1. Introduction
Mild steel is a widely used material in industry due to its low cost and recyclability. However, its application is often limited as a result of low corrosion resistance [1,2,3]. Deposition or electrodeposition of zinc coating on the surface can protect steel structures from corrosion, since the potential of zinc is less noble than that of iron and can ensure “sacrificial” protection in some environments. Zinc corrodes when exposed to harsh conditions. Dispersing metal oxide particles in the zinc matrix can help to fill up/isolate the active corrosive regions on the coating surface, forming a physical barrier which separates the aggressive species from the metal matrix [4,5,6,7]. In this regard, electrodeposited zinc-based hybrid coatings can provide better anticorrosion performance of low-carbon steel in comparison to the ordinary zinc ones [8,9].
In recent years, the development of eco-friendly systems and green synthesis methods has become a subject of extensive scientific research. ZnO nanoparticles are recognized as safe and reliable materials, which also possess antibacterial properties and find application in the food and biomedical industries [10]. The green synthesis of ZnO nanoparticles could be realized by using plant extracts [11]. However, achieving homogeneous dispersion of zinc oxide particles in the matrix of the protective coating is a challenge due to their tendency to agglomerate in liquid media [12]. One possible way to improve these particles’ dispersability is to stabilize their suspensions against aggregation by adsorption of polymers or encapsulation inside polymer shells [13]. Usage of environmentally friendly polymers will reduce dependence on synthetic substances. Chitosan (CHI) and alginate (ALG) are widely popular pH-responsive polysaccharides for biomedical applications, as they are polymers that are low in toxicity, biodegradable, and biocompatible [14,15,16,17,18,19,20,21]. CHI is a cationic polysaccharide, which is obtained by partial deacetylation of chitin, and ALG is a negatively charged polysaccharide, derived from brown algae. Both polysaccharides are also gaining interest as “green” corrosion inhibitors due to the ability of their molecules to form coordinate covalent bonds with metal surfaces [2]. CHI and ALG can form complexes (physical hydrogels) through electrostatic interaction between the oppositely charged polyelectrolyte chains [22,23,24]. Such hydrogel particles have been used as smart delivery systems in different application fields [20]. For example, CHI/ALG nanoparticles loaded with copper oxide (CuO) have been applied as alternative nanofertilizers [23]. The role of the polysaccharide shell has been to protect the CuO nanoparticles from the environment and also to prevent spontaneous leaching of Cu2+ ions into soil. Other investigations demonstrate that CHI/ALG nanoparticles loaded with CuO have also been used to improve the anticorrosion and antifouling performance of zinc coatings on mild steel in aggressive salt solutions [24].
Once the barrier layer is destroyed, the aggressive corrosive ions will penetrate through defects, causing corrosion and destruction processes to occur deep inside the coating [25,26]. The incorporation of corrosion inhibitors in the barrier layer can prolong the life time of the metal structure, providing an additional (active) layer of protection [27,28,29]. For example, the presence of pH-responsive polymeric nanocontainers with entrapped corrosion inhibitors into the matrix of an ordinary zinc coating can provide an additional “self-healing” effect to the coating. The dispersal of such nanocontainers in zinc coatings is expected to ensure controlled release of the inhibitor in as result of local changes in pH values due to swelling of the nanocontainers and diffusion of the inhibitor molecules across the polymeric shells in the event of a corrosion attack. Caffeine (1,3,7-trimethylxantine) is a naturally occurring nontoxic alkaloid that shows desirable characteristics of a good organic corrosion inhibitor since it has π-electrons and N and O heteroatoms. The CAF molecules have been found to adsorb on mild steel surface through a spontaneous chemisorption, forming protective film with hydrophobic characteristics [30]. The CAF molecules can be loaded in the CHI/ALG nanocontainers due to interaction of their positive charges (pKa of CAF is 10.4 [31]) with negatively charged functional groups of ALG and TPP [32]. The electrostatic interaction between CHI and ALG chains, on the other hand, provides an effective barrier against the spontaneous release of CAF [33,34].
Ionotropic gelation is a frequently used technique for the encapsulation of various substances, based on electrostatic interaction (complexation) between two oppositely charged species that can produce nanoparticles [35]. In the case of CHI and ALG, electrostatic interaction between positively charged amine (–NH3+) groups of CHI and negatively charged carboxylic (–COO−) groups of ALG leads to formation of CHI/ALG nanoparticles. Both polysaccharides can also interact via hydrogen bonding [23]. The interactions between CHI and ALG (in the presence of TPP as a crosslinking agent) are strong enough to form stable polyelectrolyte complexes at pH value in the range from 4 to 6 [22]. Electrostatic interactions and the formation of hydrogen bonds between the metal oxide surface and the functional amine and hydroxyl groups of CHI permit the entrapment of oxide particles into the CHI/ALG complexes [36]. Given that CAF is positively charged at pH 5, the interaction of its molecules with the negative charges of ALG and TPP can provide entrapment of CAF in the ZnO/CHI/ALG nanocontainers.
