Identification of hard grey coating on aluminium strip

[Simon55]

I have a sample of aluminium (6061-T6) that has a hard, highly adherent matt grey coating on both surfaces. I need some method of identifying the coating type. Any ideas?
What I know so far is
-The coating remains intact/adherent during a tensile test
-coating is harder than the aluminium substrate
-coating does not contain fluorine (ie no PTFE or similar)
-no primer visible microscopically on polished cross section
-thickness of 30-35 microns

 

[kenvlach]

Is it a good electrical insulator?
How much extension did you do in tension?

There's a good probability that it's hard anodize (anodic oxide). I would expect some cracking from tensile or bend tests, though. See a brief description of anodic coatings for Al 6061 & other alloys at

http://www.anodizing.org/reference_guide.html