Erosion–Oxidation Behavior of Chromized–Aluminized 9% Chromium Steel under Fluidized-Bed Conditions at Elevated Temperature

Abstract

An investigation into the erosion-oxidation behavior of chromizedaluminized 9% chromium steel has been carried out in a fluidized-bed erosionoxidation rig in air at temperatures of 550 8C to 700 8C for particle impact angles of 308 and 908, at speeds of 7.0-9.2 m s À1 . After exposure for 200 h, the meanthickness changes were determined, and the specimens were examined and analyzed by scanning electron microscopy and X-ray diffraction.The results show that the chromized-aluminized specimens experience only relatively small amounts of material loss for 308 particle impacts but greater amounts for 908 particle impacts; such angle dependence is typical of a brittle erosion process. Under both impact angles, the thickness losses increase with increase in speed, and also, in temperature up to 650 8C, but then decrease with further increase in temperature. This change in the behavior between 650 and 700 8C is due to an increase in ductility of the coating and a greater contribution from oxidation. The two coating layers (an outer Al 13 Cr 2 and an inner a-chromium layer) show similar behavior, i.e., have lower material loss rates under 308 impacts than under 908 impacts. As the oxidation rate of the a-chromium layer is greater than that of the outer layer, the oxide scale plays a greater role in the erosion process for the former, particularly under 308 impacts. These observations are discussed in terms of the thickness changes and the morphologies of the damaged surfaces, with the emphasis being put on the relative roles of growth of the oxide scales and removal of such scales (and the underlying coating) by the impacting particles.

Microstructure and elevated-temperature erosion-oxidation behaviour of aluminized 9Cr-1Mo Steel

“…As the layer of sand particle debris becomes continuous enough, such as at the speeds of 8.6 or 8.8 m s −1 , it evidently starts to provide the specimens 8 protection against further particle impacts, therefore leading to smaller thickness losses than at the lower speeds with more discontinuous particle debris layer. In previous studies on aluminium based diffusion coatings, deposition of sand residues on the specimen surfaces has been observed during the tests [20,[25][26][27][28], consistently with studies on uncoated steels [29][30][31][32][33] and other uncoated alloys [34], but very seldom such deposition has protected the target material against further particle impacts. Only in the cases where the erodent particles have been reported to contain soft constituents, such as Ca, S and K, formation of a homogeneous and protective layer of particle debris has been detected [35,36], with calcium compounds acting as a cement that binds the bed material ingredients together on the metal surface [37].…”