*4.2.2. Copper matrix composite coatings*

The corrosion behavior of electrodeposited Cu/SiO2 nanocomposite coatings, produced onto steel (OL 37) in a bath containing (or not) a surfactant agent C16H33N(CH3)3Br (CTAB) in different concentrations, was evaluated in 0.2 g L-1 Na2SO4 solution at room temperature and pH 3 [45]. A decrease of the corrosion current density for the composite deposit Cu/SiO2 was also observed in comparison with the pure copper coating, suggesting that the presence of nanoparticles had a beneficial effect in decreasing the corrosion rate as shown in Table 4. These results were confirmed by the Rp values obtained from a linear polarization experiment in the same corrosion medium. This behavior was explained by the lowering of the electrode surface area in contact with the electrolyte due to the presence of the nanoparticles and by a more finegrained compact structure of the coating.


1.141×10-7 A cm-

172 Electrodeposition of Composite Materials

composite coatings [89].

*4.2.2. Copper matrix composite coatings*

grained compact structure of the coating.

process [8].

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

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

The corrosion behavior of electrodeposited Cu/SiO2 nanocomposite coatings, produced onto steel (OL 37) in a bath containing (or not) a surfactant agent C16H33N(CH3)3Br (CTAB) in different concentrations, was evaluated in 0.2 g L-1 Na2SO4 solution at room temperature and pH 3 [45]. A decrease of the corrosion current density for the composite deposit Cu/SiO2 was also observed in comparison with the pure copper coating, suggesting that the presence of nanoparticles had a beneficial effect in decreasing the corrosion rate as shown in Table 4. These results were confirmed by the Rp values obtained from a linear polarization experiment in the same corrosion medium. This behavior was explained by the lowering of the electrode surface area in contact with the electrolyte due to the presence of the nanoparticles and by a more fine\*The composite coatings were produced from Cu(II) solutions containing SiO2 10 g L-1 + CTAB (A) 0.00 mol L-1, (B) 10-5 mol L-1, (C) 10-4 mol L-1, and (D) 10-3 mol L-1.

**Table 4.** Corrosion data for Cu and Cu-SiO2 deposits obtained by Zamblau et al. [45] in the absence and presence of different concentrations of CTAB.

The increase of CTAB concentration in the electrolyte hindered the corrosion process even more when CTAB concentration was increased from 1.0×10-5 to 1.0×10-2 mol L-1 and then to 1.0×10-3 mol L-1. As the surfactant increased the incorporation of SiO2 in the coating by modifying the surface charge of the nanoparticles (the negative initial charge turns into a positive one) and their hydrophilicity, the anticorrosive performance of the coating was enhanced [89]. The highest Rp value was obtained for a bath containing 10 g L-1 SiO2 and 1.0×10-3 mol L-1 CTAB, confirming the good effect of this additive in the electrolytic deposition of metal-nanoparticle composites [45].

The protective and anticorrosive properties of copper coatings, Cu+μSiC composite coatings (particle size of 1–2 μm), and Cu+*n*SiC composite coatings (particle size of 20 nm), produced on standardized steel Q-pane, were evaluated by potentiodynamic curves in two different environments. An acidic solution (0.5 mol L-1 Na2SO4 at pH 2) was used to evaluate the uniform corrosion rate, whereas an alkaline solution (0.5 mol L-1 Na2SO4 at pH 12) was employed to evaluate if the presence of the particles influenced the passive potential range and current [91]. Table 5 shows that, in acidic environment, the specimens coated by the Cu+μSiC composite deposit presented the most negative corrosion potential and the highest corrosion rate, expressed by the corrosion current density (jcorr = 5.0 μA cm-2). This behavior was attributed to the voids between the ceramic particles and the metallic matrix that allow the access of the electrolyte to the substrate. These voids were larger in the Cu+μSiC composite coatings than in the Cu+*n*SiC composite ones. In fact, the value of the corrosion potential of the Cu+μSiC composite coatings was comparable to the value obtained by this research group for the bare steel substrate in the same acidic corrosion medium. On the contrary, the Cu+*n*SiC composite coating samples presented no differences in the corrosion potential compared to the pure copper coatings. However, the nanocomposite samples showed a lower corrosion current density and, consequently, a lower corrosion rate than the copper coating, which might be attributed to their more compact microstructure [91].

Nevertheless, the potentiodynamic curves obtained in the alkaline environment showed no differences for the three types of coatings before their passivation. The two composite coatings


**Table 5.** Results for corrosion evaluation of Cu, Cu-μSiO2, and Cu-*n*SiO2 deposits produced by Lekka et al. [91] in acidic (pH 2) 0.5 mol L-1 Na2SO4 solution.

showed an improved passive domain in comparison to the pure copper coating, with only one, continuous, and stable passive zone. On the contrary, the pure metallic coating presented two passive regions corresponding to the formation of two different types of oxides. The voids between the SiO2 microparticles and the metallic matrix earlier mentioned seemed to cause no influence on the behavior of the Cu-μSiO2 composite coating in an alkaline environment because the compact oxide film could have covered them. The samples covered by the Cu+*n*SiC composite coatings also presented the lowest passive current density. Thus, the compact microstructure of the Cu+*n*SiC composite coatings also led to the formation of a compact and protective oxide in alkaline solutions [91].

