**4.2. Main results concerning the corrosion resistance of MMC coatings**

The comparison between the anticorrosive performance of metallic and MMC coatings was studied for several metallic matrix/ceramic particles (or nanoparticles) systems using different corrosive media [26,37,40,45,47]. Most of the results affirm that the presence of the particles increases the corrosion resistance of the MMC coating/substrate system, although it is impor‐ tant to mention that the optimum values of particle content in the metallic matrix to produce a coating with good anticorrosive performance directly depend on the metal/particle system produced and on the deposition parameters used. Moreover, the corrosion mechanism of the metallic coatings containing these particles is not completely elucidated; the process is generally related to blocking effects or to the creation of a more difficult path to the electrolyte attack in the coating [47,58,75,87]. In addition, there are also inconclusive results, as the conditions used to produce the coatings were not always adequate or the used parameters were not always completely mentioned (for example, the kind of substrate or the stirring time). Finally, the results presented here also show that the deposition parameters earlier described may have synergistic and/or antagonistic effects on the coating deposition process and on the anticorrosive properties of the coatings.

#### *4.2.1. Ni and Ni-alloy matrix composite coatings*

most used are the corrosion potential (Ecorr), which shows the nobility of the coating compared to the substrate, and the corrosion current density (jcorr), which is related to the intensity of the corrosion process. Based on the jcorr values, it is possible to calculate the corrosion rate and the coating efficiency (EfCoat) [32,52] as well as to estimate the coating porosity [82]. It is also possible to determine the Tafel anodic and cathodic slopes (βa and βc, respectively), which are related to the kinetic aspects of the anodic and cathodic electrochemical reactions [32,79].

The anodic branch of the polarization curve can also be used to study the passivity of the coatings and evaluate parameters such as the passivation potential (Epassivation), the critical

LPR is a real-time technique regulated by the ASTM G59 standard method and is based on the potential variation around the open circuit potential (OCP; typical variations approximately ±10 mV [76] to ±20 mV are used) [83,84]. The current required to maintain a specific displace‐ ment of the resting potential is directly related to the corrosion process on the electrode surface. This technique is particularly useful in aqueous systems and is applicable to obtain the polarization resistance (Rp) [75]. It is also possible to calculate the current and the corrosion rate if the values of Rp and the Tafel slopes are known [32,83]. In addition, it is possible to evaluate the porosity of the coating by comparing the Rp values of the coating and of the

EIS is a technique that provides detailed information on the electrical characteristics of the electrode/solution interface. EIS is based on the application of a small sinusoidal signal of potential (or current) to the working electrode according to a particular desired frequency range. As a response, it obtained the impedance (Z), which can be related to the opposition to the current flux in the system [32]. Important information about the charge transfer kinetics, structure, and properties of the interface electrode/electrolyte can also be achieved. The frequency range usually used for disturbing the system varies from 100 kHz to 10 MHz, using the amplitude signal in the range of 5 to 50 mV rms, depending on the studied system [32,85].

The results obtained from the EIS measurements can be used to construct diagrams repre‐ senting the behavior of the electrode in a particular electrochemical process. One of these diagrams is the Nyquist diagram, in which the real (Z) and imaginary (Z) impedance data are represented in a complex plane. The real impedance (Z) incorporates the ohmic resistance (i.e., the pure resistance that is independent of the frequency). The imaginary impedance (Z) incorporates the capacity and/or inductive reactance (i.e., the resistance that is dependent on

The Bode and phase diagrams are also used to represent the EIS results. These diagrams show the variations of the impedance modulus (|Z|) or the phase angle with the frequency, respectively. An advantage of the Bode diagram is the possibility of observing the impedance

current density for passivation (jcrit), and the pitting potential (Epitting) [32].

*4.1.2. Linear polarization resistance*

168 Electrodeposition of Composite Materials

*4.1.3. Electrochemical impedance spectroscopy*

the frequency applied to the system) [32].

substrate [82].

