Letter 27004

Patchy Hard Coating of 2024  

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We are a machine shop currently starting up an anodizing department and have had a good deal of success with Type II & Type III anodizing of 6000 Series and 7000 Series Aluminum alloys. We have separate tanks for Type II (72 deg F) and Type III (32 deg F). The sulfuric acid concentration for both tanks is keep about 18 % with the type III tank having about 3 percent MAE additive. We have a Dynapower rectifier capable of both "voltage" and "current" anodizing. The question I have relates to problems with 2024-T3. I am well aware that the copper content of 2024 tends to cause problems but I have had almost no luck in trying to develop an even oxide layer in our hard coat tank. I can achieve a uniform thickness of .0004" in our "Type II Tank" by ramping voltage slowly from 18 to 20 V over 20 minutes. Parts anodized in our type III tank do not develop an oxide layer consistently over the surface of the part. Typically, the surface of the part develops "pits" of thick coating (tan in color) with almost none on the rest of the same surface. As anodizing time proceeds, these pits become more numerous until they "fill in" the surface, yielding an overall patchy appearance with varing thickness.

I have made the following observations that might be clues to this problem.

-I am experimenting with a 76 sq in cube of 2024-T3 material
-When voltage anodizing, and slowly ramping the voltage, no current draw occurs until approx 27 volts, then current spikes to the full current limit within seconds, (100's of amps if I let it)
-When constant current anodizing at 30 A/sq ft. Voltage climbs to about 30 V within a minute than settles down to about 27 V followed by a slow climb back to 30 V over 15 minutes
-Racks are titanium with titanium bolted connection
-Similar results (poor) were acheived by be with some actual parts made from the same material, but we had one of our better outside processors do the same parts with great success.
-6063 aluminum cathodes, (about 2 sq feet of cathode area, yes i know this is too much according to recommendations but i don't know what the effect of this is, can anyone tell me?)
-I've seen "burned" (distructive dissolution) parts and I wouldn't call these burned, but not sure.
-Results seem to be the same regardless of voltage vs current anodizing. Patchy appearance

Can anyone make any suggestions that would allow me to achieve a more uniform oxide layer? What variables should I be looking at? Thanks for any assistance, would be willing to pay a consultant who could solve this problem.

Joseph G.
Finishing Engineer - Greenboro , North Carolina

Joe, you may want to try using aluminum racks. The current travels greater through the aluminum than titanium. We almost always use aluminum racks on any 2000 material. The task of hardcoating 2024 can be quite difficult, there are very few shops that will hard coat 2024 with out a disclosure agreement due to the difficulty.

Casey Weizel 
- Minneapolis, Minnesota

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