I use an aluminium counterflow wort chiller. To sanitize this I circulate the boiling wort through the chiller to clean and sanitize the inside
¿Do you recommend any acid or alkaline product for this? What can I do to make sure that is clean.
Chlorine or Caustic/Alkaline will corrode the oxide layer on aluminium; don't use them.
You could use a sanitizer that is based on Phosphoric acid or similar and another option would be an iodine solution.
You can circulate boiling water through the chiller to ensure there is no residue before storage and again before usage; that will sanitize it.
I would suggest circulating boiling WATER through the chiller first to sanitize it, rather than using the wort to sanitize it. In that scenario, anything unclean in those tubes ends up in your beer.
I usually boil some water in my kettle before the actual boil (either while the beer is mashing for all-grain, or before anything else for extract) and send that through all of my pipes and tubes and equipment, to make sure they're all sanitized. Having seen the muddy, dirty water that comes out afterward, I would NEVER want that in my beer! It's shocking what kind of stuff can build up in those dark, damp environments, even when you rinse them out again after the brew day.