Morphological evolution of oxide scales grown on ferritic steels in steam

This study is concerned with the thermally-grown oxide scales formed inside tubes in a steam boiler during normal operation, and the influence of the morphological evolution of those scales on how strains are developed and accommodated. Understanding such details is an important consideration in modeling the tendency for scale spallation or exfoliation. Overall, the scales formed on ferritic steels in steam can be simplistically described as a layer of essentially pure magnetite in contact with the steam, and a second layer of magnetite containing alloying elements (especially Cr) next to the alloy. Hematite also can develop as a third, outermost oxide layer that may be non-uniform in thickness, and penetrate inwards along grain boundaries in the magnetite layer. A peculiar variant of this morphology often reported on low-alloy ferritic steels after boiler service (but not on laboratory- oxidized specimens) has a relatively thick outer magnetite layer and multiple 'inner layers' described as decreasing in thickness as the alloy surface is approached. These inner layers appear to be repeating sets of double layers of 'pure' magnetite and magnetite containing alloying elements. The results of detailed examination of such multi-layered morphology and the implications for the mode of scale growth and failure are discussed.