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October 9, 2011

Black Anodizing Aluminum Problems

As theoretically on aluminum anodizing seem have no problem if we want to make coloring on anodizing aluminum, but actually some problem may happen unpredictable. The example problem is on 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. The pieces are putting in the anodizing tank at one time. The parts are finely grained (sanded). Our procedure is as follows:
  • After graining, then clean in a mild detergent solution.
  • Water rinse
  • Dip in caustic etch (Oakite 360L) for 2 minutes
  • Water rinse
  • Dip in deoxidizer (Oakite Deox) for 1 minute
  • Water rinse
  • Anodize in sulfuric acid (24 Baume A) at 76 - 78 degrees Fahrenheit for 55 minutes at 15 volts
  • Water rinse for 5 minutes
  • Dip in black dye (Sandoz) tank for 20 minutes at 110 - 140 Fahrenheit
  • Water rinse for 2 minutes
  • Seal in boiling water with nickel acetate solution for 15 minutes
  • Water rinse
Several problems can happen on some process:
There appear to be stains of some sort that are in/under the anodic layer. They do not rub off of the panels. Then can getting a bluish hue around the edges and holes.

Solution from smut - we are trying to clean with a scotchbrite pad and either mild detergent or naphtha.

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?
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 assuming that 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 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.