Different impellers made of nickel base Ahoy UNS NO6625 have failed afler 5000 h and 15000 h in service at high temperatures (= 500°C, 932°F) in 1994. The impellers were used in petrochemical industries. The faiiures were induced by stress corrosion cracking and stress corrosion fatigue. Material of the impellers showed a high susceptibility to intergranular corrosion and stress corrosion attack. Formation of a newly, not yet identified, super lattice within the nickel matrix has been discovered by transmission electron microscopy. The formation of the super lattice was caused by the service conditions, temperature and time. Based on these findings, influence of ageing time at 500C (932°F) on the mechanical properties and the corrosion resitance of UNS NO6625 was investigated. The specimen investigated showed strong differences in mechanical properties and corrosion behaviour in standard corrosion test solutions, as well as a strong ageing
dependence.