AC IMPEDANCE STUDY OF CU AND CU-NI ALLOYS IN AERATED SALT-WATER .1. PD COATING AND CORROSION PRODUCT STRIPPING

Electrochemical impedance spectroscopy measurements were conducted 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% NaCl solution, after vapor deposition of a Pd layer and finally after stripping the detachable corrosion products. The results indicate that for pure copper only the inner corrosion product layer impedes the corrosion reaction whereas for the more corrosion resistant Cu-Ni alloys, the porous outer corrosion product layer accounts for the majority of the corrosion resistance. Contributing factors in the case of the Cu-Ni alloys are the availability of electrons and catalytic sites within the pore structure especially in the vicinity of the inner/outer layer interface and the diffusion of dissolved oxygen within the pore electrolyte to these sites, the latter being the rate-limiting step.