Cause Analysis on Surface Damage of Coupling Studs of the Steam turbine in a 1 000 MW Nuclear Power Unit

During the period of maintenance of a 1 000 MW nuclear power unit, damage with different degree appeared on the surface of coupling studs of the steam turbine. The causes of the surface damage of the studs were analyzed by methods of macro and micro morphology observation, mechanical property testing, metallographic examination, scanning electron microscope observation, chemical composition and energy spectrum analysis. The results show that:the type of the surface damage of the studs was the pitting corrosion damage; the cause of pitting corrosion damage for studs was that corrosive elements existed in the service environment of the studs, especially the reactive anionic Cl^-.