Rust at Bottom of Support Column
If I saw that in my basement I wouldn't worry about it until the column (post) started moving.  

But if I did want to get proactive I'd get a jack and a 4x4 set up next to that column and jack it until that post is loose. Then put a new steel plate in there which should last the next 50 years.  

And since this is internet advice and might actually be followed, use two posts and two jacks, you know, for safety.  

Of course, when jacking things up you are acting against some very static forces and might end up cracking the tiles upstairs.