Anodizing: Current Density vs. Voltage

Q. We are a small job shop anodizer. After the last AAC conference, our quality manager returned talking about “current density” anodizing instead of “constant voltage” anodizing. Can you detail the procedure for anodizing using the current density method? What are the quality advantages for us to completely change our processing technique? U.K.

A. Sometimes using constant voltage control works all right. This is mainly when Type II anodizing is being done on loads that will not run very long—say 30 min or less. Usually on this type of load the coating thickness requirement is less than 0.7 mil, and the coating can form in a reasonable amount of time without the voltage decaying more than a volt or two during the run. When this approach is used, the only way there is to predict the running time, if you know the desired coating thickness, is by experience—also known as trial and error.

In the anodizing process, it is the current (amperage) that builds the anodic coating. As the coating builds up on the parts, it impedes the flow of current to the parts. If you fix the current throughout the anodizing cycle, the time required to build the desired coating thickness can be predicted. Because the amperage is fixed, the voltage is free to rise to whatever level is required to overcome the resistance to current flow as the coating builds up. If the voltage is fixed and the amperage is allowed to roam freely, the increasing resistance of the coating build-up will cause the current to gradually drop during the cycle as voltage remains constant. So, you can see that if the amperage is constantly dropping, it will take longer to get the desired coating thickness. Furthermore, there is no way to predict how long to anodize the load because the variable that builds the coating, amperage, is constantly changing. Yes, if there is an ampere-hour meter in the circuit, the load could be run to a predetermined amp-hour total, if that value is known. It would still take longer to anodize the load to the desired coating thickness, however.

By using amperage control (“anodizing by current density”) the amperage is fixed for the entire anodizing cycle, thus providing a “constant current density” for the entire length of the cycle. So, if the current is constant, the cycle time required to achieve the desired coating thickness can be calculated. This can be done with the help of the following formula(s):

Rule of 720:
Min to anodize = mils (of coating desired) × 720
amps/sq ft

Rule of 312:
Min to anodize = microns (of coating desired) × 3.12
amps/sq dm

Of course, these equations can be manipulated to give the coating thickness if you know how long you want to anodize or the current density required to achieve a certain coating thickness in a given amount of time.

Anodizing by current density is a little more trouble, because the surface area of the parts (and the rack, if aluminum) needs to be known. I believe the payoff is worth the trouble of calculating the surface area of the load. This is because the anodizing cycle is shortened, even if by only a small amount sometimes, and the anodizer actually has control of the process. The surface area can be calculated for most extrusion loads by using the perimeter stated on the print. Sometimes the part has to be measured and the surface area calculated. If surface area can be calculated to within 10 percent of the actual, it will still work fine. You will have to decide if this is worth it for your operation.