Electrochemical evaluation of mucilage and cochineal pigments as a hybrid film coating on aluminum surfaces

This work deals with the electrochemical evaluation of a hybrid coating based on mucilage and prickly pear cochineal (Dactylopius coccus) for corrosion protective applications. The Opuntia streptacantha mucilage was extracted by grinding prickly pear cochineal, and three coatings containing mucilague (CM), cochineal-mucilage (CMC), and cochineal-mucilage without Tween 80 (CMC-T) were formulated. The aluminum working electrodes (WE) were coated by immersion and then left to dry for 72 h at room temperature. The formed coats were assessed by electrochemical impedance spectroscopy, electrochemical noise, and potentiodynamic polarization curves after 24, 72, and 168 h of immersion in a 3.5 wt% NaCl solution. These electrochemical measurements were performed in triplicate to check reproducibility. The Tween 80 plasticizer-free cochineal-mucilage-based hybrid coating reveals more excellent protection against corrosion than uncoated aluminum. Optical micrographs were used to set apart the conditions of the hybrid coating after its valuation, which show the protection of the metallic surface on which it was not coated. Results showed that the hybrid coating has suitable properties as a barrier against corrosion due to its ability to block the aggressive species diffusion by trapping them in the coating structure, which prevents their contact with the metal surface. This property is attributable to better mucilage and cochineal film homogeneity. Also, it acts as a corrosion inhibitor due to its semi-permeable behavior where only water molecules flow through its pores. This hybrid coating showed an excellent corrosion-resistant behavior to be used to protect aluminum.


Introduction
The phenomenon of corrosion has significant consequences that can be broken down into two main areas: a) Economic costs: direct costs are estimated at 3.5% of Per capita gross domestic product (GDP), in addition to production stoppages, leaks, product contamination and the application of safety coefficients as indirect costs. It is considered that savings of between 20% and 30% can be obtained if the existing technology for corrosion control and protection methods are adequately applied [1][2][3]. Aluminum is one of the most widely used materials in light industrial constructions [4]. Chloride-induced corrosion is one of the common causes of metallic structure degradation. Any new mitigation alternative should be effective in several of these process stages. Similarly, methods for slowing corrosion initiation reduces corrosion rates, such as designed electrochemical corrosion inhibitors, and could be implemented to allow functional changes throughout the aluminum interface [5,6]. The application of coatings is one of the most common methods on aluminum [7]. The most representative example of

Experimental design
This work is based on a study on the behavior of organic-inorganic hybrid coatings, using mucilage and prickly pear cochineal on aluminum as a protective barrier against corrosion by chlorides. The experimental process consisted of preparing a metal surface to be coated and a blank; obtaining and characterization mucilage and grana cochineal from the nopal; formulation of the coatings, application, evaluation, and analysis of the coating on the aluminum surface in a solution rich in chloride ions. An aluminum electrode with a purity percentage of 99% was used, with an exposed surface of 1 cm 2 . It was polished with CSi abrasive paper of increasing grain size up to #320, rinsed first with distilled water and then with methyl alcohol, and dried by airflow. This procedure was carried out before the immersion coating of the metal surface, subsequent drying of the coating in the environment for 72 h, and subsequent electrochemical testing.

Obtaining mucilage, treatment of cochineal and formulation of coatings
The Opuntia streptacantha cactus, native to central Mexico, was used in this study. Cladodes aged between 2 and 3 years were considered since they contain the highest concentrations of mucilage, according to previous studies [29]. Mucilage extraction was performed according to Sepúlveda et al [30]. After removing them, both the mucilage and Dactylopius coccus (cochineal) were ground separately in unglazed porcelain mortars to a fine powder. Subsequently, the formulation of the coating was carried out with the following process: in an Erlenmeyer flask, 0.5 g of mucilage, 0.5 g of cochineal, 20 g of deionized water (18 MΩ cm-1), and 0.4 g of glycerol were added; the mixture was heated at 90°C for 5 min with constant stirring at 1,200 rpm; then it was cooled to 25°C, followed by the addition of Tween 80 (C 64 H 124 O 26 ); finally, the mixture was stirred for 5 min at 70°C and 2000 rpm. Three coatings were obtained with variations in the active element of interest (table 1): CM (coating with mucilage), CMC (coating with mucilage and cochineal), and CMC-T (coating with mucilage, cochineal, and without Tween 80).

