I just converted my 9 galllon aluminum pot to have a ball valve (successful!) to use as my HLT and after the test boil to test for leaks (again, successful!) I noticed a few small areas where it looked like the passive oxidized layer had been removed. The entire inside of the pot is that dark grey that you want, except these few small areas that now look like brand new aluminum. I did another boil today to try and build it back up in those areas and nothing, still bright and shiney.
So I guess I have 2 questions:
1) Any ideas what would have caused this and why a 1hr boil won't fix it? and
2) Given that I conducted two 1+ hour boils in the pt, are those shiney areas still oxidized, but just don't look it?