Influence of pH on the Inhibiting Characteristics of Cresol Red Incorporated in Chitosan Coatings on Zinc

Abstract

The present work focuses on the investigation of protective coatings produced on zinc from chitosan (Chit) and an anionic dye, namely cresol red. Cresol red (CR) fulfills the basic requirements to be used as a corrosion inhibitor because it possesses a relatively high molecular weight and includes in its structure oxygen and sulfur atoms as well as aromatic rings. Moreover, it is an anionic compound that can interact with positively charged chitosan to produce reinforced coatings for zinc anti-corrosion protection. The influence of cresol red as a possible corrosion inhibitor for zinc substrates was investigated either in solution or incorporated in Chit coatings. Two preparation methods for the coatings were used: (i) Chit coating impregnation by immersion in the CR solution after Chit deposition on Zn, and (ii) chitosan mixing with the CR solution before applying the dip-coating technique. Potentiodynamic polarization curves were used to determine the kinetic parameters of the corrosion process. Long-term measurements were carried out in wet/dry cyclic conditions by using electrochemical impedance spectroscopy. EIS measurements recorded in 0.2 g/L Na₂SO₄ at pH = 7 show an important increase in the impedance of the coatings occurring from the first until the fifty-fifth day in a row, in dry–wet cycles. This increase is due to the beneficial effect of CR incorporated in Chitosan and could be, at least partially, related to a consolidation of the Chit coating structure in the presence of CR by crosslinking between Chit and CR molecules. The structure of the coatings was studied, and the interactions between chitosan and cresol red were put into evidence by using FT-IR spectroscopy. Adhesion and wettability measurements were also carried out. The adhesion of Chit incorporating CR on Zn was better than that on glass substrates and reached ~99.99%, suggesting a better affinity of the chitosan coating towards the Zn substrate due to the existence of ZnO on the substrate surface. All the results show that CR could be used on zinc as a corrosion inhibitor incorporated in chitosan at basic pHs, but without taking advantage of its pH-indicating properties, which are lost due to the interactions occurring between the positively charged biopolymer and the negatively charged dye molecule. The preparation method of Chit coating impregnation with CR by immersion in the solution after deposition on Zn led to poorer results than the method in which chitosan was previously mixed with CR before applying the dip-coating technique.

1. Introduction

Zinc is a very important metal used in a wide range of applications by various industries, including civil engineering, constructions, etc., where it is often chosen as a sacrificial coating to protect iron and steel against corrosion. As zinc is an active metal that corrodes very quickly, its anti-corrosion protection is of real interest. In this regard, one commonly used method involves corrosion inhibitors, which minimize the corrosive attack of the environment and prolong the lifetime of zinc coatings.

A large number of chemical substances, especially organic, can be used as corrosion inhibitors for zinc, especially compounds containing heteroatoms such as nitrogen, oxygen, or sulfur [1,2], aromatic rings [3], or triple bonds [4]. Natural compounds were also reported [5,6], even if their efficiency is not as good as that of synthetic compounds.

In an attempt to improve the corrosion resistance of zinc, coating its surface with protective films of different natures is also a promising solution. Inorganic compounds [7,8,9,10], but also synthetic [11,12] and natural polymers [13,14,15,16] are used to modify the zinc surface by using different techniques such as sol–gel [7,8,9], electrodeposition [17], layer-by-layer deposition [18], electropolymerization [19], etc.

Among environmentally friendly polymers, chitosan (Chit) is a promising choice due to its low cost, lack of toxicity, and versatility. Chitosan (Chit) is a water-soluble, partially deacetylated derivate of chitin that can be easily applied to metal substrates as a protective coating [20] or used as a matrix for various composite materials [21,22,23]. Although there are countless arguments in favor of the production of chitosan coatings, such as thermal stability, chemical resistance, bioavailability, biocompatibility, and biodegradability, due to their loose structure, they can easily lose their barrier properties. This disadvantage can be eliminated through functionalization and the incorporation of various additives. Through its positive charge, chitosan can interact with anionic species, resulting in an efficient ionic crosslinking effect [16], leading to consolidated coatings with enhanced mechanical and insulating properties.

In our previous work [16], chitosan coatings crosslinked with indigo carmine (IC) were prepared on zinc by the dip-coating technique. The addition of IC to Chit coatings was proven to be useful in terms of active corrosion protection, with the Chit coatings acting as a reservoir for the corrosion inhibitor. The corrosion current density decreased by one order of magnitude, suggesting an ionic crosslinking of Chit.

