I've used a ~teaspoon of CarbonKiller with water for ~30 cases at a time.
But I find the same result as with dunking after lead dip annealing. The cases have to be dried inside & out(not just blown out), then coated, or they'll tarnish within a week. Checked em with a borescope..
Look with a magnifier.
The carbon inside the case, and the thin coating of case lube, or polish on the outside, act to prevent tarnishing. Then we go & remove that for some reason...
Well, put it back, or your brass will oxidize.
Does it matter?
Well, we're expending energy here to CLEAN our brass, not tarnish it.
There is another issue that should be accounted for with necks so clean; bullet galling.
Carbon works just fine to seperate the copper in your brass from your bullet jackets.
I coat bullets anyway, and many lube necks, but neither is needed -unless you ultrasonically clean the necks.
The fella that doesn't clean his brass at all is probably miles ahead of us.