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Multifunctional Coatings Based on Sn, Ni and TiO2 Aggregates with Enhanced Anticorrosion and Antibacterial Properties

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03 September 2024

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04 September 2024

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Abstract
Multifunctional coatings based on Sn-Ni materials with and without titanium oxide nanoparticles (TiO2NPs) incorporation were prepared using the electrochemical deposition technique at 70°C. TiO2NPs were dispersed in the electrolyte bath, and their influence on both surface texture, crystalline phase, and properties was investigated. Various techniques such as X-ray phase analysis (XRD), scanning electron microscopy (SEM), and energy dispersive X-ray microanalysis (EDX) were used to characterize the prepared coatings. The anti-corrosion effectiveness of the coatings was assessed using potentiodynamic polarization curves and electrochemical impedance spectroscopy in an artificial sweat medium, while the bactericidal activity of the composite coatings (the ability to induce cell death) was evaluated in accordance with ISO 27447:2009 test. The influence of the TiO2NPs on the composition, structure and properties of the deposited coatings was established. Particular attention was paid to the relationship between the anticorrosive and bactericidal properties of the coatings and their structure and composition. The synergistic effect of the various properties was investigated and discussed.
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Subject: Chemistry and Materials Science  -   Surfaces, Coatings and Films

1. Introduction

In recent years, there has been a growing demand for multifunctional coatings that combine physical, biological, chemical and mechanical properties, such as increased hardness, decorative appearance, corrosion resistance, hydrophobicity and antibacterial properties. Indeed, the changing conditions in which coatings are used require them to be multifunctional in order to meet the challenges and demands of their environment. Various approaches have been developed to produce coatings with different properties, making them multifunctional. Among these approaches, those that combine different chemical elements and controlled nanostructuring of the surface have proven to be very effective in producing multifunctional coatings. Alloys produced by electrochemical deposition have demonstrated their ability to exhibit multiple properties, which can lead to the multifunctional nature of certain coatings and their widespread use in industry.
Metallic coatings have evolved considerably in recent times, from single layer coatings to multi-layer gradient and micro-stack coatings, and from alloy coatings to composite coatings. The most common alloys used in coatings are nickel-based, specifically nickel-tin, which offer a wide range of properties such as corrosion, wear and abrasion resistance. To date, protective metals and alloys such as Zn [1], Ni-based alloys [2], Al-based alloys [3], Ti [4], Hf [5], Cr [6] and Cu [7] have been investigated for various applications, particularly for metals that are prone to corrosion.
The blending of chemical elements in the composition of an alloy is a proven approach to create synergies between the individual properties of each component, which can lead to the enhancement of existing properties or the emergence of new ones [8,9,10]. The study of alloys of different metals and their properties is one of the main thrusts of the materials field, as is their shaping in the form of coatings.
Coatings with anticorrosive, superhydrophobic and antibacterial properties [11,12,13,14] are currently in demand for certain surfaces that require constant disinfection, such as door handles, handrails, lift buttons, etc. Cu-based electrochemical coatings are well known in the literature for their antibacterial properties and decorative appearance [15,16,17,18]. TiO2 is another material widely reported in the literature for its photocatalytic and antibacterial properties in metal coatings [19,20,21]. The synthesis method and preparation conditions often determine many of the characteristics and properties of TiO2 nanoparticles, such as size, morphology, crystalline phase, degree of agglomeration, etc. [22,23,24,25]. TiO2 has been used as an inert phase in composite coatings based on Cu [15,16], Zn [26,27], Ni [28,29,30,31,32] and alloys [33,34,35,36,37] to impart photocatalytic and antibacterial properties. The influence of ultrasonic treatment on the electrodeposition of Cu-Sn-TiO2NPs nanocomposite coatings with improved antibacterial properties has been studied in the literature [15]. The effect of ultrasonic treatment and current loading on the microstructure, quantitative and qualitative composition, distribution of TiO2NP aggregates and antibacterial properties against E. coli bacteria was demonstrated. In addition, a comparative study of the effect of mechanical agitation and ultrasound on the properties of the nanocomposite coatings obtained was carried out.
Electrochemically produced Sn-Ni-based alloy coatings are widely used in metal finishing and microelectronics [38,39,40,41,42,43,44,45,46,47,48,49]. They prevent allergic skin reactions and corrosion resistance in artificial environments [43], and can replace nickel electroplating on the skin of clothing fasteners. They can also replace chromium in the decorative and lighting industries [40]. This alloy has a combination of physical and chemical properties that allow it to be widely used in industry. These include increased corrosion resistance, improved hardness, high wear resistance and many other properties [38,39,40,41,42,43,44,45,46,47,48,49]. Various electrolytes have been used to deposit Sn-Ni alloys, including fluoride chloride [34,36], pyrophosphate [39,41], sulphuric acid [42], glyconat [42] and others. Various studies have reported the influence of the kinetic parameters of Sn-Ni alloy deposition on its physicochemical and mechanical properties [40,41,42,43,44,45,46,47,48,49], as well as on composite coatings with TiO2 NPs of different phase compositions [33,34,35,36]. Visible and photoactive Sn-Ni-TiO2NPs electrolyte coatings incorporating carbon-doped particles and N- and S-doped TiO2NPs (N, S-TiO2) have been reported in the literature [34]. It has been reported that the optimum parameters for obtaining the best co-deposition rates of carbon and doped TiO2NPs are a low current density of 1 A/dm2 and a concentration of 30 g/1 under DC conditions. The highest co-deposition percentage of 3.25 wt% was obtained in the case of incorporation of N, S-TiO2NPs. TiO2 NPs synthesized by direct oxidation of titanium have been reported in the literature [35,36] and yielded a mixture of two TiO2 polymorphs, anatase and rutile, with a particle size of 30-50 nm. The effect of the TiO2NPs aggregate content on the physicochemical and biocidal properties of Sn-Ni coatings has been demonstrated [36].
The aim of the present work was to study the kinetics of electrochemical deposition of Sn-Ni-TiO2NPs composite coatings, as well as the effect of the incorporation of TiO2NPs synthesized by the hydrothermal method on their properties. The mechanism of coating formation was also investigated. The corrosion resistance of Sn-Ni-TiO2NPs coatings prepared in an artificial sweat environment and their antibacterial properties were investigated, and the relationship between the properties of the coatings and their structure, composition and roughness was established. This will allow us to evaluate the use of these coatings for public applications such as door handles, turnstiles, lift buttons, etc.