Unfortunately, as a result of external factors (e.g., heat, light, moisture, and aggressive skin), the protective effect of the coating deteriorates. For this reason, it has become important to develop functions that can be used to monitor the deterioration of coatings. This can be imagined if the coating responds to the external factors that cause the deterioration and its result becomes noticeable. There have already been some solutions for this. Fluorescence detection of Fe³⁺ [24] and Cu²⁺ ions formed during corrosion through complex formation [25]. The detection of OH- ions produced during corrosion with pH indicators can also be a solution [26].

In this context, the present work focuses on the investigation of coatings produced on zinc from chitosan and another anionic dye, namely cresol red. Cresol red (CR) has a high molecular weight and includes in its structure oxygen and sulfur atoms as well as aromatic rings; these properties recommend it as a possible corrosion inhibitor. Moreover, it is an anionic compound that could interact with positively charged chitosan to produce reinforced coatings for zinc anti-corrosion protection. The influence of cresol red embedded in chitosan thin layers as a possible corrosion inhibitor for zinc substrates was investigated in wet/dry cyclic conditions. Moreover, as CR is a pH indicator in solution, early detection of corrosion processes in its presence would be a solution to tackle corrosion in a cost-effective way. Chitosan microspheres loaded with organic compounds having pH-indicating ability have been reported in the literature [27], but it is for the first time that Cresol read incorporated in a Chitosan coating deposited on zinc was investigated from both its inhibitive and pH sensing points of view.

For this purpose, the corrosion behavior of cresol red-loaded chitosan coatings on zinc prepared by dip-coating was investigated using potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) techniques. The interactions between chitosan and cresol red were put into evidence by using FT-IR spectroscopy. Adhesion and wettability measurements were also carried out.

2. Experimental

2.1. Materials and Methods

Both zinc and glass plates were used as substrates. The zinc plates were obtained from AltDepozit (99% Zn) and the glass microscope slides from Biozyme. Medium-molecular-weight chitosan was purchased from Aldrich, Romania, cresol red from Chinoin Budapest, Hungary, 99.8% ethanol isopropanol from Nordic Chemicals, Romania and 99% Na₂SO₄ from Merck, Rahway, NJ, USA. In addition, 96% sulfuric acid was provided by Reactivul, and 99% NaOH was from Merck, Rahway, NJ, USA. All chemicals were of analytical grade and used without any purification steps.

Chitosan	Cresol Red

2.2. Coating Preparation

Before the coating preparation, the glass microscope substrates were 2 × 5 min ultrasonicated using an aqueous detergent solution, cleaned with a 10% V/V H₂SO₄ solution, isopropanol, and distilled water.

The zinc plates were polished with emery paper (different roughness: 1500, 2000, 2500, and 5000) and 2 × 5 min ultrasonicated in distilled water. Finally, 0.1 M HCl and isopropanol solutions were used to clean the surface of any remaining zinc oxide.

Two preparation methods for the coatings were used: (i) Chit coating impregnation by immersion in the CR solution after Chit deposition on Zn, and (ii) chitosan mixing with the CR solution before applying the dip-coating technique. Here, 1 w/w% chitosan solution prepared in 1 w/w% acetic acid solution was used, and the solution was prepared from chitosan powder by mixing until all the chitosan was totally dissolved and left overnight.

Method (i): chitosan coatings deposited on glass by dip-coating from chitosan solution prepared as described above were impregnated with cresol red by immersion in aqueous cresol red solution with different concentrations (10⁻⁵, 5 × 10⁻⁴, 10⁻⁴, 10⁻³, 10⁻² M). The immersion time was 10 min.

Method (ii): the cresol red powder was added to the chitosan solution in different concentrations (10⁻⁵, 5 × 10⁻⁴, 10⁻⁴, 10⁻³, 10⁻² M) and ultrasonicated for 24 h, until it totally dissolved. The coatings were prepared by the dip-coating method from the previously prepared cresol red containing chitosan solution with a withdrawal speed of 5 cm/min. The apparatus we used was a home-made dip coater machine. The coated samples were left to dry for 24 h at room temperature.

After that, the samples were left to dry, examined by sight, and then examined by optical spectroscopy.

