2005 Volume 45 Issue 3 Pages 373-379
The materials of the attachment near the last stage of low-pressure steam turbines in power plants will be exposed to a severe corrosion condition due to the concentration of corrosive chemicals produced by the alternating dry and wet environment, a phenomenon caused by the frequent shutdown and the load change of power plants. The corrosion behavior of typical low-pressure steam turbine materials is evaluated mainly by tests under dry-wet conditions. An increase in the concentration of sulfate ion and chloride ion in waters enhanced the general corrosion particularly for the 3.5NiCrMoV rotor steel. As for 12Cr blade steel, the coexistence of sulfate ion and chloride ion accelerated the general corrosion. An alternating dry-wet condition increased the maximum corrosion pit depth of 3.5NiCrMoV steel remarkably with the application of stress. The pitting corrosion potential of 3.5NiCrMoV steel showed almost the same values in all water qualities tested. However, the coexistence of Cl- and DO (dissolved oxygen) lowered the pitting corrosion potentials of 12Cr steel. From the consideration between the corrosion and the Cr content in corrosion films, it is clearly evident that the Cr content in the films controls the corrosion resistance.