Peter Beck, age 15, of Cumberland, Md., for his question:
WHY IS ELECTROPLATING USED?
Electroplating is an electrochemical process for depositing a thin layer of metal on, usually, a metallic base. Objects are electroplated in order to prevent corrosion, to obtain a hard surface or attractive finish, to separate metals for quantitative analysis, or, as in electrotyping, to reproduce a form from a mold.
Cadmium, chromium, copper, gold, nickel, silver and tin are the metals most often used in plating.
Typical products of electroplating are silver plated tableware, chromium plated automobile accessories and tin plated food containers.
In the process of electroplating, the object to be coated is placed in a solution, called a bath, of a salt of the coating metal, and is connected to the negative terminal of an external source of electricity. Another conductor, often composed of the coating metal, is connected to the positve terminal of the electric source.
A steady direct current of low voltage, usually from one to six volts, is required for the process. When the current is passed through the solution, atoms of the plating metal move out of the solution onto the cathode, the negative electrode. These atoms are replaced in the bath by atoms from the anode (positive electrode), if it is composed of the same metal, as with copper and silver. Otherwise they are replaced by periodic additions of the salt to the bath, as with gold and chromium.
In either case an equilibrium between the metal coming out of solution and the metal entering is maintained until the object is plated.
Nonconducting materials may be plated by first being covered with a conducting material such as graphite. Wax or plastic patterns for electrotype and recording disc matrices are coated in this manner.
To ensure a strong and close bond between the object to be plated and the plating material, the object must be cleaned thoroughly by dipping it into an acid or caustic solution, or by making it the antode in a cleaning bath for an instant.
To eliminate irregularity in the depth of the plate, and to ensure that the grain at the surface of the plate is of good quality and conducive to polishing, the current density (ampers per square foot of cathode surface) and temperature must be carefully controlled. Colloids or special compounds are often added to the bath to improve the surface uniformity of the plate.
Some metals, notably chromium, have poor throwing power, that is they tend to plate heavily on projections while leaving crevices or parts distant from the anode entirely bare.
Electrochemistry is that part of the science of chemistry that deals with the interrelationship of electrical currents, or voltages, and chemical reactions, and with the mutual conversion of chemical and electrical energy. In the broadest sense, electrochemistry is the study of chemical reactions that produce electrical effects and of the chemical phenomena that are caused by the action of currents or voltages.