*4.1.1. Polarization curves*

**3. Functional composite coatings with improved mechanical properties**

microstructure and can create new mechanical features for these coatings.

166 Electrodeposition of Composite Materials

fracture toughness presented by some MMC coatings [77,78].

coated samples [76].

It is well known that the mechanical characteristics of coating materials produced by electro‐ deposition are related to their microstructure, concerning the effects of grains and grain boundaries and also the preferred orientation of the deposits, which are dependent on the electroplating parameters [72–74]. The presence of particles in the electrolytic bath that are incorporated to the final electrodeposited coating will also contribute to changes in the coating

There are several works in the literature in which MMC functional coatings are produced to improve the mechanical properties of the substrate. Some of them are presented here [37,38,40,47,75] as an example, because this item alone could be the topic of another review chapter. The generally studied electrodeposition parameters, such as stirring speed or current density, are still mentioned [47,75]. For example, the electrolytic bath stirring speed affected the hardness of nickel matrix composite coatings produced with the addition of particles of CeO2 (size between 15 and 20 nm) in the electrolytic bath [47]. The hardness of the MMC coatings increased with the stirring speed, reaching its maximum value (760±80 HV) at 450 rpm. Above this stirring speed value, however, there was a reduction in the coating hardness, which would be associated with the agglomeration of the particles of CeO2 in metallic matrix. Most of the works found in the literature, however, are related to the mechanical effects concerning the presence of the particle in the coating, the amount of these particles, and their size [38,75–78]. As the MMC coatings reinforced by particles with larger sizes (μm or higher) are susceptible to the formation of defects during mechanical loading, resulting in a premature failure of the composite coatings, it is expected that coatings containing nanorange particles present superior mechanical properties. Therefore, nanocomposite coatings could overcome some limitations, such as poor ductility and elongation, poor wear resistance, and reduced

Nanohardness evaluations were performed by applying normal and lateral force (friction) on nickel metallic matrix reinforced (or not) with SiC particles measuring 10 nm, 50 nm, or 5 μm [38]. It was observed that the addition of the particles in the composite coating decreased the penetration of the Berkovich's indentation in the normal direction, compared to the nickel coating, independent of the particle size. Moreover, the smallest the particle size used, the highest was the nanohardness of the composite coatings. However, there were no significant differences among the composite coatings concerning the applied lateral force, although all of them could support homogeneously frictional force compared to the pure nickel coating [38]. Similar results, concerning the hardness measurements, were found for zinc composite coating matrix reinforced with TiO2 particles (size between 100 and 200 nm) [76]. The higher hardness of the coating was related to the fine-grained structure of the deposit. During hardness measurements, the dispersed particles in the fine-grained matrix might have obstructed the easy movement of dislocations, which was shown by the higher hardness values of composite-

Nickel MMC coatings containing 7.9 and 11.5 wt% SiC (particle size of 10 nm), obtained by PC electrodeposition, also presented nanohardness values (3.98±0.032 and 4.10±0.065 GPa,

The polarization curves are used to evaluate the behavior of the coating/substrate system in a certain medium when the potential is varied from the corrosion potential of the system to both anodic and cathodic directions [56]. When the metal is far apart of its corrosion potential (more than ±50 mV), it is said that the metal is polarized [32]. These curves are not related to one particular electrochemical reaction; instead, they show the overall effects of all reactions that occur simultaneously on the electrode [32].

The Tafel extrapolation of the straight part of the polarization curves permits the quantitative measurement of various electrochemical parameters very useful for corrosion evaluation. The 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 current density for passivation (jcrit), and the pitting potential (Epitting) [32].
