Steam-side scale morphologies associated with scale exfoliation from ferritic steel T22

Abstract

Observations of steam-formed scales on type T22 ferritic steel reheater tubes have provided some insights into the scale morphologies associated with exfoliation. Measurements indicated that as the total thickness of the adherent scales increased, the ratio of the thicknesses of the main inner (Fe-Cr spinel) and outer (magnetite) layers increased continuously to a maximum of approximately 2.5. The often-assumed thickness ratio of 1.0 did not persist after a laminated structure of pairs of spinel and magnetite layers was developed in the main inner layer. The development of such multi-laminations appeared to be associated with a decrease in the rate of growth of the main outer, magnetite layer. In this particular set of samples, exfoliation occurred in scales of total thickness greater than approximately 270 µm, and involved separation of a single pair of oxide layers of similar thickness that consisted of the original outer magnetite layer and part of the original spinel layer. The scale lost in subsequent exfoliation events at essentially the same site consisted of a further pair of layers comprising the new magnetite layer re-grown after the initial event, and a further part of the original spinel layer. Exfoliation events did not appear to involve the whole of the spinel layer existing at that time, only the equiaxed region immediately subjacent to the magnetite and a portion of the laminated structure.

High temperature oxidation of alloy 617, 740, HR6W and Sanicro 25 in supercritical water at 650°C

“…Several studies have been done on this issue [2]. Generally, due to the fact that the main steam temperature of the widely used ultra-supercritical (USC) power plant is around 600°C, a large proportion of corrosion tests on the candidate materials for USC power plants is less than 620°C [4][5][6][7][8][9]. On the other hand, high temperature vapour or supercritical water (SCW) exposure tests were done at the temperature above 700°C on the candidate materials for USC power plants by some researchers [10][11][12].…”

Numerical simulation of the failure behavior of steam‐side oxide scale considering oxide creep and physical defects

“…In practice, oxide scale is frequently subjected to the mechanical stress, such as thermal stress and oxide growth stress . Under such stress, oxide scale is susceptible to mechanical failure as they do not have the ductility of underlying metal . Scale exfoliation can cause plugging, steam starvation, and local overheating of tubes, which would even lead to tube burst in boiler .…”