The average level detected in condensate receivers is three parts per million. This is almost one thousand times greater than the five parts per billion level that can induce pitting corrosion. Through the packing of condensate pumps. The stuffing box is under a negative pressure, and air that is one third oxygen, leaks in. Valves above the water line can introduce oxygen as the velocity of the water lowers the pressure at the valve stem.
Flanges can have the same problem as valves.
Oxygen is dissolved in makeup water that was added to the boiler because of condensate leaks. Pumps with built in repellers that create a negative pressure in the pump stuffing box. Add chemicals to convert it. Hydrazine is an example. You are adding hydrogen that will combine with the oxygen to form water.