2.3. Optical Characterization

The optical properties of different chitosan coatings deposited on glass substrates were determined by UV-VIS spectroscopy. For the measurements, a homemade spectrophotometer was used. The device contains a light source from a Spekol 20 spectrophotometer. The instrument’s light beam was coupled to a homemade sample holder through an optical cable. The transmitted light was also sent to the spectrometer using another optical cable. An Ocean Optics HR 4000CG UV-VIS-NIR microspectrometer (Dunedin, FL, USA) was used to acquire the spectra. The spectrometer has the following characteristic parameters: a measurement range of 200–1100 nm, an entrance slit of 50 µm, a Toshiba CCD detector, a pixel count of 3648, and a spectral resolution of approximately 1.5 nm. The Spectra Suite (Ocean Optics) program was used for processing the acquired data, enabling baseline correction, signal averaging, and various integration times. As a reference, we used an uncoated glass sample. The integration time was constant at 70 s.

2.4. Wettability Measurements

Wettability can be determined by measurements of the contact angle and the interfacial tension [28,29]. The wettability of cresol red-containing coatings was measured in comparison with native chitosan coatings by a sessile drop method. A sessile drop method based on the European Pharmacopoeia [30] was used to determine the contact angle using a drop of 0.2 g/L aqueous Na₂SO₄ solution, used in electrochemical measurements too, with a volume of 20 µL. Measurements were carried out in a closed space saturated with the vapor of the studied electrolyte. In each case, the contact angle evaluation was followed by taking pictures every minute for 20 min. To determine the contact angle evaluation, we used a specific measuring program.

2.5. Adhesion and Coating Thickness Measurements

For determining the adhesion between the substrate and the coating, the tape peel method was used with the Elcometer Cross Hatch Adhesion Tester. This is a qualitative mechanical method of determination. For the measurements, a special cutter with multiple preset blades is needed to make sure the incisions are properly spaced and parallel. The coatings were scratched by dividing them into 49 equal cubes. After the tape has been applied and pulled off, the cut area is then inspected and rated. The adhesion of thin films is influenced by a large number of parameters; some of these parameters are the substrate/coating materials, the substrate preparation, or the coating method.

The thickness of the coatings was measured by the TROTEC BB25 instrument, (GmbH and Co. KG Deutschland), a layer measuring instrument that works according to magnetic induction or turbulent flow principles for determining the coating thickness on magnetic and non-ferromagnetic metals.

2.6. FT-IR Analysis

For FT-IR analysis, chitosan and cresol red powder were used. The cresol red containing chitosan sol was prepared, evenly distributed on a clean Petri dish, and left to dry for 2 weeks, then scraped off the glass and powdered in an agate mortar. Infrared spectra for chitosan, cresol red, and chitosan containing cresol red were recorded on a Jasco FT/IR-6800 type A Spectrometer.

2.7. Electrochemical Measurements

Electrochemical measurements were carried out using a PARSTAT-2273 single channel potentiostat (Princeton Applied Research, UK) in a three-electrode cell containing the zinc substrates (2 cm² surface area) as a working electrode, a platinum electrode as a counter, and an Ag/AgCl, KClsat reference electrode connected to the potentiostat. All experiments were made at room temperature (25 °C) with an adjusted pH.

2.7.1. Electrochemical Impedance Spectroscopy (EIS)

Electrochemical impedance spectroscopy measurements were made for samples in two different ways: firstly, the samples were kept continuously in an electrolyte solution between the measurements (wet method), and secondly, the samples were taken out of the corrosive solution after every measurement and the solution was wiped off the sample (wet–dry method), left overnight in laboratory circumstances.

EIS measurements were conducted in a 0.2 g/L aqueous Na₂SO₄ solution at four different pH values (pH = 5, 7, 8, and 9). The frequency interval of the measurements was 10⁻² Hz–10⁴ Hz at open circuit potential (OCP) measured for 60 min before EIS measurements. For pH studies, the pH of the electrolyte was adjusted with NaOH and H₂SO₄ to the corresponding levels.

2.7.2. Polarization Curves

Potentiodynamic polarization curves were recorded with a scan rate of 0.166 mV/s in a potential range of OCP ± 20 mV vs. Ag/AgCl, KCl_sat (to determine the polarization resistance) and OCP ± 200 mV vs. Ag/AgCl, KCl_sat (to determine the kinetic parameters with the Tafel method).

Corrosion current densities (i_corr) were determined using the Stern–Geary equation [31]:

$$\mathit{icorr}=\frac{b_a\times b_c}{2.3Rp\times \left(b_a+b_c\right)}$$

(1)

where b_a and b_c are the Tafel coefficients determined from the polarization curves, and Rp is the polarization resistance determined from the Nyquist impedance diagrams.

