(b).The non-homogeneity of metal surface yielded slopes in these plots ≠ −1.Also, it was observed from one-time constant behaviour that charge migration is the only relaxation mechanism.The improved corrosion inhibitory efficiency is mainly attributed to the adsorption of higher amount of PcDLC molecules at elevated concentrations on the surface of MS.

Potentiodynamic polarisation studies
The polarisation plots of MS in 1 M hydrochloric acid are demonstrated in Figure 3, for blank and different concentrations of PcDLC inhibitor.Through the usage of graph explorations of polarisation curves, the corrosion current density i corr (mA cm 2 ) can be calculated which is used to estimate the corrosion rate (C R ).The polarisation corrosion characteristics such as current (i corr ), potential (E corr ), cathodic(-βc) and anodic Tafel slope (βa), anodic Tafel slope and inhibition efficiency (η) are presented in Table 2. From the findings of potentiometric polarisation data, it is clear that the addition of PcDLC could affect both anodic and cathodic branches.At elevated concentration of PcDLC inhibitor, the rate of cathodic reduction and anodic metal dissolution is slowed.Thus, the anodic/cathodic curves move to lower current values and found that deflection values not exceeding ±85 mV with regard to blank.It is known that anodic corrosion inhibition occurs when E corr data was positive with respect to   blank, anodic and cathodic corrosion inhibitory efficiency is more when E corr values being negative to blank.In this case, the values are moving towards the positive indicating cathodic type of inhibition activity.

Contact angle (CA) measurements
The liquid sessile drop method with the Goniometer device from Tech Inc. was used to examine the hydrophobic and hydrophilic properties of the MS surface in the presence and absence of an inhibitor.A drop of water placed on the MS surface shows intermolecular interactions between the MS surface and water.As shown in Figure 4(a), the contact angle of MS in the absence of the inhibitor was 56.4°, confirming its partially hydrophilic nature.Figure 4(b), shows more hydrophobic nature with 81.3° in the presence of inhibitor.Thus, the MS surface has developed a protective layer, which increases corrosion resistance.

Surface examination (SEM)
A SEM image revealed the extent of corrosion on MS surface in an acidic media.The micrographs of the MS samples showing their surface morphology pre-and post-treatment of PcDLC inhibitor.Figure 5(a) shows the SEM image of a fresh MS sample exposed to 1 M HCl at room temperature.It was found that the exposed surface was uneven, non-uniform with pits, cracks and cavities.It is evident from Figure 5(b) that PcDLC modified MS surface is free from pits and cracks, meaning that PcDLC molecules are evenly coated on the MS surface, inhibiting corrosion in an acidic environment.
In Figure 5(c,d), the surface composition of the metal is shown with and without the PcDLC.Also, a composition table inserted in 5(c) and 5(d) respectively confirms the formation of PcDLC on the surface.The mixtures of Fe, Mn, and S oxides of iron shown in Figure 5(c) and the mixtures of oxides of nitrogen and oxides-hydroxides of iron shown in Figure 5(d) demonstrate the deposition of PcDLC in acidic solution.

Calculations involving quantum chemistry
The structure of the PcDLC molecule was optimised and its regioselectivity analysed through the Fukui dual descriptor shows the resulting structure of the optimisation calculation for the PcDLC molecule (Figures 6 and  7).While the central moiety shows a symmetric and planar set of rings, the system's symmetry is broken by its aliphatic chains.
The positive values indicate a propensity for electrophilic attacks and are plotted in red while the negative values are plotted in blue, indicating sites that are viable for nucleophilic attacks.On general inspection, it seems that the positive portion of f D is more condensed around the planar central part of the system while the negative portion is more evenly spread across  the molecule.However, when looking for the nuclei that present the largest values of f D -both positive and negative -the central ring of the system comes to prominence.Here, N10, N12, N30 and N35 are identified as the nuclei that are more likely to receive electrons from an electrophile, while C11 and C31 are the more liable atoms through which the system could donate electrons to a nucleophile.From our dual  descriptor analysis, we can conclude that C11 and C31 atoms of LC are effectively participate in the corrosion inhibition process by donating their electrons (Figures 6 and 7).

Conclusions
In the present investigation, the corrosion inhibitory efficiency of PcDLC inhibitor on MS in acidic condition (1 M HCl) was evaluated by means of electrochemical and computational studies.Experimental investigation clearly tells that PcDLC is an effective corrosion inhibiting molecule for MS in 1 M HCl medium.The inhibition efficiency (ƞ) increases with the increase in concentration of PcDLC, giving maximum inhibition efficiency in 400 ppm of PcDLC.
Polarization data shows that PcDLC is a cathodic inhibitor.EIS analysis proves that there is single charge-transfer mechanism during corrosion inhibition.SEM-EDX micrographs confirms the development of protective layer over the surface of MS.
Inhibition efficiency exhibited by PcDLC on MS surface is substantiated by DFT calculations.The presence of an electron withdrawing N-group in PcDLC attracts the π-electrons of aromatic moiety, resulting in decrease of electron density and thus showed lower inhibition capacity.

Figure 1 .
Figure 1.(Colour online) The POM images of metal-free phthalocyanine.(a) early growth of columnar texture and (b) fully grown texture.

Figure 3 .
Figure 3. (Colour online) polarization curves for MS in 1M HCl solution pre-and post-PcDLC inhibitor treatment.

Figure 4 .
Figure 4. (Colour online) Contact angle (a) absence of the inhibitor (b) presence of inhibitor.

Figure 5 .
Figure 5. (Colour online) SEM images of MS in 1.0 M HCl (a) Pre-(b) Post-PcDLC treatment of MS.EDX spectrum of (c) Pre-(d) Post-PcDLC treatment of MS.

Table 1 .
Electrochemical impedance characteristics and inhibitory efficacy for MS in 1 M HCl solution pre-and post-PcDLC treatment with varied concentrations.

Table 2 .
Potentiodynamic polarisation characteristics and inhibition efficiencies for MS in 1 M HCl solution pre-and post-PcDLC inhibitor treatment.