Morphology of Electrochemically and Chemically Deposited Metals

Morphology is probably the most important property of electrodeposited metals. It depends mainly on the kinetic parameters of the deposition process and on the deposition overpotential or current density. The morphology of an electrodeposited metal also depends on the deposition time until the deposit has attained its final form.

As already given in Chap. 1, the most frequently used form of the cathodic polarization curve equation for flat or large spherical electrode of massive metal is given by:where i, i ₀, and i _L are the current density, exchange current density, and limiting diffusion current density, respectively, andwhere b _c and b _a are the cathodic and anodic Tafel slopes and η is the overpotential. Equation (1.13) is modified for use in electrodeposition of metals by taking cathodic current density and overpotential as positive. Derivation of the Eq. (1.13) is performed under assumption that the concentration dependence of i ₀ can be neglected [1–4].

The current distribution on a macroprofile is very important in technical metal electrodeposition. In electroplating, the current distribution determines the local variations in the thickness of the coating. In electrowinning and electrorefining of metals, a non-homogeneous current distribution can cause a short circuit with the counter electrode and the corner weakness effect in electroforming. This is very important in the three-dimensional electrodes, as well as in some storage batteries. In all the cited cases, a uniform current density distribution over the macroprofile is required.

The application of a periodically changing current in metal electrodeposition practice leads to improvements in the quality of electrodeposits. Three types of current variation have been found useful: reversing current (RC), pulsating current (PC), and sinusoidal, alternating current superimposed on a direct current (AC) [1–12]. The schematic presentation of the different current regimes of electrolysis is shown in Fig. 4.1. Also, the beneficial effects of pulsating overpotential (PO) have also been discussed [3]. Even though this kind of electrodeposition at a periodically changing rate (EPCR) is important from a theoretical point of view and offers a variety of experimental possibilities, it is as yet not frequently used in metal electrodeposition practice.

Hydrogen generated during electrodeposition processes can achieve a significant influence on morphology of electrodeposited metal. This effect is especially important during electrodeposition of metals characterized by low (so-called the, like Cu) and very low (so-called the, like Ni, Co, Fe, Pt, Cr) overpotentials for hydrogen discharge [1]. In the case of Cu, hydrogen evolution commences at some overpotential belonging to the plateau of the limiting diffusion current density. The increase of the overpotential intensifies this reaction (see Fig. 1.​10a). For copper solution containing 0.10 M CuSO₄ in 0.50 M H₂SO₄, the plateau of the limiting diffusion current density corresponds to the range of overpotentials between 300 and 750 mV, and hydrogen evolution as the second reaction commences at an overpotential of 590 mV [2]. The quantity of evolved hydrogen is determined by the current efficiency for hydrogen evolution reaction, η _I,av(H₂). As presented in Table 5.1, the current efficiency of hydrogen evolution increases with an increase in overpotential.

Powders are finely divided solids, smaller than 1000 μm in its maximum dimension. A particle is defined as the smallest unit of a powder. The particles of powder may assume various forms and sizes, whereas the powders, as an association of such particles, exhibit, more or less, the same characteristics as if they were formed under identical conditions and if the manipulation of the deposits after removal from the electrode was the same [1, 2]. The size of particles of many metal powders can vary in a quite wide range from a few nanometers to several hundreds of micrometers. The most important properties of a metal powder are the specific surface, the apparent density, the flowability, and the particle grain size distribution. These properties, called decisive properties, characterize the behavior of a metal powder.

It is general experience in materials science that alloy can exhibit qualities that are unobtainable with parent metals. This is particularly true for electrodeposited alloys, mainly due to formation of metastable phases and intermediate layers. Some important properties of materials, such as hardness, ductility, tensile strength, Young’s modulus, corrosion resistance, solderability, wear resistance, antifriction service, etc., may be enhanced. At the same time, some properties that are not characteristic for parent metals, such as high magnetic permeability, other magnetic and electrical properties, amorphous structure, etc., can also be obtained. In some cases, alloy coatings may be more suitable for subsequent electroplate overlayers and conversion chemical treatments [1].

The alloy powders of the iron-group metals (anomalous codeposition) are of great interest for many industrial applications [1]. The Co-Ni powders showed significant promise for future development of hard magnetic materials, commercial batteries, catalysts, catalyzing electroplates, hydrogen-absorbing alloy anodes, and magnetoresistive sensors, being made by several different techniques [1]. Unfortunately, limited number of papers concerning Co-Ni powder electrodeposition exists in the literature [2–7]. As shown in some of these papers [4–7], the morphology and composition of electrodeposited powders were found to be sensitive to the solution composition (to the ratio of Ni²⁺/Co²⁺ ion concentration). Fe-Ni-based alloy powders are known as promising soft magnetic materials with low coercivity and high permeability [1]. Electrodeposition of Fe-Ni alloy powders was the subject of only few papers [8–12]. The influence of the composition of electrolyte (Ni²⁺/Fe²⁺ ions ratio) on the powder morphology was investigated in Ref. [8, 9]. Zhelibo et al. [10, 11] suggested a method for producing very fine Fe-Ni alloy powder by the electrolysis in a two-layer electrolytic bath, using a hydrocarbon solvent from an oil-refining fraction as an upper organic layer with evaporation at 180 °C and subsequent reduction annealing in a hydrogen atmosphere. The influence of the reduction annealing temperature [10] and the electrolysis temperature [11] on the formation, chemical and phase composition, structure, and magnetic properties of highly dispersed Fe-Ni alloy powders was investigated, and the optimal thermal conditions for the production of powders with micro-sized particles were determined [10, 11]. The effect of complexing agents (citric and oxalic acid) on the process of Fe-Ni alloy powder electrodeposition was also investigated [12]. It was shown that complexing agents influence the kinetics of powder electrodeposition as well as the morphology of the Fe-Ni powders. Finer powders were produced in the presence of citric acid in comparison with those obtained in the presence of oxalic acid [1, 12].

The term chemical deposition of metals and/or alloys from aqueous solutions is usually used to refer to the production of metallic coatings or powders of various surface morphology and properties without an application of the external current source. As explained in previous chapters in the electrochemical deposition, electrons used for the reduction of metal ions are provided by an external current source. For the chemical deposition, electrons used for the reduction of metal ions are released under specific conditions from an appropriate reducing agent. These appropriate reducing agent may include compounds such as hypophosphite (NaH₂PO₂), borohydride (NaBH₄), formaldehyde (HCOH), ascorbic acid (C₆H₈O₆), etc., or metals which are less noble than the metal aimed to be deposited. These concepts of chemical deposition will be examined in details in the following text.