The objective of the present research is to propose the preparation procedure of environmentally friendly CHI/ALG nanocontainers with entrapped ZnO nanoparticles and CAF as a corrosion inhibitor, which would be suitable for incorporation in protective zinc coatings on steel substrates. The aim is to combine the barrier effect of ZnO nanoparticles and the “self-healing” effect of CAF in order to prolong the life time of galvanized steel in aggressive Cl− ion-containing solution. Swelling of the polysaccharide parts of the CHI/ALG nanocontainers due to changes in pH value during the corrosion process will trigger the delivery of CAF and ZnO at the damaged sites to repair defects that appear in the coating. The dissolution of zinc in the corrosive medium can provide additional inhibiting effects due to the adsorption of Zn2+–CAF complexes on the steel–coating surface [37].
2. Materials and Methods
2.1. Materials
Chitosan (CHI, Mw~50–190 kDa, pKa~6.5) with a deacetylation degree of 75–85%, sodium salt of alginic acid (ALG, Mw~30–100 kDa, pKa~3.5), sodium tripolyphosphate (TPP), acetic acid, caffeine (CAF, Mw = 194.2 Da), and zinc oxide nanopowder (ZnO < 100 nm particle size) were purchased from Sigma Aldrich (Darmstadt, Germany). According to TEM analysis, about 90% of the ZnO nanoparticles have an average size of less than 100 nm [38].
2.2. Preparation of CHI/ALG Nanocontainers Loaded with ZnO Nanoparticles and CAF
The entrapment of ZnO nanoparticles in CHI/ALG complexes is performed using previously described procedure with a slight modification [23,24]. The first step is to dissolve CHI (1 g/L) in 100 mL acetic acid solution (0.03 M) by magnetic stirring for 24 h. The second step is to disperse ZnO nanoparticles (0.1 g/L) in the CHI solution under ultrasound treatment in an ice bath for 15 min. Finally, 20 mL of aqueous ALG solution (0.45 g/L) and 20 mL aqueous solution (1 g/L) of sodium TPP (as a crosslinking agent) are mixed and added to the CHI suspension with ZnO nanoparticles under magnetic stirring for 30 min. Then, the obtained suspension is homogenized via 1 min sonication. The whole procedure is carried out at pH 5 (adjusted with NaOH) in order to assure optimal conditions for spontaneous ionic gelation of CHI with TPP and complexation with ALG [23]. To load the nanocontainers with CAF, its solution (1 g/L) is mixed with ALG-TPP before addition to the CHI suspension containing ZnO nanoparticles (Scheme 1).
2.3. Characterization of CHI/ALG Nanocontainers with ZnO and CAF
CHI/ALG nanocontainers unloaded (ZnO/CHI/ALG) and loaded (ZnO/CHI/ALG-CAF) with CAF were characterized in terms of their size (hydrodynamic diameters) and surface charge density (zeta potentials). Both parameters were measured by dynamic light scattering (DLS) and laser Doppler velocimetry, respectively (Zetasizer Pro Red Label, Malvern Panalytical Ltd., Malvern, UK). The distribution of hydrodynamic diameters and zeta potentials of the nanocontainers were obtained in a 4-times-diluted initial suspension (with concentration of ZnO nanoparticles of 0.1 g/L, i.e., the concentration is 0.025 g/L) with water or zinc sulfate electrolytes used for electrodeposition of the hybrid coatings to ensure reliable DLS and zeta potential results. As a light source, a He-Ne laser was applied, and the intensity of the backscattered light was measured at 173°.
Scanning electron microscope (SEM) (INCA Energy 350 unit, Oxford Instruments, Oxford, UK), equipped with an energy-dispersive X-ray spectroscope (EDX, Oxford Instruments, Oxford, UK), was used for analysis of the surface morphology of the electrodeposited zinc coatings before and after treatment in the corrosion medium (3.5% NaCl solution).