Electrochemical techniques and test solution
A three-electrode Pyrex ® glass electrochemical cell, ASTM G-5 [31], was used for the electrochemical tests, employing a saturated calomel reference electrode (SCE) and a graphite counter electrode. All the potentials in work are referred to as the SCE, which was measured until it reached a stable value, typically 30 min. A 3.5% wt. NaCl solution was employed as an electrolytic solution for the tests, prepared with analytical grade reagents and distilled water, kept at room temperature, naturally aerated, and with a pH equal to 6. Electrochemical measurements were performed in a Potentiostat-Galvanostat model BioLogic 150.

Potentiodynamic polarization curves
For the potentiodynamic bias curves, aluminum was biased from −2.5 V to +2.5 for the value of the corrosion potential E corr at a scan rate of 1 mV s −1 . The corrosion rates were determined by relating the corrosion current density, i corr , with the polarization resistance, R p , from the Stern-Geary equation [32], for reactions controlled by charge transfer (equation (1)).
Where b a and b c are the anodic and cathodic Tafel slopes, respectively, and The Tafel slopes' determination and the potentiodynamic polarization curves were obtained in a reduced category of potentials, that is +/−60 mV, concerning the E corr , both in cathodic and anodic zones.

Electrochemical impedance spectroscopy (EIS)
A signal of±10 mV was applied to E corr , in the frequency range between 200 kHz and 0.01 Hz, obtaining 6 points per decade. Inhibitor efficiency values were calculated using equation (2): where R ct2 and R ct1 are the load transfer resistance values with and without the addition of coating, respectively.

Electrochemical noise
For electrochemical noise measurements according to the ASTM-G199 standard [33], the arrangement of two nominally identical working electrodes and one SCE reference electrode was used in a potential range of +/−2.5 for E corr . The potential and current values were recorded at a speed of 1 sample/second; potential and current series were obtained according to time, and the average standard deviations of each series were calculated.
To determine the resistance in noise, Rr, the standard deviation of the noise in current, σI and voltage, σV of each run was calculated, and equation (3) was applied (Cottis et al 1996, [34]):

Microscopic analysis
The coated and uncoated aluminum was analyzed under a light microscope at 40X magnification before and after electrochemical tests. The surfaces were photographed with a camera adapted to the microscope. Micrographs of the segments were studied, and comparisons were made.

Coating formulations
Three different coatings were made (see table 1), varying the presence of cochineal and Tween 80. The hydrocolloid obtained was placed in Eppendorf tubes to be later placed on the aluminum surface. Aluminum sections were dip-coated and then dried at room temperature to monitor coating thickness for three days. The proposed alternative was formulated based on renewable sources such as nopal mucilage and cochineal. The formulation considered adequate is the one shown in table 1. It was not possible to incorporate different proportions of the order of those added of mucilage, cochineal, glycerol, and Tween 80, because the coatings obtained were too fragile and could not be manipulated, possibly due to the interactions of the cellulosic polysaccharides with the molecules of the matrix that modify the degree of aggregation of the chains and the resistance of the polymeric network.

Fourier transform infrared spectroscopy
Fourier transform infrared spectroscopy (FT-IR) was carried out using a PerkinElmer Frontier spectrometer with the fully attenuated reflectance technique ATR using a spectral resolution of 4 cm -1 (PerkinElmer, Waltham, MA, USA).

