What are the common oxygen scavengers used for boiler water?

There is a chemical called Carbohydrazide which is used in boiler water. Carbohydrazide is a volatile oxygen scavenger, contributes no solids to the system, reacts readily with oxygen at low temperatures and pressures, and passivates the metal of the boiler system. Carbohydrazide cannot be used in applications where the steam comes into contact with food.