why weak acids are more corrosive?

Question

for weak nitric acid we req zirconium msterisl but for concentrated one ss316 or for 99% aluminium req.

Answer 1

You don't mean "weak" acid, but "more dilute". The dilute acid can have a higher concentration of H+ ion than concentrated solution, since if the concentration gets high enough there will be significant reverse reaction where H+ + A- ---->HA. The concentration of H+ ion will then be governed by an equilibrium constant.