Guide to Electroplating

Electroplating is a method of coating a metal or plastic surface with a metal. A common example is the brightwork on automobiles -- bumpers, door handles, emblems, script, etc. Much of this begins as a piece of zinc, steel or plastic. The electroplater applies an electroplate of copper, then a nickel electroplate, then chromium three deposits over one another. The result is a surface brighter and more corrosion resistant than bare metal or plastic.Electroplates are applied by immersing the object to be coated in a tank containing the proper chemicals dissolved in water. If nickel is being applied, nickel metal is one of the components of that solution.Now imagine that the part to be plated is attached to a negative electrical lead (like that on your car battery). Once it is attached to the negative electrical lead it is called a cathode.The other electrical lead, the positive (+) is in the solution. When current is turned on, the negatively charged part to be plated attracts positively charged metal from the solution (opposites attract). This continues as long as current is on, and the coating or deposit becomes thicker and thicker. But most electroplates are not very thick. One one thousandth of an inch (0.001 inch) is regarded as pretty thick.Since metal is being taken from the solution, it must be replenished. Often this is done by hanging pieces of the metal nickel, if nickel is being plated, for example, in the solution. The chunks of metal are called ANODES, and the positive electrical lead is then attached to them. They dissolve in the solution as metal is taken away by plating. So at this point we have metal being removed from the anode and deposited on the cathode, which are the parts to be plated.Since a car battery is not a good source of power for this application, platers use electrical current supplied by their power companies. But they must have DIRECT current (DC), while the power company supplies ALTERNATING current (AC). To convert AC to DC platers use a RECTIFIER. Its function is to convert AC to DC.Nearly every metal plated is plated in this same way.Before the parts ever enter the line. they are hung on a RACK. This is just a way of carrying the parts through the various solutions.Before anything is plated, the parts to be coated must be CLEANED. Platers use CLEANERS for this. They are alkaline materials that remove oils, dirt and rust.In a typical plating line the part is first immersed in a cleaning tank, then in an electrocleaning tank (uses power from a rectifier to aid in cleaning), and then into the plating tank.A typical plating tank has three copper bars suspended over its top, one connected to the negative lead from the rectifier, two connected to the positive lead. The racks of parts to be plated hang from the bar that is connected to the negative lead, the anodes (metal to be plated) from the positive bars.The solution in the tank may have to be heated or cooled. For this, platers use IMMERSION HEATERS or HEAT EXCHANGERS.The solution becomes contaminated with dirt and other particles, which would cause rough plates. To prevent this, platers use filters.In some cases the plated part is chromated. Zinc plated parts, for example, will become bluish or yellowish if they are chromated. You see these appearances on nuts and bolts you buy in a hardware store. The chromate coating is applied by dipping the zinc plated part in a tank containing chromic acid and other chemicals. The acid reacts with the zinc plating to form a zinc chromate. This is called a conversion coating, because the chromic acid solution converts the surface to zinc chromate. This coating further improves corrosion resistance. There are also black and olive drab conversion coatings.Larger parts are usually plated on racks. But if you have a million nuts and bolts to plate, you don't want to hang each of them individually on a plating rack. For this reason a plating barrel is used. The parts are dumped into a plastic barrel with holes drilled into the plastic sides. Then the barrel load of parts is immersed into the plating solution. Inside the barrel is a dangler, a piece of flexible metal that reaches down into the load of nuts and bolts to carry current to them. The current is conducted from part to part by their electrical conductivity and the whole load begins to be plated. The barrel is rotated while current is applied. The nuts and bolts become plated with zinc or cadmium or whatever is desired. This is barrel plating.If you have lots of racks or lots of barrels and you don't want to hand carry them from tank to tank you will attach them to a conveyor that moves the racks or barrels from tank to tank, immersing them in each solution for a preset time. This is conveyorized plating, which may be done from an automatic line or from a hoist line.Anodizing is an operation performed mainly on aluminum. The effect is to develop an oxide coating on the aluminum. Again, this is a conversion coating: the surface is converted from aluminum to aluminum oxide.Unlike plating, the part to be coated is connected to the anode --- not the cathode. The part is immersed in a dilute solution of sulfuric acid, current is passed, and the part is anodized an oxide coating forms. The oxide coating is harder and more corrosion resistant than bare aluminum.Some aluminum pots and pans are anodized, and door and window frames are often anodized. There are ways to make the anodizing pick up the color of brass or bronze or black, as well and almost any color of the rainbow.Electroless plating is just what the name implies. It is a way to coat one metal with another without passing current. Certain chemicals cause this to happen. The most common example you will see advertised is electroless nickel. The reason it is applied is that there are some limitations in electroplating when complex shapes are being coated. Often you cannot electroplate deep recesses and holes or interior passages. Electroless plating plates anything that is wetted by the solution.Electroless plating is more expensive than electroplating in most cases, and is slower to build up a given thickness of deposit, which explains why it is used only for certain applications.Plating on plastics. Plastics cannot be plated in the same way as metals because plastics are not electrically conductive. Thus one cannot immerse a plastic part connected to the negative lead and expect it to plate. Instead, electroless plating is applied first to get a conductive surface, then the electroless plated parts are electroplated. Many automotive parts, including grilles and all manner of decorative trim have been plated plastic.Printed circuits. The simplest printed circuit begins with a plastic board that has copper foil glued onto its surface. The circuit paths are made by printing onto the copper foil coatings that resist the etchants used. These coating or resists are applied in the shape of the circuits that must remain. Then the exposed copper is etched away and the circuit paths remain. The resist is removed and you have a rudimentary printed circuit. Some circuits or portions of them are plated with tin or tin lead to provide solderability or with gold to provide contact reliability. So this is another market for plating chemicals and equipment.Many small electronic parts are also plated with tin or tin lead or gold or other metals, to provide various electrical properties.Polish/Buffing. Generally done as a preplate operation, to smooth the surface before plating.Vibratory finishing. Put parts into a vibrator filled with tiny pebbles or other media and a cleaning solution, shake the vibrator and you will smooth the surfaces of the parts.Vacuum Coating. Put a part in a vacuum chamber, exhaust the air, heat aluminum to vaporize it inside the chamber and the aluminum vapor will travel through the vacuum and condense on the parts' surfaces. A cheap way to get a bright surface. Picture frames are made this way, as are many other bright surfaces, including mirrors.Electropolishing. Use the right chemicals in a tank and you can immerse certain metals and pass current to smooth the surface. The part is the anode and tiny high points are deplated, making the surface smoother.Water Pollution Control. After each plating step, the chemicals remaining on the surface of the plated part must be rinsed off. There are also spent plating solutions that must be disposed of. The rinse water and the spent solutions contain metals, cyanides and acids, all of which are damaging to fish and other things reached eventually by sewer water. So the EPA requires that platers not dump these chemicals into sewers or streamsThe plater has two choices: reclaim the chemicals from the rinse water, or treat the rinse water with other chemicals that remove the toxic materials and allow disposal of the remaining water.The cheapest and easiest way has been to add chemicals that neutralize the offending chemicals. The problem is that a sludge is left after the water has been treated. Sludge is becoming difficult to dispose of in landfills, because the EPA has rules about the sludges, too. Thus reclamation is becoming a more attractive option.Recovery/Recycling/Closing the Loop. A simple way to reclaim is to evaporate the water from the contaminated rinse water until the plating chemicals remaining are concentrated enough to return to the plating tank. There are a number of other ways to reclaim, but all of them find some way to reconcentrate the chemicals or to purify the chemicals so that they can be reused.