District Heating Systems-Carbonate

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"Failures due to stress corrosion cracking has occurred in the return pipes of a district heating system. The steel pipes are laid inside a casing filled with a mixture of bitumen and cork (Lebit system).Iron carbonate is present in the corrosion products of the pits. As a result of the chemical equilibrium in the pits between iron hydroxide, iron carbonate and the high pH of the water, a high carbonate concentration arose in the pits. Cracking started from the carbonate filled pits. (foto KEMA) (ref 52, 57, 59)"
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A cracked zone near the weld. (foto KEMA) (ref 52, 57, 59)
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The macro-etching technique shows the different areas in the heat-affected zone and the cracks. (foto KEMA) (ref 52, 57, 59)
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A selection of the SEM images of the corrosion deposit and a part of the crack with the analysis points indicated. . Scanning electron microscope image of a pit and a crack in a DHS tube. The dark gray spots C, D and K contain carbonate. (foto KEMA) (ref 52, 57, 59)
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Carbonate concentration in equilibrium with ferrous hydroxide and ferrous carbonate. (ref 52, 57, 59)
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Titration curve of water with 15 mmol bicarbonate and 5 mmol carbonate. (ref 52, 57, 59)
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The critical range for stress corrosion in a 1.5 Mol carbonate/bicarbonate medium indicated by Wendler-Kalsch. (ref 52, 57, 59)
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Intergranular cracks in the tensile test bar after testing in the active-passive transition. (foto KEMA) (ref 52, 57, 59)
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Decrease of in service failures after decreasing the operating pH value in the water of the Breda District Heating System. (ref 52, 57, 59)
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On nucleate boiling locations in a evaporator tube pitting and intergranular corrosion took place. Carbonate has been found on the dark places in the cross section. From these pits carbonate Stress Corrosion Cracking was found too. (foto KEMA).