2. Materials and Methods

The Sn-Ni alloy coating was deposited using a fluoride chloride electrolyte, with the ionic strength of the solution compensated by sodium chloride. The compositions of the solutions are given in the Table 1.
Table 1. Electrolyte compositions used for the different coatings studied.
Table 1. Electrolyte compositions used for the different coatings studied.
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Electrolytes were prepared using double distilled water and analytical grade NiCl2·6H2O, SnCl2·2H2O reagents purchased from Belreachim. NH4F, NaCl and HCl compounds were also purchased from Belreachim. After mixing all the electrolyte components, the pH of the electrolyte was adjusted with 0.1 M HCl and 1 g/l titanium dioxide, synthesized according to the procedure described in [25], was added to the electrolyte composition. The electrolysis was carried out at a temperature of 70°C, pH of 3, current density of 1 A/dm2 and with continuous stirring using a magnetic stirrer, and the anode consisted of nickel and tin. To avoid contamination of the electrolyte by sludge, the anode was enclosed in fabric sheaths. Copper was used as the cathode.
The kinetics of coating deposition was studied by chronovoltammetry and chronoamperometry using an Autolab PGSTAT 302N potentiostat and a three-electrode configuration consisting of a saturated silver chloride electrode as reference electrode and a platinum electrode as counter-electrode.
The most commonly used model to describe the electrocrystallization process is that of Scharifker and Hills (SH). It can be used to determine the type of nucleation by analyzing the rising parts of the chronoamperograms before the nuclei diffusion zones overlap. Depending on the slope of these parts of the chronoamperogram, two types of nucleation can be identified: instantaneous and progressive. Instantaneous nucleation corresponds to the situation where all the nucleation sites on the surface are immediately activated by the application of the potential. Furthermore, the number of nuclei remains globally unchanged during the growth process.
In the case of progressive nucleation, the number of nuclei increases progressively with deposition time. Equations (1) and (2) describe progressive and instantaneous nucleation, respectively, in the electroplating process:
    i i m a x 2 = 1.9542 t t m a x 1 − e x p − 1.2564 t t m a x 2 · i n s t a n n e o u s  
                                i i m a x 2 = 1.2254 t t m a x 1 − e x p − 2.3367 t t m a x 2 2 · p r o g r e s s i v e  
Where imax is defined as the maximum current from a current transient at a corresponding time, denoted by tmax.
The phase composition of the coatings was determined by X-ray phase analysis (XRD) using a Bruker D8 Advance AXS diffractometer. X-ray scanning was performed at a rate of one degree per minute using CuKα radiation. The position of the maxima of the analyzed lines was measured to an accuracy of ±0.05°.
The Raman spectra were measured at room temperature on a U1000 Raman microscope-spectrometer (Renishaw, UK). The spectra were excited by laser radiation with a wavelength of 532 nm (power 5 mW on the sample). Registration was carried out with a spectral resolution of 1 cm-1.
The surface morphology of the prepared coatings was studied by scanning electron microscopy (SEM) using a JEOL JSM-5610 LV microscope, while their elemental composition was determined by energy dispersive X-ray (EDX) microanalysis using a JED-2201 EDX.
Corrosion tests were carried out in an artificial sweat medium, the composition of which is shown in Table 2.
Table 2. The composition of artificial sweat electrolyte g/L.
Table 2. The composition of artificial sweat electrolyte g/L.
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Polarization curves were recorded at a sweep rate of 1 mV/s, 20 minutes after the working electrode was immersed in the artificial sweat solution to establish a steady-state potential. Corrosion currents were calculated from the intersection of the straight Tafel sections of the cathode and anode curves. Impedance spectra were recorded in the frequency range 10 kHz to 0.01 Hz with an AC voltage amplitude of 10 mV. The impedance frequency spectra were recorded 20 minutes after the sample had been kept in the test solution. The data obtained were processed using Nova 2.1 and ZView 3.2c software.
Test cultures of the health indicator bacteria Escherichia coli ATCC 8739 and Staphylococcus aureus ATCC 6538 were used to assess the antimicrobial properties of the coatings. The antibacterial properties of the composite coatings were determined according to the standard test method ISO 27447:2009 described in the literature [35]. The evaluation of the bactericidal activity of the composite coatings in terms of their ability to induce cell death was assessed using the reduction factor (RF), which was determined using the following equation (3):
FR = log(K1/K2)
Where, K1, is the concentration of viable cells in the nutrient medium with the coating examined without UV irradiation (CFU/cm3); K2, is the concentration of viable cells in the nutrient medium with the coating examined after UV irradiation (CFU/cm3).

