http://www.azom.com/article.aspx?ArticleID=1220 sounds like white rust?

Yes, I think it is that white rust.

The drums are in a dry room, but because of the warm weather it was very hot inside as well (almost 30°C) and this together with the rather humid air outside which was also inside might have caused an environement that was suitable for that rust.

Although the curtains were closed and there was no direct sunlight on the drums even the wrap moved and shows a light bubble here and there near the wrap's top or bottom edge. All because of these hot temperatures. Of course the shells are mad af two plies of cheap wood and two plies of some fiber board. So these shells may be more affectable by heath than pure wood shells and this may have caused also the wrap bubbling a little.

I have a Rogers Superten maple snare with a similar problem - the interior screws would get "fuzzy" with a kind of oxidation that went from black to green to white. I believe it had something to do with the felt lug gaskets holding moisture from the air. I solved the problem by soaking them in dawn detergent and water for 24 hours to clean them, then dried them until they were warm with a heat gun (hair dryer or air drying for 24 hours would work too), and then gave them a very light coating of household machine oil, wiping off any excess before reinstalling. Haven't had the problem since.