Air-hardening is just something that 631 and 853 do after you weld them. It means that as the metal cools down after welding it gets stronger. That is the special feature of those alloys. They heat-treat themselves in the weld-zone. It's not something you do or don't do. Some people add corrosion treatments to the insides of frames after they build them but from the factory the tubes have a kind of black residue inside them (probably oil and stuff from the mandrels, perhaps a bit of mill scale) which is pretty corrosion resistant. You wouldn't expect a frame to rust from the inside except in places where water collects. That's why this is an odd location for it to happen because water doesn't collect there. But people are saying it was stored for a long time after the failure so it may have just rusted after it broke because that exposed clean metal.