3. Results

The synthesis and formation mechanism of TiO2NPs aggregates have been described in previous works [25]. In the present work, the hydrothermal synthesis temperature was set at 170°C, and at the end of the synthesis, washing and annealing processes, a white powder was obtained. The FEGSEM characterization of the TiO2NPs aggregate powders shows a morphology in the form of porous spherical aggregates of several particles (Figure 1a). The size of the aggregates varies from 50 to 200 nm.
The crystal structure and phase of the prepared TiO2NPs aggregate powder were analyzed by X-ray diffraction. The XRD patterns obtained are shown in Figure 1b and revealed a crystal structure with peaks characteristic of the anatase phase of TiO2NPs aggregates (JCPDS No. 89-4921). This indicates that the powder is crystallized and of high purity.
The synthesized TiO2NPs aggregates were added to the fluoride chloride electrolyte for the deposition of the Sn-Ni alloy, as well as Ni and Sn monolayers, at a concentration of 1 g/L, as described in the experimental section. In order to understand the properties of the Sn-Ni alloy coating, a comparison was made with those of the Sn and Ni coatings. The electrochemical kinetics of the electrodeposition processes for the Sn, Ni and Sn-Ni alloy composite coatings were studied (Figure 2) with and without the addition of TiO2NPs aggregates to the electrolyte. The electrodeposition of Sn from a fluoride chloride electrolyte with ionic strength compensation using NaCl proceeds towards potentials more negative than -0.44 V and more positive than -0.34 V for Ni. For the Sn-Ni alloy, the cathodic polarization curve shifts towards potentials more positive than the start of electrodeposition, which is around -0.28 V, indicating a depolarization effect. The introduction of TiO2NPs aggregates into the electrolyte has virtually no effect on the evolution of the cathodic polarization curves of the coating deposition processes.
To further identify the polarization characteristics accompanying electrochemical electrodeposition reactions, the activation energy for various partial processes has been calculated (Figure 3).
A comparison of the activation energies of Sn and Sn-TiO2NPs shows that in the same potential range between -0.4 and -0.44 V, the activation energies decrease uniformly. At potentials between -0.45 and -0.55 V, the variation of the activation energy shows a very slight maximum, which can be explained by a process limitation due to diffusion processes in the case of Sn and Sn-TiO2NPs deposition, in good agreement with results published in the literature [34]. In the case of the Sn-Ni alloy coatings, a more pronounced activation energy peak is observed at -0.35 V. By comparing the dependence of the activation energy on the deposition potentials of the Sn-Ni alloy and the Sn-Ni-TiO2NPs composite, it can be concluded that the ions are discharged together in the form of intermetallic compounds, similar results have been reported in the literature [34].
The nucleation mechanism of Sn-Ni electrodeposition was studied using the chronoamperometry technique, and the potentiostatic current-time transient curves at different potentials are shown in Figure 4a. The current peak over a short period (-230...-410 mV) is due to the formation and growth of the crystalline nuclei, then the current gradually decreases due to electron transfer and tends to a flat state at the end due to the growth of the alloy nuclei. This shows the typical characteristic of a diffusion-controlled electrochemical nucleation process with three-dimensional nucleus growth and can be characterized by the Cottrell equation, which is expressed in equation (4) as follows [29]:
i = n · F · A · D 1 2 · c 0 π 1 2 · t 1 2
Where, i, is the current through the electrolyte, n is the number of electrons involved, F, is Faraday's constant, A, is the working area of the electrode, D, is the diffusion coefficient, Co, is the concentration of the species in the bulk and, t, is time. The diffusion coefficient is an important kinetic parameter used to estimate the effect of additives on the electrodeposition of Sn-Ni and can be determined from the slope of the chronoamperogram of, i, versus, t1/2, according to Cottrell's equation (Eq. 4).
It has been observed that as the potential moves towards more electronegative values down to -410 mV, the time taken to reach the maximum current on the chronoamperograms increases (Figure 4a).
A comparison of some other parameters, such as the maximum value of the transient current (Im) and the time to reach it (tm) for the electrodeposited alloy with different applied potentials is shown in Figure 4b. It can be seen that when the potential is more negative, the value of Im is higher and the value of tm is greater. A comparison of the curves obtained at different potentials with the theoretical instantaneous and progressive curves of the SH model is shown in Figure 4b. This comparison shows that the experimental curves are closer to an instantaneous nucleation model; therefore, the electrodeposition of Sn-Ni follows an instantaneous nucleation mechanism and the three-dimensional growth is controlled by diffusion [39]. In addition, the deposition potential slightly affects the Sn-Ni nucleation mechanism. However, Figure 4b shows that when t > tm, the experimental curves deviate from the theoretical instantaneous nucleation behavior due to proton reduction during Sn-Ni electrodeposition, which can occur during electrodeposition of metal ions in aqueous solutions as observed in previous work [41].
The SEM and EDX characterizations in Figure 5 show that within 2 seconds of electrolysis at a potential of -0.32 V, the nuclei begin to coalesce to form a continuous film, while the Ni content of the alloy formed is almost twice the Sn content. This could be an indication of the predominance of Ni deposition in the early stages of electrolysis.

Influence of Current Density on Cathode Current Efficiency

In complex electrolytes, the variation in cathodic current yield is due to the process characteristics of the electrode surface. The evolution of the cathodic current yield as a function of the applied current density for TiO2NPs-based alloy and composite coatings at a concentration of 1 g/L is shown in Figure 6.
It was found that as the current density increased from 0.5 to 4 A/dm2, the cathodic current efficiency for the Sn-Ni alloy decreased from 99.8% to 53.9%. With the introduction of TiO2NPs into the electrolyte composition, an approximately linear dependence of current efficiency on current density was observed. For a composite coating with TiO2NPs, the current efficiency of the coating is much higher at high current densities, which may be due to an increase in electrolyte diffusion and a significant change in the composition of the coating phase. To confirm the latter hypothesis, the crystal phase and elemental composition of Sn-Ni alloy coatings at different current densities were investigated and the X-ray diffraction results are shown in Figure 7.
Figure 7 shows that an increase in current density to 2.5 A/dm2 is accompanied by a significant change in the phase composition of the coatings. It is also interesting to note that the Ni4Sn intermetallic compound did not form at current densities above 2.5 A/dm2, whereas a Sn peak was observed for coatings deposited at current densities of 1.5 and 2 A/dm2. It should be noted (Figure 8) that the Sn and Ni contents of the alloy also increase slightly with current. At current densities of 2 A/dm2 and above, the Ni and Sn contents in the coating tend towards a 50/50 ratio.
Increasing the cathodic current density slightly affects the structure of the coatings and also leads to changes in the Sn/Ni ratio. The maximum Ni content in the alloy composition is observed at a current density of 2 A/dm2 and amounts to 50.2 wt%, the minimum content is observed at a current density of 1.5 A/dm2 and amounts to 46.06 wt%. These results show that an increase in cathodic current density induces Ni and Sn deposition in equivalent proportions. Another hypothesis to explain the behavior of current efficiency as a function of current density is the increase in the specific surface area of the electrode. Under the influence of the electric field resulting from the applied electrodeposition potential, TiO2NPs covered with ionic adsorbates migrate towards the cathode surface where they are adsorbed. As a result, the positively charged particles adsorbed on the cathode surface increase the active surface area and provide more nucleation sites for Ni2+ ion reduction [50].

The Influence of TiO2NPs Concertation on Structure and Composition of Deposited Coating Based on Sn-Ni Alloy

