Heat Flux Corrosion

Heat flux corrosion is generally the result of a number of unfavourable factors, such as too high heat flux, too low mass transport or too high fraction of steam in the tubes, presence of deposits on the surfaces and inadequate water quality. The influence of certain impurities in the boiler water upon corrosion can be thoroughly studied with experiments in autoclaves.

1499 experimental boiler
1 Mw boiler for corrosion experiments at KEMA (the Netherlands).

In order to be able to include the different dynamic factors in the general investigation of steam generation corrosion -heat flux, two-phase flow of the water-steam mixture and formation of deposits -N.V. KEMA has an experimental boiler installation.(Fig. 1).

See for the results paper nr: 11 and 61.

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SEM picture of the pock shown in foto 1503. (foto KEMA, refd 61).
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SEM picture of the pock shown in foto 1504. (foto KEMA, refd 61).
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SEM picture of detail of the pock shown in foto 1508. (foto KEMA, refd 61).
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SEM picture of detail of the pock shown in foto 1507. (foto KEMA, refd 61).
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SEM picture of the dehydrated rust-layers in foto 1525. (foto KEMA, ref 61).
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SEM Picture of a pock in a boiler tube. (foto KEMA, ref 61)
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SEM Picture of a detail of foto 1527. (foto KEMA, ref 61)
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1499.jpg
Experimental 1 Mw boiler for corrosion experiments at KEMA (the Netherlands). With this boiler a very extensive research was performed on boiler or heat flux corrosion, erosion-corrosion and even experiments at super-critical conditons. (foto KEMA, ref 11, 13, 21, 23, 27, 28, 40, 47 and 61).
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Pockshape deposits (resulting from the nucleate boiling process) show often a donut shape. Even the flow direction of the water can be noticed. (in both pictures from left to right). (foto KEMA, ref 61).
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1505.jpg
The 3 types of deposits under different heat flux conditions: a) pimples, b) feather-shape deposits and c) homogeneous deposits. (foto KEMA, ref 20,61).
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Pockshaped deposits as a result of nucleate boiling in two phase flow condition in relative good boiler water. (foto KEMA, ref 11).
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Scanning electron microscope picture of the homogeneous deposit layer with a depositfree path behind an obstacle. See foto 1505 case c. (foto KEMA, ref 11).
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Pockshaped deposits on a Zircaloy fuel element in a Boiling Water Reactor. Nodular corrosion takes plac at the pellet interfaces. (foto KEMA, ref 61).
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1550.jpg
"Model for the formation of salt deposits in a porous layer of iron oxides. The salts are conducted to the bottom of the steam tunnel, where they precipitate when their solubility is exceeded. (foto KEMA, ref 11). "
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Foto's a and b: SEM pictures of the porous oxide layer. Steam chimneys are present. Foto's c and d: Optical microscopic pictures of the cross sections . (foto KEMA, ref 13, 61).
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Two tube halves: (a) heat irradiated part, and (b) nonirradiated part. The loose oxide layer on both tube halves has been removed by water rinsing. On tube part with heat flux (a) a network of salts is noticed under the loose oxide layer. (foto KEMA, ref 13, 61).
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Longitudinal section through tube 57/ 145/ 99. On the top of the 1 to 2 micron thick magnetite layer are a dark isotropic salt layer (40 microns thick), an anisotropic salt layer (80 microns thick) and a porous layer of hematite and magnetite. Right: Scanning electron microscopic picture showed that the elements Mg, S and Fe were found in the dark salt layer. In the outer layer is, in addition, Ca found. (foto KEMA, ref 11).
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1556.jpg
Scanning electron microscopic picture showed that the elements Mg, S and Fe were found in the dark salt layer. In the outer layer is, in addition, Ca found. (foto KEMA, ref 11)
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1557.jpg
Detailed pictures of the dark salt layer on top of the thin magnetite layer in tube 57/145/98-100. The pearlite structure of the steel can be recognised in the dark-grey salt layer, which points to attack of the magnetite by the salt layer.(foto KEMA, ref 11).