CrMo is chromoly, a steel alloy with Cr (Chromium) and Mo (Moly-something... grab a periodic table..) and it can rust, just as stainless steel can rust. How do you know that your CrMo bars don't rust? I have CrMo bars that haven't rusted, but that doesn't mean that it can't rust.

Usually there is some kind of corrosion prevention used, such as paint for your frame, but if the frame or parts are really important to you and/or expensive, something like "framer saver" would be prudent.

I thin pretty much everything with steel can rust and most other metals and/or alloys that may or may not have steel will corrode in some way. Except for that steel tower in the Middle East somewhere that has been outside for more than a thousand year... I think the Arabs somehow made a true rust-proof steel. Too bad the art has been lost, like Greek fire.