Use of a Metallic Complex Derived from Curcuma Longa as Green Corrosion Inhibitor for Carbon Steel in Sulfuric Acid

Universidad Autonoma del Estado de Morelos, CIICAp, Av. Universidad 1001-62209, Cuernavaca, Morelos, Mexico Universidad Autonoma del Estado de Morelos, CIQ, Av. Universidad 1001-62209, Cuernavaca, Morelos, Mexico Universidad Autonoma del Estado de Morelos, FCQeI, Av. Universidad 1001-62209, Cuernavaca, Morelos, Mexico Universidad Autónoma de Chihuahua, Facultad de Ciencias Químicas, Circuito No. 1, Campus Universitario 2, Chihuahua, Chihuahua C.P. 31125, Mexico Universidad Autónoma de Chihuahua, Facultad de Ingeniería, Circuito No. 1, Campus Universitario 2, Chihuahua, Chihuahua C.P. 31125, Mexico Universidad Nacional Autónoma de Mexico, Av. Universidad S/N 62209, Cuernavaca, Morelos, Mexico


Introduction
The use of organic inhibitors, those which contain oxygen, nitrogen, sulfur, or carbon, is one of the most commonly used methods to combat the corrosion of metals, which causes huge economic loses and severe accidents in the industry [1][2][3][4][5][6]. This is because compounds with π-bonds generally exhibit good inhibitive properties due to their interaction with the metal surface [7][8][9]. The way they decrease the corrosion rate is by the formation of a compact barrier onto the metal surface by adsorbing on the metal surface and blocking the active sites by displacing water molecules. However, organic inhibitors are toxic, highly expensive, and environmentally unfriendly. Thus, many efforts have been made in the last few years towards the use of compounds found in plants including seeds, roots, flowers, etc. since they contain phytochemical compounds which incorporate effective corrosion inhibitors [10][11][12][13][14][15][16][17].
Curcumin is one of the main phenolic compounds present in the Curcuma longa rhizome which has been used in Asia since ancient times. The powder obtained from the rhizome of Curcuma longa has many uses including as a spice, food colouring, and preservative, among others. In addition to India, it is also cultivated in China, Japan, and Burma. The obtained powder from Curcuma longa has a peppery bitter flavour and is the main ingredient to make the spice known worldwide as curry [18][19][20][21]. Due to the potent antioxidants contained in Curcuma longa, it is used in folk Indian medicine to help combat illness such as digestive problems and fever and reduce cholesterol, and recently, some anti-Alzheimer, antimutagenic, anticarcinogenic, antimicrobial, and antiviral qualities have been found.
Curcuma longa extract has been used as a corrosion inhibitor for steel in different environments due to the presence of antioxidants [22][23][24][25]. Alternatively, since curcumin is the most abundant compound found in Curcuma longa, some of its derivatives have also been evaluated as green and ecofriendly corrosion inhibitors for metals because of the existence of inhibition aromatic rings which block the active surface sites to reduce the corrosion of steel in corrosive solutions [26][27][28][29]. For instance, Fouda and Elattar [26] evaluated three curcumin derivatives, namely, 1,7-bis-(4hydroxy-3-methoxy-phenyl)-hepta-1,6-diene-4-arylazo-3,5dione (I-V), as green corrosion inhibitors for brass in 2.0 M nitric acid by using weight loss and electrochemical measurements. They found that these curcumin derivatives had a corrosion efficiency which increased with their concentration but decreased with an increase in the testing temperature. In another research work [27], Rajendran et al. evaluated the corrosion inhibition efficiency for pure aluminium in artificial sea water solution by using weight loss tests and potentiodynamic polarization curves. Their results showed that the curcumin corrosion efficiency was as high as 98% and the corrosion current density value, I corr , was decreased by one order of magnitude. Finally, Kandias et al. evaluated different curcuminoids extracted from Curcuma longa as green corrosion inhibitors for carbon steel in 1.0 M sodium chloride solution by using gravimetric and electrochemical tests [28]. They found that the curcuminoid corrosion efficiency increased with their concentration, bringing a reduction in the I corr value from 180 to 34 μA/cm 2 . Thus, the goal of this work is to evaluate the use of a new curcumin derivative as a green corrosion inhibitor for 1018 carbon steel in 0.5 M sulfuric acid solution. The reason for this is the high corrosion rates found in the metallic components, made mainly from carbon steel, in the pickling and decaling industry, where acids such as nitric, hydrochloric, and sulfuric are widely used and the use of corrosion inhibitors is compulsory.

