The electrolytic processes can be used only in the case of a limited number of metals and the factors governing deposition processes are: 1. Throwing Power 2. Nature of Electrolyte 3. Heating of the Bath (Temperature) 4. Nature of the Metal on which Deposit is to be made 5. Addition of Agents 6. Electrolytic Concentration 7. Polarisation and its Effects on Electro-Deposition.
Factor # 1. Throwing Power:
Throwing power is the ability of the electrolytic bath to produce an even deposit on the article of irregular shape. In fig 5.5 is shown an article ABCD having an irregular shape. The potential between any point on the article and the anode is the same.
But the surface AB is twice as from the anode as the surface CD with the result that the resistance to the current path in the former case is about twice as much as in the, later case. Hence the thickness of the deposit on the surface AB will be smaller.
Better result can, however be obtained by increasing the distance between the object and the anode so that the relative variation in resistance between the anode and the different parts on the object is reduced. The voltage drop at the cathode surface also influences the deposition. The solutions of cyanides of metals usually give greater throwing power the solution of sulphates.
Factor # 2. Nature of Electrolyte:
Smooth deposits are obtained from solutions having complex ions e.g. cyanides. Silver from nitrate solution forms a coarse deposit while from cyanide solution is forms a smooth deposits. Therefore, the formation of smooth deposit largely depends upon the nature of the electrolyte used.
Factor # 3. Heating of the Bath (Temperature):
In many electro-deposition processes there is a requirement that the bath should be heated to a high temperature. This can be achieved by using steam or electric heating. At moderate temperatures, the deposits are good.
Factor # 4. Nature of the Metal on which Deposit is to be made:
This factor influences the growth of crystals since it is believed that the operation of crystals is in continuation of these in the base metal.
Factor # 5. Addition of Agents:
Addition of sulphuric acid to the copper sulphate solution reduces the resistance of the electrolyte and saves a certain amount of power. Addition of glucose to zinc sulphate is essential for satisfactory deposition of zinc. The quality of deposit is improved by adding organic compounds such as gums, rubber, alkalis and sugar etc.
Factor # 6. Electrolytic Concentration:
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Increase of concentration of the electrolyte tends to give better deposit because by increasing the concentration of the electrolyte, high current density can be achieved.
Factor # 7. Polarisation and its Effects on Electro-Deposition:
The rate at which the metals are deposited depends upon the current density. If the current density is low, the deposit is coarse and crystalline and at higher current densities, the deposit will be uniform.
But if the current density is more than the require limit, it will result in electrolysis of water and hydrogen will deposit on the cathode. The hydrogen evolved blackens the base metal which diminishes the rate of metal deposition. This phenomenon is called polarization. Blackening effect of the base metal can be reduced by agitating the electrolyte.