Abstract
Data on the corrosion behaviour of pure Zn, Zn-0.4%Fe, Zn-0.6%Co and Zn-12%Ni coatings deposited from an alkaline cyanide-free solution, studied by salt spray test and anodic polarization curves in a nearly neutral NaCl solution are presented. The Zn-12%Ni and Zn-0.6%Co alloys show higher corrosion resistance than the two other systems. An attempt has been made to correlate the observed corrosion resistance of these two coatings with the electrochemical behaviour of anodically formed passive layers on them in alkaline solutions, investigated by Electrochemical Impedance Spectroscopy. Analysis of the electrochemical impedance data in the frequency range 1 Hz–10 kHz obtained for anodic passive films on the four systems in 0.1 M NaOH at different potentials in the passive region suggest that the passive films on Zn-Ni and Zn-Co present deviations from the ideal Mott-Shottky behaviour which were attributed to the introduction of deep donor levels by the doping elements Co and Ni. These doping elements seem to play an important role in the anodic control of the corrosion process which involves ionic transport processes through the oxide films.
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