Rippling in Oxide Layers

Increasing flow resistance rippling formation of the oxide layer is sometimes observed in high pressure supercritical boilers, resulting in a lower boiler efficiency. In 1968 Grosskraftwerk Mannheim (GKM) started a detailed research project on operational boiler tubes (Schuster, 1971; Schoch et al., 1972). A few years later, the University of Stuttgart joined GKM in a study in greater depth the results of which have been published too (Pfau et al., 1978).
In the German study flow resistance had increased to approx. 50% due to rippling in
the oxide layer on the tube wall across the direction of flow. Ripple height was 1-5 ltm
and ripple intervals were 30-100 pm. Ripples in the samples appeared as undulations
in the oxide layer. The steel oxide front was straight and there was no corrosion
below the thinner oxide areas.

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Example of the rippling pattern in the oxide layer.

KEMA has started a program to complement the German research and focuses on
the effects of steel quality and heat flux. One experiment was done under
supercritical and one under subcritical conditions. Because of experience gained
from the German studies, a variety of steel and tube types were used in the KEMA project.

See paper nr 28.

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Example of deposition rippling. Magnification 500 x.
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Example of deposition rippling from a power station boiler. Cross section. (foto KEMA ref 28)
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Eroded magnetite crystals from the top of the deposition ripples in Figures 1201 and 1202. (foto KEMA ref 28)
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SEM picture of erosion corrosion rippling patterns on steel 15Mo3, downstream a weld bead (magnification: x 100), Right: cross section of the sample. (foto KEMA ref 28 and 60).
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Erosion-corrosion rippling of heat-loaded (A and C) and non heat-loaded (B and D) specimens from 14Mn4 steel. Magnifications x 100. (foto KEMA ref 28)
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"Rippling pattern in 14Mn4 (1D6). A: sample from cased tube (magnification x 100). B: sample from inside cased area. Deposition rippling in A (cased area) and erosion-corrosion rippling in B (outside cases=d area). (foto KEMA ref 28)"