The finned evaporator tube of medium pressure evaporator leakage occurred after the no. 1 unit of a combined-cycle gas turbine (CCGT) power station was put into commercial operation for only 40 days (including 24 days of operation and 19 days of shutdown and standby). Cut off a section of failure pipe for a thorough inspection. Macro inspection found that severe corrosion occurred in some local areas of some pipe sections, which led to the obvious thinning of pipe wall thickness, leading to perforation and leakage. Metallographic test and scanning electron microscopy (SEM) showed that the metal materials around the leakage points was qualified and the microstructure was normal. The corrosion products around the leakage points were mainly iron oxide which were generated before the unit had been put into commercial operation. The root cause was that the pH value of water used in the hydraulic test during the commissioning of the unit was as low as 9.2, and after the completion of the hydraulic test, the filled water remained in the boiler for a month and a half before being drained. Researches indicated that if carbon steels were prolonged immersed in demineralized water with a lower pH value, it would inevitably occur severe local corrosion which was due to pitting induced by the activation of inclusions in the steel. After pitting corrosion induced by active inclusions, no obvious pitting corrosion occurred in the surrounding inclusions. Accompanied with the extending of corrosion spots that had been formed, the activation zone continued to expand. Along with the formation of surface rust spots, corrosion developed along the inclusion boundary to the depth of the matrix, and finally caused the perforating and leaking of the tubes. The inner pitting corrosion of fin-tube of evaporator in HRSG could be avoided as long as the relevant provisions of “Guidelines of chemical supervision for combined cycle power plants” (DL/T 1717-2017) were strictly implemented during normal operation of boiler, standby maintenance and water pressure test after overhaul.