A Drainpipe Leakage Caused by Under-Deposit Corrosion in a Gas–Steam Turbine Combined Cycle Power Plant

A drainpipe of a gas–steam turbine combined cycle power plant leaked after served for only 64 months. The chemical compositions, metallographic structure and mechanical properties were qualified, and therefore, the leakage was not caused by material quality. The comprehensive analysis of leakage morphology and deposition revealed that the leakage was caused by under-deposit corrosion. The decarburization of metallographic structure was not detected, which indicated that H generated by cathodic reaction has not occurred, so that the under-deposit corrosion caused by H+ was ruled out. The deposition was the iron oxides. The substances (mainly Na3PO4) used for chemical cleaning absorbed in deposition resulted in occluded alkaline electrolyte and damaged the protective film of pipe wall, and electrochemical corrosion process caused the drainpipe failure subsequently. The process and mechanism of pipe wall thinning and leakage in the working medium without heating concentration and corrosion substances were revealed. The preventive suggestions were also proposed for such a failure.