Theory and Practice of Metal Electrodeposition

Electrodeposition refers to a film growth process which consists in the formation of a metallic coating onto a base material occurring through the electrochemical reduction of metal ions from an electrolyte. The corresponding technology is often known as electroplating. Besides the production of metallic coatings, electrochemical metal reduction is also used for the extraction of metals starting from their ores (electrometallurgy) or for the reproduction of molds to form objects directly in their final shape (electroforming). In most cases, the metallic deposit thus obtained is crystalline; this process can therefore be called also electrocrystallization; this term was introduced by the Russian chemist V. Kistiakovski in the early twentieth century.

The electrodeposition of metals or alloys occurs within a spatial region of finite thickness at the interface (or, more precisely, an interphase) between the growing material and the solution. The structure of this region, in particular the distribution of ions, solvent molecules and other uncharged species, and the resulting distribution of electric charges and potential, has an important bearing on the interface energy of the system, the nature and rate of charge transfer processes, and on the processes of nucleation and growth of metallic crystals. In general, charge separation occurs at this interface as a result of the different nature of the mobile charges in the two regions considered: electrons in the solid and ions in the electrolytic solution. The electronic charge distribution in the electrode extends into the solution farther than the charges generated by the ionic cores, and this excess of charges must be balanced by an opposite charge in the electrolyte. In the simplest approximation this separation of charges can be thought of as a parallel arrangement of opposite charges; for this reason, this region is also called double layer.

The thermodynamic conditions of equilibrium for an isolated biphasic system include the equality of the chemical potential in the two phases.

The concentration of electroactive ions c _i which determines the overall kinetics of the electrode process differs in general from the bulk ion concentration. This is a consequence of the location of ion discharge, which is at a potential ψ′, different from the bulk solution potential, as it have been noted earlier. Additionally, at high overpotentials, the rate of discharge is very fast and ion reduction is limited by the rate of transport to the electrode surface. The steady state rate of the electrochemical process is in this case exactly equal to the rate of transport of the electroactive species from the bulk.

In the process of metal electrodeposition metal ions are reduced at the substrate, forming adsorbed atoms that diffuse on the substrate surface; these adatoms will eventually contact other adatoms, forming atomic clusters that may be stable or unstable. Unstable clusters will eventually disappear, while stable clusters will be able to grow, finally forming the film.

Electrocrystallization proceeds through the nucleation of stable clusters, followed by the attachment of adatoms to these clusters. The kinetics of attachment of incoming atoms to a flat macroscopic surface does not differ from that occurring during the growth of supercritical clusters about four times larger than the critical size; with smaller clusters on the other hand the attachment rate decreases, as discussed in Chap. 5. A rigorous description of the growth process however requires a more specific discussion, since the growing surface is characterized by various geometric and energetic peculiarities, which will be considered in this chapter.

The growth rate, composition, microstructure and properties of an electrodeposited metal or alloy are mainly determined by the potential and current distribution at the electrode. It is therefore essential to understand the phenomena that determine potential and current distribution in order to control the film characteristics. For example, an inhomogeneous current distribution may result in a non-uniform film thickness, with possible negative impact on the functional properties of the coating.

At a rough electrode surface, the local current densities at peaks and valleys are different even though the macroscopic current distribution over a given surface region is completely uniform. The conductivity of the electrode material is usually much larger than that of the electrolyte. In this case, the electric field in the vicinity of the interface is non-uniform: the equipotential surfaces reproduce the surface topography and the local current lines must be perpendicular to the surface (Fig. 8.1). The resulting local field pattern depends on the ratio of the conductivities of the electrolyte and the electrode, on the characteristics of the surface profile and also on the hydrodynamic conditions. In turn, any non-uniformity of the electric field results in a corresponding non-uniformity of the current density.

In the absence of faradaic processes the concentration of reacting species in the electrochemical cell is uniform, and no diffusion layer is formed at the electrode. After imposing an applied potential or a current to the electrode of interest, the concentration profile of the various electroactive species changes with time until it is stabilized by the balance of faradaic, diffusional and convective processes. A stationary diffusion layer with a definite thickness is eventually set up when the applied potential or current is constant in time; this situation was discussed in the previous chapter. Under non-steady state electrochemical conditions, which include in particular those cases where the current or potential vary with time, also the diffusion layer evolves with time. In this chapter, various time-dependent electrochemical phenomena will be considered, focusing particularly on their influence on metal electrodeposition. The concepts thus developed will also be applied to the study of electrode kinetics.

The theory of codeposition of two or several metals is of particular interest, as the current trend in engineering and technology supports the replacement of individual metals by their alloys, which usually feature a wider spectrum of properties. This is particularly true in electroplating technology. In addition, the theory of codeposition is relevant even when individual metals are deposited, as the latter also contain codeposited metallic impurities; the problem of deposit purity is also partially an issue of alloy formation.

Practically all the chemical species present in an electroplating solution can eventually be incorporated as impurities in electrochemically deposited metals. However, the mechanism and kinetics of their incorporation by the growing deposit can differ widely; similarly, the qualitative relationships between the concentration of impurity atoms in the solution and in the deposit and also their chemical state may vary significantly. For example, the relative content of the elements in the deposit can be higher than in the solution, or the same quantity can be lower by several orders of magnitude, depending on the mechanism of incorporation.

Electrodeposition is a technology for the production of metals, alloys or composite films, characterized by a unique simplicity of implementation, low capital cost, and high versatility. This technique has been in use since the nineteenth century for the deposition of decorative films, as well as coatings that impart better corrosion resistance, or improved mechanical or wear properties. During the last 40 years however, electrodeposition has been applied to various aspects of manufacturing in electronics, leading to revolutionary advances; with an increased understanding of its fundamentals, it is being utilized today in an ever wider set of applications. In this chapter we discuss the set of materials that can be electroplated from aqueous solutions, their applications, and the methods involved in their manufacturing.

A wide range of metals and alloys can be electrodeposited from aqueous solutions. These include most of the mid- and late transition metals, the precious metals, and some of the 4p, 5p and 6p elements. Some early transition metals (V, Mo, W) as well as the elements P and B can be co-deposited only simultaneously with the deposition of the iron group metals. Highly reactive elements cannot be electrodeposited due to their low redox potentials; the dissociation of the solvent is highly competitive with the reduction of these elements and the electrode cannot be made to assume the potential required for their reduction.

The physical, chemical and mechanical properties, and consequently the performance of electrodeposited metals and alloys usually differ from the reference data reported for the pure metallurgical samples. These variations are due to two main factors: the peculiarities in the structure and microstructure of electrodeposited materials and the presence of impurities. In the case of alloys, the discrepancy between the actual phase structure of the electrodeposit and the equilibrium phase diagram plays an important role as well.

The properties of electrodeposited metals deviate to a greater or lesser extent from the standard values reported for pure bulk materials obtained by metallurgical methods (for example, crystallized from the melt), which usually exhibit a structure close to the equilibrium one. These deviations are mainly due to a higher density of structural imperfections and the presence of impurities (among these hydroxides, sulfides or organic substances) uncharacteristic of the corresponding bulk metals. In particular, the observed variation of deposit properties appear to be related to the grain boundaries, more precisely to the volume of defected region localized near the boundaries, the presence of precipitates or impurities—in particular oxides and hydroxides—and finally to the structural features of these regions, for example, the angular disorientation of neighboring crystals.