The inhibition efficiency of the coatings was determined using the following equation:

$$\mathit{IE}=100\times \frac{i_{\mathit{corr}}^o-i_{\mathit{corr}}}{i_{\mathit{corr}}^o}$$

(2)

where ${\mathrm{i}}_{\mathrm{corr}}^{\mathrm{o}}$ is the value of the corrosion current density of the uncoated Zn sample, while i_corr is the corrosion current density of coated systems.

3. Results and Discussion

3.1. Chitosan Coatings Impregnated with Cresol Red

Both glass and zinc substrates were used to investigate CR incorporation in the chitosan layer. As expected, the visual examination of the glass samples coated with chitosan and impregnated with CR by immersion put in evidence different color intensities, depending on the CR solution concentration (Figure 1a), proving the presence of the compound in the chitosan layer in an increasing amount, proportional to solution concentration. The impregnation with 10⁻⁵ M was invisible to the naked eye and is not shown in Figure 1. The incorporation of CR in the Chit coatings deposited on zinc is less visible than on glass (Figure 1b). Nevertheless, a concentration dependence was also noticed, reflecting the impregnation of the chitosan coatings with CR.

3.2. Optical Characterization

In order to establish the proportionality between the CR concentration and the color intensity of the coatings, three glass samples on which the color of CR was visible to the naked eye were analyzed with UV-Vis spectroscopy.

It can be observed from Figure 2 that, for cresol red, the spectrum exhibits two peaks, one larger at 442.01 nm and one smaller at 590.16 nm. The literature data [32] mention a peak appearance in the visible region of the cresol red spectrum in solution: at 430 nm at pH 4.5 and at 573 nm at pH 12. It can therefore be said that the maximum absorption of cresol red is influenced by the pH of the medium. The differences between the literature data reported for cresol red and those from our experiments could be attributed to the different pHs used and to the fact that in the first case, the values correspond to CR in solution, while in the later case, the values were recorded on chitosan coatings deposited on glass sheets. The contribution of the glass sheets used in our experiments should also be taken into consideration.

Moreover, the small second peak that appears is due either to the fact that the pH distribution within the chitosan-containing CR layer is not uniform or to the coexistence of the protonated form and the unprotonated one. Nevertheless, a linear dependence of the absorbance on the CR concentration in the layer is observed, (y = 113.18x − 0.0005, R² = 0.9997) proving its incorporation in chitosan (see inset in Figure 2).

3.3. FT-IR Measurements

Taking into account that CR is an anionic compound that could interact with positively charged chitosan, FT-IR spectroscopy was used to provide evidence of possible interactions between the two components of the coatings.

As it can be observed from Figure 3, the chitosan spectrum showed the characteristic 3500–3300 cm⁻¹ band caused by the hydroxyl and amino groups’ O--H and N--H stretchings. Bands at 2920 and 2870 cm⁻¹ resulted from C--H stretching. Bands for C = O stretching at 1646 cm⁻¹ (amide I), N--H bending at 1601 cm⁻¹ (amide II), and C--N stretching (amide III) at 1323 cm⁻¹ were visible. The peaks at 1423, 1385, and 1073 cm⁻¹ are CH₂, CH₃ bending, and C--O stretching signals. For the chitosan spectra, the peaks seem to be weak, but when it is alone, the spectra are similar to those found in the literature [32,33].

The Chit/CR_10⁻³ M spectrum showed strong OH stretches at about 3250 cm⁻¹ in chitosan, which decreased highly because an etheric bond was formed between cresol and chitosan.

The C-O-C stretching from ether shows up in the region 1050 cm⁻¹. The chitosan-cresol complex showed a band at 1450 cm⁻¹ for C-H bendings and C-C bands at 1500 cm⁻¹ from chitosan. Also, C-H bendings from the aromatic ring are present at 1050 cm⁻¹.

As it will be demonstrated by the electrochemical measurements, the interactions between Chit and CR could explain the behavior of Chit-CR coatings in zinc anti-corrosion protection.

3.4. Influence of pH

It is well known that in aqueous solutions, CR changes color from yellow (in acidic media) to magenta (in basic solutions). In order to verify if CR still behaves as a pH indicator when it is incorporated in chitosan layers, glass samples covered with chitosan loaded with Cresol red 10⁻³ M were contacted with Na₂SO₄ solutions having different pH values: 5 (left) and 8.2 (right). Chit/CR_10⁻³ M in contact with a Na₂SO₄ solution of pH 5 changed to yellow, and the sample at pH = 8.2 showed a specific dark pink color (Figure 4).

