Electrolyte

Electrolyte is the electrical conductor in which current is carried by ions rather than by free electrons (as in a metal). Electrolyte completes an electric circuit between two electrodes. Upon application of electric current, the positive ions in the electrolyte will move toward the cathode and the negatively charged ions toward the anode. This migration of ions through the electrolyte constitutes the electric current in that part of the circuit. The migration of electrons into the anode through the wiring and an electric generator and then back to the cathode constitutes the current in the external circuit. The metallic ions of the salt in the electrolyte carry a positive charge and are thus attracted to the cathode. When they reach the negatively charged workpiece, it provides electrons to reduce those positively charged ions to metallic form, and then the metal atoms will be deposited onto the surface of the negatively charged workpiece.

Different metals may need different types of electrolyte. The composition and properties of the electrolyte is very important for the coating quality. Types of electrolytes include water solutions of acids, bases, or metal salts, certain pure liquids. In addition to metal salts, electrodeposition electrolytes usually contain a number of additives for various purposes. Some agents are used to increase electrolyte conductivity (supporting electrolytes). Inorganic and organic salts, acids or alkalis are used to increase the electrolytic conductance .Other additives may be used for increasing bath stability (stabilizers), activating the surface (surfactants or wetting agents), improving levelling or metal distribution (levelling agents), or optimizing the chemical, physical, or technology properties of the coating. These coating properties include corrosion resistance, brightness or reflectivity, hardness, mechanical strength, ductility, internal stress, wear resistance, or solderability.

The equilibrium electrode potential is the electrical potential of an electrode measured against a reference electrode when there is no current flowing through the electrode. It is also called open circuit potential (OCP).

The equilibrium potential between a metal and a solution of its ions is given by the Nernst equation as follows:

E = E⁰ - RT/nF lna

where E⁰ is the standard electrode potential, which is a constant characteristic of the material of the electrode; R the gas constant (8.3143 J/mol.K); T the absolute temperature (K); F the Faraday constant; n the valence change; a the activity of the metal ion. In approximation, the concentration of the metal ion can be used instead of the activity. The equilibrium is dynamic with metal ions being discharged and metal atoms being ionized, but these two effects cancel each other and there is no net change in the system. For the realization of metal deposition at the cathode and metal dissolution at the anode, the system must be moved away from the equilibrium condition. An external potential must be provided for the useful electrode reactions to take place at a practical rate; this external potential may have several causes.

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