Materials and Methods
2.1. Inhibitor Synthesis. The inhibitor used was bis[1,7-bis(4hydroxy-3-methoxyphenyl)-1,6-heptadiene-3,5-dionato-κO,κO ′ ]bis(butyl), a Sn-containing metallic complex derived from curcumin. The synthesis procedure started with the curcumin obtained from Curcuma longa bulbs described elsewhere [29]. Briefly, Curcuma longa, which was purchased from a local market, was dissolved in ethanol (1 L). The solvent was removed by using an evaporator, then the solid was recrystallized in ethanol (80 mL); in the end, it was filtered to obtain an orange-red solid which was dried completely. Once the curcumin was obtained (Figure 1), 0.6 g (1.62 mmol) of curcumin was dissolved in 25 mL of methanol and stirred into a ball flask, and after this, 0.2 g (0.80 mmol) of dibutyltin oxide was added. Following this, and after the complete material dissolution (approximately 15 minutes), the reaction was carried out under reflux during 7 hours. After this, the mixture was allowed to cool down, and a red solid, metallic complex was obtained by complete evaporation of the solvent. The metallic complex characterization was made through three spectroscopic techniques including Infrared, 1 H Nuclear Magnetic Resonance, and Mass Spectrometry as detailed in [29]. According to Priyadarsini, this metallic complex is soluble in organic compounds, but insoluble in water or in sulfuric acid [30].

Testing Solution.
As testing solution, 0.5 M sulfuric acid (H 2 SO 4 ) was prepared by using analytical grade reagents. For the weight loss tests, specimens were exposed to this electrolyte containing different concentrations of inhibitor during an exposure time of 72 hours by triplicate at room temperature, i.e., 25°C. After this, specimens were cleaned to remove the corrosion products and to obtain the weight loss per unit area, ΔW. For this, the following expression was used: were m 1 is the weight of the specimen corroded in the absence of the inhibitor, m 2 is the weight of the specimen corroded in the presence of the inhibitor, and A the exposed area of the specimen. Inhibitor efficiency, I.E., was obtained as follows: where ΔW 1 is the weight loss of the specimen without inhibitor and ΔW 2 is the weight loss with inhibitor. Selected corroded specimens were observed in a low vacuum scanning electronic microscope (SEM).