2.4. Electrodeposition of Hybrid and Ordinary Zinc Coatings
The newly developed hybrid zinc coatings with ZnO/CHI/ALG nanocontainers, both unloaded and loaded with CAF, were electrodeposited on mild steel samples/plates with a working area of 4 cm3 (2 cm × 1 cm × 0.1 cm) from sulfate solution (pH 4.5–5.0) containing 150 g/L ZnSO4.7H2O, 30 g/L NH4Cl, and 30 g/L H3BO3 (chemicals were purchased from Valerus Ltd., Sofia, Bulgaria), as well as two additives (authors’ own products elaborated in the Institute of Physical Chemistry)—AZ1 as a wetting agent (50 mL/L) and AZ2 as a brightener (10 mL/L). The composition of the mild steel substrate was as follows (wt.%): C–0.05–0.12; S ≤ 0.04; P ≤ 0.35; Mn—0.25–0.5; Cr ≤ 0.1; Si ≤ 0.03; Ni ≤ 0.3; Cu ≤ 0.3; As ≤ 0.08; Fe balance. Electrodeposition was carried out with ultrasonic agitation (in ultrasonic bath with a voltage of 220–240 V, power of 580 W, and frequency of 50/60 Hz) in a glass cell containing 600 mL electrolyte volume at ambient temperature and a cathodic current (DC current) density of 2 A/dm2 achieved using soluble zinc anodes. The concentration of both types of nanocontainers (unloaded or loaded with CAF) in the starting zinc electrolyte used to obtain the hybrid coatings was 0.025 g/L. The thickness of all coating types obtained was ~12 μm. After taking out the coatings from the electrolyte, they were dried in air.
The two types of hybrid coating obtained are designated as follows:
Hybrid zinc coating with incorporated ZnO/CHI/ALG nanocontainers (ZnO);
Hybrid zinc coating with incorporated ZnO/CHI/ALG-CAF nanocontainers loaded with CAF (ZnO CAF).
2.5. Corrosion and Electrochemical Characterization
The protective ability of the investigated coatings was evaluated using potentiodynamic (PDP) polarization curves and polarization resistance (Rp) measurements. PDP is a well-known and widely applied accelerated method. The Rp measurements were taken over a prolonged time interval aiming to receive more reliable information. The reference electrode was a Saturated Calomel Electrode (SCE) (Hanna Instruments, Woonsocket, RI, USA) and a platinum plate was used as the counter electrode. The state of the tested sample during the PDP test (scan rate of 1 mV/sec.) was checked visually by the “naked eye”, and after the appearance of bare steel substrate, the anodic polarization was stopped.
The Rp measurements were taken at certain time intervals and have a duration of 40 days. The experimental data obtained between the individual samples show an approximate error of ±10%.
Electrochemical processes like cathodic electrodeposition and anodic dissolution were investigated via cyclic voltammetry (CVA) studies. These were conducted in the potential interval between −1.4 V and 0 V, with a scan rate of 10 mV/sec in the starting electrolytes for the electrodeposition of ordinary or hybrid zinc coatings.
The electrochemical and corrosion tests were realized with a PAR “VersaStat 4” device.
2.6. Water Contact Angle Measurements
An Easy Drop DSA20E apparatus (Krűss GmbH, Hamburg, Germany) was applied to measure the water contact angle (WCA) of the hybrid coatings at ambient temperature. A computer dosing system creates a sessile droplet of distilled water (10 μL) which is deposited onto the sample surface. Computer analysis was used to calculate the WCA using DSA1 software (v 1.92–05). The analyzed data were averaged from 5 measurements for each sample.
2.7. Test Media and Reproducibility
For the realization of the corrosion experiments, a model medium of 3.5% NaCl solution (pH~6.7) was used. Electrochemical CVA tests were carried out in the starting electrolytes to obtain results on the hybrid coatings and of ordinary zinc. The presented results are averages from the data received from five samples per type, i.e., either ordinary zinc or hybrid zinc coatings.
3. Results
3.1. Characterization of Size Distribution and Charge Density of CHI/ALG Nanocontainers
Figure 1 presents the results from the size distribution and zeta potential measurements for suspensions of ZnO/CHI/ALG and ZnO/CHI/ALG-CAF nanocontainers, respectively. As it can be seen, the nanocontainers have positive zeta potential values. This is a reasonable effect from the excess CHI in both types of CHI/ALG nanocontainers. The distribution of the hydrodynamic diameters of the nanocontainers is also presented in Figure 1. The standard deviations (shown in Table 1) reflect a certain instability of the suspensions. Two families of nanoparticles can be registered. The larger-sized particles can be associated with aggregates from CHI/ALG nanocontainers (hydrodynamic diameters of around 300 nm), while the smaller ones can be associated with non-aggregated structures. The results are compatible with hydrodynamic diameters obtained for CHI/ALG nanocontainers with entrapped CuO nanoparticles, which have been prepared using the same procedure at similar conditions (pH 5.2 and a CuO concentration of 0.1 g/L) [23,24]. The loading of CAF does not considerably change the size and zeta potential values of the nanocontainers (Table 1), although a high encapsulation efficiency for CAF is measured in the nanocontainers (see below). One possible explanation for this might be that the entrapment of CAF leads to the formation of more compact structures due to additional interactions between the components of such a complex system.
