Characteristics of scale layers on steels in fluidized bed combustor service

The characteristics of the scale deposit layers that form on and protect steel surfaces eroded-corroded at elevated temperature, i.e. the composition distribution, morphology, deformation, thickness, porosity and mechanical integrity, were determined and related to metal wastage under various laboratory test and in-service conditions. It was found that in the outer part of the scale layer either straight bed material or a mixture of it and iron oxide can form. Near the scale-metal interface the bed material content of the layer was markedly decreased and consisted primarily of iron oxide. The gradation of the constituents through the layer were determined using line scan and X-ray maps. The distribution of iron and calcium had a linear gradient through an inner layer near the substrate interface. The iron, calcium and sulfur in the outer layer were neither linear nor could they be correlated with the distance from the outside surface to the scale layer-metal interface. The composition distribution depended on both the chemical and mechanical processes that occurred. In the inner layer the chemical process predominated and in the outer layer only the mechanical process occurred. The erosion-corrosion (E-C) scales that formed under the test conditions used appeared to be relatively ductile, even at room temperature. The microhardness of the E-C surface layers could not be correlated with the E-C metal wastage. However, the single-particle impact behavior of a preformed layer could be related to the E-C metal wastage.