Corrosion of copper pipes by boiler steam cooling water

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This post was last edited by gaodi913 on 2015-10-23 at 11:45. The steam with a pressure of 3.82 MPa, generated by one of our 35-ton boilers, is used to power a generator set. However, as steam and steam condensate pass through the brass tube bundles of the unit’s two-stage steam ejector, it always causes leakage in these brass heat exchange tube bundles. Working principle of the two-stage steam ejector: The condensed water generated as a result of the work done by the steam turbine inside the copper tube bundle of the two-stage steam ejector (with a water temperature of 30–60°C) comes into contact with the new steam ejected from the nozzles of the ejector (with a steam temperature of 400°C and a pressure of 1.2–1.4 MPa), causing condensation. This leads to a rapid contraction of the vapor phase, thereby creating a vacuum. The location of the leakage is: at the junction of the flanged tube sheet and the tube bundle ； Middle and top of the tube bundle. After tube bundle corrosion, the outer wall turns black, accompanied by white pitting corrosion and resulting corrosion spalling; the corrosion spalling does not appear as black powder. I would like to ask the experts here, what causes this?