Nickel is the most studied metallic matrix to produce composite coatings with improved corrosion properties, and different ceramic particles and nanoparticles were used to this purpose. The corrosion resistance of nickel matrix composite coatings using α-Al2O3 particles (smaller than 1 μm; 98% purity) as the second phase on steel substrate, compared to a standard nickel coating produced onto the same substrate, was studied [87]. The corrosion evaluation data of the coating/substrate systems (from polarization curves and electrochemical impe‐ dance experiments) were performed in 0.5 mol L-1 Na2SO4 solution with different exposure times. Despite the physical (defects and dislocations) and chemical heterogeneities (the presence of nickel oxides or impurities), Al2O3 particles in the nickel coatings disturbed the electrochemical electrode reactions. As a result, the corrosion process was enhanced in some parts of the coating, whereas the phenomenon was inhibited in other parts. Therefore, no significant differences were observed concerning the anticorrosive behavior of the standard nickel coating and of the composite coating for the data obtained in the first days of exposure. Although the corrosion rates of both coatings increased with the exposure time, the corrosion resistance of Ni/Al2O3 composite coatings was always higher than the standard nickel coating in Na2SO4. Because of the adsorption of water, OH ions, and oxygen, the surfaces of both coatings were covered with a thin layer of nickel oxides and hydroxides causing an increase in Rct during the first 3 days of the experiment [87].

The EIS measurements showed that this result was related to the fact that the resistance of the electrolyte in the pores of the composite coating was much lower than in the pores of the standard nickel coating. It means that the presence of Al2O3 particles in the voids of the coating increases its porosity and the discontinuity of the passive layer formed on its surface. In the cracks and pores of the passive layer, the electrolytic solution is in contact with either nickel or dielectric Al2O3 particles. However, the passive layer on the surface of the Ni/Al2O3 coating was tighter possibly due to the more finely crystalline structure of the composite coating in comparison with that of the nickel standard coating. As a result, the pores and cracks in the passive layer on the composite coating were sufficiently small to reduce the diffusion of metal corrosion products from their inside effectively, and they built up in the pores, blocking them and decreasing the corrosion process [87]. After 14 days of exposure in the corrosive medium, the corrosion rate of the Ni/Al2O3 composite coating was three times lower than that of the nickel coating.

Ni/CeO2 composite coatings were produced on steel from an acid electrolyte containing CeO2 nanoparticles using square-wave pulse current mode under different stirring speed conditions and at 45°C. The corrosion behavior of these coatings was evaluated in a 3.5 g L-1 NaCl solution and compared to that observed from a nickel coating in the same medium [47]. More positive corrosion potentials and higher Rp values were obtained for the composite coatings (ranging from 3.54×10³ to 9.772×10³ Ω) compared to those verified for the nickel coating (2.05×10³ Ω). Once more, the presence of the inert particles in the coating was consid‐ ered important in the improvement of the corrosion resistance of composite coating because they may have acted as a physical barrier to the propagation of defects. The dispersion of the nanoparticles in the metallic matrix formed corrosion microcells, which facilitated the anodic polarization, resulting in the inhibition of pitting corrosion and in the promotion of uniform corrosion of the coating. If the crystallites in the coating remained on their nanometer size, the corrosion process could be explained considering that the electrolyte must travel a tortuous path to reach the substrate and this path is longer in the Ni/CeO2 composite coatings than in the nickel coating [47].

The increase in the stirring speed until 450 rpm increased the CeO2 particles in the nickel matrix coating, as well as the Rp values measured for these coatings, indicating a relationship between the content of particles in coating and its anticorrosive performance. Further increase in stirring speed, however, decreased both the particle incorporation and the Rp value, showing that it must be an optimum stirring speed condition to produce coatings with high corrosion resistance [47]. However, the effects of the pulse plating and the deposition temperature on the corrosion performance of the coating/substrate system were not evaluated.

