You can use zinc chromate in a spray can, its effective, cheap, and easy to apply... Its not highly resistant to scratches. I would wash the tube with conversion coating first, then rinse with water per directions.
Other option is to go get a can of self etching primer and spray this on... This stuff may be avaliable at an auto parts store or else at a paint and body supply store... It is usually about 14-16 a can