Every once in a while it is a good thing to run some diluted vinegar (diluted with water) through your steam iron this will remove rust and mineral build-up.
Cleaning a Clothes Iron
Removing Rust from Metal?
Steam Iron Doesn't Produce Steam
Clean Your Iron with a Dryer Sheet
Removing Melted Plastic from a Stove Burner?
Cleaning Steam Iron of Calcium Deposits
Removing Rust from Cast Iron
Cleaning Heat Stains on Kitchen Stoves?