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Corrosion of carbon steel tubes used under boiler water conditions is possibly accelerated by unexpected flow conditions. Mechanisms of flow accelerated corrosion (FAC) are ordinary explained by the dissolution of oxide films formed on carbon steel surfaces, mass transport of metal ions, dissolved oxygen, and water chemicals. These mechanisms have, however, not been precisely proved or reproduced at an experimental level and actual service. In this study, corrosion or erosion-corrosion tests for carbon steel and low alloy steel were conducted in a batch test container at pH 9.0 and 9.2 and temperatures of 393 and 413 K under flow conditions with rotating a gear-type rotor. In order to clarify the corrosion mechanisms, the total corrosion loss of carbon steel was separated as iron mass in oxide films and dissolved iron in the solution. The steel surfaces, debris, and particles suspended in the solutions were observed and analyzed by scanning electron microscopy (SEM, EDX). It was supposed that the partial dissolution of iron from the steel surface and suspended debris occurred followed by the mechanical removal of oxide filmsthat formed on the steel surface, from observations of exposed steel surfaces and cracks on the oxide film surfaces. The dissolution rate of iron depended on the properties of oxide films and the flow conditions. [ABSTRACT FROM AUTHOR]