The color change in solutions as a function of pH is due to reactions (Figure 5).

Unfortunately, the color change of chitosan coatings incorporating CR deposited on zinc is less visible. Besides the dark color of the zinc substrate, it is possible that the incorporated CR does react with a zinc surface covered with a thin layer of ZnO, so that the functionalities acting in pH indication are blocked and the color change is no longer visible. ZnO interaction with CR was reported in the literature [33], as a function of pH. At acid pHs (<7), ZnO and CR molecules are both positively charged, so repulsion occurs. At neutral pHs, (7–8.2) ZnO is still positive, while the CR molecule is neutral or negatively charged, which results in a weak interaction between the two compounds. Finally, at strong basic pHs (>8.2), ZnO becomes negatively charged, which results in a strong repulsion between ZnO and CR.

3.5. Electrochemical Measurements

3.5.1. Polarization Curves

To put in evidence if there is any effect of CR on zinc corrosion, potentiodynamic polarization curves in the absence and in the presence of different concentrations of CR-containing solutions were recorded on bare Zn. The semi-logarithmic polarization curves of Chit and Chit/CR-coated Zn samples are presented in Figure 6 and their characteristics in

Table 1. Kinetic parameters of the corrosion process of Zn and of Zn/Chit in solutions of different concentrations (10⁻⁵ M to 10⁻¹ M) at pH 7.

Sample	Ecorr (mV vs. RE)	icorr (µ/Acm−2)	ba mV/dec	/bc/ (mV/dec)	IE (%)
Zn
Zn	−942.5	20.67	112	389	-
Zn/CR 10−5 M sol	−708.9	18.49	99	622	10.54
Zn/CR 10−4 M sol	−736.9	15.99	73	301	22.64
Zn/CR 10−3 M sol	−773	4.78	82	191	76.87
Zn/CR 10−2 M sol	−855.4	9.44	141	194	54.32
Zn/Chit
Zn/Chit	−892.7	22.93	110	745	-
Zn/Chit_CR 10−5 M sol	−1131.8	18.41	139	439	19.71
Zn/Chit_CR 10−4 M sol	−1242.5	12.94	134	403	43.56
Zn/Chit_CR 10−3 M sol	−784.4	11.25	70	208	50.93
Zn/Chit_CR 10−2 M sol	−808.8	18.17	67	213	20.75

.

As it can be observed from Figure 6a, in solution, CR has a beneficial effect on Zn corrosion resistance, as even at small concentrations of the organic compound, a serious decrease in the corrosion current density is noticed, from 20.67 µ/Acm⁻² for bare Zn to 4.78 µ/Acm⁻² in the presence of 10⁻³ M CR (Table 1).

Encouraged by this result, potentiodynamic polarization curves were recorded for Zn coated with chitosan in CR solutions of different concentrations (Zn/Chit_CR 10^−x sol). The results are depicted in Figure 6b, and the kinetic parameters, including the current density (i_corr) and the corrosion potential (E_corr), are presented in Table 1.

When Cresol Read was added to the solution, the Ecorr of Zn was shifted toward positive values, suggesting that an ennoblement of the surface took place. Surprisingly, even if all values of i_corr are smaller in the presence of CR in solution for both Zn and Zn/Chit than in its absence, it seems that the Chit layer does not offer convenient protection to Zn when CR is present in solution. Thus, the CR inhibiting effect is less effective when a Chit film is interposed between the solution and the zinc substrate, suggesting that the CR inhibiting effect is somewhat blocked after its immobilization in Chit. It should also be mentioned that high concentrations of CR are not beneficial, which is not surprising, as for any specific inhibitor in any given medium there is an optimal concentration (in this case, 10⁻³ M).

In an attempt to improve the protective effect of the biopolymer layer in the presence of CR, another preparation method for the coatings was used. The chitosan solution was prepared, and cresol red was added in the same concentration previously mentioned. This mixture was ultrasonicated until a uniform solution was received, then deposited on Zn by the dip-coating technique. The zinc samples coated with this type of layer were named Zn/Chit_CR_10^−xM coat (10^−x = CR solution concentration). Next, corrosion tests of the coated zinc samples were carried out in Na₂SO₄ solutions of various pHs, with the results being presented in Figure 7 and

Table 2. Kinetic parameters of the corrosion process on Zn, Zn/Chit, and Zn/Chit/CR_10⁻³ M coat samples at different pH values. Electrolyte: Na₂SO₄ with pH 5, 7, 8, and 9, respectively.

