Nothing that I know of, although it certainly has an oxidation layer that has to be cleared (obviously).
One thing I learned the hard way, having worked in ‘the business’ a while ago, is to be absolutely sure that the flux you use is fresh, as in recently purchased at a high-volume outlet, as using old flux is not any different than using no flux at all.
“”””One thing I learned the hard way, having worked in ‘the business’ a while ago, is to be absolutely sure that the flux you use is fresh, as in recently purchased at a high-volume outlet, as using old flux is not any different than using no flux at all.””””
I’ve used old flux kept in my plumbing van for years in a hot climate and never had a problem.
Being a repair plumber, my can of flux with the hole in the lid and a brush sticking out would be used for years and the old backup cans that had been stored in the van for years then being opened.
I'm just a bit puzzled because I've never had any problem soldering to copper wire even several years old (using std. electronic flux if soldering electronics and such) unless the copper wire was visibly darkened at least a little by oxidation.
Granted that said copper wire is usually stated to be "low oxygen" or "oxygen free", and especially the "oxygen free" types seem to stay new in appearance / not visibly oxidize longer.
In this case the inside surface of the tubing LOOKS new, but you are saying it is still oxidized, right? Maybe some copper alloys are prone to a "clear" surface oxidation layer that the "oxygen free" wire alloys are not? Or maybe "low oxygen" copper doesn't add surface oxidation so quickly?
I do know sanded Aluminum can surface re-oxidize quickly, and even though looking great it has to be re-sanded before soldering. (Using aluminum flux, of course.)
“”””having worked in ‘the business’ a while ago””””
What did you do in the plumbing business, or whatever business it was?