The process of depositing a coating of one metal over another metal or non-metal electrically is called the electrodeposition. It is used for protective, decorative and functional purposes and includes such processes as electroplating, electroforming, electrotyping, electrofacing, electrometallisation, electrodeposition of rubber and building up of worn-out parts for repairs.
The factors, on which the quality of electrodeposition depends, are given below:
Factor # 1. Nature of Electrolyte:
The formation of smooth deposit largely depends upon the nature of electrolyte employed. The electrolyte from which complex ions can be obtained, such as cyanides, provides a smooth deposit.
Factor # 2. Current Density:
Electrodeposition depends upon the rate at which crystals grow and the rate at which fresh nuclei are formed, therefore, at low current densities the deposits are coarse and crystalline in nature. The deposit of metal will be uniform and fine-grained if the current density is used at rate higher than that at which the nuclei are formed. In case the rate of formation of nuclei is very high due to very high current density, there is a chance that the limiting value of the electrolyte is exceeded. At such instances, the deposit will be spongy and porous.
Factor # 3. Temperature:
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A low temperature of the solution favours formation of small crystals of metal; and a high temperature, large crystals. In some cases this is very marked, a difference of only 15°C resulting in a 50% decrease in strength of the metal deposited. On the other hand, high temperature may give beneficial results due to (a) increased solubility of the salts, permitting greater metal concentration and higher current densities; (b) increased conductivity, which also permits higher current densities and reduces the tendency to form trees; (c) decreased occlusion of hydrogen in the deposited metal, which in many cases is the case of bad deposits. Since both (a) and (b) tend to decrease crystal size, they may in some cases counteract the tendency of temperature alone to increase the crystal size.
Factor # 4. Conductivity:
The use of a solution of good conductivity is important from the standpoint of view of economy in power consumption and also because it reduces the tendency to form trees and rough deposits.
Factor # 5. Electrolytic Concentration:
Higher current density, which is necessary to obtain uniform and fine-grain deposit, can be achieved by increasing the concentration of the electrolyte.
Factor # 6. Additional Agents:
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The addition of acids or other substances to the electrolyte reduces its resistance, as already mentioned. There is another class of additional agents which takes little or no direct part in the chemical reactions but influences the nature of deposit, sometimes even making an otherwise unworkable process into one of practical importance. Such additional agents are glue, gums, dextrose, dextrin, gelatin, agar, alkaloids, albumen, phenol, glycerin, sugar, glucose, rubber etc. The crystal nuclei absorb the additional agent added in the electrolyte. This prevents it to have large growth and thus deposition will be fine-grained. For obtaining satisfactory deposit of zinc from zinc sulphate solution addition of glucose or certain types of sugar is necessary.
Factor # 7. Throwing Power:
This is the ability of electrolyte to produce uniform deposit on an article of irregular shape and is one of the most important characteristics of plating or deposition bath. The distance between the various portions of cathode and anode will be different due to irregular shape of the cathode. Due to unequal distance, the resistance of the current path through the electrolyte for various portions of the cathode will be different but the potential difference between the anode and any point on the article to be plated (cathode) will, of course be the same and the result will be that the current density will be more on the portion nearer to anode and it will cause uneven deposit of the metal.
Throwing power can be improved in two ways—firstly by increasing the distance between the anode and cathode and secondly by reducing the voltage drop at the cathode surface. In some cases decrease of current density causes a decrease in voltage drop at cathode, leaving more voltage available for overcoming the resistance of the electrolyte, thus tending to counteract any change in current concentration. This is the reason that solutions of the cyanides of metals usually have a better throwing power than solutions of the sulphates.
Factor # 8. Polarization:
The rate of deposition of metal increases with the increase in electroplating current density up to a certain limit after which electrolyte surrounding the base metal becomes so much depleted of metal ions that the increase in current density does not cause increase in rate of deposition. Use of current density beyond this limit causes electrolysis of water and hydrogen liberation on the cathode. This hydrogen evolved on the cathode blankets the base metal which reduces the rate of metal deposition.
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This phenomenon is called the polarization. Blanketing effect can be reduced by agitating the electrolyte. With reverse current electroplating, in which at regular intervals plating current is reversed for a second or so, sufficient electron concentration is established around the base metal and the polarization effect becomes negligible even with very high overall speed of plating.
The other advantages of reverse current plating are:
(i) unsound and inferior metal is depleted during reverse current period and flat level surfaces are produced,
(ii) Metal surface is brightened causing elimination of buffing or polishing operation.