Identifying AL alloys

[bomscho]

After the great results of switching to TI racks, I think the last real bug to iron out of my process is alloy-based.

I buy my AL sheet, plate, rod etc from scrapyards, which don't discriminate between alloys. But they all have the backing on them still.

Which are the best alloys for ano (form best coating, take dye best) and how can you identify them? The protective plastic is always a different color and opacity so I figure that might be a clue? - color coded plastic?

Thanks!

[acidrain]

The alloys I get are never covered with plastic. I wouldn't depend on color coded protective plastic.
The alloy that smuts the least, and ano's the best (in my opinion) is 6061.
I had made a thread on how to identify alloys, but I see it was closed, and a lot of the info is gone.
From memory:
The easiest way to tell what the alloy is, is by the color and tenacity of the smut after etch.
6xxx smuts very little.. just a light gray.
5xxx is vey similar to 6xxx, and anodizes the same.
2xxx makes a light black smut that is not easily wiped.
7xxx makes a deep black smut that is easliy wiped.
Alloys as seen:
7075 6061
5056 2024


Also, (see below) the same alloys ran to .75 mil per 720 rule. As you can see, the 2xxx and 7xxx don't need to be run to the full 1 mil for black. In fact, if you run those alloys to 1 mil at 68-70F, desolution will begin. For 2xxx and 7xxx, I always cut the ano time to 75% of 720 rule calcs.
Hope this helps.

[gcdrummer02]

sticky this!!

[bomscho]

This has been very helpful. My few remaining problems WERE in fact caused by poor alloy identification. I've been using almost exclusively 7xxx, it turns out.

Does anyone find that certain alloys dye strangely? For example, despite 5 or 6 efforts to get 7xxx dyed black to acidwash+overdye with burgundy, the burgundy always wipes out the black in less than a minute--but turquoise can overdye black easily.

Plus, when some rinsewater collected on the bottom of a drying anodized piece (I know, bad), the part that had been wet longest wouldn't take any black dye, but was fine absorbing turquoise. I'm guessing that black is very pore-sensitive.