Edge defects/discoloration (pic inside)

[blackcote]

I've been trying to get these parts to look nice for some time now, but my newbie knowlege doesn't seem to get me much of an answer I need. When I first tried this part only the side that I buffed took an anodize finish. So I thought there was a clear paint on it. But paint stripper didn't do squat. So I used the Anodize stripper and it took it right off. It came out so so but had some different color variations and didn't match the piece it was going to be mated with. So I stripped it again and repolished it cleaned and de-ox and this time I get what you see in the pic. I thought this would be caused by not so great prep work but It seems to get worse each time I try. What else can cause this? Can machining heat do something to the aluminum? The alloy is unknown billet aluminum. Can it be the unknown alloy? It seems odd that it's only on the edges if it was related to the alloy. I also get these spots which I think is attributed to the stripper causing pits maybe? (upper left of the pic)
Feel free to browse som other pieces here http://www.blackcote.com/pics/

[Fibergeek]

It looks like whatever was on it originally hasn't been completely removed, an outside radius is an odd place to see this though. I'm assuming that your rinsing between steps is complete and through. The edges like that can get work hardened if they were under severe mechanical stress during whatever machining was done, too bad you don't know where its been.

Maybe M_D has seen this before and can shed some light on it.

[M_D]

I can’t give any definite answers. I have had occasional spots (steaks is more accurate in our case, similar to what your part edges look like) that didn’t dye well, which we have found was from inadequate degreasing.

In the upper right corner of your picture, I can see the effects of stripping. There are what appear to be shallow, flat-bottomed valleys.

The light colored pin spots look like ones I have seen with 2000 series alloys, both from commercial places and ones we have done. It happens (to us at least) when multiple pieces of 2011 series are racked together, and dyed for extended periods. I have never seen it happen when individual pieces are dyed alone, or when multiple parts are done at the same time on separate hanging wires.

Maybe your part is a 2000 series alloy, and maybe not. Here is some information that might help you to determine if it is a 2000 series alloy or not. When 2000 series is dipped in a caustic, either for cleaning, stripping, or pre-anodize etching, it will build a layer of smut at a much quicker rate than 6061. We have manufactured identical parts in both alloys, and occasional mixed 1 or 2 together by accident. After a couple of minutes in hot cleaner, the 2000 series parts stick out like a sore thumb because they get a dark stain. The same thing happens in the stripper or etching solution, except it is a smut that will wipe off. We use sodium hydroxide for both etching and stripping, but have separate tanks for each purpose. So, if you have some known alloys on hand, you might compare the reaction compared to your trouble part. While I am on the subject, 6063 is significantly more resistant to getting smutty than 6061, or any other alloys we have experience with.

I have a hunch that your problems on the edges could be dissolution. Is it lighter there right after dying, or does it show up more after sealing?

[Fibergeek]

M_D,
Interesting technique for a quick determination if an unknown alloy is 2xxx series or not. I see now how it would be simple and effective. Perhaps you could expound on it a little? I've faced unknown alloys before (and I'm sure I'm not alone) and I could have avoided some unnecessary trouble if I would have though of this and applied it.
Very shrewd.