It is known that a high ionic strength of the plating bath leads to particle aggregation because of the decreased electrostatic repulsion between the particles, which is an obvious obstacle for the incorporation of well-dispersed particles into the metal matrix during electrodeposition of the hybrid coating on the metal surface [39]. The results from the hydrodynamic diameter and zeta potential measurements performed in the starting zinc solution are presented in Table 1. The average sizes of the nanocontainers are shown to increase and their zeta potentials decrease due to the high ionic strength of the zinc electrolyte. Interestingly, the hydrodynamic diameters of both types of nanocontainers increase only slightly (from ~325 nm to ~350 nm), notwithstanding that zeta potentials decrease from +37 mV to +6 mV. This is consistent with earlier results reported by Huang et al. [40], who reported high colloidal stability of suspension from CHI-TPP nanoparticles in the presence of 1.5 M NaCl. Due to the electrolyte screening of the electrostatic repulsion between likely charged chains inside a hydrogel, the addition of salt can cause de-swelling of the particles and a decrease in their dimensions [41]. The results also indicate that the nanocontainers remain positively charged, which means that they will move to the cathode via electrophoresis during the electrodeposition process of zinc on the steel surface.
3.2. Encapsulation Efficiency and Release of CAF from CHI-ALG Nanocontainers
One objective of this investigation is to evaluate the entrapment efficiency of CAF in the ZnO/CHI/ALG-CAF nanocontainers. The experiment was carried out at pH 5 with the solution used to prepare the nanocontainers loaded with CAF. The concentration of CAF is measured in the supernatant obtained after centrifugation of the suspension at 15,000 rpm for 1 h at 4 °C, using UV-vis spectroscopy (Figure 2—Error of ~5%, i.e., ±1–2 mg/L). The supernatant was diluted 5 times before measuring to improve the quality of the results. The calibration curve for CAF was obtained at λ = 275 nm (inset in Figure 2). The encapsulation efficiency is evaluated by comparing the amounts of CAF in the supernatant and the initial amount of CAF used for the preparation of the ZnO/CHI/ALG-CAF nanocontainers. This concentration is 164 mg/L, which corresponds to an 83.6% encapsulation efficiency (the initial CAF concentration is 1 g/L).
The corrosion of galvanized steel has been found to decrease the pH value in the region close to the zinc layer due to metal hydrolysis [42]. A local decrease in pH near the corrosion defects in the hybrid zinc coating is expected to facilitate CAF’s release from the pH-responsive polysaccharide nanocontainers. Therefore, the amount of CAF released is determined under neutral pH conditions (pH ~ 7, adjusted using NaOH) and under slightly acidic conditions (pH 4 and pH 5, adjusted with acetic acid). The latter conditions are found to be more suitable for CAF to provide good corrosion inhibition for mild steel [43].
The released amounts of CAF were measured after 24 h and 48 h in the model corrosion medium containing 3.5% NaCl at the above-mentioned pH values. In this experiment, after centrifugation of the initial suspension for 1 h (at 15,000 rpm and 4 °C), the first obtained supernatant was discarded and replaced by 3.5% NaCl solution. Every sample was sonicated for 1 min, and the amount of CAF released was measured after 24 h in the supernatant, obtained through centrifugation of the suspension under the above conditions. This procedure was repeated, and cumulative amounts of the released CAF were calculated (Figure 2). The results show a slight increase in the amounts of CAF released at pH 4 and pH 7, compared to pH 5, which was used during the preparation of the nanocontainers. According to the results presented in [22], at pH > 6.5, most amine groups of CHI are uncharged and most carboxylic groups of ALG are ionized. Thus, the repulsion between the carboxylic groups leads to an increase in the swelling of CHI/ALG complexes (hydrogel nanoparticles). Decreasing the pH below 5.5 also causes swelling due to the increased repulsion between the ionized amine groups of CHI. Taking into account that excess CHI is present in our nanocontainers, it is reasonable to obtain larger amounts of released CAF from the ZnO/CHI/ALG-CAF nanocontainers at pH 4 compared to pH 5 and pH 7, respectively.
The results presented in Figure 2 indicate that relatively small amounts of CAF are released from the ZnO/CHI/ALG-CAF nanocontainers during the first 48 h in the model corrosion medium. Very possibly, when immersed in the 3.5% NaCl solution, the nanocontainers (hydrogel particles) are dehydrated and their shrinkage leads to a closer contact between entangled polysaccharide chains [44]. The interaction between polymeric molecules increases, and they form more compact structures that block the release of free CAF. However, the obtained amounts of released CAF might still impede the corrosion processes in the hybrid zinc coating to a great degree under neutral and acidic conditions. For example, Rajam et al. [35] have shown a 48% inhibition efficiency of CAF for the protection of carbon steel in well water (pH 8.5, after 3 days’ immersion) in the presence of Zn2+ ions—50 mg/L CAF and 50 mg/L Zn2+.
