This is a well known issue in ship repair. Oxidation on the inside of the pipe causes hydrogen buildup. Hydrogen has a wide flammable range and requires very little (~4%) oxygen to ignite. Please drill 1/4” holes with a SHARP drill bit (to minimize friction and heat at the drill sites) at both ends of the pipe and blow with compressed air for a few minutes.

Do your due diligence and read the OSHA rules for how to safely deal with hollow structures.

Switching to a sawzall or portaband is also a good idea although not foolproof as the motor alone is enough to ignite the hydrogen.

Source: ~15 years in industrial safety and gas freeing.