Polarization curves
The polarization curve was measured to examine the corrosion kinetics of aluminum samples in a 3.5% wt. NaCl solution, in the absence and presence of coatings. Figure 1 presents the polarization curves of Al (blank, uncoated sample), CM, CMC, and CMC-T samples at 298 K. The electrochemical parameters obtained by Tafel analysis are listed in table 2 as the backup information. It can be seen that the addition of CM, CMC, and CMC-T markedly changed the slope of the polarization curves in figure 1. Namely, the oxidation-reduction reactions are restricted by incorporating the CM, CMC, and CMC-T coatings. T in the NaCl solution. This effect implies that the coatings hinder the hydrogen evolution reactions. However, CM, CMC, and CMC-T suppression phenomena are also transferred to the cathodic zone, indicating that oxidation-reduction reactions are blocked in coatings. CMC and CMC-T coatings show a more significant reduction in corrosion potentials, E corr , especially for CMC.
For the Al, CM, and CMC-T cases (Kelly et al, 2003), when applying the reverse sweep, the curve that returns along the same path or for a lower current value establishes that the material does not present a tendency to localized corrosion [35], that is, that the increase in the current is not due to localized corrosion. However, some other anodic reaction since the area remains constant. However, in the case of CMC coating, the return of the curve is from the front, which indicates that the layer does not break but reaches the potential oxidation reaction of water to occur. This reaction is supported by the extent of the highest corrosion potential observed for the back scan due to the slowing of anodic dissolution kinetics.
In table 2, corrosion potential (E corr ), corrosion current density (I corr ), polarization resistance (R p ), and corresponding inhibition efficiency (IE) values for different inhibitor concentrations are given as electrochemical parameters. Different inhibitor concentrations calculated inhibition efficiencies from equation (1). Although the anodic and cathodic curves are not symmetrical, the cathodic curves also have regions with a linear ratio between the potential and the logarithm of the current density and zones called Tafel regions. By extrapolating such linear segments from the anodic and/or cathodic curves, and intersecting them to E corr , the value of i corr for the metal in an aggressive medium can be determined.
The results for each sample were calculated as the mean±standard deviation of the triplicate determinations. The percentage error of the current density was found using equation (1), and the corrosion current density (i corr ) was calculated to validate the data obtained from the curves of the linear polarization resistance and the Tafel slopes of the cathodic and anodic zones. The Stern-Geary coefficient (B) is related to the anodic (b a ) and cathodic (b c ) Tafel slopes.
After performing the corresponding calculations to find the theoretical values of the parameters by applying equation (1), the error percentage is calculated by taking into account the equation (6).
Theoretical value Experimental value Theoretical value 100 % % 6 The highest efficiency was the CMC-T coating according to the determined parameters, followed by the CMC and CM. The lowest current density was for the CMC-T. The best behavior against corrosion in environments with chlorides in the study case was for the coating that includes cochineal and mucilage in its formulation without the presence of Tween 80.

Electrochemical noise
The corrosive system is immediately observed in figure 2, which shows the electrochemical noise resistance, R n , response for the different approaches, where the behavior of each of them is immediately observed. Through electrochemical noise techniques and potentiodynamic polarization curves, it was shown that the presence of the coating in the analyzed system revealed a rapid action on metallic surfaces. For the unprotected aluminum, we observe the resistance series, R n , with a small amplitude of the transients due to the speed of generation of the natural film, and it is protected in an accelerated manner. In the first 24 h, the behavior of CMC began with greater amplitude of the transients, generating a more significant attack than the other samples; however, the amplitude reduces without reaching the behavior of Al. In comparison with the results obtained in R p , it is shown that the sample presents an active corrosion potential (more negative). The behavior in the CM and CMC-T systems is very similar with a subtle attack where the transients present small amplitudes when passing. The surface activity is more active over time, showing transients with greater amplitude in short intervals.
Comparing with Al they are very similar, clearly corroborated with the R p technique.

Noise resistance
The resistance values were determined for each of the cases studied (equation (3)): Al, CM, CMC, and CMC-T, as well as in the different immersion times. Figure 3 shows the values obtained, corroborating what is shown in the graphs (figure 2) regarding the behavior of the resistance series. The cases studied with coating presented higher resistance values at 24 h than uncoated aluminum. During this investigation, the behavior of aluminum presents an increased advance of the immersion time with a maximum of 96 h and maintains a similar magnitude in the value at 168 h. In the cases of the CM and CMC coatings, the maximum values of resistance over the uncoated aluminum were at 24 h. During the hours of immersion, these values were reduced, the lowest presented at 168 h. The higher resistance values are present for the case of the CMC-T coating. The higher Table 2. Parameters obtained from polarization curves for aluminum immersed in NaCl solution at 3.5% wt. with different coatings. resistance values are present for the case of the CMC-T coating. It indicates the contribution of mucilage, cochineal, and the removal of polysorbate 80 (CMC-T), improves the behavior and resistance in noise resistance values, when compared with aluminum without cover.

Location index
The location index equation (4) is a parameter that evaluates the noise variation in current and compares it with the mean value. This index was calculated through the ratio of the standard deviation in the current (σ i ) and the root mean square of the current (I rms ) [36].
i Irms 4   Thus, removing the Tween 80 component from the coating formulation caused a more significant effect on surface corrosion resistance phenomena. For CMC and CMC-T samples at 96 h, when performing the analysis, there is a considerable decrease due to superficial phenomena such as the protection of the coating and generating that the series current density time series present a smaller amplitude; this is observed in figure 2. The resistance time series for these two systems are observed as a continuous straight line, indicating this process and allowing the modification of the corrosion phenomenon different from the initial one. Subsequently, the resistance time series presented greater amplitude in the transients obtained, therefore, the increase in the value of the IL for these cases.