The introduction of TiO2NPs into the electrolyte also contributes to a change in the composition of the coating phase.
The XRD results in Figure 9 show the presence of Cu in the structure of the Sn coatings. This may be due to the diffusion of Sn into the Cu substrate structure and the formation of intermetallic compounds. TiO2NPs peaks are absent from the diffraction pattern, which can be explained by the relatively low concentration of TiO2 in the composition of the electrolytes used to prepare the coatings. Figure 10 shows the diffractograms of the Sn-Ni alloy and the Sn-Ni-TiO2NPs alloy-based composite coating.
The XRD results in Figure 10 show that the Sn-Ni alloy is formed by a number of unstable intermetallic compounds such as NiSn, NiSn2, Ni3Sn2, Ni4Sn. When TiO2NPs are introduced into the electrolyte, the intensity of the main NiSn metastable peak changes. This can be explained by a slight change in the Sn content associated with a change in its solubility. The structure of the prepared coatings was also studied by SEM and the results are shown in Figure 11.
The introduction of TiO2NPs into the electrolyte leads to changes in the texture of the prepared coatings. It has been shown that in the case of Sn-TiO2NPs deposition, more pronounced facet structures are observed compared to Sn deposition (Figure 11a,b). The Sn-TiO2NPs coating is inhomogeneous with the local formation of large Sn-TiO2NPs about 20 µm larger than the Sn grains (Figure 11a) and larger than the size of the TiO2NP aggregates (Figure 1a). This may be due to the hindered deposition of the Sn coating when the large TiO2NPs aggregates are deposited on the substrate surface, as well as the rapid diffusion of Sn into the Cu substrate with the formation of new intermetallic compounds. In the case of Ni, when the TiO2NPs are introduced into the electrolyte, the deposit has a uniform texture with fewer pores (Figure 13d). The Sn-Ni alloy coating is characterized by the formation of small spheroidal crystallites (Figure 11e). When TiO2NPs are introduced into the electrolyte composition during the deposition of the Sn-Ni alloy, an increase in the size of the spheroids on the surface is observed, reaching diameters of the order of 10 μm larger than the size of the TiO2NPs aggregates (Figure 1a), which is the signature of local inclusion of TiO2NPs in the coating (Figure 11f). The elemental and phase compositions of the prepared alloys and coatings are given in Table 2.
Table 2. Composition of different prepared coatings based on Sn, Ni, Sn–Ni alloy without and with TiO2NPs.
Table 2. Composition of different prepared coatings based on Sn, Ni, Sn–Ni alloy without and with TiO2NPs.
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For single metal coatings, the Cu content reaches 100%. In the case of Sn-TiO2NPs, the coating is characterized by an irregular structure and the intercrystallite contain Cu (Figure 11d). In addition, it has been shown that when TiO2NPs at a concentration of 1 g/L are introduced into the Sn and Ni deposition electrolytes with ionic strength compensation, the TiO2NPs are incorporated into the Sn and Ni coatings with a composition limit of 0.73 and 0.13 wt.%, respectively. For the Sn-Ni alloy coating, the Sn and Ni contents are 51-56 wt.% and 48 wt.%, respectively. In the case of the composite alloy coating, TiO2NPs are not present in the coating composition. This may be due to their very inhomogeneous inclusion in the coating composition in the form of agglomerates of TiO2NPs aggregates. As the EDX analysis did not reveal the presence of TiO2NPs in the coating, an XRF analysis was carried out for confirmation (Table 3).
Table 3. Elemental content of all coatings.
Table 3. Elemental content of all coatings.
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Table 3 shows that Ti was not detected in the Sn-Ni TiO2NPs based coating at a TiO2NPs concentration of 1 g/l in the electrolyte. However, by increasing the TiO2NPs concentration to 2 g/l in the electrolyte, the Ti content in the coating increases to 0.12 wt.%, a signature that TiO2NPs have been incorporated into a Sn-Ni TiO2NPs based coating. To further investigate the presence of TiO2NPs in the Sn-Ni TiO2NPs coating, Raman experiments were carried out and the spectra are shown in Figure 12. Analysis of these spectra provided an estimate of the TiO2NPs content.
In the Raman spectra corresponding to Sn-Ni alloy based composite coatings with the presence of TiO2NPs in the deposition electrolyte, peaks were observed in the region of 150, 395, 512, 634 cm-1 (Figure 12c,e), which are characteristic of TiO2NPs (Figure 12a). This confirms the inclusion of TiO2NPs in the composition of Sn-Ni alloy based composite coatings and that the embedded TiO2NPs increase with the concentration of TiO2NPs in the electrolyte. In addition, these results show that Raman measurements are more sensitive to the presence of TiO2NPs in Sn-Ni alloy based composite coatings at concentrations lower than 1g/l.
Furthermore, the introduction of TiO2NPs into the composition of the deposition electrolyte for Sn, Ni and Sn-Ni coatings was observed to have an ambiguous effect on the surface roughness of the prepatterned coatings (Figure 13).
With the introduction of TiO2NPs into the deposition electrolyte composition, Sn and Ni coatings become rougher. However, for a Sn-Ni alloy based composite coating, the microroughness decreases slightly. This can be explained by the uniform incorporation of TiO2NPs into the alloy phase deposited over the entire surface, as well as the absence of TiO2NPs agglomeration in the deposition electrolyte. Table 4 shows the surface microroughness for the different coatings studied.
Table 4. The main indicators of the microroughness of the surface of different prepared coatings.
Table 4. The main indicators of the microroughness of the surface of different prepared coatings.
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It was found that by introducing 1 g/l TiO2NPs in the composition of the electrolyte for the deposition of Sn and Ni (with compensation of the ionic strength by a NaCl solution), the various parameters quantifying microroughness (Ra, Rmax and Rz) increased by almost three times. In the case of composite coatings based on Sn-Ni-TiO2NPs alloys, the opposite effect was observed, i.e. a decrease in roughness. The dependence of the microroughness of Sn-Ni coatings on the deposition current density was also investigated and the results are shown in Figure 14.
The roughness results shown in Figure 14 (a) indicate that maximum values of Ra, Rz and Rmax were obtained at a current density of 2 A/dm2 while minimum values were observed at a current density of 3 A/dm2. The irregular dependence of microroughness on current density may be related to a significant change in phase composition and degradation of coatings deposited at current densities above 3 A/dm2. In the case of TiO2NPs-based composite coatings, the evolution of microroughness was studied at current densities of 1 and 4 A/dm2 and the results are shown in Figure 14 (b). It can be observed that an increase in the current density of Sn-Ni-TiO2NPs coatings leads to an increase in the roughness parameters Ra, Rmax and Rz by a factor of 2 to 3. These results are in good agreement with those obtained from the SEM characterization. In fact, the textures observed from the SEM characterization show an increase in the size of spheroids on the surface, which is a signature of local inclusion of TiO2NPs in the coating.
Surface roughness is a characteristic that has a major influence on the properties of the coating, particularly its optical and wetting properties. The physical interaction of a liquid with the surface of a solid or other liquid determines the wetting properties of that surface, and the degree of wetting is characterized by the wetting contact angle, θ, which is the angle formed by the plane tangent to the interfacial surfaces bounding the wetting liquid, with the vertex of the angle lying on the line of separation of the three phases. Depending on this angle, a distinction is made between hydrophilic (θ < 90°) and hydrophobic (θ > 90°) surfaces. The value of the contact angle (θ), excluding the effect of gravity, depends solely on the molecular properties of the solid surface and the phases in contact with it. Figure 15 shows a diagram of the effect of current density on the wetting angles of Sn-Ni alloy coatings.
In Figure 15a the results show that the variation in contact angle is inversely proportional to the variation in surface roughness. For coatings with the lowest roughness, obtained at a current density of 2.5 A/dm2, the maximum value of the wetting angle was observed to be 78°±1.2. For Sn-Ni alloy-based coatings with higher roughness, obtained at a current density of 2 A/dm2, the wetting contact angle reached minimum values of around 61°±1.3°. Furthermore, the introduction of TiO2NPs into the composition of the deposition electrolyte of Sn or Ni or Sn-Ni based coatings contributes to a modification of the wetting contact angle (Figure 15b).
As well as affecting surface roughness, the introduction of TiO2NPs also contributes to changes in the composition and structure of the coatings. TiO2NPs introduced into the electrolyte composition for the deposition of Sn-based coatings result in a 1.4-fold decrease in contact angle. For Ni and Sn-Ni alloy coatings, the introduction of TiO2NPs into the electrolyte composition increases the contact angle by 2 and 1.1 times respectively. The dependence of the Sn-Ni alloy wetting contact angle on the microroughness of the coatings studied was determined. The optimum current density of 1/dm2 was used to obtain a wetting contact angle of about 65±1.2°. The dependence of the TiO2NPs incorporation on the contact angles of the prepared coatings was evaluated. It was found that the introduction of 1 g/l TiO2NPs into the composition of the Sn-Ni alloy deposition electrolyte imparts hydrophobic properties to the surface.

