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Anodizing Polarity Problem

I am hard anodizing an 6061 T6 aluminum cylinder. Just the inside. I used latex on the outside to prevent any buildup. Used 40 amp. density, temp. was 40° F., I used a 3/8" aluminum rod for the cathode, sulfuric 1 to 3 water. Problem: I got confused and switched the anode and cathode connection and anodized the rod perfectly with a .0005 build exactly as calculated. It was a medium grey.

So, I put it in the lathe and took it down to aluminum again. The cylinder upon examination showed no change of any kind that was visible. I put it all back in and tried again. Same time (18 minutes) and I get absolutely no anodize!

Can anyone tell me what is going on ? Is there some kind of change because the cylinder was a cathode ? The anodize was very hard when cutting it off with a carbide tip tool. Also, I used dry ice in Ethylene Glycol to get the temp. down. I used a .010 sheet aluminum to set the cylinder on and ran it up one side to the (negative D.C. in error, but reversed it to positive and nothing happens). I do get fizz on the rod (now negative) but not on the cylinder. Thanks to anyone for their help in advance.

Jack Clodfelter
- Lakewood, Colorado USA
2000

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My first guess is that you are just setting the part on a piece of aluminum that is positively charged. This is an inadequate contact. Even tho it is touching, it is gradually anodizing the contact point which is an insulator. You are probably anodizing the 0.010 sheet rather than the part.

I would rig it with an aluminum all thread, or threaded rod and clamp the part between a ba and two dogs or clamps that are tightened down with nuts. The triangle bar with the part on one of the sharp edges will make a good contact that will not arc; a piece of pvc block will keep the cathode from touching the bottom bar.

You did not mention the diameter of your bore. You may need a larger cathode.

James Watts
- Navarre, Florida
2000