This is a very basic question and here is one most general answer. 98% & up H2SO4 in principle forms a tight & adherent film on Fe surface by a reaction controlled process . This is basically stable Iron Sulphate . This forma a layer and prevents further reaction of Tank surface with acid. Ofcourse this is for static storage as this sulphate protection will get damaged with velocity and turbulances. Whearas in dilute acids this layer is not stable and due to exothermic reaction during dilution this layer breaks and thus dilute acid tanks normally rubber lined or lined with super performance 100% solid paints.