Shield says it grants immunity to corrode damage, but doesn't... what's up?

I picked up an Alkaline Turtle Shield that says in green text Grants immunity to corrode damage, so I took it for a spin in Caustic Caverns. I tried running through the caustic lake to get the badass challenge I Bet I Can Make It, thinking it would be a breeze with this shield equipped, but it didn't protect me. So next I fought some mutated varkids which I think spew corrosive material at you, and it also didn't protect me from them. What's up? Is the shield specifically for protection from corrosive weapons and barrels only?