Electrochemical
Techniques. Specimens measuring 6.00 mm in length were cut and encapsulated in commercial polymeric resin, abraded with 600-grade emery paper, washed, and degreased with acetone. Employed electrochemical techniques included potentiodynamic polarization curves and electrochemical impedance spectroscopy (EIS). For this, a three-electrode electrochemical glass cell was used with a saturated calomel electrode (SCE) and a graphite rod as the reference and auxiliary electrodes, respectively. Before starting the experiments, specimens were immersed into the solution during an exposure time of 20 minutes to allow the free corrosion potential value, E corr , to reach a steady state. For polarization curves, scanning started in a potential value of 800 which is more cathodic than the E corr value, and it was scanned in the anodic direction at a scan rate of 1 mV/s up to a potential value of 400 mV which is more anodic than E corr . An ACM potentiostat was used for this purpose. Corrosion current density values, I corr , were calculated by using the 2 International Journal of Corrosion Tafel extrapolation method. EIS measurements were carried out at the E corr value by applying a perturbation signal with an amplitude of 15 mV peak to peak in a frequency interval of 0.01-20,000 Hz. In this case, a PC4 300 Gamry potentiostat was used.
Parameters of interest included global hardness (η) [41], electronegativity (χ) [42], electrophilicity (ω) [43], dipolar moment (μ) [44], and the Highest Occupied Molecular Orbital (HOMO) and Lower Unoccupied Molecular Orbital (LUMO) [34][35][36][37][38][39][40][41][42][43][44]. In addition to these parameters, Fukui functions were calculated [45] by using the following equations [45]: for an electrophilic attack, ð3Þ 3. Results and Discussion 3.1. Inhibitor Characterization. Figure 2 shows the infrared spectra for the metal complex and the materials used for its synthesis (dibutyltin oxide and curcumin). The different absorption peaks for the carbonyl groups are observed. Hence, the absorption peaks for the carbonyl groups pres-ent in the curcumin are shown at 1601 and 1625 cm -1 , while these groups were shifted towards lower wave numbers (1588 and 1617 cm -1 ) for the metallic complex. Thus, it was noticed that the bonding of the tin metal centre to the curcumin carbonyl groups made the carbon-oxygen double bond weaker due to the formation of the metallic complex. The 1 H NMR spectrum for the obtained metallic complex is shown in Figure 3. The signal for H1 was shifted to δ = 5:7 ppm as a double as a consequence of the tin coordination; this signal was observed as a singlet for free curcumin. The H3 signal is observed at δ = 6:32 ppm as a double because of the coupling with H4. The aromatic part shows signals at the lower field; for instance, the signal for H6 is shifted to δ = 6:90 ppm as a singlet, H10 is shifted to δ = 6:96 ppm as a doublet, and H9 is shifted to δ = 6:78 ppm. These values are similar to those observed for curcumin because the interaction of the tin atom is carried out in the carbonyl groups. The remaining signals of the hydrogens correspond to the observed hydrogens to the butyl groups. A summary of the shift and constant coupling of the hydrogen atoms for the metallic complex is shown in Table 1. Finally, the mass spectrum of the metallic complex is shown in Figure 4, where it can be seen that the molecular weight corresponding to this complex m/z = 969 g/ mol was not observed. However, some fragments were detected due to the presence of tin. For instance, a peak in m/z = 763 was assigned to the breaking of a curcumin molecule, and the most abundant peak was observed at 307 which is the curcumin molecule with the loss of the methoxy groups.    Table 2. In all cases, it is evident that the inhibitor efficiency increases with its concentration, which is due to the adsorption of the metallic complex on the steel surface. The fact that the metal surface area covered by the inhibitor θ increases with its concentration supports the idea that the decrease in the corrosion rate is due to the inhibitor adsorption on the steel surface to form a protective corrosion product film due to the existence of heterocyclic compounds in the inhibitor such as shown in Figure 1 [18][19][20][21]. A visual examination of the specimen corroded without an inhibitor showed a severely corroded steel, whereas the corrosion in the presence of an inhibitor corrosion rate of 100 ppm was visibly reduced because of the formation of protective corrosion products on top of the steel surface.
The Sn-metallic complex reduces the steel corrosion rate due to its adsorption onto the steel surface. The interaction between the inhibitor molecules and the steel surface is provided by the adsorption isotherms. As indicated above, it is assumed that θ is related to the inhibitor efficiency, which was obtained from the weight loss experiments. Different adsorption models exist, including the Langmuir, Temkin, Frumkin and Flory-Huggins. As can be seen from Figure 5, the adsorption Frumkin isotherm had the best data fit, with an R 2 = 0:89, which is given by where C inh is the inhibitor adsorption, K ads is the adsorption isotherm, and f is the interaction coefficient, which is positive for attraction and negative for repulsion. The adsorption constant, K ads , and the standard free energy of adsorption (ΔG ads ) are related according to the following equation: where R is the universal gas constant and T is the absolute temperature. From Figure 5,     positive value for f indicates that there is an attraction between the steel surface and the inhibitor, whereas the relatively low value for K ads indicates a weak interaction between the inhibitor molecules and the steel surface; on the other hand, the negative value for ΔG ads indicates a spontaneous process. The ΔG ads value of -25.85 indicates that the type of adsorption between inhibitor molecules and the steel surface is weak and physical in nature, which involves electrostatic interaction between the inhibitor and the charged metal surface [46][47][48]. Figure 6 displays the polarization curves for 1018 carbon steel immersed in 0.5 M H 2 SO 4 solution containing different concentrations of the Sn-containing metallic complex as a corrosion inhibitor. Various parameters such as E corr , I corr , anodic, and cathodic Tafel slopes (β a and β c ) are reported in Table 3. It is clear that in all cases, regardless the inhibitor concentration, the addition of the Sn-metallic complex results in a marked decrease in both cathodic and anodic current density values, but in a more marked way for the cathodic one. Alternatively, polarization curves displayed an active behaviour, without evidence of the formation of a passive layer. In the cathodic region, there is no clear differentiation between the two cathodic oxygen reduction and hydrogen evolution reactions. However, both cathodic reactions can be clearly differentiated with the addition of the inhibitor. The E corr value was only marginally affected, since it fluctuated between -460 and -480 mV as shown in Table 3. Both anodic and cathodic Tafel slopes were affected by the addition of the inhibitor, thus confirming that the Sn-containing metallic complex is acting as a mixed type of corrosion inhibitor, affecting, therefore, both oxygen reduction and hydrogen evolution reactions. Thus, we can say that the cathodic reactions are retarded by blocking the active sites of the steel where oxygen and hydrogen can be adsorbed, whereas the anodic reaction is retarded by the formation of iron complexes due to the presence of a long pair of electrons in the organic compound structure [49][50][51].