EIS analysis was performed in aqueous NaOH (1 mol L-1) and HNO3 (1 mol L-1) solutions to evaluate the anticorrosive properties of Ni/TiO2 nanocomposite coatings. The coatings containing different TiO2 contents, (A: Ni-3.9 wt% TiO2, B: Ni-6.5 wt% TiO2, and C: Ni-8.3 wt % TiO2) were produced onto steel from an acid Ni(II) bath. The charge transfer resistances of pure nickel and Ni-TiO2 coatings [49] are shown in Table 3.

nickel coating and of the composite coating for the data obtained in the first days of exposure. Although the corrosion rates of both coatings increased with the exposure time, the corrosion resistance of Ni/Al2O3 composite coatings was always higher than the standard nickel coating

coatings were covered with a thin layer of nickel oxides and hydroxides causing an increase

The EIS measurements showed that this result was related to the fact that the resistance of the electrolyte in the pores of the composite coating was much lower than in the pores of the standard nickel coating. It means that the presence of Al2O3 particles in the voids of the coating increases its porosity and the discontinuity of the passive layer formed on its surface. In the cracks and pores of the passive layer, the electrolytic solution is in contact with either nickel or dielectric Al2O3 particles. However, the passive layer on the surface of the Ni/Al2O3 coating was tighter possibly due to the more finely crystalline structure of the composite coating in comparison with that of the nickel standard coating. As a result, the pores and cracks in the passive layer on the composite coating were sufficiently small to reduce the diffusion of metal corrosion products from their inside effectively, and they built up in the pores, blocking them and decreasing the corrosion process [87]. After 14 days of exposure in the corrosive medium, the corrosion rate of the Ni/Al2O3 composite coating was three times lower than that of the

Ni/CeO2 composite coatings were produced on steel from an acid electrolyte containing CeO2 nanoparticles using square-wave pulse current mode under different stirring speed conditions and at 45°C. The corrosion behavior of these coatings was evaluated in a 3.5 g L-1 NaCl solution and compared to that observed from a nickel coating in the same medium [47]. More positive corrosion potentials and higher Rp values were obtained for the composite coatings (ranging from 3.54×10³ to 9.772×10³ Ω) compared to those verified for the nickel coating (2.05×10³ Ω). Once more, the presence of the inert particles in the coating was consid‐ ered important in the improvement of the corrosion resistance of composite coating because they may have acted as a physical barrier to the propagation of defects. The dispersion of the nanoparticles in the metallic matrix formed corrosion microcells, which facilitated the anodic polarization, resulting in the inhibition of pitting corrosion and in the promotion of uniform corrosion of the coating. If the crystallites in the coating remained on their nanometer size, the corrosion process could be explained considering that the electrolyte must travel a tortuous path to reach the substrate and this path is longer in the Ni/CeO2 composite coatings than in

The increase in the stirring speed until 450 rpm increased the CeO2 particles in the nickel matrix coating, as well as the Rp values measured for these coatings, indicating a relationship between the content of particles in coating and its anticorrosive performance. Further increase in stirring speed, however, decreased both the particle incorporation and the Rp value, showing that it must be an optimum stirring speed condition to produce coatings with high corrosion resistance [47]. However, the effects of the pulse plating and the deposition temperature on

the corrosion performance of the coating/substrate system were not evaluated.

ions, and oxygen, the surfaces of both

in Na2SO4. Because of the adsorption of water, OH-

170 Electrodeposition of Composite Materials

in Rct during the first 3 days of the experiment [87].

nickel coating.

the nickel coating [47].


**Table 3.** Charge transfer resistance for pure nickel coating and Ni-TiO2 nanocomposite coatings (A–C) obtained from EIS data [49] after fitting by the equivalent circuit model

The Nyquist diagrams obtained from the experiments carried out in NaOH showed a capac‐ itive loop at high frequencies followed by an almost straight line at low frequencies, suggesting that the corrosion mechanism was controlled by both charge transfer and diffusion processes. The corrosion rate was affected by the interdiffusion of Ni2+ and OH- ions, confirming that there should be an effect related to the diffusion of these ions in the coating. The Rct values for the composite coatings exposed in this medium were improved with the increase of the TiO2 nanoparticle content in the coating, because they probably decreased the diffusion process [49].

