All nozzles and blading shall be of hard; corrosion and erosion resisting alloy steel suitable for the specified steam. In the low pressure stages where the moisture percentage and the blade peripheral speeds are high, additional protection against erosion by way of hardened surface of blades or strips of erosion and corrosion resistant material such as satellite, secured to the leading edge, shall be provided. The method of fixture of the blades to the rotor shall have been satisfactorily proved by long use of operation. Care shall be taken to ensure that the natural frequency of the rotor blades is compatible with the rotational frequency of the machine.