Steel really likes to corrode, so unless something has been done to stop that it will eventually find enough moisture somewhere to corrode a little. However, steel bars are usually quite chunky and can for most applications well take some surface rust. Usually, once the available moisture is used up corrosion will stop.
Try the following: Remove end cap, lean bike over, tap/shake a little, see how much that falls out. If what falls out is a fine powdery stuff and not much of it(teaspoon or less) ride on and don't worry about it. If what falls out consists of large flaky bits, and quite a lot of it(tablespoon) - consider replacing bar.
If you really want to you can pull both end caps off, thread a steel wire through the bar, pull a rag through the bar a couple of times and then apply the rust proofing agent of your choice. Either bike specific frame saver or something from your local car parts store.