In the experiments performed in HNO3, there was an observed discrete capacitive loop at high frequencies of the Nyquist diagram. An inductive loop was also observed at low frequencies, which was attributed to the relaxation process of H+ and NO3- species adsorbed on the electrode surface [49]. As observed in the experiments carried out in NaOH medium, the increase in the TiO2 content in the coating caused an increase in the charge transfer resistance of the composite coating. However, the Rct values obtained in acidic medium were significantly smaller than those verified in alkaline medium [49]. These results suggested that the acidic medium may have damaged the coating or that the corrosion mechanism has changed in this medium.

The presence of different concentrations of sodium dodecyl sulfate (SDS) in the electrodepo‐ sition bath to produce nickel/α-Al2O3 and its influence on the anticorrosive performance of the coating produced onto steel in NaCl (3.5% m/v) was also studied [8]. Although the use of a surfactant may enhance the incorporation of inert particle in the metallic matrix, it also has some disadvantages, as they can be incorporated in the metallic matrix and change the mechanical properties of the deposit (e.g., internal stress and fragility). This is why the concentration of surfactants in the deposition bath should be very well controlled [88]. The nanocomposite coatings produced using the optimum SDS concentration (125 g L-1) contained higher levels of alumina nanoparticles in the metallic matrix. This coating presented a more positive corrosion potential (Ecorr = -0.209 VAg/AgCl) and a lower corrosion current density (jcorr = 1.141×10-7 A cm- ²) than the bare substrate (Ecorr = -0.488 and jcorr = 4.832×10-6 A cm-2). It was considered that the incorporation of such low reactive nanometer alumina particles played an important role in improving the corrosion resistance of the coating/substrate system, as they apparently filled the pores and micropores of the coating and thereby reduced the corrosion process [8].

The corrosion performance of Ni-Co alloy matrix composite coatings containing different contents of SiC nanoparticles was evaluated [37]. The coatings were produced onto copper substrate from acid Ni(II) and Co(II) sulfamate solutions using DC and different stirring speeds. The polarization curves of both the composite coatings and the Ni-Co alloy coatings were performed in a 0.5 mol L-1 NaCl solution. It was found that the composite coating produced at 0.03 A cm-2 contained 3.2 wt% SiC and that this coating showed a lower corrosion current density and a more positive corrosion potential (Ecorr = -0.33 VSCE, jcorr = 7.9×10-3 A cm-2) compared to the results of a Ni-Co alloy coating (Ecorr = -0.39 VSCE, jcorr = 3.98×10-2 A cm-2). The better result obtained by this nanocomposite coating was attributed to a decrease in the size of the defects related to the incorporation of SiC nanoparticles, which is useful for creating a tortuous path to the corrosive medium attack the substrate, preventing pitting corrosion and enhancing the corrosion resistance of these nanocomposite coatings [37]. However, the results concerning Ni-Co/SiC coatings containing other SiC contents were not mentioned in the work.

Composite coatings of Ni-Zn matrix containing TiO2 nanoparticles (Degussa P-25 anatase, 25 nm) as the strengthening phase were produced onto a mild steel (DIN C25-AISI 1025) substrate using ultrasonic-assisted electrodeposition process [89]. These coatings were electrochemically evaluated in a natural 3.5 wt% NaCl solution using EIS measurements at the OCP. The results showed that higher Rct values were obtained for samples produced using 108 and 216 W cm-2 of ultrasonic powder densities, indicating a better anticorrosion performance of these coatings. On the contrary, the Rct values of coatings produced without using the ultrasonicassisted process were smaller than that obtained for the alloy Ni-Zn coating. The authors claim that the use of ultrasonic vibration together with electrodeposition might have improved the coating nanoparticles uniform dispersion and hence improved the anticorrosive ability of the composite coatings [89].
