Spray an anti-corrosive spray on the rusted portion of the metal sprinkler head. Read the label to ensure it is also a rust remover that will work on metal.
Allow the spray to penetrate the rust for at least 30 minutes.
Grasp the sprinkler head with pliers and turn it counter-clockwise to see if it has become loose.
Spray the anti-corrosive spray again and allow it to penetrate until it unscrews.