Since the industrial and vintage looks are trendy, and there is a great focus on sustainability, you should give your old metal furniture pieces (such as cabinets or chairs) a new lease of life.
Especially if the piece is vintage, it is a real treasure that you shouldn't alter or over-restore. Don't paint it, as the interesting metal patina (the colour change caused by oxidisation) would be lost under the new paint.
Just rub off the worst patches of rust gently with a fine-grade sandpaper. Next, apply a clear metal sealer as a top coat to stop the rusting and oxidisation process. The staff at major DIY stores should be able to advise you.
Finally, find a way to use it in a manner that plays up its history, or use it in a different way. If it's an office drawer cabinet, display it in your living room, using it to hold books and family photos. Continue the heritage theme by accessorising this corner with vintage-style accessories and slightly "battered" or distressed furniture.