PURPOSE: To enable it to prevent any erosion without deforming a rotor by forming a corrosion-resistant surface treated layer on the outer circumferential part of a flange-form projection formed in a gland part of the rotor being opposed to both front and rear glands.
CONSTITUTION: A flange-form projection 10 is formed in a gland part 1a of a rotor 1 being opposed to a front gland 4 and a rear gland. Then a corrosion- resistant surface treated layer 11 is formed in the outer circumferential part of this projection 10. Since surface treatment is applied to the outer circumferential part of the projection 10, a thermal effect due to this surface treatment is not directly exerted on a shaft part of the rotor 1, so that such a possibility that the rotor 1 might be deformed is brought to notching. In addition, the rotor 1 is protected by the surface treated layer 11 and therefore erosion is preventable.