Presumably you mean steel POST (like piece of pipe) filled with concrete, not steel beam.
Depends - if surficial rust, just power wire brushing then rusty metal primer followed buy a couple spray can coats of finish paint might solve it for quite a while - Rustoleum would be my preferred brand of off-the-shelf products.
If heavily rusted so it has lost a lot of metal or it heavily flaking, then probably replacement is called for.
You would need to Search the List for vendors with good reviews and ratings - then call them, Unless you use the SnapFix App, AL does not have a mechanism for you to post your issue and ask vendors to contact you - wopuld not work because vendors can't spend all their time checking to see if someone has posted asking for a specific service they provide. Technology for this is a bit down the road still.
Your Search the List category for this would be Foundation Repair or possibly Builder - Garage/Barn/Shed. Be sure they have done this before, make them explain exactly how they intend to support the beam while removing and replacing the post (and hammering a couple of 4x4's under it is NOT enough - they should be able to expain how they will support the beam AND temporarily fasten the supports so they cannot kick out if bumped, and also how he intends to repair/replace the bottom connection for the post, depending on whether the base bolts are rusted out or not.
