Black Dye problems/Smears and Streaks

[Easttex]

Finally, my real job is slowing down a bit and I can get back to anodizing...I just mixed up a fresh batch of black dye and tried it last night. The parts came came out with a reddish bronze look to them with out the slightest hint of black (See black die problem pics in the anodizing album.) The parts appeared to be taking die well in the run but after two minutes in my drying cabinet the black die vanished. Parameters are as follows: 4.5 amps/sq "...73* I hit par at 71 minutes. The Die tank was at 120* for 25 minutes.

I had another piece in the run that was polished rather than bead blasted, it had some different problems.... I died it light blue on the rear of the body to clear on the front. Along the whole body it seemed to have faint streaks, spots, and smudged areas on it (See smears and streaks pics in the anodizing album.)

As I mentioned my chemicals have been covered dormant for about three months, my job is very seasonal. All my tanks have lids on them and are in a clean environment.
Thanks for the help,
Easttex

[neilfj]

I couldn't find your pics, so much of this will be guessing at possible issues.

I don't think it is the age of the solutions. I just finished a test with cast A356-T6 using solutions I mixed almost 9 months ago and they all worked just fine (except for the sealer which is only good for a few weeks). And, my environment doesn't sound as clean as yours does.

First, we've found that sometimes, under certain conditions, PAR doesn't result in ideal results, so use of the 720 rule is recommeded. The rule is:
720 divided by current density times desired thickness = anodizing time.

Your temp. of 73F is within the recommended range of 70-75F, so that doesn't appear to be an issue.

Based on the information you gave, 720 / 4.5 x ? = 71 min
This would give you an approx. thickness of .44 mils (11.2 microns). Although .44mils is usually good for dyeing, sometimes it depends on the alloy, which you didn't mention.

In my experience, when the black dye appears bluish, it is either that the surface isn't retaining the dye due to oversized pores, or the layer isn't thick enough. To better narrow this down, what concentration of electrolyte are you using? What is the size of the piece? What was the amp. setting? What was the peak voltage reached and provide an indication of how quickly it rose? Did any of the dye leach out during sealing?

Also, the recommended temp for the dye tank is 140F. I found a dramatic improvement in dyeing once the temp was raised from 120 to 140F. This could contribute to the problem.

Like I said, I can't find your pictures, but streaks, spots, smudges are usually the result of incomplete cleaning. It can also be caused by incomplete rinsing after each step, or failure to keep the work wet. For instance, if you used the SP Degreaser at 180F, and didn't rinse it completely (including any holes, threads, etc) it will cause streaks. Since some of these holes/crevices are difficult to rinse and they tend to hold the solution and this solution tends to weep out of the holes and causes streaks. Also, if you allow the workpiece to dry, it can cause streaks. What I usually do after rinsing, I use a separate bucket with distilled water and let the part sit in the bucket, completely submerged until the temp of the workpiece drops before moving to the next step.