Electroplating Theory 1
Most inorganic (non living matter) and some organic chemical compounds in a molten state or dissolved in water become ionized, that is, their molecules become disassociated into positively and negatively charged components which have the ability to conduct an electric current through the medium in which they are suspended. If a pair of electrodes (positive and negative terminals, such as in a car battery) are placed in a solution of an electrolyte (chemicals dissolved in a medium such as water) and a source of direct current is connected between them, the positive ions in the solution will move toward the negative electrode and the negative ions will move toward the positive electrode (like in a magnet). Upon reaching the electrodes the ions will gain or lose electrons and be transformed into neutral atoms.
Electroplating is performed when the workpiece is placed in an electrolyte, usually called a bath, that has the coating metal dissolved in it. These metals are called salts. A salt is the product of an acid or base and a metal. Thus if you put nickel in sulfuric acid, you get nickel sulfate, if you put nickel in hydrochloric acid, you get nickel chloride. Copper introduced to cyanide, a strong base, will produce copper cyanide. The goal in electroplating is to tear apart these salt molecules, liberate the metal component, and deposit it on the workpiece. The two atoms, i.e., nickel and sulfur in a molecule of nickel sulfate (it also contains oxygen) are held together by opposite electrical charges (opposites attract). Dissolving nickel sulfate in water and introducing a positive and negative current into the water will, if sufficiently strong, overcome the attraction of the nickel and sulfur to each other and attract the nickel to the cathode (negative terminal) and the sulfur molecule would be attracted to the anode (positive terminal) except that it is so reactive or unstable, it will combine with other elements in the bath before it makes it over to the anode.
Examples of electroplating metal salts include nickel sulfate, nickel chloride, chromium trioxide, cadmium oxide. These are obtained in powder or liquid form and are mixed into the plating bath water to create the electrolyte.
The workpiece is connected to the negative terminal of an external source of direct current electricity or cathode. This is said to make the workpiece cathodic. Why is direct current only used in electroplating? Alternating current reverses polarity many times a second. The metal ion would be alternately attracted and repulsed to and from the workpiece and little or no plating would take place.
The positive side of the current is supplied to the bath by insertion of the anode. In electroplating, a steady direct
current of low voltage, usually 1-6 volts is used. When the current is passed through the solution or electrolyte, ions of the plating metal, i.e., nickel, chromium, deposit out of the bath and onto the cathode, or workpiece. These ions are replaced in the bath by periodic additions of the metal salt to the bath or by corrosion of the anode. A brief example: if the salt copper sulfate is dissolved in water, it disassociates into positive copper ions and negative sulfate ions. When a voltage is applied to the electrodes, the positive copper ions move to the negative electrode, are discharged onto it as metallic copper. The sulfate ions, now that they are separated from the copper ions, are unstable (highly reactive) and combine with the hydrogen in the water of the solution to form sulfuric acid and oxygen. Such action caused by an electric current is called electrolysis. The quantity of material transformed at each electrode is proportional to the quantity of electricity passed through the electrolyte.
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