The axle tube itself is about 1/4" thick so very unlikely to be corroded seriously. However the spring perch (which is welded to the axle tube) is much thinner and open at one end only which allows mud in but not out. It is probably this which has rusted to the point where the weight of the vehicle has crushed it. The solution is to remove the axle, cut off the remains of the old perches, fabricate a pair of new ones and weld them on. You could buy new perches to suit the diameter of the axle tube or make your own like I did.