A. What Is Electroplating?
Electroplating is a process in which a metal is deposited onto a metallic substrate. The plated metal is normally of a thickness less than 0.002 in. and in most applications approximately 0.0001-0.0003 in. Metals are plated to afford the substrate properties that it would not otherwise have, such as improved corrosion resistance, aesthetic appeal, greater abrasion resistance, improved surface hardness, changed electrical characteristics, and adjusted dimension to tolerance, as well as imparting other desired properties.
The greatest use of plating is probably plating zinc on steel, thereby providing both a corrosion-resistant surface and one that is attractive in appearance. Such articles as nuts, bolts, washers, wire goods, castings, and numerous stampings are processed in this manner. Another extensive use of plating on steel is decorative chrome; this usually comprises a triplate of copper, nickel, and chromium. This finish has found widespread use on automotive and houseware applications where both a corrosion-resistant and an attractive surface must be provided.
Many metals other than steel are electroplated. Metal substrates, such as aluminum, brass, copper, and zinc, are plated to provide various desired properties.
Electroplating of plastic articles is finding increasing acceptance in industry. Items fabricated of molded plastics, such as automotive grilles, taillight assemblies, trim, and numerous household appliances, as well as plumbing fixtures, are being electroplated. A special process is required whereby the plastic part is metabolized to make it conductive so that it can be plated to the desired finish.
The design engineer is able to select from numerous substrates to fabricate a part, considering such factors as strength, application, and cost. The part, once fabricated, can then be finished by electroplating or by other related processes to provide the desired properties and appearance.
B. Fundamental Principles
The process used for electroplating is similar for all metals. The part to be processed is cleansed of all surface soil and then activated. The activated metal is placed immediately into plating solution where the actual electroplating is to be achieved. The exact plating cycle and various solutions used will vary somewhat depending upon the base metal and the type of plating to be done.
In the actual electroplating operation the parts to be plated are made cathodic in the plating solution. In a simplified operation these parts are suspended by a hook or wire (Fig. 25.1) from a cathode bar. Low-voltage direct current is supplied by a rectifier and is connected by a cable or buss bar to both the anode and cathode circuits in the plating tank. The anode and cathode bars are fabricated of copper and are positioned on, but insulated from, the tank rim. These bars are designed to carry the weights of the anodes and cathodes as well as to conduct the current that is applied.
The anodes are electrodes that are suspended in the plating solution. They are normally of the same metal that is to be plated, that is, copper anodes for copper plating nickel anodes for nickel plating. During the plating operation the anode dissolves to replenish the solution of metallic ions deplated by metal deposition on the cathode. This type of anode is known as a soluble anode. Certain situations, however, require an insoluble (inert) anode whose only purpose is to provide current to the electrolyte. Examples of typical insoluble anodes are carbon for certain processes and lead anodes for chromium plating. Insoluble anodes are used in situations where the electrolyte would dissolve the anode at a rate faster than the metal would be plated out of solution or in situations where the cost of a pure anode would be prohibitive.
During plating low-voltage direct current is supplied to the electrolyte by a rectifier. Generators are also used from time to time but have largely been replaced by the newer rectifiers (see Sec. IX). Depending upon the process, the current supplied by the rectifier is in the range of 6-12 V, with current density depending upon the surface area of the cathode. Typical current densities for many plating operations are 20-100 A/ft2 area to be plated. Hence a cathode area of 10 ft2 would require a rectifier capable of 200- 11000 A. In actual production plants, rectifiers with a capacity of 5000-20,000 A are quite usual.
Fig. 25.1 Schematic diagram of a typical electroplating tank setup. Low-voltage direct current is supplied by a rectifier. (A) Plating tank. (B) Plating solution. (C) Anodes.
(D) Cathode (work to be plated). (E) Rectifier
The plating solution, that is, the electrolyte, is generally an aqueous solution that contains dissolved salts of the metal to be plated and other chemicals as may be required. Some electrolytes are quite simple in nature and may contain as few as two ingredients. Acid copper plating, for example, uses a solution of only copper sulfate and sulfuric acid. Other electrolytes, however, may be quite complex. The solution for nickel plating may contain as many as six to eight ingredients, such as nickel sulfate, nickel chloride, boric acid, sulfuric acid, and wetting agents, as well as several organic addition agents.
These electrolytes are conductive media that allow the current to pass from anode to cathode. As direct current is applied to the anode (Fig. 25.2), metal is dissolved and ionized. The current causes these positively charged metallic ions to migrate toward the cathode, where they are reduced to the metallic state and deposited onto the cathode.For the most part the anode and cathode act independently of each other in the electrolyte. The simplified reaction also causes the migration of negatively charged ions to the anode where they are neutralized by the positive charge. By-products that are caused by this electrolysis include the release of oxygen at the anode and hydrogen at the cathode. Due to the complexity of some electrolytes numerous other side reactions may also occur.
Most plating electrolytes can be operated over an indefinite period of time and seldom if ever require discarding. As the plating process continues and various chemicals are consumed, replenishment is required in order to maintain the electrolyte within the prescribed concentration limits. The plating solution may become unbalanced during continued operation due to
Due to these factors it is important to have the electrolyte analyzed on a periodic basis with chemical additions or purification steps taken as may be required.