pH	Sample	Ecorr mV vs. RE	icorr µAcm−2	/bc/ mV/dec	ba mV/dec	IE (%)
5	Zn	−1025	38.89	105	140	-
	Zn/Chit	−921	25.53	-	79	34.35
	Zn/Chit_CR_10−3 M coat	−917	17.10	926	73	56.02
7	Zn	−942	20.67	112	106	-
	Zn/Chit	−856	19.00	110	745	8.07
	Zn/Chit_CR_10−3 M coat	−922	11.17	59	476	45.96
8	Zn	−887	32.35	642	88	-
	Zn/Chit	−928	14.03	-	59	56.63
	Zn/Chit_CR_10−3 M coat	−784	8.95	804	63	72.98
9	Zn	−1219	47.75	31	111	-
	Zn/Chit	−880	9.13	-	63	80.87
	Zn/Chit_CR_10−3 M coat	−730	7.21	424	26	84.90

.

Similar to the case when CR was present in the solution, the Ecorr values of Zn samples coated with CR incorporated in Chit are shifted toward positive values, suggesting that an ennoblement of the surface takes place. The inhibiting effect of CR is more pronounced in the anodic branches of the polarization curves of the samples (Figure 7), suggesting that CR acts more as an anodic inhibitor. Looking at the results from Figure 7 and Table 2, it can be concluded that CR diminishes the corrosion current density at all pHs, but basic pHs seem to be more favorable than neutral or acidic ones, where chitosan is vulnerable and can be destroyed. At pH 9, CR decreases the corrosion current density of Zn and of Zn/Chit by one order of magnitude, and at a concentration of 10⁻³ M, it acts as an efficient corrosion inhibitor. A significant positive shift in the corrosion potential is also observed, especially at pH 9, suggesting an ennoblement of the coating in the presence of Chit and CR.

The explanation of the coating behavior at this pH could be the interactions taking place between the Zn surface (naturally covered with ZnO) and CR, whose electrical charge changes as a function of pH. Thus, at basic pHs (>8.2), ZnO is negatively charged, which results in a strong repulsion between ZnO and CR, which is also negative. The intercalation of the Chit layer between ZnO and the dianionic CR molecules reduces the repulsion between them, and at the same time, the Chitosan protective layer is consolidated through a crosslinking between Chitosan and CR, as confirmed by FT-IR measurements. The possibility of interactions between Chit and propane-sulfone rings was mentioned in the literature [34]. The formation of sulfopropyl chitosan derivatives at a pH exceeding 8 involves the engagement of the amino group of chitosan in a nucleophilic substitution on the benzene ring, leading to the opening of the sulfopropyl ring and the formation of sulfonic groups (see Scheme 1).

Tafel slope can provide an important reference for exploring the reaction mechanism, especially in clarifying the reaction rate-determining steps and reaction paths. The results suggest that the corrosion mechanism differs from uncoated to coated zinc and, moreover, from one pH to another. It should also be mentioned that cathodic values should be regarded as less precise than the anodic ones, taking into account the horizontal shape of the cathodic branch in some cases due to the oxygen diffusion step that takes over the charge transfer.

It should be mentioned that the pH indicator property of CR is also affected by its incorporation in the Chit coating. CR becomes less active due to its immobilization in the biopolymer. Thus, the color change is not observed, and CR does not act as a pH indicator, its activity being partially ruined due to its incorporation in the coating. Nevertheless, at pHs > 8.2, it has a beneficial effect on the corrosion resistance of coated Zn, acting as a corrosion inhibitor when used in proper concentration.

3.5.2. Electrochemical Impedance Spectroscopy

Influence of pH

To gain a deeper insight into the corrosion process, electrochemical impedance spectroscopy (EIS) measurements were carried out on zinc, Zn/Chit, and Zn/Chit_CR_10⁻³ M coat samples at the same pH values as in the case of polarization measurements. The corresponding Bode plots of the impedance are presented in Figure 8.

As expected, all /Z/_0.01Hz values are higher when Chit and CR are present and are clearly the largest at pH 9, meaning the coatings have the best corrosion resistance (

Table 3. /Z/_0.01Hz values extracted from Bode curves presented in Figure 8 at different pHs.

