Just curious. Will scandium or titanium rust?

...not like steel. But all metals do oxidize to some extent - some much more slowly than others.

What S&W calls a "Scandium" frame is really an aluminum alloy frame with traces of elemental scandium mixed in (less than .05%). Think marketing trade name rather than chemical composition. It does improve the engineering properties of the alloy, but there's very little actual scandium involved.
The alloy can't rust, because there's no iron. Like other aluminum alloys, any surface oxidation that forums acts as a protective layer and prevents further degradation, so it‘s virtually maintenance free.

__________________
"I also cook."

The third post is correct, however as far as the above, certain metals do NOT oxidize(react with oxygen). Gold is one metal that stands out as not reacting with almost anything.