Electrochemical impedance spectroscopy (EIS)
EIS technique was used to obtain electrochemical characteristics of the corrosion process; also, it was employed to separate the contributions of different phenomena that interfere with and control the corrosion process of aluminum with and without the coating. EIS Nyquist diagrams of the different systems under study, shown in the graphs (figure 5), were obtained to carry out a quantitative analysis of the impedance spectra. These were simulated with the analogous circuit shown in figure 4.
The system's EIS results without organic coating can be presented with physical elements in an equivalent circuit. The circuit in figure 4(b) comprises a resistor, R s , representing the solution resistance, and a constant phase element, CPE ct , connected in parallel with the charge transfer resistance R ct . This circuit is associated with  uncoated aluminum in redox reactions in aggressive media. The analogous circuit shown in figure 4(b) generally describes the process at the metal-solution interface. Figure 4(a) shows the resistance of the solution represented by R s , the charge transfer resistance by R ct , and a double layer capacitance by CPE ct. These are associated with the film formed by the organic coating, in addition to a resistance linked to the coating R coat , connected in parallel to a constant phase capacitor CPE coat that represents the heterogeneity of the surface that is produced by the dissolution of aggressive species [38]. Impedance parameters were determined by a half-circle suitable method by EC-Lab software (EC-Lab ® ) using a simple-randomized numerical generator and are presented in table 4. EIS measurements were performed on a BioLogic SP-150 potentiostat by applying a ±10 mV signal around the E corr value at a frequency interval between 10 kHz and 0.5 Hz. To obtain coating efficiency values, they were calculated using equation (5): where R ct2 and R ct1 are the load transfer resistance values with and without the addition of the coating, respectively. The values of the constant phase elements obtained for the best fit of the experimental data to the analogous circuit of figure 4 is shown in table 4. It is observed that the coat values for the CPE coats obtained for the different coatings under study depending on the immersion times are very similar (about 0.75-0.85) and can be mainly attributed to the heterogeneity of the electrode surface. In the case of CPE ct , it is essential to note that all the calculated n ct are comparable in magnitude, which validates the supposition that these elements are associated with species diffusion [39]. The CMC-T system has higher values of resistance and efficiency of organic coating as a function of inversion time than the other systems studied CM, CMC, and the white (without coating).
In the case of CPE ct , it is essential to note that all the calculated n ct are similar in magnitude, which validates the assumption that these elements are associated with species diffusion [39]. The CMC and CMC-T systems present higher values of resistance and efficiency of the organic coating as a function of the inversion time than the studied CM system and the reference without coating. It is pointed out that the R s values of different coatings under study are minor compared to a without covering system due to the microstructure in systems with mucilage is denser and presents holes that provide a diffusion path of the electrolyte to the recovering/substrate interface [40][41][42].

Microscopy
The morphology of the coated and uncoated aluminum samples in a 3.5% wt. NaCl solution was observed with a metallographic microscope and a 50X objective (brand, model) ( figure 6) before immersion. In the electrolyte and after 168 h of immersion. Subsequently, cleaning was carried out by sonication in deionized water and drying with air. According to the micrographs in figure 6, corrosion products are visible in larger surface areas for uncoated aluminum, with no corrosion damage on the coated metal surfaces. However, hybrid coatings exhibit surface degradation and chloride ions can diffuse through the coating surface using the porosity of the coating as a pathway, causing loss of the coating by delamination.
When the electrolyte reaches the cladding/metal interface, electrochemical reactions begin. All formulations act as a barrier, reducing the diffusion rate of chloride ions and, therefore, localized corrosion damage on the metal surface is reduced. The CMC-T formulation exhibits more acceptable barrier properties, less coating surface damage, and smaller coating-free zones, showing no evidence of coating degradation and excellent adhesion to the metal substrate after testing.

