Rust is oxidation and almost all metals will oxidize. Titanium certainly does oxidize. If it didn't , it would not be as corrosion resistant as it is. I'll explain that in a little bit.
When pure metal atoms are exposed to an environment with oxygen , they almost immediately form an oxide of that metal on the surface. Iron atoms form one of 3 types of oxides. Aluminum forms an oxide and titanium forms an oxide. It is virtually impossible to prevent the oxidation of pure aluminum and titanium because the thermodynamic driving force for them to oxidize is huge.
The reasom iron rots away and the aluminum and titanium don't is because of the nature of the oxide that each forms. The oxide that forms on aluminum and titanium is tightly bonded to the metal it forms on. It also does not allow oxygen through to oxidize the pure metal under it. Aluminum is the same. These types of oxide films are call "passivating" films. If the Ti and AL didn't readily oxidize to form these films , it would corrode easily. Iron oxide does not form a tightly adhered oxide film that prevents oxigen from getting through. Therefore after the initial thin layer of oxide forms on iron , the base metal continues to oxidize below this initial film. The iron oxide film does not "passivate" the surface of the base metal.
Stainless steel has Ni and Chrome in it. When it is exposed to an oxidizing environment , it forms a layer of chrome oxide. Chrome oxide is a passivating film to some extent.
The big issue with salt water comes from the salt which is NACL primarily. Aluminum for example , forms the nice protective aluminum oxide film . The problem is that the NACL in water is a source for CL ions. These ions can cause the protective aluminum oxide film to break down by what is called a reduction reaction which is the exact opposite of an oxidation reaction. So the protective film is broken down and then both oxygen and the chloride ions can get at the base metal. The biggest problem is still not general corrosion of the surface. What happens is these metals will tend to form pits. The pit is almost like drilling through the material. very little metal is actually oxidized compared to what happens on the outside surface but when the pits form , all the oxidation takes place in that tiny little area of the pit. pits will cause aluminum and stainless to become porous with very little amount of metal corroded.
Titanium falls into the category of the stainless and aluminum in that it has the protective layer but that layer is subject to pitting. Titanium is much more resistant to pitting from chloride ions and so it appears to not be oxidizing at all.
Other than inert metals like gold , the metals that form a tightly adhered passivating film are the most corrosion resistant. I believe all of these passivating metals will be susceptable to some degree of pitting that varies depending on the exact negative ion available , such as the chloride ion in salt water.
the reason some "corrosion resistant" materials corrode under the rings and thread of guides is because of a phenomenon called concentration polarization. The builk of the fully exposed metal is at whatever level of polarization (galvanic current) determined by that metal and the solution (salt water or even air with moisture) the part that is wrapped has trapped some water and ions under it. This means that the whole exposed area of the guide is at one galvanic potential while the area under the thread is at a different galvanic potential because of the concentration of ions trapped below the thread. This potential difference leads to very accelerated corrosion of the metal under the thread. In addition to the concentration polarization , there is also a very large anode to cathode area causing the smaller area under the threads to corrode even faster.
anyway , that's a little about corrosion and why sea water is such a problem , even for metals like aluminum and stainless steel.