Scale and Corrosion
Currently, after outages, many boiler and pre-boiler systems are filled with aerated water, sometimes containing
high ppm concentrations of ammonia and hydrazine. This water, which is exposed to air in vented storage tanks, contains high
concentrations of carbon dioxide, carbonates, and oxygen. The practice of dosing the tanks with ammonia and hydrazine may reduce
the oxygen concentration, but it increases, by about 10 times, the concentration of carbonate due to the formation of ammonium carbonate.
While this water is being pumped into the system and heated during startups, the ammonium carbonate breaks down, elevating the
concentration of carbonic acid and reducing the pH in the feedwater and boiler. In room-temperature laboratory samples, an alkaline
pH is misleading because it does not represent the actual pH at the temperatures in the boiler.
Filling the boiler and preboiler with
aerated water results in large variations of pH and oxygen concentrations. These variations influence the solubility of iron
and copper oxides; solubility is high at low pHt (pH at temperature) and low at higher pHt. During startup and normal operation,
CO2 is driven out and the oxides which dissolved after boiler fill and warm-up then precipitate and form deposits. A high concentration
of oxygen and low pH during the startup period enhances corrosion of the economizer and other components, generating additional corrosion
products, which later deposit.
Exhaustion of Condensate Polishers
Adding aerated makeup during startups and operation causes exhaustion
of condensate polishers by carbonates and elution of already exchanged impurities. This is especially a problem in steam cycles
with high makeup requirements, such as in cogeneration cycles.