In which environments is titanium more corrosion-resistant?

Where titanium truly "refuses to rust" comes down to two things: enough oxygen to keep its film patched, and enough chloride to never break through it. The sweet spots look like this:

1. Seawater – from room-temperature spray to 120 °C brine, chloride-soaked surf or abyssal depths; hulls and submersibles stay untouched for decades.
2. Oxidising acids – nitric up to 65 %, wet chlorine, sodium hypochlorite: the harsher the oxidiser, the thicker the self-grown film and the safer the metal.
3. Urban or coastal air – salt mist, acid rain, sulphur films-nothing more than a rinse for titanium.
4. Neutral chloride solutions – NaCl, FeCl₃, CuCl₂, pH 4–11, even at 90 °C cycling: no pitting, no crevice attack.
5. Hot, dry oxygen – air or low-pressure wet chlorine up to 400 °C; the oxide mends itself faster than it spalls.

Fluoride, oxygen-starved strong acids (conc. HCl, HF, HBr) or high-temperature reducing environments are the exceptions-there titanium needs alloying to survive.