The cathodic efficiency and consequently the thickness of the Cu-μ(γ-Al2O3) coatings produced from pyrophosphate bath decreased with the increase (in modulus) of the cathodic potential (*E*) independent of the previous stirring time (*t*) used [61]. Thinner coatings can directly influence the anticorrosive performance of the coatings. On the contrary, Figure 6 indicates that there may be a joint influence of both parameters, *E* and *t*, in the coating composition, although none of them alone presented a significant influence.

**Figure 6.** Cu and Al2O3 contents (wt%) in the composite coatings produced from baths containing CuSO4 0.02 mol L-1, K4P2O7 0.90 mol L-1, and γ-Al2O3.

Although there was no direct relationship between the applied cathodic potential and the γ-Al2O3 content in the coating, the increase in this parameter also seemed to increase the dispersion of the particles in the coating, as shown in Figures 7 and 8, for the coatings produced after 5 h of stirring at (800 rpm).

showed an improved passive domain in comparison to the pure copper coating, with only one, continuous, and stable passive zone. On the contrary, the pure metallic coating presented two passive regions corresponding to the formation of two different types of oxides. The voids between the SiO2 microparticles and the metallic matrix earlier mentioned seemed to cause no influence on the behavior of the Cu-μSiO2 composite coating in an alkaline environment because the compact oxide film could have covered them. The samples covered by the Cu+*n*SiC composite coatings also presented the lowest passive current density. Thus, the compact microstructure of the Cu+*n*SiC composite coatings also led to the formation of a compact and

**Table 5.** Results for corrosion evaluation of Cu, Cu-μSiO2, and Cu-*n*SiO2 deposits produced by Lekka et al. [91] in

**Coating Ecorr (VAg/AgCl) jcorr (μA cm-2)** Cu -0.053 0.7 Cu-μSiO2 -0.456 5.0 Cu-*n*SiO2 -0.013 0.3

The cathodic efficiency and consequently the thickness of the Cu-μ(γ-Al2O3) coatings produced from pyrophosphate bath decreased with the increase (in modulus) of the cathodic potential (*E*) independent of the previous stirring time (*t*) used [61]. Thinner coatings can directly influence the anticorrosive performance of the coatings. On the contrary, Figure 6 indicates that there may be a joint influence of both parameters, *E* and *t*, in the coating composition,

**Figure 6.** Cu and Al2O3 contents (wt%) in the composite coatings produced from baths containing CuSO4 0.02 mol L-1,

protective oxide in alkaline solutions [91].

acidic (pH 2) 0.5 mol L-1 Na2SO4 solution.

174 Electrodeposition of Composite Materials

K4P2O7 0.90 mol L-1, and γ-Al2O3.

although none of them alone presented a significant influence.

**Figure 7.** Surface morphology of Cu/γ-Al2O3 coatings produced at -1.20 VSSE from pyrophosphate bath previously stir‐ red for 5 h at 800 rpm [61].

**Figure 8.** Surface morphology of Cu/γ-Al2O3 coatings produced at -1.50 VSSE from pyrophosphate bath previously stir‐ red for 5 h at 800 rpm [61].

It was expected that a coating presenting a well-dispersed second phase and a high amount of micrometric γ-Al2O3 particles presented the best anticorrosive performance. However, these effects had no direct influence on the corrosion performance of these coatings in 0.5 mol L-1 NaCl, and no significant differences could be noted among their Ecorr, as shown in Table 5 and Figure 9, whereas the jcorr values increased when the applied potential became more negative. Moreover, the anticorrosive performance of these coatings, in terms of jcorr, was worse than the values obtained for the pure copper coating (5.27 μA cm-2). The thin layers produced under the deposition conditions could have probably masked the effects of the deposition parameters on the anticorrosive performance of these coatings. In addition, the micrometric size of the γ-Al2O3 particles could have contributed to create defects and voids in the coatings, enhancing the corrosion process, confirming the results earlier shown for Cu-μSiO2 coatings [91], and showing that the size of the particle is an important parameter to produce composite coatings with high anticorrosive characteristics.


**Table 6.** and Ecorr values of composite coatings Cu/γ-Al2O3 in 0.5 mol L-1 NaCl. The coatings were produced at -1.20 and -1.50 VSSE after previous stirring for 5 h of at 800 rpm [61].

**Figure 9.** Polarization curves of Cu/γ-Al2O3 coatings/steel substrate systems in 0.5 mol L-1 NaCl.

Cu/boehmite nanocomposite coatings [AlO(OH) Disperal P2, 25 nm size; Sasol®] was produced onto steel substrate (AISI 1020) by DC electrodeposition using j = 7.0 and 21.0 A m-2 from a pyrophosphate bath, containing (or not) allyl alcohol as an additive agent, under 1000 and 1300 rpm of stirring speed [61]. The allyl alcohol has been reported in the literature as a brightening agent and relieving stress in the production on Cu coatings and Cu-Zn alloy [92]. The corrosion resistances from these coatings were obtained using the linear polarization experiments (Rp). The results showed that the presence of allyl alcohol improved the corrosion resistance of the composite coatings in 0.1 mol L-1 Na2SO4 compared to those produced from baths without this additive under the same deposition conditions [61]. For example, the composite coating produced at 1000 rpm and 7 A m-2 presented Rp = 2956 Ω, when a bath without allyl alcohol was used, and Rp = 2.0×107 Ω, when a bath containing this additive was employed. This behavior can be related to the effect of this type of compound, usually used to refine the coatings in an electrodeposition process and favor the formation of layers without cracks or defect, improving the corrosion resistance [92].
