a tip...
when you take off a radiator, always immediately turn it upside down to carry out of the room. That way, the valve holes (or in your case the split) will normally be at the top, and unable to dribble sludge on your carpet. Black sludge is pretty well impossible to remove, and will stain most things, including rough skin and plastics.
A radiator in good condition will withstsand enormous pressure, so your almost certainly rusted from the inside. This is usually caused by a plumbing fault that allows air, or fresh oxygenated water, to enter the system.
An old system, with no corrosion inhibitor, will eventually rust away, but usually it will stop working due to sludge and corrosion blockages before then.
Assuming that all your other radiators are equally old, and have had the same quality of water circulating through them, they are likely to be equally rusty inside, so you should plan to replace them before they leak, and to get an experienced heating engineer to diagnose the (probable) leak or pumping-over problem that caused it, and give the system a through clean to remove the corrosion sludge that is probably clogging your boiler and making it bang.
If you recently had a modern boiler fitted to old radiators, the cause might already have been cured, but the radiators will still be corroded and weakened.