To the best of my knowledge there is nothing but air inside the copper heat pipes of a tower style cpu heatsink. I have the original version of the Thermalright Ultra 120 that I purchased a few years ago and it is working properly. I have not noticed anything out of the ordinary. I do not know if there is some sort of protective coating on the interior or exterior surfaces of the heat pipes.
Last August I did a case mod with a black and copper theme. I made two small copper panels for the case. After cleaning and prepping the panels I used vinegar to etch the panels. Then I sprayed them with a high gloss clear coating. The copper panels show no signs of tarnishing.
There's a lot more than air - if it were just air, they wouldn't work nearly as well. They actually use a liquid at low pressure so the heat causes it to evaporate. This gas condenses on the cool end of the heatpipe, depositing the heat there, and transporting far more heat than just a pure copper tube would.