STUDY OF PASSIVE FILM FORMATION ON CU-NI 70/30 ALLOY IN SEAWATER AT HIGH-TEMPERATURE

The corrosion mechanism of Cu-Ni 70/30 alloy in quiescent seawater at 80-degrees-C with a dissolved oxygen content of 3.0 ppm was studied with electrochemical and free corrosion tests. The passive films formed on free corroded specimens after an exposure time of 10 min-360 h were analysed with XPS. Free corrosion tests show that Cu-Ni alloy undergoes copper-selective dissolution; the kinetics of the dissolution process are governed by a transport-limited mechanism. Electrochemical tests show that the overall corrosion process is under cathodic control and that the reduction of dissolved oxygen is the most important parameter of the corrosion process. X-ray photoelectron spectroscopy shows that the composition of the passive films changes by changing the exposure time. The main components of the passive films formed on metallic surfaces are Ni compounds after the shorter exposure time and Cu compounds after a longer exposure time.