Do those spells have any effect on creatures which don't have armor? What about on creatures that apparently wear armor (e.g. goblins)? Are those spells similar to Skyrim's Marked for Death dragon shout (causes the target to receive an armor penalty), where even creatures are affected and may even get a negative armor rating causing them to receive extra damage?
It affects any creature that wears lootable armours exactly how it affects the player. So for goblins and other creatures with armour on their model but no actual armour items, it has no effect.
My source is personal experience. I'm the author of a mod that adds a pitcher plant ingredient with corrosion as one effect, so I had plenty of time to observe this during development, testing, and normal play.