Q: What causes bipolar electrodes and when might they be used in a plating process? X.C.
A: Bipolar electrodes always seem to be proposed as the cause of a problem when there does not seem to be any other possible explanation. They can be a real problem. In my opinion, the best and most direct discussion on this issue can be found in Lawrence Durney's SOLUTION 4.1 Software, Kushner Electroplating School, www.platingschool.com, 714-632-1374. (Disclosure: As of November, 2008, I am no longer affiliated with the Kushner Electroplating School.) I have excerpted a few paragraphs from the software program below:
"In unlined conductive tanks, the distance from the anodes to the tank wall plus the distance from the tank bottom to the bottom parts on a rack may be less than the distance from the anodes to the work. The bipolar electrode which then forms may supply excessive current to the lower parts on the rack. The cure is to insulate the tank, or insert a shield between the anodes and the tank wall, or on the bottom of the tank. Wire-reinforced glass is frequently used for this purpose.
"In a lined tank, parts that fall into the tank and are allowed to remain there may form a bipolar electrode with the same effect described above. Additionally the portion of the part that becomes the anode may dissolve, contaminating the solution. This is a frequent problem when nickel plating zinc-base die castings, resulting in severe contamination of the solution with copper and zinc. The cure is to regularly remove any parts that fall off the racks.
"On automatic plating machines, a rack being lowered into the tank may temporarily become a bipolar electrode until it makes contact with the cathode bar. Hazing, clouding, or even peeling of the area that served as the collector during this period may result. If this condition is suspected, provisions should be made for 'live' entry of the rack into the solution, i.e. a connection with the cathode bar should be made before entry."
When may a bipolar electrode be put to good use? Again, ther following is an excepted paragraph from the software program:
"A passive electrode placed between the parts to be plated (cathode) and the anode. The end closest (also called the "collector") to the anode becomes negative and is plated, collecting current which flows through the electrode to the end nearest the cathode making it an anode which then functions the same as the internal anode described above.
"The essential requirement for the functioning of this type of electrode is that the sum of the distance from the anode to the collector plate plus the distance from the bipolar anode to the area to be plated is less than the distance from the anode to the parts. Bipolar anodes have the advantage over auxiliary anodes that no direct contact with the anode bar needs to be made. They have the disadvantage that the metal plated on the collector plate is wasted unless it can be stripped off and used as anode material.