Polarization Curves.
On the other side, the I corr value was dramatically decreased with the addition of 1000 ppm of inhibitor for almost two orders of magnitude. Inhibitor efficiency was calculated by using: where I corr and I corr/inh are the corrosion current density values obtained in the absence and presence of the inhibitor, respectively. From the data given in Table 3, we can see that inhibitor efficiency increases with an increase in the inhibitor concentration; its highest value was reached with the addition of 1000 ppm of inhibitor, which indicates that more inhibitor molecules are adsorbed onto the steel surface with an increase in the metal surface area covered by the inhibitor.

EIS Results.
The corrosion behaviour of carbon steel in sulfuric acid with and without the presence of the Sncontaining metallic complex was studied with the aid of EIS studies, and the obtained results are shown in Figure 7. It can be seen that Nyquist diagrams display a single, depressed, and capacitive semicircle at high and intermediate frequency values, followed by an inductive loop at lower frequency values for inhibitor concentrations lower than 800 ppm. The presence of an inductive loop indicates that the corrosion process is controlled by the adsorption/desorption of some intermediate species on the steel surface. On the other hand, for an inhibitor concentration of 1000 ppm, Nyquist data display a capacitive loop at high and intermediate frequency values, followed by an increase in the real impedance value while the imaginary values remain constant at lower frequency values, which is due to the accumulation of all kinds of species at the metal/solution interface and increases the total impedance value [52,53]. The presence of the capacitive loop at high frequency values indicates that the corrosion of carbon steel in sulfuric acid is mainly controlled by the charge transfer and the formation of a protective film on the steel surface. The imperfect semicircle is generally attributed to the frequency dispersion as a result of roughness of the metal surface, grain boundaries, impurities, and distribution of surface active sites. The shape of the loops was not altered by the addition of the metallic complex, which indicates that this inhibitor reduces the steel corrosion rate without affecting the corrosion mechanism. On the other hand, Bode diagrams, Figure 7(b), showed a single peak and, thus, one time constant only, for the uninhibited   International Journal of Corrosion solution and inhibitor concentrations lower than 800 ppm, whereas at an inhibitor concentration, this peak is broadened to a wider frequency interval, indicating the presence of two time constants, due to the presence of a protective corrosion product film. All EIS spectra were analysed by fitting the experimental data to an appropriate equivalent electric circuit to find the parameters, which are described as being consistent with the experimental data. Figure 8 depicts the proposed electric circuits used to simulate the EIS data. In this figure, R s is the solution resistance, R ct is the charge transfer resistance, CPE dl is a constant phase element related to the double electrochemical impedance, R f is the resistance of the film formed by the corrosion products, CPE f is the constant phase element related to its capacitance, L is the inductive element, and R L is its resistance. CPE dl and CPE f are placed instead of an ideal double layer capacitor, C dl , and film capacitance, C f , to take into account the dispersion effects due to surface roughness and other surface heterogeneities as mentioned above. The impedance of the CPE, Z CPE , is given by [54] where Y 0 is the admittance, i = −1 1/2 , ω the angular frequency, and n a physical parameter which gives interphase properties of the working electrode such as roughness and inhibitor adsorption. Resulting parameters from the use of circuits given in Figure 8 are shown in Table 4. Data given in this table show that the charge resistance value, R ct , increases whereas the CPE dl value decreases with an increase in the inhibitor concentration. Results given in Table 4 are in  complete agreement with the weight loss and polarization  data, Tables 2 and 3, respectively. An increase in R ct is normally associated with a decrease of the dissolution of the steel because the Sn-metallic complex is adsorbed on its surface by a gradual displacing of the water molecules and the formation of a protective film. This film will isolate the steel surface from the electrolyte and decrease its dissolution rate. Alternatively, the decrease in the CPE dl value is attributed to a lowering in the dielectric constant value, ε, which is caused by the adsorption of the inhibitor molecules which have a lower local dielectric constant value or to the increase in the double electric layer thickness [55] according to the expression of the Helmholtz model for the calculation of the double electric layer capacitance, C dl : where ε 0 is the vacuum electrical permittivity, δ the electric layer thickness, and A the surface area.  7 International Journal of Corrosion 3.5. Surface Characterization. After corrosion tests, surfaces were analysed with a scanning electronic microscope, SEM, and micrographs of corroded specimens in the absence and presence of inhibitor are shown in Figure 9. It can be seen that the corrosion product film formed in the absence of an inhibitor, Figure 9(a), shows porous microcracks and some other defects which do not prevent the contact between the electrolyte and the steel. Unlike this, the formed film in the presence of the inhibitor, Figure 9(b), is much more compact, with a much lower amount of porous and microcracks, avoiding the contact between the corrosive solution and the steel surface.  