Concentrated sulphuric acid is virtually all molecular meaning that you have whole molecules of H2SO4 rather than individual ions of hydrogen and sulphate which is what is in dilute sulphuric acid. This is because dilute sulphuric acid contains water which dissolves some of the H2SO4 dissociating the acid.
The hydrogen ions in dilute sulphuric acid is then what attacks metals corroding them which is dangerous for storage and transportation as the acid can leak out. Hence in order to transport dilute sulphuric acid you want to store it in polymer or glass containers but this is stupid because these containers are obviously struturally weaker than steel containers which is why we don't transport dilute H2SO4.
Concentrated sulphuric acid however contains no hydrogen ions but whole molcules of acid. Therefore it undergoes a different reaction with steel: it instead forms an insoluble iron sulphate layer with steel. This layer actually prevents further attack.
Sulphuric acid reacts vigorously with water in a very exothermic reaction hence contamintion of container or transport tankers must be avoided.
i hope that helps :/
