Corrosion of Carbon Steel in Flowing Pure Water under High Temperature and High Pressure Conditions

Abstract

The all volatile treatment (AVT) has been currently adopted for once-through boilers in Japan. Recently, the introduction of oxygenated treatments, namely combined water treatment (CWT) and neutral water treatment (NWT) are being examined with the intention of reducing the pressure loss in the water line. In this study, corrosion tests were conducted on carbon steel in a jet-in-slit testing apparatus under high pressure at elevated temperatures. The mass loss of specimen and damage depth were used as the index of corrosion damage. It was revealed that AVT is superior to others in that the mass loss as well as the damage depth of specimen was smallest in this water. This is because the corrosion product film generated on the specimen surface is of the magnetite. It is vested with enough mechanical strength not to be removed from the metal surface even at those place where the intensity of turbulence and the shearing stress of the fluid flow is locally raised. Thus, the occurrence of so-called erosion-corrosion is almost prevented. In CWT environment, the erosion-corrosion did not occur but radial deep ditches were formed on the surface near the periphery of the specimen where the flow velocity of water was lower than it was at the central part. This might be attributed to the enhanced oxygen supply in the central part of specimen which brought about the generation of sound passivation film there. On the surrounding surface, in contrast, the oxygen supply was poor so that the passivation film was not complete, which must be linked with the formation of the ditches apparently under the influence of fluid flow. In the NWT environment, the most inferior result was obtained in the both aspect of mass loss and damage depth.