Coating thickness
An ultrasonic thickness measuring tool (Bruker NDT model) was used to determine the coating thickness. Ten measurements were taken at 10 random points on the coated surface, and the average value was calculated; the coating thickness obtained was 18.75±0.01 μm for CMC, 22.12±0.01 μm for CMC, and 21.16±0.01 μm for CMC-T. Three test items of each formulation were evaluated to ensure reproducibility. According to the values obtained, the coating thickness is homogeneous. Figure 7(a) shows the infrared spectrum (FT-IR) of the mucilage extracted from the nopal (CM). The band at 3255 cm −1 is attributable to carbohydrate O-H bonds of the hydroxyl groups [41]. The C-H stretching vibrations at 2930 cm −1 , and small bands at 2882 and 2850 cm −1 are due to C-H and -CH 2 stretching of the pyranose and the carboxylic groups, respectively [41]. The C-C stretching bonds are visible at 2100 cm −1 . The COO-antisymmetric and symmetric stretching vibrations of carboxylic acid salts of the mucilage were visible from 1615 to 1512 cm −1 [42]. The band at 1400 cm −1 is due to H-C-H bending vibrations. The peaks from 1321 to 988 cm −1 are closely related to the presence of -COOH groups, aromatic proteins, phosphoric groups, and some polysaccharides in pyranose ring conformations, such as the mannose and glucose [43,44]. On the other hand, the absorption band at 853 cm −1 corresponds to β-D-glucose, while the bands observed at 671 and 550 cm −1 have been assigned to the N-H and O-H out-of-plane vibrations . In this sense, figure 7(b) shows the interaction effect between the cochineal dye and the mucilage extract. The cochineal FT-IR spectrum (C) shows an absorption band at 3350 cm −1 associated with the -OH stretching mode. The bands between 2930 and 2855 cm −1 are due to C-H, -CH 2 , and -CH 3 stretching vibration modes. The band at 1642 cm −1 is from the C=O stretching of the anthraquinone middle ring, while at 1565 cm −1 is observed the C=C stretching of the I ring, the C-O-H scissoring vibration, and the C-H out-of-plane bending. The C-C stretching from I and II aromatic rings and the glucose C-H deformation is present at 1463 cm  band located at 884 cm −1 . The bands that arise between 758 to 532 cm −1 are due to the rocking C-H, out-of-plane bending C-OH, out-of-plane wagging C-H, and several skeletal vibration modes of aromatic systems [45][46][47][48]. Table 5 lists the main vibrational modes for the mucilage and the cochineal dye. In addition, the composite formed from the cochineal dye with the mucilage (CMC) shows a different behavior to the CM and the C, indicating a new interaction state between both components. Finally, the effect of removing the polysorbate 80 from the CMC sample is also shown in figure 7(c) (red curve: CMC-T). In this case, a small intensity increment occurred in the bands associated with the CH vibrational modes. This increment could be related to an increment in the polymer composite's crystallinity by removing the polysorbate 80.

Conclusion
In this work, we found that high molecular weight organic-inorganic coating is an effective corrosion barrier for pure aluminum in NaCl at 3.5% by weight, with higher efficiency with the coating with mucilage, CM, CMC, and CMC-T. And cochineal without polysorbate 80, CMC-T. The polarization curves showed that the proposed coatings function as mixed-type inhibitors with a more significant effect on the electrochemical reactions in the cathodic branch. From the duration of the corrosion tests, the corrosion potentials, Ecorr, of the coatings were more positive than with the uncoated material. The resistance value, Rp, of the coating, first decreased and then gradually increased, indicating that the protective effect of the coatings is gradually significant. The presence of coatings on the metallic surface decreases the current density. The electrochemical noise results correlate with the polarization and EIS curves and obtain the localization index, which interprets a better behavior against corrosion by the CMC-T coating. The EIS measurements indicated that the corrosion process was under charge transfer control, agreeing with the efficiency values obtained using polarization curves and EIS electrochemical techniques. IR spectra showed the effect of combining the cochineal with the mucilage, having a close molecular affinity, which helps to improve the structural strength of the organic covering. Combining these compounds in the coatings protects the metallic surface and reduces the attack of chloride ions. Environmentally friendly water-based coatings with fewer volatile organic compounds with low environmental impact were obtained.

Acknowledgments
The researchers show gratitude to Universidad Autónoma del Estado de Hidalgo for the facilities and financial support provided.

Data availability statement
The data that support the findings of this study are available upon reasonable request from the authors. Table 5. Infrared vibrational modes of the mucilage (CM) and the cochineal dye (C).

Data availability statement
The data generated and/or analysed during the current study are not publicly available for legal/ethical reasons but are available from the corresponding author on reasonable request.

Disclosure statement
The researchers disclosed that no possible conflicts of interest.