Cathodic Metal Deposition

Cathodic deposition (electrocrystallization) of metals is the basic process in electrometallurgy and electroplating. 

Two types of metal deposition are distinguished that on the same metal and that on a substrate (matrix) consisting of another metal. In the latter case a number of special features can arise. 

Underpotential Deposition of Metal Atoms Because of the energy of interaction between a foreign substrate and the adsorbed metal atoms formed by discharge, cathodic discharge of a limited amount of metal ions producing adatoms is possible at potentials more positive than the equilibrium potential of the particular system, and also more positive than the potential of steady metal deposition. 

Incorporation of Metal In certain cases, metal atoms, after their discharge, can penetrate into the substrate metal, forming alloys or intermetallic compounds in the surface layer and down to a certain depth. This effect has been known for a long time in the discharge of metals at liquid mercury, where liquid or solid amalgams are formed. In 1968 B. Kabanov showed that an analogous effect is present in metal ion discharge at many solid metals. 

Epitaxy In many cases, the structure of the deposit will duplicate that of the substrate when the crystallographic parameters of the metal being deposited are not 

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FIGURE 4.3 Migration (/ ) and diffusion (Jj) fluxes of anions and cations in cathodic metal deposition from a symmetric binary solution. 

In applied electrochemistry, reactions are very common in which a new phase is formed (i.e., gas evolution, cathodic metal deposition, etc.). They have a number of special features relative to reactions in which a new phase is not formed and in which the products remain part of the electrolyte phase. 

Two types of reactions producing a new phase can be distinguished (1) those producing a noncrystalline phase (gas bubbles liquid drops as, e.g., in the electrolytic deposition of mercury on substrates not forming amalgams), and (2) those producing a crystalline phase (cathodic metal deposition, anodic deposition of oxides or salts having low solubility). 

Experience shows that in the deposition of a number of metals (mercury, silver, lead, cadmium, and others), the rate of the initial reaction is high, and the associated polarization is low (not over 20 mV). For other metals (particularly of the iron group), high values of polarization are found. The strong inhibition of cathodic metal deposition that is found in the presence of a number of organic substances (and which was described in Section 14.3) is also observed at mercury electrodes (i.e., it can be also associated with the initial step of the process). 

Plasma electrochemical reactions have been studied by chemists for a surprisingly long time, with the first report on cathodic metal deposition at the free surface of a liquid electrolyte with free electrons from a plasma dating back to 1887 [1], long before the plasma state had been named by Langmuir in 1928 [2], A short survey of past work with more conventional liquid electrolytes is also included in this chapter. 

To raise this problem is a favourite remark from members of the audience at meetings on organic electrochemistry, and the answer is a never-failing I don t know, and I know of no way of finding out . However, since pH-control in the layer near the electrode appears to be crucial in, e.g., cathodic metal deposition, several methods have actually been developed for the direct measurement of pH near... 

From the derivation of the above equation and after setting the result as equal to zero, we can obtain the peak potential for the cathodic metallic deposition and its current density, pc and /p, respectively ... 

The equilibrium electrode potential for cathodic metal deposition according to the equation... 

In electrolytic metal plating processes, short circuits can occur due to dendrites or nodules growing out from the cathode surface until they reach the anode. This allows current to pass between anode and cathode without any cathodic metal deposition, thus reducing current efficiency. Short circuits can also occur due to misalignment of cathodes and anodes to the extent that they contact each other, as well as from delaminating or self stripping deposited metal bridging the gap between anode and cathode. 

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