7000 series parts dye, but 6000 series won't

[sswee]

Acidrain,

You said the black was very old. Can you say roughly how old and was it used extensively over that time. I was curious if it is the dye, wether it's from use or time.
SS

[acidrain]

This last batch of dye is only about 6mo old, but it was added to dye that was about 1yr old. I use it maybe once a month, but haven't used it in the last couple months. It's a three gallon batch that has seen a total of maybe 10 sq. ft. of parts.
I just checked the ph thinking that maybe I dragged acid into it during my last bunch of acid wash effects experiments, but it is actually fairly neutral on the scale at 7.5ph. By the way, all of those last experiments came out fine... a nice deep black.
My new dye will be here early this coming week, and I'll re-ano again & report how it went.
This is worse than screwing with somebody elses stuff... these parts go to both mine and my son's brand new paintball guns which we haven't even got to shoot yet! Aghhhh

[acidrain]

Here's an update to my problem...
Recieved the new dye, and really only wanted to change one thing at a time in order to pinpoint the problem, but I changed two things; new dye and no soda water rinse after ano.
In another thread, MD offered his bad experiences that was traced back to the soda rinse.
I just really wanted to get these parts done, and done they are! Thankfully, the parts did come out OK.
When I have some time, I will run a couple of control experiments to pinpoint the exact cause. I really think it was just old dye.

[mcaswell]

When trying to discover what went wrong, you can NEVER later TWO things at once.

One thing at a time, will get you the answer much quicker.

[acidrain]

Originally Posted by caswell

When trying to discover what went wrong, you can NEVER later TWO things at once.

One thing at a time, will get you the answer much quicker.

True, but it won't get the parts anodized if you change the wrong one thing.
I'm sure you can sympathize... these parts have already been ano'd, stripped, and re-ano'd three times. This was the forth ano.
OK,I promise to back up to where I was before.
Here is how the experiment will go... I'll ano normally, and split the batch four ways...
batch 1) soda rinse, new dye.
batch 2) soda rinse, old dye (the control).
batch 3) water rinse, new dye (as was resently successful).
batch 4) water rinse, old dye.

[acidrain]

Here is an update to this problem...
I finally found some time to run some experiments.
I made two ano runs this morning on a set of identical 6061 samples. Both runs were done exactly as previously explained (6amps CD, 70F, with aggitation), only one set was ran for 1.5hrs (.75 mil target), and the other at 2hrs (1 mil taget). All the samples were dyed for exactly 15min @ 140F.
Here are the variables...
First run for 1.5 hrs:
#1 rinsed in baking soda water, then dyed in the old black dye.
#2 rinsed in distilled water, then dyed in the old black dye.
#3 rinsed in baking soda water, then dyed in the new black dye.
#4 rinsed in distilled water, then dyed in the new black dye.
Second run for 2 hrs:
#5 rinsed in baking soda water, then dyed in the old black dye.
#6 rinsed in distilled water, then dyed in the old black dye.
#7 rinsed in baking soda water, then dyed in the new black dye.
#8 rinsed in distilled water, then dyed in the new black dye.

Every sample took the dye a little differently. The finished colors ranged from a silvery dark blue-black to a nice deep true black. Listed from worst to best, here are the results:
#1
#3
#2
#4
#5
#7
#6
#8 (the only one that came out really black)

So, the big surpise is that rinsing in baking soda water inhibits the dye. Thats too bad, because it really works good at eliminating acid carry-over and consequential streaking from holes and crevices.

Not too surprising is the longer ano times result in a deeper color. This either means that the dye (at least black dye) does penetrate ano layers thicker than .75mil, or that I'm not building an ano layer as thick as I think I am.

Finally, the Black dye I was using had become old (both PH levels are basically the same in the old and the new, and both were accurately mixed with distilled water).

So all is well now... new black dye, longer ano times, and eliminating the baking soda rinse has fixed the problem.

[sswee]

Appreciate the info. Since you originally brought up the problem, I have stopped using the neutralizer rinse tank and just use a RO rinse tank only. I had parts dye different tints but could not pin point a problem, just attributed it to a difference in the coating. Now, with your info, I am rethinking the situation.

One thing on your longer run times. Most likely the dye is not penetrating deeper but the pore size is getting larger and giving a better color.

[Sid03]

I believe some black dyes do need a thicker ano layer for proper dying. Might wanna look into that

[M_D]

The dye does get weak with age, we have found. We have tried to strengthen it by adding more new dye to the old and it seems to work to an extent, but it does seem that the dye reaches a point where you have to add about as much new dye as if you were making a completely new batch and it doesn't seem to stay good for long. We haven't done any side by side control experiments, so I wouldn't swear this is true, but it does appear to be true.

As far as thickness and dye qualities, there may other factors that make it look like a thicker coating dyes better. I know to a degree, a thicker coating is better. Except once a coating gets so thick, it becomes harder to keep growing it. What often happens is the coating gets softer, or more porous (dissolution). You need a certain amount of dissolution to dye the part nicely. As long as it doesn't get too bad, that softer coating dyes faster and darker. It's just a matter of the degree of dissolution, too little is bad for dying, and too much is bad also. There are ways to make a thinner coating more porous so it dyes ok. A higher acid concentration, and/or warmer electrolyte are two ways. A lot of commercial anodizers routinely use a thinner coating that looks good, for them time is money so for the most parts the less time in the tank the more loads they can run. So they have an incentive to get the parts to look acceptable in the least amount of time with as little resources used in the way of electricity and electrolyte degradation. Just something to think about.

Also, if I remember correctly, at one point I washed dye out of an unsealed part with strong soda water and it worked as well as bleach. I don't have much time to play around with experiments because of time constraints, so I haven't tried it to find out for sure. Some day, if I think of it when it is handy and I have some spare time, I'll do a little experiment unless somebody here tries it first and reports back. Anymore, I don't do much anodizing at all. If I get involved, it is just to help trouble shoot or advise the employees, or when I am doing R&D.

[M_D]

I was going to comment on the soda rinse versus the plain water rinse and acid carry over. This is how we do it, not that it is the best way, but it works for us. We have a rinse tank next to the anodizing tank, and when parts are anodizing we submerge the rack(s) and depend on the parts agitate the racks and/or let them set for a few minutes. Naturally, the acid will build up, and even with a fresh water change you will still have a mildly acidic water left on the parts. We the use a spray rinse (hose) to to try and remove vitually all the acid. The rinse tank gets changed out for new rinse water frequently.