Corrosion Properties

Since these coatings can modify objects in massive contact with human skin, evaluation of their corrosion resistance in artificial sweat was carried out using electrochemical impedance spectroscopy. The impedance spectra in an artificial sweat medium (pH=7) are shown in Figure 16.
From the Nyquist and Bode diagrams in Figure 16, it is clear that in the absence of TiO2NPs in the composition of Sn coatings, higher resistance values are observed. The charge transfer resistance (low-frequency region) of Sn coatings is 1.40·104 Ω·cm2, however, with the introduction of TiO2NPs, the resistance decreases to 7.90·103 Ω·cm2 (Table 5). An opposite behavior was observed with the introduction of TiO2NPs into the electrolyte composition for Ni deposition, an increase in the charge transfer resistance of the coating from 8.3·103 to 2.10·104 Ω·cm2 was observed. In the case of Sn-Ni alloy-based coatings, with the introduction of TiO2NPs, charge transfer resistance decreases from 4.25·104 to 3.7·104 Ω·cm2. The impedance spectra were analyzed using the equivalent circuit shown in the insert of Figure 16a. The parameters obtained from simulation of the impedance spectra using the equivalent circuit are shown in Table 5.
Table 5. Parameters of the equivalent circuits of the impedance spectra for coatings Sn, Ni, Ni–Sn and Sn–TiO2NPs, Ni–TiO2NPs, Sn–Ni TiO2NPs obtained in an artificial sweat medium.
Table 5. Parameters of the equivalent circuits of the impedance spectra for coatings Sn, Ni, Ni–Sn and Sn–TiO2NPs, Ni–TiO2NPs, Sn–Ni TiO2NPs obtained in an artificial sweat medium.
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Introducing TiO2NPs into the electrolytes when depositing Sn and Sn-Ni coatings reduces the charge transfer resistance at the interface, whereas For Ni coatings, the introduction of 1 g/L TiO2NPs into the electrolyte composition doubles the charge transfer resistance. For all the coatings studied, the introduction of TiO2NPs into the deposition electrolyte composition results in a slight decrease in the exponent n1 of the CPE1 element, with values between 0.7 and 0.91, indicating the capacitive nature of the element.
To further assess the anti-corrosion properties of the prepared coatings, the potentiodynamic polarization technique was applied, and the results obtained are shown in Figure 17. The results analysis using Tafel equation yield the corrosion current and potential.
The introduction of TiO2NPs into the deposition electrolyte of the Sn-based coating induces a shift in corrosion potential (Ecorr) by 70 mV towards the electropositive region, from Ecorr (Sn) = -0.42 V to Ecorr (Sn-TiO2NPs) = -0.35 V, whereas this shift is only about 40mV in the case of a Ni coating. The Ecorr increased from 0.14 V for the Ni coating to 0.18 V for the Ni-TiO2NPs coating. In the latter case, corrosion currents (Icorr) are 4.84 10-6 A/cm2 and 4.87 10-5 A/cm2 respectively. Figure 17c shows that in the case of the Sn-Ni alloy coating, the introduction of TiO2NPs into the electrolyte during deposition of the Sn-Ni alloy coating induces a shift of the Ecorr by 300mV towards the electronegative region, taking the Ecorr from 0.09 V (Sn-Ni) to -0.21 V (Sn-Ni-TiO2NPs), the corresponding Icorr are 1.82·10-4 A/cm2 and 2.63·10-5 A/cm2 respectively. The results of the main corrosion process parameters in an artificial sweat environment with a pH of 7 are presented in Table 6.
Table 6. Electrochemical parameters of corrosion of the studied samples.
Table 6. Electrochemical parameters of corrosion of the studied samples.
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The Ecorr of Sn- or Ni-based coatings changes insignificantly with the introduction of TiO2NPs into the electrolyte composition, while in the case of Sn-Ni-based coatings, the corrosion current decreases 6.9-fold. The results of electrochemical impedance spectroscopy of the coatings in a 3% NaCl solution are shown in Figure 18. It can be seen that the Nyquist diagrams obtained for all the coatings studied in the NaCl medium are characterized by a capacitive semicircle in the high- and mid-frequency region.
In the case of Ni-based coatings it can be seen in Figure 18a,b, that the introduction of TiO2NPs into the electrolyte used for deposition, induces a higher increase in charge transfer resistance from 1.80·104 to 4.0·104 Ω·cm2. Whereas, the Bode diagrams (Figure 18d) show a slight increase in charge transfer resistance from 1.10·104 to 1.20·104 Ω ·cm2, in the case of Sn-based coatings with embedded TiO2NPs (low-frequency region). For Sn-Ni alloy-based coatings (Figure 18e,f), the introduction of TiO2NPs into the deposition electrolyte, the charge transfer resistance decreases from 1.20·105 to 9.0·104 Ω cm2. The parameters obtained from the equivalent circuit analysis of the impedance spectra are shown in Table 6.
Table 6. Parameters of equivalent circuits used to analyze the impedance spectra of different prepared coatings (Sn, Ni, Sn-Ni and Sn-TiO2NPs, Ni-TiO2NPs, Sn-Ni-TiO2NPs) in solution 3% NaCl.
Table 6. Parameters of equivalent circuits used to analyze the impedance spectra of different prepared coatings (Sn, Ni, Sn-Ni and Sn-TiO2NPs, Ni-TiO2NPs, Sn-Ni-TiO2NPs) in solution 3% NaCl.
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The introduction of TiO2NPs into electrolytes for the deposition of Ni and Sn coatings increases the resistance to charge transfer at the interface. For a Sn-NiTiO2NPs composite coating with 1 g/L TiO2NPs in the electrolyte composition, the charge transfer resistance indicators decrease by a factor of two. For nickel coatings, the introduction of TiO2NPs into the composition of the deposition electrolyte decreases the exponent n of the CPE1 element slightly, but for Sn-TiO2NPs and Sn-Ni-TiO2NPs this exponent increases. The n1 index is in the range from 0.61 to 0.82, which indicates the presence of defects in the coating.
The results of potentiodynamic polarization curves are presented in Figure 19 for different prepared coatings. Based on these results, the corrosion currents and the corrosion initiation potential were calculated.
Analysis of the potentiodynamic polarization curves showed that the corrosion potential of the Sn-based coating (Figure 19a) is -0.22 V, and that the introduction of TiO2NPs into the electrolyte composition for deposition resulted in a 110 mV shift in potential towards the electronegative potentials region, reaching a value of -0.33 V. It can also be observed that the corrosion current is 1.03·10-4 A/cm2 and 6.48·10-5 A/cm2 for Sn and Sn-TiO2NPs, respectively. In the case of Ni coatings, it was observed that the corrosion potential is 0.05 V, but when TiO2NPs nanoparticles are introduced into the deposition electrolyte, the corrosion initiation potential shifts by 230 mV towards the electronegative potentials region and reaches a value of -0.18 V. The corrosion currents corresponding to the Ni or Ni-TiO2NPs based coatings are 2.39·10-5 A/cm2 and 2.18·10-5 A/cm2 respectively (Figure 19b). Figure 19c shows that TiO2NPs introduced into the Sn-Ni alloy deposition electrolyte shifts the corrosion initiation potential by 100 mV towards the electropositive potentials, from 0.08 V to -0.02. Furthermore, the corrosion currents are 1.01·10-4 A/cm2 and 9.94 10-6 A/cm2, for Sn-Ni and Sn-Ni-TiO2NPs respectively. The main parameters characterizing the corrosion process are shown in Table 7.
Table 7. Electrochemical Tafel parameters of corrosion of the studied coatings.
Table 7. Electrochemical Tafel parameters of corrosion of the studied coatings.
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The introduction of TiO2NPs into the deposition electrolyte compositions of all coatings was accompanied by a more or less pronounced decrease in corrosion current. The corrosion current of Sn-Ni alloy and Ni-based coatings does not change significantly with TiO2NPs in the deposition electrolyte composition. In the case of Sn coatings, the corrosion current decreases by a factor of 1.6.
According to these results, obtained both in an artificial sweat medium and in a 3% NaCl solution, the different coatings prepared show different trends in terms of anticorrosion properties when TiO2NPs are added. Indeed, TiO2NPs incorporated in Sn, Ni and Sn-Ni coatings modify their anticorrosion properties, which can be explained by their surface wetting properties and their chemical composition. As shown by SEM surface characterization, the surface texture changes with the introduction of TiO2NPs, which consequently influences their roughness and wetting properties. It is well known in the literature that a surface with an increased contact angle reduces the area of contact with the liquid on its surface, which can lead to a reduction in the surface area in contact with the aggressive medium and therefore its corrosion [51,52]. Bearing this in mind, the results obtained can be explained as follows.
