Cadmium Plating
Cadmium plating is used more generally on aircraft parts than any other plating method. It is a generally practice to cadmium plate all steel parts small enough to fit in the bath, prior to painting. Welded tubular fuselages, engine mounts and landing gears are not cadmium plated because it is impractical. Steel parts are cadmium plated to increase their corrosion resistance. Cadmium plating does not improve the paint adherence to the surface but resists corrosion itself. In fact, it is sometimes difficult to make paint stick to cadmium plated surfaces unless they are kept exceptionally clean. Parts made from copper or its alloys are frequently cadmium plated in order to reduce the electric potential between those parts and adjacent steel or aluminium parts. Cadmium lies between iron and aluminium in the galvanic series, both of these, in turn, are far removed from copper. Aluminium can also be cadmium plated but such plating is seldom done because there is a better treatment available, known as the anodic oxidation process. Cadmium plating is an electrical process carried out at a low voltage not exceeding 12 volts. The cadmium is deposited directly on the surface without the necessity of a preliminary coating of another metal. The cadmium deposit must be adherent and without blisters, porosity or other detects. A coating 0.0005 inch thick is usually specified except on threads, where a minimum coating 0.0002 inch thick is required. Parts plated in this manner will withstand a 250-hour salt-spray test without showing evidence of corrosion of the the base metal. It is customary to select cadmium plated samples at random periodically and submit them to a salt -spray test to check the quality of the plating that is obtained in production. Before putting parts in the cadmium-plating bath, they must be thoroughly cleaned by pickling or sandblasting. Pickling is preferable. Parts with more than 0.60% carbon should not be sandblasted. It is also essential to remove all particles of sand by immersing the part in a dilute solution of hydrofluoric acid. Copper, brass and bronze parts must be pickled in a sulfuric acid solution prior to plating. All parts should be immersed for at least 30 seconds in a solution of sodium cyanide immediately before plating. The plating solution consists of sodium cyanide, cadmium oxide and caustic soda, dissolved in water at room temperatures. If a bright plating is desired, a brightener, such as hide glue or molasses, is added to the bath. The work to be plated is suspended by hooks or racks from the cathode bus bar and is completely immersed in the solution. Cadmium anodes are used. A voltage between 4 to 6 volts is required for this method of cadmium plating. Another method, called the “barrel plating” method, requires 8 to 12 volts. In this method the work to be plated is placed in a perforated barrel which resolves during the plating operation.The thickness of the cadmium plating deposit is dependent upon the time and the current density. Increasing either the time or current will increase the thickness of the coating obtained. The physical character of the coating is also determined by the rate of formation as controlled by the current density. A coarse, soft deposit is obtained with a low current, while a stronger current produces a fine-grained, hard deposit. A high current results in a “burnt” deposit. The ideal time and current density for any particular set of conditions must be established to obtain the desired hardness and appearance. For a cadmium deposit of 0.0005 inch the following combinations of time and amperage may be used to obtain satisfactory results. The amperage given is per square foot of surface to be plated. It should be noted that the current is inversely proportional to the time…..