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topic 5936

Black Anodizing Problems


We are having problems black anodizing some parts. The material is .090" 5052 aluminum. The part size is 5 x 8 with about 30 - 40% of the panels cut away. We are putting 40 pieces in the anodizing tank at one time. The parts are finely grained (sanded). Our procedure is as follows:

  1. After graining, we clean in a mild detergent solution.
  2. Water rinse
  3. Dip in caustic etch (Chemetall Oakite 360L) for 2 minutes
  4. Water rinse
  5. Dip in deoxidizer (Oakite Deox) for 1 minute
  6. Water rinse
  7. Anodize in sulfuric acid (24 baumé A)at 76 - 78 degrees Fahrenheit for 55 minutes at 15 volts
  8. Water rinse for 5 minutes
  9. Dip in black dye (Sandoz) tank for 20 minutes at 110 - 140 Fahrenheit
  10. Water rinse for 2 minutes
  11. Seal in boiling water with nickel acetate solution for 15 minutes
  12. Water rinse
We're having several problems:
  1. There appear to be stains of some sort that are in/under the anodic layer. They do not rub off of the panels.
  2. We are getting a bluish hue around the edges and holes.
  3. Smut - we are trying to clean with a scotchbrite pad and either mild detergent or naphtha.
Any ideas to alleviate any of these problems would be greatly appreciated!
Trey Hooper
- Baltimore, MD USA


About the smut- do you manually clean and handle the pieces in between the anodizing processes?

If you're still having problem with the smut even after deox then, you may have problem with the deox solution or you need to adjust your deox time. What about your rinse process, is it enough? Have you tried at 70 deg, 30 min anodizing ?

Dado Macapagal
- Toronto


>We're having several problems:

>There appear to be stains of some sort that are in/under >>the anodic layer. They do not rub off of the panels.

Have you attempted to strip and re-anodize the parts? If so, after stripping the parts down, take a piece of mild scotch-brite pad and go over the part. Don't press too hard on the material when you're rubbing, but generally speaking this can do the trick. If that still doesn't work consider some alternative form of "sanding" the material. By sanding I am assuming you mean with a sanding belt of determined grit to achieve a certain grained finish. What grit you using? Have you considered a lighter/heavier grit to possibly eliminate this problem? Are you using varying grits, such as, 200 then 400? Its been years since I've been around polishing and buffing myself, but I seem to recall we used a pretty heavy belt, like around 120, it did a really nice job cutting the material, left it with a heavily satined grain, and when anodized really showed through well.

>We are getting a bluish hue around the edges and holes.

How are these parts cut? Are they stamped? Were they laser cut? How did your customer get their end product that you have? This is probably the biggest determining factor here. What you're seeing is likely a stress in the material from the manufacture of the parts. Its not something that can be seen before the anodizing. But because of the anodizing and the potential stress thats been employed to make the parts, you may have varying coloration around the holes and the sides because of this.

Its not common, I'll say that, but it does exist. Micro burrs on the edges can also contribute to some odd looking streaks and comet tails around the edges.

>Smut - we are trying to clean with a scotchbrite pad and either mild detergent or naphtha.

So many people have their own personal smut and no one persons seems to be able to relate to another persons in terms of conditions and what the stuff looks like or acts like. Is it slimy, is it chalky, does it have a pattern to it? There's really no way I could figure out what to do with the smut without having a part of two to fix (or break :-) ) myself.

I've seen some unwanted layers that are smoky in appearance from a sealing bath. Generally these can be rubbed with a terry cloth to a nice, smooth, beautiful bright finish. This generally happens with a fresh seal bath and the subsequent first few loads that leave the seal. After that, it disappears.

I wish everyone out there had access to a digital camera, even a really cheap one, to snap pictures of some of these problems, now that there, would be a revolution for the forums (note to Tom/Ted, ever consider allowing pictures to be added to a post to show the problems in question? -- I know the file sizes might be a problem, but a decent sized picture, properly fixed only takes up a few K of space and it would help those in question out tremendously, as well as those that want to provide answers).

Good Luck to you Trey, if by chance you do have the ability to digitize a photo of the parts and you'd like me to check them out you can e-mail them to me. Maybe that would help me, to help you.

Matthew Stiltner
- Toledo, Ohio

Ed. note: You are right, pictures would help, and we encourage inquirers to attach pictures of their problems.

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