3.3. Surface Morphology of Hybrid Zinc Coatings
Figure 3 presents SEM images of hybrid zinc coatings obtained via the electrodeposition of zinc and ZnO/CHI/ALG nanocontainers before and after immersion in the model corrosive medium. The surface of the non-treated coating consists of large irregular (close to spherical) agglomerates and relatively homogeneous (more compact) structures among smaller particles (Figure 3A). At higher magnification (inset in Figure 3A), one can see that most of the small particles have a size close to that measured by DLS in the zinc sulfate electrolyte before electrodeposition on the steel substrate. The surface morphology of this coating after 40 days’ immersion in a neutral corrosion medium (3.5% NaCl solution) is shown in Figure 3B—a flatter surface can be seen compared with the non-treated coating. However, some well-expressed defects (deep holes) also appear as a result of the corrosion attack. At higher magnification (inset in Figure 3B), the surface of the treated coating is quite different (more homogeneous) from the non-treated one. Given our previous investigations on this medium, the reason for this observation is the presence of a layer of the corrosion product zinc hydroxide chloride (ZHC) [45].
The surface morphology of the hybrid zinc coating with ZnO/CHI/ALG-CAF nanocontainers is demonstrated in Figure 4(A.1). The image is more homogeneous, although some defects (spots of large nearly spherical particles and cracks) are also observed as in the above coating. At higher magnification (Inset in Figure 4(A.1)) the homogeneous part of the surface consists of smaller agglomerates compared to the coating not containing CAF. The smaller roughness of this coating can be attributed to the direct adsorption on the steel surface of CAF molecules, released from the nanocontainers during electrodeposition of the hybrid zinc coating. According to de Souza et al., CAF molecules can adsorb on mild steel surface through a spontaneous process forming a protective film with hydrophobic characteristics [30].
Figure 4(B.1) shows surface morphology of the corrosion-treated hybrid coating with ZnO/CHI/ALG-CAF nanocontainers. This micrograph also demonstrates a more uniform coating compared to that of the treated samples without CAF, although some cracks also appear after immersion in the corrosion medium. At higher magnification, small defects can be observed on the flat coating surface.
The morphological characteristics of the above-presented hybrid coatings differ considerably from those of the coating obtained by co-electrodeposition of CHI/ALG nanocontainers with entrapped CuO nanoparticles and zinc under similar conditions without ultrasound agitation [24]. The surface of this coating was homogeneous, containing a lower proportion of smaller particles below the detection limit of the equipment used for EDS analysis. The result can be explained by the sedimentation of the larger agglomerates in the electrolyte solution, which prevented their incorporation into the matrix of the zinc coating. Obviously, the ultrasound-assisted electrodeposition of the present ZnO/CHI/ALG nanocontainers (with or without CAF) causes the formation of heterogeneous surfaces of the hybrid zinc coatings. In previous studies, such influence of the ultrasonic agitation has been attributed to cavitation bubbles, which cause the formation of strong “microjets” of electrolyte-containing particles and the development of a heterogeneous surface of the coating [46,47].
The usage of ultrasound treatment during electrodeposition of the present hybrid coatings promotes the formation of surfaces with higher particle contents. Precise evaluation of the number of ZnO nanoparticles entrapped in the matrix of the present zinc coatings is not possible, since zinc also belongs to the matrix. However, the EDS analysis shows an increased amount of C, N, and O atoms in the CAF-containing coating compared to the one without CAF (Figure 4(A.2), Table 2), which can be regarded as indirect evidence for the nanocontainers’ (particles’) deposition on the coating surface. EDX measurements were also performed to evaluate the coatings’ composition after 40 days’ immersion in a 3.5% NaCl solution (Table 2). The results presented in Table 2 show a slight decrease in the amount of C and N and a significant increase in O compared to the non-treated coating with CAF, which is related to the metal’s oxidation during the corrosion process (Figure 4(B.2), Table 2).
3.4. Cyclic Voltammetry Measurements
The results of CVA investigations of ordinary zinc as well as of ZnO/CHI/ALG and ZnO/CHI/ALG-CAF nanocontainers are demonstrated in Figure 5. These tests are performed in the starting electrolytes which are being applied for the electrodeposition of the coatings. It is observed that the electrodeposition of ordinary zinc occurs at a much higher cathodic current (−0.05 A) compared to both hybrids. The cathodic currents of the hybrids are practically equal (about −0.01 A). It can be calculated that the cathodic electrodeposition peak of zinc is about five times higher.
