Abstract
The all volatile treatment (AVT) of boiler feed water where the dissolved oxygen content is maintained at levels of less than 7 ppb and the hydrogen ion content at pH 8.5-9.6, is typically used for the majority of once-through boilers in Japan. In the past several years, some extraordinary high rate of corrosion damage have been reported to occur on the inside wall surface of tubes which carry the AVT boiler feed water at elevated temperatures. In this study, the effect of pH on the corrosion rate of carbon steel at 120°C in flowing AVT feed water was examined with the goal of elucidating the mechanism which operated for this type of unusual corrosion. A jet-in-slit test apparatus was used, in which the so-called erosion-corrosion as well as corrosion accelerated by the gap in flow velocity could be reproduced on a model system. At pH 9.0, the specimen surface, after a 40h test was covered with a thick corrosion product layer of magnetite (Fe3 O4) to an extent that the so-called erosion-corrosion, as well as the corrosion accelerated by the flow velocity gap, had occurred. At pH 10.0 the entire surface had the original metallic polish even after 40h of flowing test liquid, suggesting that a perfect passivation film had been deposited on it. At pH 9.5, the major portion of the specimen surface was covered with a passivation film, while numerous narrow dark lines radiated from the center of the specimen. These lines were revealed to be grooves filled with magnetite which appeared to be produced at a high rate (over 1mm/y) by a type of oxygen concentration cell corrosion. In conclusion, AVT boiler feed water, at pH 9.5, should be avoided.