In the case of the Sn coating, the decrease in corrosion resistance can be explained by the decrease in the contact angle and therefore an increase in the surface area exposed to corrosion. Furthermore, the increase in corrosion resistance of the Ni-based coating after the incorporation of TiO2NPs is due to the synergistic effect between the increase in contact angle, and therefore minimization of the contact surface with the aggressive solution, and the anticorrosive property of the chemical element Ni. In the case of the Sn-Ni based coating, it was observed that the corrosion resistance decreased after the introduction of TiO2NPs, despite a relatively high contact angle compared to the other coatings and the presence of the chemical element Ni. This can be explained by the appearance of porosity in the coating after the incorporation of TiO2NPs, which can facilitate the penetration of the aggressive solution through the coating, reaching a maximum surface area susceptible to corrosion.
Antibacterial properties are strongly recommended for surfaces in public areas that come into contact with human skin. TiO2NPs is a material with high photocatalytic properties and, when used in coatings, imparts antibacterial properties. The antibacterial properties of the coatings prepared in this work have been investigated and are presented in Table 8.
Table 8. Antibacterial activity of samples in relation to the test culture St. aureus ATCC 6538.
Table 8. Antibacterial activity of samples in relation to the test culture St. aureus ATCC 6538.
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According to the antibacterial test presented in Table 8, the prepared coatings show bactericidal activity with an RF parameter >2; and show that under UV light they can reduce the concentration of microorganisms by 4 to 5 orders of magnitude. Furthermore, the antibacterial activity of Sn-based coatings is due to the presence of copper with antibacterial properties in the coating composition, as Sn has no antibacterial properties. Figure 20 shows the antibacterial properties of the coatings studied in relation to the St. aureus disease test bacterium.
Experiments have shown that for all coatings, the introduction of TiO2NPs into the composition of the deposition electrolytes, and consequently into the coatings, leads to an enhancement of the biocidal properties with and without UV radiation.
Table 9 shows that the introduction of TiO2NPs (1 g/l) into the deposition electrolyte for the deposition of Sn, Ni, Sn-Ni coatings leads to an increase in the antibacterial properties in the presence of UV radiation for an E. coli ATTC 8739 strain, but with a lower efficacy than that observed against the St. John's bacterium. Electrochemical composite coatings with TiO2NPs were shown to have the best biocidal properties against gram-positive and gram-negative strains. They can be widely used on objects that are accessible to humans in public environments.
Table 9. Results of the antibacterial activity of samples against E. coli ATTC 8739 test culture.
Table 9. Results of the antibacterial activity of samples against E. coli ATTC 8739 test culture.
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In order to assess the antibacterial properties of a solid surface, several factors must be taken into account, such as the properties of the surface in terms of hydrophobicity, roughness and chemical composition, as well as the bacterial species. In fact, the proliferation of bacteria on a solid surface is affected by the configuration and roughness of the surface, which has a strong influence on the adhesion of bacteria to the solid surface and their viability. Bacterial attachment to solid surfaces is considered to be the first step in bacterial growth and proliferation. It has been widely demonstrated that poor bacterial adhesion to the solid surface leads to bacterial death. Therefore, weakening bacterial adhesion is considered one of the indirect approaches to prevent bacterial proliferation. In fact, surface patterning and roughness can increase the hydrophobic character of the surface, causing bacteria to repel and weakly adhere to the solid surface, leading to their death.
Another approach to the surface to prevent bacterial proliferation, which is considered to be direct, is the chemical composition of the surface. The metallic surfaces of nanoparticles (NPs) such as zinc, aluminum, nickel, zirconium, cerium, magnesium and titanium dioxide have antibacterial properties [53,54,55,56,57,58]. In particular, the photoelectrochemical and photocatalytic properties of TiO2 nanoparticles [58,59] have been shown to enable them to generate electron-hole pairs that can react with water or oxygen to produce reactive oxygen species (ROS) such as superoxide anions, hydrogen peroxide and hydroxyl radicals. The latter species can penetrate bacterial cell walls and cause cell death by disrupting cell function [60,61,62,63,64,65,66]. Several studies in the literature have reported that TiO2 has strong antibacterial and antifungal activity against many Gram-positive bacteria such as Staphylococcus aureus and Gram-negative bacteria such as Escherichia coli [67,68].
In addition, Ni nanocrystals have been reported to have antibacterial activity against Gram-positive/negative bacteria and bacterial spores such as Staphylococcus aureus (S. aureus) and Escherichia coli (E. coli). Although the mechanism of the antibacterial effect of Ni nanocrystals is not yet fully understood, it has been reported that during the interaction between the Ni surface and bacteria, Ni2+ leaching has been detected, which penetrates the bacterial membrane and generates reactive oxygen species, leading to disruption of its function and bacterial death [69]. In the case of the Sn surface, it has been reported to exhibit weak antibacterial activity when exposed to Staphylococcus aureus (S. aureus) or Escherichia coli (E. coli) bacteria [70].
To understand the antibacterial test results of the coating prepared with and without TiO2NPs, it is necessary to consider the synergistic effect of the two factors influencing the antibacterial activity of the solid surface mentioned above. In fact, there is a synergistic effect between the surface repulsion and weakening of adhesion due to the hydrophobicity of the surface on the one hand and the antibacterial properties of the chemical elements of the solid surface on the other. In the case of Sn with low antibacterial activity, the observed result could be explained by the hydrophobicity of the surface leading to weak adhesion of the bacteria and their death at a later stage. When TiO2NPs are incorporated into the Sn coating, although the contact angle decreases from 89° to 63° and therefore the adhesion of bacteria to the surface increases, the antibacterial activity of TiO2NPs offsets this effect, explaining the small variation in RF from 2.3 to 2.6.
For Ni and Sn-Ni coatings, the strong increase in RF after incorporation of TiO2NPs could be explained by the synergistic effect of the antibacterial properties of Ni and TiO2NPs rather than the poor adhesion of bacteria to the surface. For the Ni coating, although the surface is hydrophilic (contact angle of 32°) and bacteria need to adhere well, the Ni guarantees the antibacterial activity of the coating. Furthermore, for the Sn-Ni coating after the incorporation of TiO2NPs, the contact angle shows little variation (Figure 15) and therefore a similar bacterial adhesion strength, the antibacterial activity observed is mainly due to the synergistic effect between that of Ni and that of TiO2NPs.
These results clearly show that Sn, Ni and TiO2NPs are crucial elements to be used for the preparation of coatings with high performance anti-corrosion and antibacterial properties, as Sn-Ni-TiO2 exhibits the best antibacterial activity with an RF of 3.4 and the best anti-corrosion performance with an Ecorr more shifted (0.08V) towards the positive potential, a lower Icorr of 9.95-10-6 A/cm2 and a higher charge transfer resistance of 25067 Ω·сm2.

