The inside of the tank is most likely rusted. Does the weld around the nut have any undercut? Picture in your mind's eye the tank slowly rusting from the inside out, making the tank walls ever thinner and thinner. Logically, it would crack and leak at the thinnest part.
The tool repair shop here has a tank that was never drained and consequently rusted from the inside out. One day it exploded and went through the ROOF of the house that was being built. Also I was working on a compressor with a pet cock that was broken. It still had a full charge of air on it and I stupidly didn't pull the pressure relief valve. I thought I could just screw it out enough to bleed the air off then replace it with a new one. Well it blew off in my hand and injected bits of tank particulate and rusty water into my skin. That little stunt earned me an hour and a half with a doctor digging around in my hand with miscellaneous pointy objects. You know the saying "You can't fix stupid"? Well you can, with PAIN or DEATH.
My advice... buy a new tank. Heck, buy a new compressor.