kill 25 enemies with A corrosive weapon.
You can tell if your weapons and grenade modes have specific elemental effects as they have an icon on them:
• Red Flame - Incendiary
• Blue Lightning Bolt - Shock
• Yellow Explosion - Explosive
• Green Radioactive Symbol - Corrosive
The barrels are also colour coded:
• Red/Orange - Incendiary
• Blue - Shock
• Yellow - Explosive
• Green - Corrosive