Recently I anodized two reels. All end plates and pillars (6061 aluminum) look right. The feet (7075 aluminum) look right. Of the 20 screws (all 7075 aluminum), 14 look right but 6 of them are dark and dingy. What happened? Each reel frame was anodized as an assembly, so the dark screws did not get a different process
I had to put this aside and work on other parts while I thought about it. After a few days, I had a theory. I had bought 10 rods (0.25 inch diameter) of 7075 to make screws. By the time I did these reels, I was on the 4th rod and had had no problems. Part way through making these 20 screws, I started on rod 5. Was this rod really 7075, or perhaps something that does not anodize well, like 2024?
So I made small anodizing samples from each of the 5 rods yet unused. I also made a sample from a 6061 rod, and one from the partially used rod (#5) that may have been the source of the 6 bad screws. I marked the samples with a file so I could match them to the rods.
And when done anodizing the samples, I had 2 that were dark and 5 that were correct. The two dark ones were from rod #5 and from one of the 5 unused rods.
So now I knew which rods I could use to make replacement screws. The new screws all have proper appearance after anodizing.
My conclusion is the the vendor sent me 8 rods of 7075 and 2 of something else.
Update 12 Sept 2012: Further tests on the unknown material posted 5 Sept 2012.