International Journal of Corrosion assumed that the inhibitor is most likely to be an electron donor with respect to the metallic surface, the electron density distribution of the HOMO is of particular significance [56]. The HOMO and LUMO orbital distributions of the Sn-metallic complex, which we will call the SnX molecule from now on, are shown in Figure 10. Table 5 indicates that the negative value of HOMO for the Sn-containing metallic complex indicates a greater efficiency of inhibition of the compound. The hardness obtained value for the SnX is an important property to measure molecular stability, and inhibition of a molecule was 1.33 eV. Furthermore, the value for the dipolar moment is an important indicator which had a value of 13.02 for SnX also. Electrophilicity, ω, on the other hand, denotes the electron-accepting capability of the molecule where a value of 6.97 eV was obtained. Table 5 gives data for parameters such as IP, EA, χ, and η [57,58]. Further analysis of Fukui functions of the SnX molecule are shows in Figure 11 indicating that there is a nucleophilic attack in the C41 atom whereas the electrophilic attack occurs in the O10 atom. It has been found that the sites with electrophilic attack are places with enhanced HOMO energies; conversely, LUMO sites are most susceptible to suffer nucleophilic attack [57]. Figure 8: Electric circuits to simulate EIS data for 1018 carbon steel immersed in 0.5 M H 2 SO 4 in the presence of the Sn-metallic complex in concentrations of (a) lower than 800 ppm and (b) 1000 ppm.    International Journal of Corrosion 3.6.2. Protonated Molecule. The optimized structure of the SnX molecule in its neutral state contains more than one active centre for protonation according to the electronic density observed in HOMO that corresponds of the electrophilic attack ( Figure 11). Thus, the protonation site of the molecule was determined through the Fukui functions. According to the Fukui function, the most susceptible site for an electrophilic attack in the SnX molecule is located on the O10 atom. Once the molecules are protonated, the electronic parameters of the protonated inhibitor molecule such as HOMO, LUMO, ΔE, dipole moment, and reactivity parameters are shown in Table 6. Additionally, the optimized structure, HOMO, and LUMO are shown in Figure 12. The HOMO value of the protonated SnX molecule is -5.80 eV. The dipolar moment value, μ, in the protonated inhibitor had a value of 38.03 Debye. The electronegativity value, χ, measures the     Figure 13: The calculated molecular electrostatic potential (MEP) map: the colour range (blue more positive, red more negative) oscillates for (a) neutral SnX, -1.041e-1 from 1.041e-1, and (b) protonated SnX, -9.952 from 9.952. Atomic colours represent gray (C), white (H), red (O), blue (N), and dark-blue-square (Sn). International Journal of Corrosion capacity of an atom to attract electrons [58] which is commonly considered in corrosion studies. Therefore, high values of χ indicate better attraction properties, and that value for the protonated SnX molecule was 4.40 eV; according to Kaya et al. [59], electronegativity is one of the parameters that provide important clues about corrosion inhibition efficiency of any chemical species. On the other hand, Figure 13 shows the molecular electrostatic potential (MEP) of the neutral and protonated molecules, where it can be seen that all molecules have a green region, namely, both molecules are in the intermediate zone of the electrostatic potential.

Conclusions
A Sn-containing metallic complex derived from Curcuma longa, bis[1,7-bis(4-hydroxy-3-methoxyphenyl)-1,6-heptadiene-3,5-dionato-κO,κO′]bis(butyl), was evaluated as a green corrosion inhibitor for 1018 carbon steel in sulfuric acid. Adsorption of the metallic complex corrosion inhibitor produced a decrease in the corrosion rate of steel, and it was found to follow the Frumkin adsorption isotherm. The metallic complex affects the kinetics of the corrosion processes, and its inhibition efficiencies increased with an increase in the inhibitor concentration. Thermodynamic parameters revealed that the inhibitor is spontaneously adsorbed. Negative values for ΔG ads indicate a spontaneous adsorption process of this metallic complex on to the steel surface. SEM analysis showed that the inhibition of corrosion by the Sn-metallic complex is due to the formation of a physically adsorbed film constituted by inhibitor molecules and corrosion products on the metal surface. Theoretical calculations demonstrated the relationship between inhibitor efficiency and its electronic properties.

Data Availability
There is no data availability.

Conflicts of Interest
The authors declare that they have no conflicts of interest.