In wikipedia I read that the structure of the cubesat are made of aluminum alloys of 7075, 6061, 5005, and 5052. Is this the surface layer of the satellite or it is necessary to cover it with kapton tape?
Typically the outer layer of the spacecraft is chosen for thermal control reasons and for compatibility with UV exposure and atomic oxygen (depending on your altitude).
Kapton as an outer layer has better ratio of emissivity to absorptivity so it runs cooler.
White paint runs cooler still if you get a good low outgassing epoxy that's UV stable (these specialty paints are expensive but Lord Industries sells them (e.g. A276).
If your cubesat is mostly solar cells on the outside then these will dominate and your temperature will be close to room temp on average and the small amount of anodized or alodined surface won't matter.