4. Conclusions

In this work it was found that the electrochemical deposition of monolayers and Sn-Ni alloy proceeds according to an instantaneous nucleation mechanism with ion transfer as the limiting step. Furthermore, it was observed that the introduction of TiO2NPs does not affect the potentiodynamic polarization curves, but significantly modifies the phase composition and structure of the formed coatings. The deposition current density of the alloy was also found to have an influence on the coating yield and the elemental and phase composition of the coatings studied.
For coatings deposited at a current density of 0.5 to 1.5 A/dm2, Sn predominates in the coating composition up to 55% by weight. The anticorrosion properties of SnNi alloy-based coatings in an artificial sweat medium were investigated. It was found that the introduction of TiO2NPs at a concentration of 1 g/l into the deposition electrolyte led to a significant reduction in the current density at the origin of the corrosion inhibition.
It has been found that the introduction of TiO2NPs into the composition of the deposition electrolytes and, consequently, of the coatings produced, leads to an increase in the biocidal properties, both with and without UV radiation, which will allow these coatings to be used on objects in the public domain that come into frequent contact with people.

Author Contributions

Conceptualization, (A.C) and (A.T); methodology (H.P); validation, (A.T) and (Y.X); investigation, (M.V) (H.P) and (O. A); data curation, (O.A); writing—original draft, (H.P) and (V.B); formal analysis, (M.V), (H.P) and (V.B); writing—review and editing, (A.T) and (X.Y); supervision, (A.C) and (A.T). All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by grant of the Ministry of education of the Republic of Belarus, grant number 24-038 «Electrochemical production and physical-chemical properties of nickel-cobalt alloy».

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data presented in this study are available on request from the corresponding author due to (specify the reason for the restriction).

Acknowledgments

The authors acknowledge the Ministry of education of the Republic of Belarus for supporting the achievement of this work.

