Corrosion behaviors of carbon steel and low alloy steel under batch type flow test environments

Corrosion of carbon steel tubes used under boiler water circumstances 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 are, however, not precisely proved or reproduced in 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, temperatures of 393 and 413 K under flow conditions by rotating a gear type rotor. In order to clarify the corrosion mechanisms, total corrosion loss of carbon steel was separated as iron mass in oxide films and dissolved iron into the solution. The steel surfaces, and the debris and particles suspended in the solutions were observed and analyzed by scanning electron microscopy (SEM, EDX). As results, it was supposed that the partial dissolution of iron from steel surface and suspended debris occurred as followed by the mechanical removal of oxide films formed on the steel surface, from the 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 also on flow conditions.