Metal Finishing Guide Book

2011-2012 Surface Finishing Guidebook

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Page 403 of 707

lined steel, stainless steel, lead lined wood, fiberglass-lined concrete, and plastic tanks have all been used in the past. A metallic tank can be used as the cathode, but adequate distance between the work and the tank must be maintained to pre- vent shorting. Some problems are experienced using metal tanks. For instance, the anode-to-cathode ratio is generally out of balance; also, since the entire tank is an electrical conductor, uneven current flow is possible leading to uneven oxide thickness formation. This uneven oxide formation causes wide color vari- ations in organically dyed materials and is not generally recommended. Generally, the use of inert materials in the construction (or lining) of the anodize tank is recommended. PVC, polypropylene, or fiberglass are good inert materials for this application. Cathodes can be aluminum, lead, carbon, or stainless steel. Almost all new installations are using aluminum cathodes because of their ability to reduce the energy requirements of the process. Because of the better conductivity of aluminum, the anode-to-cathode ratio becomes extremely important. It has been found that an anode-to-cathode ratio of approximately 3:1 is best for most applications. Cathode placement is also of vital importance. It is recom- mended that the cathodes be no longer (deeper) than the work being anodized. Placement of the cathodes along the tank sides should be such that they extend no further than the normal work length. For example most 30-ft long tanks can only handle 28-ft lengths; therefore, the cathodes should be posi- tioned at least 1 ft from either end of the tank to keep the work material from "seeing" too much cathode and anodizing to a thicker oxide on the ends. The depth of the cathodes in the tank should not exceed the normal depth of the work being processed. If the cathodes extend deeper into the tank than the parts being anodized, there will be excessive oxide growth on the parts in the lower portion of the anodizing tank. This will result in color differences in the oxide and subsequently colored parts. The correct alloy and temper for aluminum cathodes is vital, 6063 or 6101 Cathodes alloys in the T-6 or T-5 condition are best. The overaged T-52 temper should nev- er be used! Cathode material should be welded to an aluminum header bar using 5356 alloy welding wire. Bolted joints are not recommended due to the possibility of "hot joints." Employment of aluminum cathodes has done much to improve the overall quality of anodized finishes in all areas of application. Temperature Control This is one of the most important factors influencing the properties of the anodic oxide and must be closely controlled to produce consistent quality. The temperature should be held to plus or minus 2O F. Most installations have some means of temperature control, since large amounts of heat are generated in the anodizing process. Lead cooling coils have been used in the past, but newer plants use external heat exchangers. The external heat exchanger has been found to be more effi- cient in cooling the solution while offering additional agitation. Again, as mentioned above, the presence of other metals in the tank, in conjunction with the aluminum cathodes, can cause undo electrical problems. One of the added benefits of using a heat exchanger is agitation. Proper placement of the intake and outlet piping can insure good agitation as well as min- 402

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