Hi, Iqor

As you write that the column gets rust just above floor level. So below are some methods to repair it
1.) Rust of column should be removed by steel sand paper and look that rust is only on outer surface, if so then you may paint the surface and for further protection you may need to paint the surface periodically.
2.) If rust is found some deeper in the surface, clean all the rust from the effected portion by sand papering and washed with kerosene oil. After cleaning, cement concrete should be filled in the effected portion of steel column and finish it according to your liking.
3.) If the rust is found so deep and there are holes on the surface of steel column, you should weld extra steel stripes around the effected portion of the column. In this case the steel strips should be welded above and below 6 inch from the rusted portion of the column. After this the portion should be filled with cement concrete and finish it according to your choice.
 
Rust in column happens due to not doing proper painting on the steel. When we clean floor with wet cloth than upper layer of the paint removed slowly with time and steel column get rusted. To avoid this all steel members should be painted periodically above the floor level. 

In conclusion above tips will help you get your problem solved.

Enjoy, Thanks, Goodluck.