Acidic gadgets do 2/4/6 damage, but also reduce the armour of their target by the same amount.
Apply the damage first, then the armour reduction. So, if you throw an acid bomb at a Dalek (Armour 10) and get a Fantastic Success, you do six damage, but then reduce the Dalek’s armour by six to 4. Your next bomb will be a lot more damaging.