*2.1.1. Particle concentration in solution*

The particle concentration in the electrolyte may affect the deposition process, changing the ratio metal/particle in the coating and its grain size and causing variations on the coating properties. The incorporation rate per volume of the particles in the deposit is an increasing function of the concentration of particles in the electrolyte suspension [23,41] and is a param‐ eter often used to control the amount of particles in the coating [27,28]. However, as shown in several studies, it is evident that the amount of particles in the deposit does not grow infinitely but reaches a limit value [5,27,42], which depends on the deposition conditions. The concen‐ tration of particles in the electrolyte can also result in problems relating to the homogeneity of their suspension, agglomeration, and precipitation [27,28].

Composite coatings of Zn-SiO2 were produced in the presence of N,N-dimethyldodecylamine and there was a direct trend in the increase of the amount of incorporated particles up to 100 g L-1 of silica particles in the solution [6]. Beyond this concentration, however, the increasing incorporation response is oscillating. This behavior was related to the fact that any concentra‐ tion beyond 100 g L-1 of particles might be sufficiently high to induce localized agglomeration, which could lead to uneven distribution of particles in the coatings.

The TiO2 particle concentration (5.0, 10.0, and 15.0 g L-1) in the electrolytic bath also influenced the content of these particles in a zinc matrix composite coating [43]. Although the increase of particle concentration in the bath elevated the content of particles in the coatings, concentra‐ tions higher than 10.0 g L-1 caused a decrease in the codeposited particle content in the metallic matrix. This effect was explained by the agglomeration of particles in the coating due to their poor wettability [43].

The dependence of TiO2 nanoparticles (1.0, 1.6, 10.0, and 16.0 g L-1) added to different concen‐ trations of Zn(II) electrolyte (0.5, 0.3, and 0.1 mol L-1) on the grain size of the Zn matrix was also investigated [44]. It was observed that the grain size of the metal matrix decreases with the increase of the nanoparticles added to the bath, which was related to changes in the nucleation and growth processes of zinc crystals due to the presence of these particles. Similar results were obtained for nickel deposition in the presence of SiC nanoparticles [44] and for copper codeposition with SiO2 nanoparticles [45]. These results are in agreement with the literature, which relates this grain refining effect to the nanoparticle abilities of providing more nucleation sites and, consequently, decreasing the velocity of the crystal growth process [45].