Conflicts of Interest

The authors have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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Figure 1. (a) FEGSEM images of TiO2 aggregates obtained at synthesis temperatures of 170°C, insert the corresponding high magnification, (b) XRD pattern of TiO2NPs aggregates prepared at synthesis temperatures 170°C.
Figure 1. (a) FEGSEM images of TiO2 aggregates obtained at synthesis temperatures of 170°C, insert the corresponding high magnification, (b) XRD pattern of TiO2NPs aggregates prepared at synthesis temperatures 170°C.
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Figure 2. Cathodic polarization curves of Sn, Ni and Sn–Ni electrodeposition with and without the addition of TiO2NPs at temperature of 70°С as indicated.
Figure 2. Cathodic polarization curves of Sn, Ni and Sn–Ni electrodeposition with and without the addition of TiO2NPs at temperature of 70°С as indicated.
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Figure 3. Dependence of the activation energy on the potential of different coatings with and without the addition of TiO2NPs in the electrolyte as indicated.
Figure 3. Dependence of the activation energy on the potential of different coatings with and without the addition of TiO2NPs in the electrolyte as indicated.
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Figure 4. (a) Chronoamperograms of the Sn–Ni alloy deposition at various potentials, (b) Typical current-time transients for the electrodeposition of a Sn-Ni alloy from a fluoride-chloride electrolyte (T=70°С) at different applied potentials, V: 3) -0.250; 4) -0.310; 5) -0.320; 6) -0.390. Theoretical representations of the SH models in cases 1 - instantaneous nucleation and 2 - progressive nucleation.
Figure 4. (a) Chronoamperograms of the Sn–Ni alloy deposition at various potentials, (b) Typical current-time transients for the electrodeposition of a Sn-Ni alloy from a fluoride-chloride electrolyte (T=70°С) at different applied potentials, V: 3) -0.250; 4) -0.310; 5) -0.320; 6) -0.390. Theoretical representations of the SH models in cases 1 - instantaneous nucleation and 2 - progressive nucleation.
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Figure 5. (a) SEM image of the electrode surface after Sn-Ni electrodeposition over 2 s at applied potential of - 0.32 V, and (b) the corresponding EDX.
Figure 5. (a) SEM image of the electrode surface after Sn-Ni electrodeposition over 2 s at applied potential of - 0.32 V, and (b) the corresponding EDX.
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Figure 6. The current efficiency versus the current density for Sn–Ni and Sn–Ni−TiO2 coatings.
Figure 6. The current efficiency versus the current density for Sn–Ni and Sn–Ni−TiO2 coatings.
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Figure 7. XRD patterns of coatings based on Sn-Ni alloy at different current densities.
Figure 7. XRD patterns of coatings based on Sn-Ni alloy at different current densities.
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Figure 8. Elemental analysis of Sn−Ni coatings at different current densities.
Figure 8. Elemental analysis of Sn−Ni coatings at different current densities.
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Figure 9. Diffractogram of different prepared coatings as indicated (a) Sn, (b) Ni, (c) Sn– TiO2NPs and (d) Ni–TiO2NPs.
Figure 9. Diffractogram of different prepared coatings as indicated (a) Sn, (b) Ni, (c) Sn– TiO2NPs and (d) Ni–TiO2NPs.
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Figure 10. Diffractograms of (a) Sn−Ni and (b) Sn–Ni TiO2NPs coatings.
Figure 10. Diffractograms of (a) Sn−Ni and (b) Sn–Ni TiO2NPs coatings.
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Preprints 117127 g011
Figure 11. SEM images of different prepared coatings (a) Sn, (b) Sn–TiO2NPs, (c) Ni, (d) Ni–TiO2NPs, (e) Sn–Ni and (f) Sn–Ni TiO2NPs, at current density of 1A/dm2.
Figure 11. SEM images of different prepared coatings (a) Sn, (b) Sn–TiO2NPs, (c) Ni, (d) Ni–TiO2NPs, (e) Sn–Ni and (f) Sn–Ni TiO2NPs, at current density of 1A/dm2.
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Figure 12. Raman spectra for TiO2NPs (a), Sn-Ni (b) and composite coatings based on Sn-Ni alloy with different concertation of TiO2NPs in electrolyte (g/l): 1 (c), 2 (d) and 5 (e).
Figure 12. Raman spectra for TiO2NPs (a), Sn-Ni (b) and composite coatings based on Sn-Ni alloy with different concertation of TiO2NPs in electrolyte (g/l): 1 (c), 2 (d) and 5 (e).
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Preprints 117127 g013
Figure 13. Microroughness of (a) Sn, (b) Ni and (c) Sn-Ni coatings with and without TiO2NPs.
Figure 13. Microroughness of (a) Sn, (b) Ni and (c) Sn-Ni coatings with and without TiO2NPs.
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Figure 14. Microroughness of the surface of Sn-Ni coatings (a) at different current densities, (b) Sn-Ni-TiO2NPs coating (CEC) at current densities 1 and 4 A/dm2. .
Figure 14. Microroughness of the surface of Sn-Ni coatings (a) at different current densities, (b) Sn-Ni-TiO2NPs coating (CEC) at current densities 1 and 4 A/dm2. .
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Figure 15. Contact angle of the (a) Sn-Ni coating at different current densities, (b) studied coatings, 1 – Sn, 2 – Sn–TiO2NPs, 3 – Ni, 4 – Ni–TiO2NPs, 5 – Sn–Ni and 6 – Sn–Ni–TiO2NPs.
Figure 15. Contact angle of the (a) Sn-Ni coating at different current densities, (b) studied coatings, 1 – Sn, 2 – Sn–TiO2NPs, 3 – Ni, 4 – Ni–TiO2NPs, 5 – Sn–Ni and 6 – Sn–Ni–TiO2NPs.
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Figure 16. Nyquist and Bode diagrams of different prepared coatings as indicated in an artificial sweat medium (a) Sn and Sn-TiO2NPs (Nyquist), (b) Sn and Sn-TiO2NPs (Bod), (c) Ni and Ni-TiO2NPs (Nyquist), (d) Ni and Ni-TiO2NPs (Bod) and (e) Sn-Ni and Sn-Ni-TiO2NPs (Nyquist) and (f) Sn-Ni and Sn-Ni-TiO2NPs (Bod). The (a) insert correspond to Equivalent circuit of an electrochemical cell for modeling impedance spectra. Rs - solution resistance, CPE1 - capacitive type constant phase element, R1 - charge transfer resistance at the interface, CPE2 - diffusion type constant phase element, R2 - diffusion resistance.
Figure 16. Nyquist and Bode diagrams of different prepared coatings as indicated in an artificial sweat medium (a) Sn and Sn-TiO2NPs (Nyquist), (b) Sn and Sn-TiO2NPs (Bod), (c) Ni and Ni-TiO2NPs (Nyquist), (d) Ni and Ni-TiO2NPs (Bod) and (e) Sn-Ni and Sn-Ni-TiO2NPs (Nyquist) and (f) Sn-Ni and Sn-Ni-TiO2NPs (Bod). The (a) insert correspond to Equivalent circuit of an electrochemical cell for modeling impedance spectra. Rs - solution resistance, CPE1 - capacitive type constant phase element, R1 - charge transfer resistance at the interface, CPE2 - diffusion type constant phase element, R2 - diffusion resistance.
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Figure 17. Potentiodynamic polarization curves of the corrosion process in an artificial sweat media of different prepared coating as indicated.
Figure 17. Potentiodynamic polarization curves of the corrosion process in an artificial sweat media of different prepared coating as indicated.
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Preprints 117127 g018
Figure 18. Nyquist and Bode diagrams of different prepared coatings in 3% NaCl solution as indicated (Ni and Ni-TiO2NPs, Sn and Sn-TiO2NPs, Sn-Ni alloy and Sn-Ni-ТiO2NPs).
Figure 18. Nyquist and Bode diagrams of different prepared coatings in 3% NaCl solution as indicated (Ni and Ni-TiO2NPs, Sn and Sn-TiO2NPs, Sn-Ni alloy and Sn-Ni-ТiO2NPs).
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Preprints 117127 g019
Figure 19. Potentiodynamic polarization curves of the corrosion process in a solution of 3% NaCl for prepared coatings as indicated. (a) Sn and Sn-TiO2 coatings, (b) for Ni and Ni-TiO2 coatings and (c) Sn-Ni and for Sn-Ni-TiO2 coatings.
Figure 19. Potentiodynamic polarization curves of the corrosion process in a solution of 3% NaCl for prepared coatings as indicated. (a) Sn and Sn-TiO2 coatings, (b) for Ni and Ni-TiO2 coatings and (c) Sn-Ni and for Sn-Ni-TiO2 coatings.
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Figure 20. Antibacterial properties in terms of the bacteria St. aureus test of different prepared coating as indicated with and without UV exposition.
Figure 20. Antibacterial properties in terms of the bacteria St. aureus test of different prepared coating as indicated with and without UV exposition.
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