Pump with corrosion resistant shaft and rotor surfaces

An apparatus comprising:

• a multi-stage dry vacuum pump shaft and a vacuum pump member removably mounted on said shaft for rotation therewith, said shaft and pump member located at least partially within a sealed pumping chamber provided in operation with a source of fluoride, and said shaft and pump member having respective contact surfaces that face each other and are positioned relative to each other to allow the formation of two layers of corrosion products between the contact surfaces, wherein a first layer of corrosion products comprises a first type of fluoride and is formed from the corrosion of the shaft, and a second layer of corrosion products comprises a second type of fluoride and is formed from corrosion of the pump member, and wherein the first and second layers of corrosion products are substantially incompatible with each other so that joining of said shaft and pump member at said contact surfaces by corrosion is substantially prevented, and wherein said pump member is a claw or roots rotor and the contact surface thereof is defined by a bore in which the contact surface of said shaft is received when said rotor is mounted on said shaft.