7000 series parts dye, but 6000 series won't

[acidrain]

HELP!
I'm really scratching my head on this one...
OK, I made a run last night with a mix of alloys and colors. All but one of the parts is 6000 series, with the one oddball 7000 series part.
The parts were to go three different colors... lime green, bordeaux red, and black. The one oddball 7000 series part goes black.
So I make my run the same as I always do, and the green and red parts turn out great, but only the one 7000 series part took black dye. The rest of the black parts turned out blue gray. What the hell!!!
At first I thought the black dye was giving me problems (it's quite old). I didn't notice that the one part turned out OK until today, and that's when I realized it was the one 7000 series part.

So here is my setup:
3:1 battery acid mix
69F acid temp
connections checked at beginning of the run & OK
10A CC PS
111.65 sq. in of parts run at 4.6 amps for 90min
voltage climbed quickly to 11.5v, and stayed there for the entire run
All of the parts were prepared the same, run at the same time, rinsed and dyed at 140F, yet only the 6000 series black parts wouldn't take dye.

I didn't seal the bad parts, and this morning thought that maybe it was an error in my calculations... usually in the last 15min of my run the voltage begins to drop by .1 or .2 volts, but this time it stayed steady. So as an experiment, I removed the dye with dilute bleach, rinsed well, then followed up with 3:1 acid. With all of the color removed, and the parts still attached to thier wires, I went another 30min at the same 6A CD, then rinsed and dyed at 140F. The results were exactly the same... blue-ish gray.

Does 7000 series alum. take dye better than 6000 series?
Is there a problem anodizing at 6A CD when dying black?
Anybody else experienced this? What was the solution?
I'm going to have to strip the parts and start over now, and I don't want to waste my time with bad dye, but why would the dye work on 7000 series if it was bad?

[sswee]

I don't have an answer for your particular problem right now. I'll have to give it some thought. I can tell you that most of the parts I do are 6061 and I've never had any problems taking dye. I try not to mix material grades if I can keep from it because of the different voltage curves in Fibergeek's charts. They indicate to me that the different grades grow the coating in different ways possibly allowing some parts to rob current from the others. This is just my perception on the process. SS

[rclint]

I would aslo keep the grades sorted out while anodizing. I run about 25 parts 6061 series with perfect results, Olive Drab, I rund a small 3 part test of the same material and dyed blue, also perfect results.

I have some cast that I think is 7000 series, and it anodizes and takes dye great also, but I have never mixed the parts in the bath.

Clint

[acidrain]

Sswee, you may be on to it. I have more black dye coming, but the more I think about it, the more I'm convinced it's something to do with the process, or the mixture of alloys. The curious thing is that the other colors came out good.
In an effort to sort this out (even though it may be more work should it flop again) I should strip and re-ano the 6000 series parts using the same process, CD, and dye as before (leaving out the 7000 series part). If they turn out good, that will tell me there is definately a problem mixing alloys.
Please add anything else you guys can think of. I really don't want to do this a third time. -Thanks!

[mcaswell]

I seem to remember Fibergeek posting something about mixing alloys in the same ano batch. Suggest you hunt through his postings.

[Sid03]

I run mixed batches, but I did a good bit of investigating beforehand. The alloys do ano at difference rates. When i mix them, both alloys turn out with the same thickness as they would have if I anodized individually. The key is knowing what rate your new alloy ano's at.......maybe it needs to run longer then the rest.

[acidrain]

You know, I'm actually only guessing at the alloys... the only reason I know the one is different is because there was smut on it after stripping, and my experience has been the 6000 series leave no smut. It's impossible to get accurate info from the manufacturer. For example, there were two of the same parts, same manufacturer in that run... one came out smutty, and the other did not. I get the feeling they make 'em out of whatever they have on hand.
I'll run the bad 6000 series parts tonight using the same settup as an experiment, and let you know how it goes. Thanks, keep the thoughts coming!

[sswee]

This is what I have found so far:

[quote=Fibergeek]Yes its possible, but you should avoid it if you can. No two alloys have the same anodizing characteristics. Mixing alloys in the same batch forces some sort of compromise, favoring the one with the most surface area. Try it and see if both alloy samples anodize to your satisfaction.

[quote=M_D]The 2XXX series aluminum can be difficult, it has copper in it which has a larger negative effect on anodizing than about any other aluminum alloying element I am aware of. I think you will find that the 2024 will anodize to the optimum level faster than the 7075, I have found that it is more sensitive to longer anodize times, and should be avoided. The current capacity is different and the voltage-curve will not be the same as other aluminum.
They were on pg. 15 or 16 of posts made by Fibergeek.

[acidrain]

Thanks sswee, I'll let you know how the "control" experiment turns out.

[acidrain]

Well, it was not the mixed batch of 6000 and 7000 series parts that caused the problem...
I just got done re-anodizing the parts that did not take dye.
As a control to pinpoint the problem, I change only one thing in this experiment... I ano'd only the 6000 series parts, and left out the 7000 series parts. Everything else was ano'd exactly as before. The thought was that the mix of parts caused the problem, but this batch turned out exactly as before. Bummer... now I have to strip and start over again, but it was an effort toward knowledge, so it's not completely wasted.
I have brand new dye coming. I hope that bad dye is the problem, because if the next batch turns out bad (with new dye), I'll be totally stumped.