The electrodeposition of ZnO/CHI/ALG nanocontainers begins also at more positive potential value (−1.15 V) compared to ZnO/CHI/ALG-CAF (−1.22 V) and ordinary zinc (~−1.20 V). This means that the process proceeds more easily, i.e., the electrodeposition of this coating is depolarized. Contrary to this, the cathodic electrodeposition of ZnO/CHI/ALG-CAF nanocontainers is overpolarized and occurs at more negative cathodic potentials, which means that it occurs with greater difficulty. As observed from Figure 5, the cathodic process of the ordinary zinc coating is placed between the CVA curves of both hybrid coatings.
The anodic area of the CVA tests presents the course of the dissolution process of the investigated samples. It is well known that the presence of anodic parts confirms the existence of a material that has been electrodeposited during the cathodic polarization (i.e., this is proof that more than just the hydrogen evolution process occurs). It is obvious that the zinc dissolution peak is the greatest one (~0.07 A at −0.73 V), which indicates the presence of a greater electrodeposited mass. The anodic peaks of both hybrid coatings are lower, i.e., ~0.02 A for the coating with ZnO/CHI/ALG and ~0.01A for the coating with ZnO/CHI/ALG-CAF nanocontainers. The inset in Figure 5 demonstrates these peculiarities in more detail.
As already commented [45], these results can be explained keeping in mind that the ZnO/CHI/ALG nanocontainers incorporated in the zinc matrix are larger in size compared to the zinc ions and seem to physically block (“barrier action”) the steel surface during the electrodeposition of the zinc, impeding in this way the appearance of the hybrid coatings.
3.5. Potentiodynamic Polarization Curves
The potential polarization method makes it possible to investigate the corrosive and electrochemical behaviors of different samples under conditions of external polarization. Figure 6 presents the experimental results of both hybrid coatings—ZnO/CHI/ALG and ZnO/CHI/ALG-CAF—as well as of the ordinary zinc coating in the test medium of 3.5% NaCl. As can be seen, the corrosion potential of the ordinary zinc coating is situated at more negative values compared to both hybrid coatings, i.e., 1.07 V (see Table 3 below). The corrosion potentials of both hybrid coatings are practically equal (−1.04 V). All the tested coatings differ in their corrosion current values. For example, the Icorr of the ordinary zinc coating is the highest—1.60 × 10 −5 A·cm−2. The same parameter for the ZnO/CHI/ALG coating is 1.25 × 10−5 A·cm−2, and for the ZnO/CHI/ALG-CAF coating, it is 1.20 × 10−5 A·cm−2. These results (summarized in Table 3) demonstrate the positive influence of the incorporated nanocontainers on the corrosion parameters, where the presence of the inhibitor leads to better results.
The anodic curve of the ordinary zinc gradually increases after the corrosion potential area, with a maximal value of about −0.8 V. Zinc fully dissolves at more positive potentials of about −0.6 V, leaving the low-carbon-steel surface practically unprotected. The newly developed hybrid coatings show better expressed protective behavior; i.e., their anodic curves are longer (up to −0.45 V). In addition, the same curves demonstrate lower anodic current density, which means that the dissolution process is more difficult.
3.6. Polarization Resistance Measurements
The newly developed coatings were immersed in the model test medium for a prolonged period of 40 days in order to evaluate their corrosion resistance and protective ability (Figure 7). It is obvious that the ordinary zinc coating has the lowest polarization resistance value during the whole test period—its Rp is about 2–2.5 times lower than those of the hybrid zinc coatings, i.e., about 750–850 ohm·cm2. The maximal value for this coating was registered on the 25th day of immersion.
The ZnO/CHI/ALG-CAF sample presents the highest Rp during the whole period, except at the beginning. Its maximal value is observed on the 15th day, and at the end of the test, it is ~1250 ohm.cm2. The hybrid zinc coating not containing CAF has similar behavior, showing slightly lower Rp values. The obtained experimental results confirm the positive effect of the incorporated nanocontainers (with or without inhibitor) in the metal zinc matrix.
3.7. Water Contact Angle Studies
The information from water contact angle measurements concerning the relationship between hydrophobicity and corrosion resistance of the samples is somewhat contradictory. It is known that this characteristic is determined from the intrinsic chemical propeties as well as from the surface microstructure and topgraphy. However, most researchers accept the thesis that higher hydrophobicity is more favorable from a corrosion point of view.
Water contact angle data for both hybrid samples are shown in Figure 8. The obtained results demonstrate that both surfaces are hydrophobic since the contact angle values are greater than 90o. However, the surface of the ZnO/CHI/ALG-CAF sample is more hydrophobic, which can be regarded as a sign for its better corrosion resistance and protective ability in the model test medium.
