At one point in my Navy life I was an Inert Ordnance Certification Officer. That means it was my job to determine and document if ordnance training devices were truly inert. We were trained by EOD personnel but perhaps the 1st thing we learned is that there was no standards, up to that point, for safeing ordnance. In the case of rounds like you have, Guy, we would shake them and listen for something like powder slopping around. For the HE heads, we would look for obvious signs of deactivation, such as holes or hollow basis. A blue ban on the warhead indicated an inert training rd too. But in most cases we did not have live rds that had been deactivated. It was considered to dangerous. Our training ordnance was usually made that way, from the factory or depot
If the projectiles appear to never having been removed and you cannot see the base, or obvious external signs of deactivation, I would certainly consider these to be live
As for the corrosion, I would suspect there is enough residue inside that has reacted with either moisture or air and produces a corrosive gas that is attacking the case. We had this happen to some pyrotechnic cartridges for the AN/M8 pistol that had been deactivated from live rds and someone from NAD Crane told us to spray WD40 into the holes. However, it either did not work or the damage had gone to far. They were eventually replaced with new inert rds from Crane. These cases were aluminum, not brass or steel.
Don’t know whether this helps or not but just trying to pass on some stuff your tax dollars spent for me to learn!