Going back to my aircraft engineering days, all nitrogen bottles were supplied with a lock charge of "dry" nitrogen at about 50psi to prevent the ingress of impurities and moisture and we only released that charge immediately before fitting the bottle. I try to keep my bottles charged at all times, but of course I can't be sure that the compressed air supplied by the site is as dry as possible. I've seen corrosion in bottles before, but usually steel ones, Usually corroded aluminium would give a white powder and copper a green residue, so it could be a mould mixed with aluminium corrosion.
