Role of the Corrosion Product Film in the Corrosion Protection of Cu-Ni Alloys in Saltwater

摘要

Measurements were conducted to determine the impedance of the electrochemical interface on rotating disk specimens of pure copper and commercial 90-10 and 70-30 copper-nickel alloys over a 28 day period while immersed in an aerated aqueous 3.4% sodium chloride solution. A sinusoidal voltage was applied to the specimen, and the in-phase and out-of-phase components of the current were measured as a function of frequency using a technique called Electrochemical Impedance Spectroscopy. Specimens were exposed at a rotation speed of 1000 RPM, although brief measurements were conducted from 500 to 2000 RPM at the end of the exposure period. Under circumstances where a cupric hydroxy-chloride corrosion product formed, copper experienced an increase in corrosion resistance with time which could not be correlated with a specific growth law. When the hydroxy-chloride was not present, the corrosion rate of copper was constant over time. Copper-nickel alloys also experienced increasing corrosion resistance with increased exposure time. On all of the alloys tested, the enhanced corrosion resistance was due to a process associated with the specimen surface, such as diffusion through the corrosion product layer or limited catalysis of the cathodic reaction, rather than by diffusion through a Nernst-like fluid boundary layer.