Bill, May I suggest that you take the tanks to a local welding shop. They will be able to tell just by looking if they are aluminum or stainless steel. They will also have the proper chemical (possibly carbon tetracloride) to remove the oxygen from the air inside the tanks and prevent explosion during the welding process.
Caution! You could wash them out with water for two days and they would still explode. Definitely NOT radiators! Rick 611GC
