The chemical difference between titanium and steel is mainly that titanium has a much higher reactivity with oxygen and nitrogen, the main constituents of air.
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.
> 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.
My understanding is that rust fails to protect iron the same way. Is that right? If so, why the difference?
Yes, it is right. The difference is that in the case of aluminium and titanium (but also stainless steel), the oxide grows in a uniform way, covering all the metal. These protective layers are very thin and act as barriers stopping oxygen from reaching the metal underneath.
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.
Once rust starts, it is porous & flaky and allows more oxygen to infiltrate and hit the next layer of iron. The reason it is porous & flaky is due to creating a mix of FeO and Fe2O3 which have different crystal structures so it doesn't create a nice protective barrier.
Rust can protect iron in that way, bluing is a common process to create a protective rust coating. However rust is fragile and often flakes off thus allowing the process to continue. Other metals their oxide is strong enough to protect the pure inner layers.
This depends on the alloy involved as well. In general though rust is not a good iron protection.
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.