Abstract
Corrosion characteristics of the materials of rotors and blades for low-pressure turbines in a steam power-generating plant were evaluated by using condensed-water evaluation equipment, which is capable of reproducing the environment of the vapor condensation process for testing, constant-strain testing equipment in the dry-and-wet-alternating condition and low-cycle-fatigue testing equipment. The condensed-water evaluation revealed that impurity ions in steam such as sodium, sulfate and chloride were separated in the initial condensation process, i. e. at the initial stage of condensation, from vapor into the initial condensate. An empirical equation obtained from the measured concentrations of impurities in water vapor and condensates suggested that the concentration of impurities in early condensate can be as high as 50ppb in the environment of an industrial plant. Low-cycle-fatigue life decreased with increasing the concentration of impurities. Meanwhile, the kinds of impurity ions and their mixing exhibited no superposition effect on their concentration. Impurity ions were found to shorten the fatigue life of the material far more than dissolved oxygen. Constant-strain testing of roter material using double U-bend specimen showed that dry-and-wet-alternate and wet-and-air alternate conditions increased crack susceptibility. This could be related to their infiltration and accumulation of impurity ions in interstices and the influence of chloride ion was the strongest.
