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?
The shields that grant immunity to any element, doesn't grant you immunity against base damage, it grants immunity against the elemental add-on damage bonuses, in addition to the damage-over-time debuff. With an immunity-shield, you will still take base damage, and you will still have a hard time against strong/higher-leveled enemies.