**2. Electrochemical codeposition of MMC coatings**

Electroplating or electrodeposition is an electrochemical process that occurs at the interface between a conductive material (electrode) and a conductive solution (electrolyte). As a whole, the process can be explained as the result of applying an electrical potential to the electro‐ chemical system by an external source, and the consequent flow of electric current through the electrode-electrolyte interface can be measured (chronoamperometry) [29–31]. On the contra‐ ry, an electric current can also be applied, and the generated potential difference is then the measured variable (chronopotentiometry) [32]. In the electrolytic cell of an electrodeposition process, the cathode is generally the working electrode, which will be coated during the procedure, whereas the anode is the counter-electrode. The third electrode is the reference electrode used to monitor the working electrode potential [30,33–35].

**Deposition process**

150 Electrodeposition of Composite Materials

**Substrate Metallic**

**matrix**

**Table 1.** Examples of MMC coatings produced by different deposition processes.

**2. Electrochemical codeposition of MMC coatings**

reliability to prevent corrosion.

**Second phase Main features Ref.**

to the manual one;

adjusted

Although all of the aforementioned processes may be used to produce functional MMC coatings, the most widely used production method is electrochemical codeposition, also known as electrocodeposition, which consists of incorporating nanoparticles or nanowires (generally nonmetallic ones) intentionally added to the electrolyte to the metallic matrix during the electrodeposition process. This technique has been under investigation for several decades, and some authors have proposed models to explain the codeposition phenomenon of the particles during the formation of a cathodic deposit by electrodeposition [6,8,11,23–25]. This topic is still up to date because the process is more complex than the traditional electrodepo‐ sition and no commercial electrochemical baths have been developed so far for industrial production of these types of composite coatings. The main parameters that affect the process (e.g., solution pH, stirring speed, and current density) were related; however, there is no consensus in the literature concerning their effects in the nanoparticle content in the coating and in the anticorrosive performance of the coating/substrate system [5,6,8,11,12,17,23,26–28]. It is also necessary that the nanoparticles be maintained suspended and dispersed (nonag‐ glomerated) in the electrolyte during the deposition process; otherwise, the precipitation of the nanoparticles occurs and causes the loss of control of the electrochemical codeposition process. Therefore, it is important to present a more fundamental review of the parameters that involve the electrodeposition process of these coatings to obtain a better understanding of the electrochemistry codeposition phenomenon and its consequence on the anticorrosive properties of the composite coatings. Consequently, it will lead to an improvement of the processes for the development of new functional nanocomposite coatings and increase their

Electroplating or electrodeposition is an electrochemical process that occurs at the interface between a conductive material (electrode) and a conductive solution (electrolyte). As a whole, the process can be explained as the result of applying an electrical potential to the electro‐ chemical system by an external source, and the consequent flow of electric current through the

with the automatic process compared


The main advantage of this technique is the production of coatings with thickness varying from a few layers up to 40 μm, relatively free of pores. Compared to the plasma processes (PVD or CVD), it is a less expensive technique and can be conducted at room temperature, normal pressure, and high deposition rate [34,36,37]. In addition, it is economically important because even thin-layer coatings produced by electrodeposition can offer adequate protection to the substrate, avoiding excess of electrodeposited metal [29,34]. The most common coatings produced by this process are the metallic ones: nickel, chromium, copper, zinc, tin, brass, silver, and cadmium [31].

The electrodeposition technique also allows the production of coatings composed by a second phase dispersed in a metallic matrix, producing MMC coatings. This second phase can be an organic or inorganic compound or even a metal particle suspended in the solution [8,34]. This is the main difference between the usual electrodeposition of metallic ions and the electrode‐ position of MMC coatings (electrochemical codeposition): instead of using a pure ionic soluble solution as the electrolyte, particles of nonconductive, semiconductive, and/or conductive nature are suspended in electrolyte solution [11,26,38–40]. Thus, during the codeposition process, both the metal matrix and the particles are deposited on the substrate, producing the MMC coatings [26,38,39].

The greatest challenge faced by those who study the codeposition of nanoparticles in MMC coatings by electrochemical codeposition seems to be the development of a methodology to deposit a sufficient amount of particles to promote the desired improvements in the properties (anticorrosive characteristics, mechanical resistance, etc.) of coatings compared to those obtained with the pure metallic coating. Additionally, it is also necessary to prevent the agglomeration of the particles in the electrolyte solution [23].

A better understanding of the electrocodeposition process is obtained by the knowledge of the process and mechanisms that involve the metal/particle codeposition. Several parameters influence the deposition process, such as applied current density (or potential), concentration of particles in the bath, size of the particles, stirring speed of the suspension, time of previous stirring, solution pH, bath temperature, and electrical nature of the particles. The stability of the suspension of particles to be added to metallic matrix also affects the codeposition process. Although these parameters will be presented in this revision separately, it is important to point out that several of them usually present mutual interactions and these effects can also influence the final results (the amount of codeposited particles, for example) as well as the final prop‐ erties of the coating.
