Most Helpful Girl
That appears to be the natural patina of the sculpture. If it was meant to stay as a steel color, it would have been sealed.
But you can use products like naval jelly and steel wool, cleaners like CLR that are known to dissolve rust, steel brushes.
Of course, follow all safety precautions like doing this in a ventilated area, using goggles, a breathing mask appropriate for the chemical you choose and to prevent rust flakes from being inhaled. Put down a chemical resistent tarp.
The clerks at your hardware store will help you.
Then when you get all the rust off, you'll want to apply sealant so it doesn't just go right back to rusty. Your hardware store clerk can recommend what is best.
Most Helpful Guy
If the pieces are separate so you don't need a ten gallon bucket full , I know that pool acid works very well and it's cheap. You'll have to watch it closely if the metal is thin. I use it all the time to remove rust. Follow up with some light oil after you rinse the pieces and let them drip off , dab them with a rag here and there and you should be able to hit them with some clear coat or paint etc. Spray paint is oil based anyway so I've never had any problems spraying over something I put light oil like WD40 on.