Metal Finishing/Plating

Metal Finishing
Metal finishing is where an object is coated with a thin layer of metal through electrolytic deposition. The article to be plated is the cathode in an electrolytic bath with an anode as the other terminal. A low-voltage current is passed through the solution, which electrolyzes and plates the cathode with metal to a desired thickness. There are various metals used for plating: nickel, cadmium, zinc, copper, tin, tin/lead, antimony/lead, lead, etc. The end products can range from screws and circuit boards to car parts and jewelry.

Chromium Plating
Chromium plating is an electrochemical process in which a uniform layer of chromium is deposited on a cathode. In most plating operations, the anode is made of the material which is to be plated. But chromium plating is the exception because the anode is an inert lead alloy which merely supplies electrons to the solutions. These electrons react with the chromium in the solution to deposit on the cathode. Pure chromium anodes are too expensive and impractical to use. Therefore, a lead or lead/antimony alloy is used and because of its inert properties, these anodes do not need to be replaced as often as other plating anodes.

Decorative Chrome Plating
Decorative chrome plating is the first type of chromium plating where a very thin layer of chromium is deposited on the surface of an object. The object can be metallic or non-metallic (i.e. plastics). The actual thickness of chromium can range from 0.00001 - 0.0002 inches.

Hard Chrome Plating
Hard chrome plating is where an object requires its physical properties to come from the plated chromium layer. Hard chrome plating is used when hardness, smoothness, chemical inertness, resistance to wear, corrosion resistance and wetting are important. The thickness of chromium deposited on the object will be greater than 0.00005 inches.

Tin/Lead Plating
Anodes made from tin and lead can also be used when the objective is to plate tin or lead. In this process, the anodes are soluble and will actually dissolve on to the electrolytic solution and then be plated on to the cathode (object). The composition of the anode depends on the desired composition of the plated material. The best plating anodes are of high-purity compositions with a fine, even-grain structure. High purity anodes reduce sludging, increase anode efficiency and closely control the alloy deposit. Impurities such as antimony, arsenic, and bismuth are known to cause heavy sludge to form in the electrolytic cell. This sludge decreases the efficiency of the plating operation and also wastes valuable tin and lead in the process. The sludge is also a hazardous waste that must be disposed of properly. Sludging is minimized by using anodes made from Fry's LSC (Low Sludge Characterization) process. The main uses for tin/lead plating are parts which need corrosion protection and for solder preparation.

Cathodic Protection
Cathodic protection is a corrosion prevention mechanism which allows the "sacrificial" anode to corrode while the metal being protected Is made into the cathode by either a galvanic or impressed current. Lead anodes are not the most commonly used material for this application. Due to the corrosion resistance of lead, however, lead alloys are popular in marine/seawater environment cathodic protection. Typical alloys are 6Sb/1Ag/93Pb and 0.5Sb/0.5Bi/99Pb (experimental).

Sacrificial anodes such as aluminum, magnesium, and zinc are used to protect metal such as steel corrosion. When high current is a concern (100+ amps), then metals with a higher conductivity level are required such as 1Ag/99Pb for protection. This is often used in high chloride equipment such as power plant condensed water boxes.