I’m not convinced that passivating a titanium cup would have much if any effect. Chemicals like citric acid remove iron, and there shouldn’t by any appreciable amount of iron on the surface to begin with. I also don’t know whether the undesired (to me) green tea reaction is with titanium metal or with titanium dioxide.
It could be interesting to experiment with anodized titanium. Apparently, one can fairly easily build up moderately thick oxide layers with various properties.
My attempts to anodize have been exclusively with aluminum, using primitive if not directly stupid methodology. The results were trivial, with a formidable mess.
Anodization is really awesome when done properly. At the risk of exposing my inner moron, I must admit I was not aware that titanium was a candidate.
It's so easy at home! You probably have a suitable acid in either your kitchen, medicine cabinet or garage. Lots of safe materials that can go down the drain, and they give slightly different results. Just degrease well for an even finish.
(I suppose the biggest expense is 9volts or a power supply but I am a guitar player so I just use my mostly spent pedal ones)
I used a few 9v batts and PhDown. I didn't tweak the voltage for any specific colors, but just aimed for a protective coating. The results were disappointing and probably produced more value in hydrogen than anodization. I might try again if I ever craft something else of aluminum that requires protection.
It could be interesting to experiment with anodized titanium. Apparently, one can fairly easily build up moderately thick oxide layers with various properties.