Pitting andStress Corrosion Cracking of Conventional and High Purity LP Turbine Rotor Steel

摘要

Service experience and laboratoryinvestigations showed that corrosion pits are oftenthe initiation sites of stress corrosion cracks. Sincenon-metallic inclusions are the most likely sites ofpit formation the behaviour of a high purity steel interms of pitting and stress corrosion crack initiationwas compared with conventional LP steels.With electrochemical methods and with exposure testsat open circuit conditions the pit initiation andgrowth behaviour was investigated. In order to studythe crack initiation loaded specimens were exposed tovarious environments. The spectrum of corrosion mediaranged from very aggressive environments such ashighly concentrated caustic solutions to mild media such as deaerated steam.The high purity steel exhibited a significantly higherpitting resistance than the conventional steels. Therole of pitting as nucleation sites of stresscorrosion cracks in different media has beendemonstrated. Consequently the high purity steel hada certain advantage if this type of crack initiationmechanism was predominant. In corrosion systems wherea films rupture mechanism causes crack initiation thebehaviour of HP and conventional steel is similar.It was also shown that transient conditions can lead to early crack initiations.