First, the nitric acid test can only be done once the wafers are scrapped, you cannot use nitric acid on good wafers.
The Nitric acid test is a simply way to see if the we have damage to the passivation that exposes top metal, if there is damage to passivation that exposes top metal then we have a potential for field failure, the reason is our metal lines are copper and copper will migrate and short between copper lines, so it is imperative that we don’t have top side damage causing cracks with penetrate to top metal. The fuming nitric acid will attack the copper traces if there are any cracks or damage to the passivation which penetrates to top metal, so the acid will etch the metal and optically we will be able to see the damaged metal, where we would not be able to see the damaged to small cracks in the passivation without top metal being damaged. The purpose of this evaluation is to understand, if wafers have scratches on some die, are their other die that scratches are not detected that were damaged and have passivation damage.
I hope this helps, please let me know if you need any additional clarifications.