Mechanical behavior of erosion-corrosion scales on steels as characterized by single-particle impacts

Samples of 2.25Cr-1Mo (less than 0.5 Si) and 2.5Cr-0.55Mo-1.4Si steels were eroded-corroded at 450 and 650 °C using fluidized bed combustor bed particles at velocities of 10 and 20 m s^-1. The steel with higher silicon content showed significantly lower metal loss rates under all conditions. The samples were subsequently subjected to single-particle impacts using spherical WC particles at velocities around 50 m s^-. The impact response of the scales could be explained in terms of a combination of substrate hardness and scale morphology effects but could not be consistently related to the superior erosion-corrosion resistance of the steel with higher silicon content. All scales were composed of oxidation product and deposited bed material erodent. Samples eroded-corroded at 450 °C had denser, more mechanically stable scales which could be associated with the generally lower erosion-corrosion rates at this temperature. At 650 °C the scales were more loosely packed, especially at the lower erosion-corrosion velocity, which resulted in apparent ductility by permitting them to densify under impact. Scales were either segmented or continuous in appearance. Thick continuous scales maintained their integrity under the lower velocity conditions of the erosion-corrosion tests, thus leading to low metal losses, but spalled catastrophicaliy under the single impacts. Segmented scales spalled in smaller pieces under single impacts. It is proposed that the segmented scales would exhibit significant failure under low velocity conditions, thus providing less protection to the steels than continuous scales under similar conditions.