4. Discussion
Zinc coatings can provide protection to steel through “sacrificial” or “barrier” mechanisms. These coatings have a higher electrochemical potential (their standard potential is more negative) than the underlying steel, allowing them to slow down (or prevent) corrosion. On the other hand, the dissolution of zinc in neutral corrosive media leads to the appearance of a protective “barrier” layer of low-solubility corrosion products—ZnO, Zn(OH)2, Zn5(OH)8Cl2.H2O, etc.—which separates the steel substrate from the corrosive environment.
Dispersing ZnO oxide nanoparticles in the zinc matrix can block some active regions (defects) in the coating, thus forming an additional protective barrier against the corrosive agents from the aggressive medium. Hybrid zinc coatings combine inorganic and organic components to achieve synergistic corrosion protection. Integration within of these coatings with the corrosion inhibitor CAF is also expected to ensure their “self-healing” capacity.
We found that the incorporation of ZnO/CHI/ALG nanocontainers in the matrix of ordinary zinc coatings positively influences their anticorrosion behavior in the presence of chloride ions. Given the composition of these hybrid coatings, we can suggest that “sacrificial protection”, “barrier”, and “self-healing” effects are possible mechanisms by which these coatings provide corrosion protection to mild steel. In particular, the “self-healing” process is possible through adsorption of the ZnO/CHI/ALG nanocontainers onto the steel surface (due to the presence of N and O heteroatoms in the CHI molecules, which are in excess in the nanocontainers). When the corrosion begins, the adsorbed nanocontainers will interact with Fe2+ ions to form a protective layer on the steel. CHI chains in the nanocontainers can also form complexes with Zn2+ ions, thus enhancing the amount of the deposited zinc during electrodeposition of the hybrid coatings.
The Zn2+ ions can form complexes with CAF as well. According to [35], the adsorption of Zn2+–CAF complexes onto the steel surface may convert them into protective Fe2+–CAF complexes, while the released Zn2+ ions will combine with OH− ions to form a protective product, Zn(OH)2. With the increase in the immersion time, more corrosion inhibitor is released and adsorbed onto the steel surface, and it could be expected that the “self-healing” process will begin to dominate. Finally, since both types of ZnO/CHI/ALG nanocontainers (with and without CAF) positively affect the corrosion protection of steel, we can speculate that the hybrid zinc coatings consist of iron complexes with CHI and/or CAF, mixed with the above-mentioned corrosion products with low solubility values.
To evaluate the inhibition efficiency (IE%) of CAF in controlling the corrosion of mild steel in a Cl− ion-containing medium, we compare results from polarization resistance measurements applying the following equation:
IE (%) = (1 − R0p / Rp) × 100(1)
where R0p and Rp are the values for the polarization resistance of the samples with hybrid coatings obtained in the absence and in the presence of CAF, respectively (see Figure 7).The inhibition efficiency increases by about 25% after 30 days’ immersion in the model corrosion medium for the sample with incorporated ZnO/CHI/ALG-CAF nanocontainers compared to the sample not containing CAF. When comparing the results with ordinary zinc coating, however, the IE% values increase to ~30% and ~47%, respectively, for the samples with hybrid coatings in the absence and in the presence of CAF. This means that the hybrid coatings presented above might impede to a great extent the corrosion of mild steel under neutral conditions compared to ordinary zinc coating.
5. Conclusions
An environmentally friendly approach for the preparation of pH-responsive nanocontainers of the corrosion inhibitor CAF, based on CHI/ALG complexes with entrapped ZnO nanoparticles, was applied. The nanocontainers showed high encapsulation efficiency for CAF at pH 5 in water solutions (~85%) and a relatively small CAF release in a neutral corrosion medium containing 3.5% NaCl.
SEM analysis revealed that incorporation of nanocontainers with CAF into the matrix of the ordinary zinc coating led to the formation of a more homogeneous surface compared to the coating without CAF, which was attributed to direct adsorption of the CAF released during the electrodeposition of this coating onto the steel surface.
Incorporation of pH-responsive polysaccharide nanocontainers with entrapped ZnO nanoparticles and corrosion inhibitor CAF into the matrix of ordinary zinc coatings was found to show better corrosion resistance and protective ability compared to pure zinc coating. These novel hybrid coatings seem to be suitable candidates for improving the long-term anticorrosion performance of mild steel in a neutral corrosive medium, combining positive effects such as sacrificial protection, barrier formation, and self-healing properties.
