Erosion corrosion wet steam

Carbon steel is generally non resistent to erosion corrosion: pH value, steam-water ratio, local flow rate and chemical composition of steel are important.
Suspended iron oxides in steam-water circuits may cause trouble in both conventional and nuclear power stations. This happens rather often; e.g., corrosion in conventional boiler evaporators, corrosion in PWR steam generators and deposition of iron oxides on fuel elements in nuclear reactors.

BWR bocht
A pipe behind the BWR water separator was constructed by welding bent steel plates. One of those plates had been attacked severely.

In view of these problems, it is recommended that the iron content in steam water circuits be kept as low as possible. This iron comes from erosion-corrosion, the major sources of which are water separators, wet steam pipes, preheaters, and evaporators.
Erosion-corrosion depends on water chemistry, water velocity, and chemical composition of the steel.

The research at KEMA focused on the relationship between the chemical composition of steel and erosioncorrosion resistance. This relation-ship has been neglected in the past because water chemistry and velocity were thought to be the dominant factors.

See for the results of the studie the following papers:
65, 60, 47, 46, 45, 27, 21 and 17.

 

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"Two eroded corroded switch levers of feed water pumps. (photo KEMA / ref 65). On the left: Switch lever after 10.000 hr operation (2.83) Cr=0.01, Cu=0.01, Mo=0.005 and C=0.09 On the right: Switch lever after 40.000 hr operation (2.108).Cr=0.025, Cu=0.03, Mo=0.02 and C=0.06. (photo KEMA ref 65)."
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A pipe behind the BWR water separator was constructed by welding bent steel plates. One of those plates had been attacked severely. The other plates, both upstream and downstream, had not been attacked. The corroded plate showed a different chemical composition. The GKEMA of the corroded steel was 91.5, whereas the values of the uncorroded plates were much lower 72.1 (foto KEMA ref 47 and 65)
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Inside (upper) and outside (down) pictures of the separator. This phenomenon can be explained in terms of electrochemical polarisation of the corrosion reaction by diffusion of hydrogen through the steel wall. There where the millscale on the outside of the wall was not present anymore erosion-corrosion was started on the inside. (photo KEMA ref 47 and 65).
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The high-pressure water separator of the BWR also had severe erosion-corrosion. see Figure 8. Low Cr, Cu, and Mo contents were measured in the steel, resulting in a GKEMA of 87.6. Besides, there is a remarkable phenomenon in this failure. The so called "tiger skin" pattern of erosion-corrosion on the inside of the pipe was also present on the outside. See picture 1011. (foto KEMA ref 47 and 65).
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* One of the most obvious cases is the erosion-corrosion of a plate in a water separator of a PWR. See Figure 9. Two plates were welded together. Only one plate was corroded, whereas on the other non-corroded plate the corrosion products had deposited. The horse- shoe pattern of erosion-corrosion is very obvious. The corroded plate and the non- corroded plate had GKEMA of respectively 91.4 and 58.4. (foto KEMA ref 47 and 65)
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* The preheater of a PWR had to be renewed because of severe erosion-corrosion. The preheater was dismantled row by row and 24 tubes were analysed The 24 tubes had been made from 2 different heats (GKEMA of 85.5 and 79.0). It could be seen that in one row some tubes were corroded more severely than other tubes adjacent to them. The most severely attacked tubes were found to have the higher GKEMA .(foto KEMA ref 47 and 65)
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* There was severe erosion-corrosion in the economiser tube bends of a waste-heat boiler. Steaming occurred in the economiser. This could also be concluded from some failed tubes where erosion-corrosion occurred on the outer radius and steam blanketing on the inner radius. The water film had come loose from the inner radius surface where coarse magnetite crystals had formed. Analysis of the steel shows the erosion-corrosion resistance of this steel to be low, because of the high GKEMA (90.2). (foto KEMA ref 47 and 65)
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* In the KEMA experimental boiler, erosion-corrosion occurred in a steam pressure drop vessel in which steam expanded and where the water steam jet hitted a buffle plate in the vessel. See Figure 6. After 10 years of operation, there was a leakage in the buffle plate and the vessel wall. The steel plate (10 mm thick) and the vessel wall (5 mm thick) were corroded and a leakage occurred. The GKEMA of the steel plate was 74.1. For such situations (a buffle plate for protecting the vessel to the wet steam flow) steel with a lower GKEMA is required.(foto KEMA ref 47 and 65)
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SEM pictures at different places at the eroded corroded surface of a mild steel specimen after 100 h of exposure in wet steam. (foto's KEMA / ref 22).
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Test equipment on behalf of erosion corrosion. (foto KEMA ref 60).
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One of the test rooms with a tested sample.(foto KEMA ref 60).
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A number of steam sieves suffered erosion- corrosion. Chemical analyses showed that three different steel heats were applied. The calculated GKEMA were 81.1 , 80.8 and 88.3. The last heat was corroded the most. (foto KEMA ref 47)
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A spindle from a condensate effluent valve of a low pressure preheater was coated only partly with a chromium steel (9% Cr). see Figure 4. It might be considered that on the top of the spindle the coating was not necessary because in general there are no corrosion failures. But because of the too low Cr-equivalent the carbon steel was corroded. (Sample nr. 2.114). Cr=0.02, Cu=0.05, Mo=0.02 en C=0.01. The GKEMA was 83.1. (foto KEMA ref 47 and 65).
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Influence of Cr, Cu and Mo on the weight loss. (foto's KEMA / ref 22).
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Frequency distribution of the weight losses, as measured in the erosion-corrosion tests. (foto's KEMA / ref 22).
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Example of droplet erosion in a turbine (foto KEMA ref 60)
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The equilibrium of magnetite with Fe2+, FeOH+, Fe(OH)2 and HFeO2-. The solubilities depend from temperature, pH value and the H2 content. (foto KEMA ref 60)
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The solubilities of magnetite (straight lines) at a pH2=1mbar, derived from the at pH2=1bar measured values from Sweeton en Baes, and the derived solubilities (van den Hoven) in 3 different types of water at pH2=1mbar (lines A, B en C). (foto KEMA ref 60)
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Geometry factors for erosion corrosion according to Keller (1974). (ref 60)
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Relative material loss for Cr alloyed steels according to research of Ducreux at EDF(1982). (ref 60)
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Influence of temperature on material loss because of erosion corrosion ,research of Heitmann (1982). (ref 60)
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There was severe erosion-corrosion in a control valve housing, resulting in a leakage. This steel had a high weight loss in the erosion-corrosion test, 107 mg. The GKEMA was 91.8. (foto KEMA ref 47)
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Two tubes of a backpressure unit were received; one showed severe erosion-corrosion, and the other had only some corrosion spots. The erosion-corrosion GKEMA for the uncorroded tube (under) was much lower than that of the corroded tube (60.9 and 85.7). (foto KEMA ref 47)
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The preheater of a power station suffered local corrosion near the support plates where the flow velocity is high. The GKEMA was 90.6. Coarse magnetite crystals were found adjacent to the corroded area. Precipitation of such coarse iron oxide crystals is often observed in case of erosion-corrosion. A second tube with a higher GKEMA (71.4) did not show any corrosion. (foto KEMA ref 47)
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Slight erosion-corrosion was observed on thin tubes in an economiser of a waste heat boiler. The GKEMA was very high, 90.6. (foto KEMA ref 47)