Sample	/Z/0.01Hz (Ω)
	pH 5	pH 7	pH 8	pH 9
Zn	260	1384	915	65
Zn/Chit	1747	1393	2064	459
Zn/Chit_CR_10−3 M coat	2333	1568	2388	7140

). Moreover, the Zn/Chit_CR_10⁻³ M coat exhibited the largest /Z/_0.01Hz value, possibly due to the smaller permeability of the Chit-CR layer, which could lengthen the pathway of the electrolyte. At the same time, it could be seen that the shape of the impedance curves at pH 9 is different from that of the diagrams recorded at lower pHs, suggesting a change in corrosion mechanism, which is in accordance with the results extracted from the polarization curves.

Long-Term Measurements

Further investigation performed over time reveals interesting behavior (Figure 9). It can be seen that Nyquist impedance diagrams exhibit capacitive behavior with a small inductive loop. According to the literature, the inductive loop originates from the coverage relaxation of the adsorbed species ZnI, ZnII, and ZnOH formed during the corrosion process [35].

A study carried out at pH 7 demonstrates that on the first day of immersion, the Zn/Chit_CR_10^−x M coat samples exhibit a smaller capacitive loop than the Zn/Chit sample, but the situation improves significantly after several dry-wet cycles.

As it can be observed from Figure 9, an important increase in the coating impedance recorded in 0.2 g/L Na₂SO₄ at pH = 7 in dry-wet cycles takes place from the first until the fifty-fiveth day in a row. This increase is related to the beneficial effect of CR incorporated in Chitosan and could be, at least partially, related to a change in the Chit coating structure in the presence of CR due to the interaction between Chit and CR molecules, as revealed by the FT-IR spectra (Figure 3). It suggests a crosslinking of Chit by CR anion, which is a process leading to a consolidated coating. The increase in impedance in time could also be due to the formation of corrosion products at the interface, which exert a protective effect [36], and/or to a reaction of CR with the main reactive corrosion products during wet/dry cyclic conditions.

Moreover, it is also possible that SO₄²⁻ ions penetrate the Chit structure over time and consolidate the coating even more. This possibility is mentioned in the literature: SO₄²⁻ anions slowly interact with chitosan NH³⁺ groups to form ionic bridges between the polymer chains [37]. This also points to a consolidation of the coating over time.

In order to better understand the underlying mechanism at the interface, the electrical equivalent circuits (EC) were fitted to EIS data for Zn/Chit_CR_10⁻³ M coat samples recorded in time. The obtained values are presented in

Table 4. Electrochemical parameter values for Zn/Chit_CR_10⁻³ M coat samples obtained by fitting equivalent circuits to EIS diagrams from Figure 9.

Days	R (kΩcm2)	R1 (kΩcm2)	Q1 (μSsn)	n	R2 (kΩcm2)	C (μSsn)	Rp (kΩcm2)	L (kH)	R3 (kΩcm2)
1	0.400	1.49	40.690	0.602	2.02	6.37	3.51
2	0.476	2.52	8.696	0.748	2.57	5.14	7.66
5	0.705	10.22	5.591	0.650	3.34	1191	13.56
8	0.662	17.01	3.229	0.691	6.89	576.4	23.90
18	0.327	108.80	0.887	0.781	-	-		913	263.1
22	0.703	116.00	0.763	0.793	-	-		1243	213.8
23	0.771	114.50	0.776	0.788	-	-		957	211.0
24	0.348	112.50	0.762	0.789	-	-		781	229.7
31	0.837	136.90	0.687	0.794				947	236.9
32	0.893	146.4	0.711	0.778				2938	437.7
55	0.340	159.5	0.597	0.826				4487	573.4

and the fitted EC in Figure 10. For the first 8 days, the appropriate EC was found to be R(Q1(R1(CR2))), where R stands for the electrolyte solution resistance, Q1 and R1 are attributed to the coating, and C and R2 represent the charge transfer at the metal/coating interface (Figure 10A). From the polarization resistance value Rp obtained as a sum of R1 and R2, one can observe that over time, this parameter shows an increasing tendency, suggesting an increasing anti-corrosion resistance. However, the corrosion process cannot be neglected, and the accumulation of the formed species as a result of metal oxidation changes the mechanism. Thus, as a result of fitting to the EIS data for the days that follow after the 8h, the obtained EC is R(Q1(R1(LR3))), where L represents the system’s inductive behavior (Figure 10B). As already mentioned, the inductive loop originates from the coverage relaxation of adsorbed species ZnI, ZnII, and ZnOH formed during the corrosion process [35] and reflects the instability of the system.

The analysis of Bode plots (Figure 11) also reveals that the impedance modulus /Z/_0.01Hz for both Zn/Chit and Zn/Chit_CR_10⁻³ M coat samples is larger after 55 days of dry–wet cycles than in the first immersion day, and the /Z/_0.01Hz value is the highest in the case of Zn/Chit/loaded with CR. This also points to a consolidation of the coating over time.

