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4200 |
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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4201 |
A cracked zone near the weld. (foto KEMA) (ref 52, 57, 59) |
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4202 |
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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4203 |
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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4220 |
Carbonate concentration in equilibrium with ferrous hydroxide and ferrous carbonate. (ref 52, 57, 59) |
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4221 |
Titration curve of water with 15 mmol bicarbonate and 5 mmol carbonate. (ref 52, 57, 59) |
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4230 |
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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4231 |
Intergranular cracks in the tensile test bar after testing in the active-passive transition. (foto KEMA) (ref 52, 57, 59) |
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4250 |
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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