There are several options I would consider, if in your situation (but I'm not telling you what to do, as my suggestions are only general in nature and subject to site limitations I may not be aware of):
If there's only one isolated hole in the "post" (with a hammer and icepick, poke and tap around as much of the perimeter as you can, to make sure there aren't additional holes/potential holes), you could consider performing a composite repair using concrete around the base of the column, if space allows. After temporarily bracing the adjacent door headers/wall loads, drill 3 or 4 sets of 1/2" through-holes through the column. Then install 2 short #4 rebar stubs in 2 sets of the holes, projecting outside the pipe's exterior (the remaining open holes are for concrete to flow inside of the column). Construct a temporary C-shaped wood form, anchored to the floor and braced plumb, after installing vertical #4 bars in each corner, held in place with tie wire and #4 horizontal stub bars. Then pour a good concrete sleeve (Quikrete 5000 or equal) around the column, using a fairly fluid (but not too wet) mix. Beat on the forms to consolidate, and beat on the column above the forms to coax concrete into it. Allow at least 4 days' cure time, then strip the forms and temporary braces. Done. And the concrete will never rust.
If there's not enough room for the above concrete sleeve idea, consider having a small steel fab shop or handyman with fabrication ability make you a 2-piece pipe sleeve 2' or 3' long (split vertically), just large enough to fit tightly around the outside of the existing column. Have him weld it up to itself (make sure he's qualified to weld in the vertical position) in place around the column, using either single-vee or single-bevel groove welds, full penetration, oriented at the sides for access. Wet things (nearby combustible wall components) down with a hose before he starts, and keep a fire extinguisher handy if the adjacent wall or sheathing start to smolder. A continuous fillet around the top to finish it off, a good coat or 2 of paint to prevent future corrosion, and it's done.
If the column is too far gone, consider just replacing it entirely with a new section of pipe, preferably slightly thicker-walled than what's there now. You could have your fab shop weld a top and bottom bearing plate on the ends, with some holes in the plates for lag screws (top) and concrete anchors (bottom) to hold it in place. Make sure of the total vertical measurement you need, or things will get difficult installing it. If you do screw up in measuring, too short is far better than too long, as steel shims can work wonders. Don't forget the header/wall braces, and the paint.
In case the existing column has more than one rusty bad hole (in the vernacular, we call it major section loss), consider wedging in a few vertical 2 x 4 braces on either side of it as a purely temporary fix. Will help you sleep better at night, until the permanent repairs are completed.
Last edited by BridgeMan; 10-08-2011 at 11:01 AM.