3.6. Adhesion Measurements

Adhesion plays an important role in governing the performance of the coatings. The dependence of film integrity on adhesion forces is important from the viewpoint of the performance of the coatings. Thus, their durability is also determined by their adhesion to the substrate, since this determines the ease of removal. The adhesion was calculated with the formula:

$$\mathrm{Adhesion} (\%)=100\times \frac{\mathit{Total} \mathit{number} \mathit{of} \mathit{cubes}}{\mathit{Total} \mathit{number} \mathit{of} \mathit{riped} \mathit{cubes} \mathit{by} \mathit{tape} \mathit{peel} \mathit{method}}$$

The results of the peel-off method applied in the case of Zn and glass samples coated with Chit without or with CR led to the results presented in Figure 12 and

Table 5. Results of the adhesion measurements. Total cubes: 49.

Substrate	Coating	Number of Ripped Cubes by the Tape Peel Method	Adhesion Ratio
Zn	Chit	0	~99.99%
Zn	Chit_CR_10−3 M coat	0	~99.99%
Glass	Chit_CR_10−3 M coat	4	~91.84%

.

The adhesion of Chit incorporating CR was better on Zn than on glass substrate. It is confirmed once more that adhesion depends strongly on the physical and chemical properties of the substrate as well as its morphology. The nature and strength of the binding forces between the two materials in contact clearly determine the adhesion of the coatings to the substrate. The ZnO existence on Zn substrates plays a certain positive role in the Chit_CR layer adhesion.

3.7. Wettability Measurements

A more hydrophobic surface is beneficial for the anti-corrosion protection of metals. The hydrophobicity of the coatings increased slightly after CR incorporation in Chitosan, as proven by the increase in contact angle (Figure 13). This result differs from other situations reported in the literature, when a slight decrease in hydrophobicity was noticed after the introduction of indigo carmine molecules in chitosan coatings [16]. However, the differences between these values and those noticed for Chit containing CR are not very large and are probably due to the different water-absorption properties of the involved compounds and to the swelling and/or reorientation of the surface groups of the coatings. Moreover, the different film-forming conditions also had an influence on the surface porosity and, hence, on the wettability of the coatings.

4. Conclusions

After analyzing the results, several conclusions can be drawn:

• Cresol red acts as a corrosion inhibitor of zinc corrosion when used in solution in different concentrations; this effect is smaller when CR is embedded in chitosan due to the blocking of its functional groups during incorporation.
• The preparation method of Chit coating impregnation with CR by immersion in the solution after deposition on Zn led to poorer results than the method in which chitosan was previously mixed with CR before applying the dip-coating technique.
• FT-IR measurements carried out on chitosan powder embedding CR provided evidence that anionic CR interacts with positively charged chitosan. The pH indicator property of CR is affected by its incorporation in the Chit coating. CR becomes less active due to its immobilization in the biopolymer. Consequently, no color change is observed, and CR does not act as a pH indicator.
• The examination of the potentiodynamic polarization curves indicates that corrosion at higher pHs of zinc coated with Chit and CR is slower than in neutral or acidic media, where Chitosan is more vulnerable and can be destroyed. At pH 9, CR decreases the corrosion current density of Zn and of Zn/Chit, and in proper concentrations (10⁻⁴ M–10⁻³ M), it acts as a corrosion inhibitor.
• The chitosan coatings containing cresol red prepared by method (ii) were monitored in dry–wet cycles for 55 days. EIS measurements recorded in 0.2 g/L Na₂SO₄ at pH = 7 show an important increase in the impedance of the coatings occurring from the first until the fifty-fifth day in a row, in dry–wet cycles. This increase is related to the beneficial effect of CR incorporated in Chitosan and could be, at least partially, related to a consolidation of the Chit coating structure in the presence of CR by crosslinking between Chit and CR molecules.
• A change in the corrosion mechanism at pH 9 is observed from the shape of the impedance diagrams.
• The adhesion of Chit incorporating CR on Zn was better than that on glass substrates and reached ~99.99%, suggesting a better affinity of the chitosan coating towards the Zn substrate, probably due to the existence of ZnO on the substrate surface.

Concluding, CR could be used on zinc as a corrosion inhibitor incorporated in chitosan at basic pHs, but without taking advantage of its pH-indicating properties, which are lost due to the interactions occurring between the biopolymer and the dye molecule.