Why does rust mist break ice armor?

[Bananadine]

I think "magical armor" in this context means "armor forged via Construction magic". Destruction works on Caelian Iceclads, for instance.

Maybe the rust mist is warm....

...No, wait, rust mist affects furs as well. It probably affects every kind of default armor--turtle shell, leather, coral, whatever. It's just a generic "corrosive" mist, not specifically a rusting mist. That "rust" is referred to so much may be seen as a minor mistake in the game's textual descriptions. (Or, if you like, you can consider it a bug in the mechanical action of the spell. I think mechanics trump text, myself.)