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Present day electroplating has become a well-established branch of metal finishing. Electroplating is a multidiscipline of engineering, mechanical and electrical, in co-ordination with applied chemistry.
In the early days of electroplating the industry started with wooden vats, D.C. generators, experience and ‘rule of thumb’ methods of process control. Over the years new metal finishes have been introduced. Automatic plant has been developed to cope with the increased volume of parts to be finished and to control the process, ensuring a constant quality of finish. Increased uses of metal (steel, stainless steel) and various plastics have been seen in the making of equipment for the finishing shop.
Present day finishing shops offer a wide and varied range of finishing processes: ion and gas plating, high speed selective plating, anodizing and electroplating on aluminum. Various electroless finishes cover a wide range of engineering requirements. Various alloy platings are carried out, such as gold cobalt, which gives a hard thickness of gold. There is also brass plating for electroplating safety pins, and components which have various rubbers bonded to them.
Over the last fifteen years development has been carried out on the plating of plastics. The technology of printed boards in the electronics industry has added impetus to the development and many plastics can be successfully electroplated. With the various processes involved in electroplating and current requirements of health and safety, appropriate precautions must be undertaken to avoid accidents and reduce pollution of the environment. These are dealt with in one of the following chapters.
With most of the processes used in model engineering there is ready-made equipment sold on the market, obvious examples being lathes, milling and drilling machines, along with the materials, metals and plastics. In comparison, with the electroplating process there is very little choice available on the market, apart from kits for electroplating. This is due to their limited use as compared to the machining and fabricating operations in model engineering and in small workshops generally.
The other main reason, however, is the degree of availability of the chemicals. Certain chemicals are restricted, and restrictions are placed on them in transit. The electrical equipment needed can be adapted from other sources – electrical test equipment, Avometers, or battery chargers or large capacity electrical cells. If desired a permanent rig can be made. This is useful for a continuous volume of components that have to be finished. A wiring diagram is included in the chapter on the supply of current.
With regard to the tanks required, this is dependent on the size of the component to be electroplated. A useful size is the 5 liter plastic ice cream container. These are useful for most pre-treatment and electroplating solutions. For warm or hot solutions, ways and means of heating the solution may be considered, such as fish tank heaters, or, if using a stainless steel or mild steel tank, a gas ring or electric hot plate may be used.
Chemical glass beakers made of heat-resisting glass may be used and can be heated on an electric hot plate or over a Bunsen burner with a suitable stand and gauze. This equipment can be purchased at most laboratory equipment suppliers.
The model engineer must decide on what size and volume of components he wants to electroplate, and what finishes he wants to use. These points will have to be considered, whether he wants a rudimentary or a more substantial electroplating facility. The other relevant factors are the space available, cost, and the convenience of using the equipment. For example, considering one finish for similar size components and a steady volume, it would suffice to have a simple facility of an alkali cleaner, a pickle made of diluted acid, with a rinse tank containing cold water or preferably running water. It could be made even simpler for certain components by giving them a scour with abrasive powder, then rinsing in cold water.
After these pretreatments, the components are electroplated in whatever electrolyte is chosen.
For an electrical supply a 12 volt battery, or a battery charger of 12 volts or 6 volts, may be used.
At the other extreme, for varied components and large volume, one could use an elaborate line of pretreatment and rinse tanks, with a line of electroplating tanks all combined with the custom-built rectifiers, heaters and agitation. The cost of this would be considerable, and there would be the problem of disposing of effluent and spent chemicals.
PRINCIPLES OF ELECTROPLATING
The fundamental laws of electroplating are based on Faraday’s two laws. These state:-
(1) The weight of metal deposited is proportional to the quantity of electricity passed.
(b) For the same quantity of electricity, the weight of metal deposited is proportional to its electro-chemical equivalent.
These two laws need a little explanation to understand their implications. This is best provided by defining the units. In law 1, the weight (w) is in grams or ounces and the quantity of electricity is in coulombs, which is amps (a) x time (seconds) (t).
Therefore w is proportional to a x t.
In law 2, the electro-chemical equivalent is defined as the weight an element will replace or combine with eight parts by weight of oxygen in a reaction.
The valency is defined as the number of atoms of hydrogen which will combine or replace one atom of an element, in this case a metal.
The relationship between atomic weight and electro-chemical equivalent is: atomic weight = electro-chemical equivalent x valency. An example is nickel, atomic weight = 58.71, valency 2. Therefore electro-chemical equivalent
The Faraday as a unit is defined as a quantity of electricity which is (amps x time), which will deposit 1 gram equivalent of the metal.
The figure for 1 Faraday is 96,500 (amps x time) or coulombs. Now 1 ampere hour is 1 amp x 3600 seconds, or 3600 coulombs. So in 1 ampere hour the weight of a metal deposited will be:
