But there’s also the base chemistry: titanium doesn’t behave like steel, and the chemical differences are why it is such a pain to work with, not inexperience.
Like with aluminum, this high reactivity is masked in finite products made of titanium, because any titanium object is covered by a protective layer of titanium dioxide.
What is worse in titanium than in aluminum is that titanium has a low thermal conductivity, so a small part of the titanium can become very hot during processing, which does not happen with aluminum, where the remainder of the aluminum acts like a heatsink.
The hot spots that exist on titanium during processing, which do not exist on aluminum during processing, make titanium much more susceptible to reacting with the air or even to starting a fire.
Titanium, even as "commercially pure", has a much higher strength than aluminum, which requires higher forces for machining and increases even more the chances for overheating.
My understanding is that rust fails to protect iron the same way. Is that right? If so, why the difference?
In case of iron, oxidation occurs at different points on the surface and the oxide layer initially leaves most of the metal exposed. The oxide is also not effective at stopping oxygen, so the rust layers keeps growing until it forms flakes that fall, exposing more of the metal. The process repeats until all the metal is consumed.
This depends on the alloy involved as well. In general though rust is not a good iron protection.