Anodes are the movable bars that hang suspended from the anode bus bar. They supply the positive current to the bath. They are strategically placed about the workpiece to provide it with sufficient current to plate. Anodes are usually constructed of a pure grade of the plating metal and are intended to corrode into solution at a rate equal to the metal that is removed from the solution and deposited onto the workpiece. If this occurs, the bath is said to be in balance. Chromium is an exception. Chromium anodes are not used; rather insoluble lead anodes are used. This requires that periodic additions of chromic acid be made to the bath to replenish it. Anodes also come in other shapes such as balls and “popcorn”. These small anodes are placed in a metal basket and bagged. Controlling the amount of anode surface is easy as balls can be added or removed as needed.
In electroplating texts, frequent mention is made of cathode efficiency and anode efficiency. When metal salts as used in electroplating are dissolved into the bath, they ionize. An ion is an atom that has either gained or lost electrons in its outer shell. In this state, it will seek to restore its normal electron complement. The cathode supplies negative current to the bath and can be thought of as a source of electrons. The metal ions will be attracted to the cathode (workpiece) so as to restore their normal electron balance. Upon reaching the workpiece, they receive the balance of their lost electrons and their orbit interlocks with the orbits of the workpiece atoms’ orbits and thus attach to the workpiece. An example is copper. If copper sulfate is dissolved into a bath, the copper disassociates from the sulfur and in the process, loses two electrons. It is attracted to the cathode (workpiece) in order to restore its normal electron state. Upon reaching the workpiece, it picks up two electrons and attaches to the workpiece. At the same time, an atom of copper from the copper anode which supplies the positive current to the bath, dissolves from the anode into the bath and leaves behind on the anode two electrons that travel up the anode to the power supply and complete the circuit. This atom of copper, actually an ion of copper, repeats the same process. The rate of deposition onto the cathode can be mathematically calculated. If the rate of deposition is less than it theoretically should be, the cathode is said to be less than 100% efficient. If the anode gives up atoms at a rate less than what it theoretically should, it is said to be less than 100 % efficient. You can see that if the anode is more efficient relative to the cathode, more metal ions will enter the solution than are taken from it. If the cathode efficiency exceeds that of the anode, more ions will be removed from the solution than are deposited into it.
If anode efficiency exceeds cathode efficiency, rough deposits may result. If cathode efficiency exceeds anode eff iciency, inadequate plating may result. The amount of current used and the amount of cathode/anode surface area in the bath is very important to get and keep a bath in balance. Generally speaking, the square footage of cathode surface in the bath should he equal to the square footage of anode surface in the bath. However, if anode efficiency is less than cathode efficiency, then more anode surface area than cathode surface area will be necessary to keep the bath in balance, Conversely, if the cathode efficiency is less than anode efficiency, fewer anodes will be required. To calculate the amount of anode surface area required, you need to know the current requirements (amperes per square foot of workpiece surface) as discussed previously. This is always given in the bath formulas. As also discussed previously, you would take the amps per square foot requirement times the actual square footage of the workpiece. Then, you need to know what size of anode(s) will supply the current requirement. An example is chromium. To calculate the number of anodes to use, remember that 12 square inches of lead are required to carry 1000 amperes of current. So if you have a 4 X 1/2 inch anode, it has 4 times 1/2 or 2 square inches 2 square inches of surface. To carry 1000 amperes, you would need 2 X 6, or 12 anodes.
When you purchase your anodes from a supplier, you will need to ascertain the current carrying capacity of different size and types of anodes. Work with your supplier on this point.
Anodes are often supplied in balls that are suspended into the solution in titanium baskets. The amount of anode surface supplied to the bath is regulated by merely adding or removing balls. Another method, is to have an anode made partly of the plating metal, and partly of an insoluble metal, such as lead. Anodes may form a sludge on them that must be cleaned periodically. Because of the impurities generated by the anodes, they are often bagged. These bags, though permeable to the bath solution, trap the sludge and other impurities. The bags must be cleaned at frequent intervals so the solution may flow freely and reach the anodes.
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