Electroplating has, over recent decades, evolved from an art to an exact science. This development is seen as responsible for the ever-increasing number and widening types of applications of this branch of practical science and engineering. Some of the technological areas in which means and methods of electroplating constitute an essential component are all aspects of electronics: macro and micro, optics, opto-electronics, and sensors of most types, to name only a few. In addition a number of key industries such as the automobile industry (that uses for example chrome plating to enhance the corrosion resistance of metal parts) adopt the methods even where other methods, such as evaporation, sputtering, chemical vapor deposition (CVD) and the like are an option. That is so for reasons of economy and convenience. By way of illustration it should be noted that that modern electroplating equips the practitioner with the ability to predesign the properties of surfaces and in the case of electroforming those of the whole part. Furthermore, the ability to deposit very thin multilayers (less than a millionth of a cm) via electroplating represents yet a new avenue of producing new materials.
Fig. 1. Schematics of an electrolytic cell for plating metal "M" from a solution of the metal salt "MA".
Electroplating is often also called "electrodeposition", and the two terms are used interchangeably. As a matter of fact, "electroplating" can be considered to occur by the process of electrodeposition. Electrodeposition is the process of producing a coating, usually metallic, on a surface by the action of electric current. The deposition of a metallic coating onto an object is achieved by putting a negative charge on the object to be coated and immersing it into a solution which contains a salt of the metal to be deposited (in other words, the object to be plated is made the cathode of an electrolytic cell). The metallic ions of the salt carry a positive charge and are thus attracted to the object. When they reach the negatively charged object (that is to be electroplated), it provides electrons to reduce the positively charged ions to metallic form. Figure 1 is a schematic presentation of an electrolytic cell for electroplating a metal "M" from an aqueous (water) solution of metal salt "MA".
To further illustrate the foregoing, let us assume that one has an object made of one of the common metals, like copper, and that it has been properly pre-cleaned. want to
By now it should be evident that electrodeposition or electroplating should be defined as the process in which the deposit of a (usually) thin layer (of metal) is formed "electrolyticly" upon a substrate (that is often, but not always, also a metal). The purpose of such process may be to enhance or change the substrate's appearance and/or attributes (such as corrosion resistance). Examples are the deposition of gold or silver on jewelry and utensils, and the deposition of chrome on automobile parts. Electroplating is performed in a liquid solution called an electrolyte, otherwise referred to as the "plating bath". The bath is a specially designed chemical solution that contains the desired metal (such as gold, copper, or nickel) dissolved in a form of submicroscopic metallic particles (positively charged ions). In addition, various substances (additives) are introduced in the bath to obtain smooth and bright deposits. The object that is to be plated is submerged into the electrolyte (plating bath). Placed usually at the center of the bath, the object that is to be plated acts as a negatively charged cathode. The positively charged anode(s) completes the electric circuit; those may be at opposite edges of the plating tank, thus causing film deposit on both sides of the cathode. A power source in the form of a battery or rectifier (which converts ac electricity to regulated low voltage dc current) is providing the necessary current. This type of circuit arrangement directs electrons (negative charge carriers) into a path from the power supply (rectifier) to the cathode (the object to be plated). Now, in the bath the electric current is carried largely by the positively charged ions from the anode(s) toward the negatively charged cathode. This movement makes the metal ions in the bath to migrate toward extra electrons that are located at or near the cathode's surface outer layer. Finally, by way of electrolysis the metal ions are removed from the solution and are deposited on the surface of the object as a thin layer. It is this process to which we refer as "electrodeposition".
Another note of caution: one should not increase the cathodic potential (and current) too far to avoid parasitic reactions that may occur beyond "over-potential reactions" in Figure 2. In cases of practical applications be it electrorefining, electrowinning, or plating the practitioner is interested only in the weight of metal deposited on the cathode. Any current causing other changes is considered "wasted". Of course, according to Faraday's law the overall amount of chemical change produced by any given quantity of electricity can be exactly accounted for. Thus we define the current efficiency as the ratio between the actual amount of metal deposited to that expected theoretically from Faraday's law. In other words, the ratio of the weight of metal actually deposited to the weight that would have resulted if all the current had been used for depositing is called the cathode efficiency, and it is desirable to keep it as close to 100% as possible.