*2.1.4. Stirring speed*

favor the codeposition of the particles. This range, however, depends on the studied particle

For example, the greatest amount of α-Al2O3 nanoparticles was incorporated to a copper matrix at low current density values (between 1 and 2 A dm-2), whereas, outside this range, the content of particles in the coating significantly decreased [11]. This dependence between the content of nanoparticles in the coating and the current density is explained by the mechanism by which the particles are captured. The increase in the particle content in the coatings occurs in the region where the reduction of the metallic ions is under charge transfer control and where the reduction of adsorbed cations on alumina is the determining step of the deposition rate. As the reduction of metal ions occurs under diffusion control at current densities higher than 2 A dm-2 (the hydrodynamic conditions are not mentioned), the codeposition of alumina particles

It was also observed that increasing current density caused a decrease in the initial rate of incorporation of SiO2 particles in the zinc matrix; however, at current densities approximately 30 A dm-2, an increase in SiO2 deposition rate was noted, mainly for the particles with the highest size (2 μm) [6]. Moreover, the regions in which the particle incorporation increased or decreased markedly with current density were sensitive to particle size. The authors suggested that the deposition process was controlled by mass transfer until the maximum value of current density, where the codeposition process was favored. Otherwise, the process was controlled by the particle adsorption on the substrate, and a further increase in current density resulted in the rapid deposition of the metallic matrix and less particles were included in the coating [6]. Similar results were obtained for composite coatings of Ni-Co alloy matrix containing SiC

The literature reports that this relationship between the current density and the amount of incorporated particles also influences the microstructure of the produced MMC coatings [47,49]. For example, the Ni/Al2O3 coatings produced at current density values of 10 A dm-2 from an acid sulfamate-based bath (pH 4.3) presented a more refined microstructure than the coatings produced at lower current density values [50]. It is an important feature, as the microstructure of the coatings can be consequently related to their corrosion resistance.

In addition, the mode of applied current [continuous (DC) or pulsed current (PC)] may also influence the microstructure and corrosion properties of the MMC coatings. Composite coatings of copper matrix and β-SiC particles were produced under DC and PC conditions [51]. The coatings containing β-SiC particles deposited under DC conditions showed grains more refined than those observed from the copper coating. However, this coating presented voids between the incorporated particles and the metal matrix, which influenced the anticorrosive properties of the composite coating, as evidenced by the corrosion tests performed in 0.5 mol L-1 Na2SO4 solution (pH 2). These coatings showed less resistance to both uniform and localized corrosion compared to the pure metal coating. On the contrary, the coatings produced using PC presented a more compact microstructure and, as a consequence, showed elevated resistance to uniform corrosion, similar to pure copper coating and greater than the coatings

and on other deposition parameters, such as bath composition or stirring.

gradually decreases with increasing current density [11].

particles with 50 nm of diameter [37].

154 Electrodeposition of Composite Materials

produced under DC [51].

The electrolyte stirring speed plays an important role in any electrodeposition process, as it favors the transport of metallic ions to the electrode, increasing the deposition rate [52]. In the electrochemical codeposition process of MMC coatings, however, the influence of this parameter is even greater because it affects the homogeneous dispersion of the particle in the suspension and controls the frequency of collision between the particles and the cathode. Furthermore, the stirring speed of the electrolyte influences the mechanism of particle deposition onto the cathode surface as well as the time they will remain adsorbed [27,41].

In general, if the stirring speed of the bath is too slow, it prevents the complete dispersion of the particles, and their sedimentation during the deposition process cannot be avoided. On the contrary, if the stirring speed is too high, the particles do not have enough time to be adsorbed to the substrate surface, resulting in a low amount of particle incorporation [5]. Moreover, under excessive high stirring speed values, the amount of particles transported to the cathode is too large to be trapped by the matrix growth, which causes the collision of the free particles (those particles that have not been adsorbed or incorporated yet) with other particles that are reaching to the cathode. These collisions result in a decrease in the incorpo‐ ration rate [5].

Therefore, this parameter should be optimized to avoid both particle sedimentation and the removal of the particles, which are already in the adsorption phase on the cathode. Nonethe‐ less, the stirring speed range that must be used to achieve the MMC coatings is not a consensus in the literature and it seems to depend on the kind of particle, the metallic ions and bath composition used, as well as the cell configuration and volume [30,47,53,54].

The incorporation of particle α-Al2O3 in a Co-Ni matrix from a Ni(II)/Co(II) sulfamate acid bath containing the suspended particle was studied, under magnetic stirring varying between 40 and 160 rpm [12]. The volume fraction of Al2O3 particles in the composite coating (Vp) increased with stirring speed and reached a maximum value (approximately 8 vol%) at 100 rpm, decreasing with further increased stirring. The authors suggested that the codeposition of Al2O3 particles in the Co-Ni alloy was apparently controlled by the particle transfer up to 100 rpm. A further increase the stirring speed could have displaced the particles spontaneously adsorbed onto the surface of the cathode, causing a reduction in the Vp values of the codepos‐ ited particles.

Another study [47] showed that there is a maximum value of particles of CeO2 (between 15 and 20 nm) incorporated to nickel matrix when the stirring speed was varied (100, 250, 350, 450, and 550 rpm). The highest quantity of particles in the matrix was obtained at 450 rpm; above this value of stirring speed, the content of CeO2 decreases. This phenomenon was explained by the collision of particles on the cathode under high values of stirring speed. Moreover, the increase of stirring speed also enhanced the turbulence in the flux suspension, which could have removed the weakly adsorbed particles from the cathode surface [47]. Similar results were obtained by other research groups [15,49,54] for different nickel composite coatings.