Conceptualization, T.R. and N.B. (Nikolai Boshkov); methodology, K.K. and N.B. (Nelly Boshkova); validation, N.B. (Nikolai Boshkov) and T.R.; formal analysis, K.K.; investigation, N.B. (Nelly Boshkova) and K.K.; resources, N.B. (Nikolai Boshkov); data curation, N.B. (Nikolai Boshkov); writing—original draft preparation, T.R.; writing—review and editing, T.R. and N.B. (Nikolai Boshkov); supervision, N.B. (Nikolai Boshkov); project administration, N.B. (Nikolai Boshkov). All authors have read and agreed to the published version of the manuscript.
The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding author.
Research equipment of the Distributed Research Infrastructure INFRAMAT, part of the Bulgarian National Roadmap for Research Infrastructures, supported by the Bulgarian Ministry of Education and Science, was used for some investigations in the present study.
The authors declare no conflicts of interest.
Footnotes
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Enlarge this image.Scheme 1. Preparation of the ZnO/CHI/ALG nanocontainers loaded with corrosion inhibitor CAF, followed by electrodeposition of hybrid zinc coating on steel substrate.
Enlarge this image.Figure 1. Hydrodynamic diameter (d) distributions and zeta potential values for the CHI/ALG nanocontainers with ZnO nanoparticles unloaded (1) and loaded (2) with CAF.
Enlarge this image.Figure 2. Cumulative release of CAF from ZnO/CHI/ALG-CAF nanocontainers in 3.5% NaCl solutions at pH 7, pH 5, and pH 4. Inset: Calibration curve for CAF. Error: ~5% (±1–2 mg/L).
Enlarge this image.Figure 3. SEM micrographs of hybrid zinc coatings containing ZnO/CHI/ALG nanocontainers unloaded with CAF before (A) and after (B) 40 days’ immersion in 3.5% NaCl solution.
Enlarge this image.Figure 4. SEM micrographs and EDS spectra for hybrid zinc coatings with ZnO/CHI/ALG nanocontainers loaded with CAF before (A.1,A.2) and after (B.1,B.2) 40 days’ immersion in 3.5% NaCl solution.
Enlarge this image.Figure 5. CVA investigations of ordinary zinc (Zn) and of both types of hybrid coatings with nanocontainers unloaded (ZnO) and loaded with CAF (ZnO CAF).
Enlarge this image.Figure 6. PD polarization curves of the coatings in the test medium of 3.5% NaCl solution. Reference electrode–saturated calomel electrode.
Enlarge this image.Figure 7. Polarization resistance measurements of the zinc (1) and both hybrid coatings without CAF (2) and with CAF (3) in a 3.5% NaCl solution. Error of ±10%.
Hydrodynamic diameters (d1, d2, and d3), indices of polydispersity (PI), and zeta potentials (ζ) of CHI/ALG nanocontainers in water and starting zinc solutions.
| Parameter | ZnO/CHI/ALG | ZnO/CHI/ALG-CAF | ZnO/CHI/ALG | ZnO/CHI/ALG-CAF |
|---|---|---|---|---|
| ζ | +38.9 ± 0.9 mV | +36.9 ± 1.0 mV | +6.4 ± 7.9 mV | +6.0 ± 2.8 mV |
| d1 | 307 ± 17 nm | 324 ± 16 nm | 349 ± 8 nm | 353 ± 15 nm |
| d2 | 127 ± 20 nm | 88 ± 13 nm | 13 ± 1.8 nm | 15 ± 2.1 nm |
| d3 | - | - | 76 ± 10 nm | 120 ± 18 nm |
| PI | 0.25 | 0.31 | 0.78 | 0.68 |
Chemical composition (EDS measurements) of hybrid zinc coatings (wt%) with and without CAF before and after treatment in 3.5% NaCl solution—error of ±10%.
| Elements | ZnO/CHI/ALG | ZnO/CHI/ALG | ZnO/CHI/ALG-CAF | ZnO/CHI/ALG-CAF |
|---|---|---|---|---|
| C | 13.0 | 6.6 | 15.5 | 11.1 |
| N | 0.0 | 0.0 | 0.6 | 0.2 |
| O | 2.5 | 22.5 | 5.6 | 20.6 |
| Na | 7.6 | 10.1 | 4.7 | 8.9 |
| Cl | 0.0 | 8.0 | 0.0 | 4.4 |
| Fe | 0.5 | 1.5 | 0.9 | 2.1 |
| Zn | 76.4 | 51.3 | 72.7 | 52.7 |
| Total | 100.0 | 100.0 | 100.0 | 100.0 |
Electrochemical parameters obtained from
| Sample | Icorr, A·cm−2 | Ecorr, V |
|---|---|---|
| Zn | 1.60 × 10−5 | −1.07 |
| ZnO/CHI/ALG | 1.25 × 10−5 | −1.04 |
| ZnO/CHI/ALG-CAF | 1.20 × 10−5 | −1.04 |
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; Boshkova, Nelly; Radeva, Tsetska; Boshkov, Nikolai