| Recently Zn-Co alloy coating has been studied because minute additions of cobalt to zinc decrease its rate of corrosion. The corrosion behavior of electrodeposited Zn-Co alloy coatings with various contents of Co was compared with pure Zn coating using electrochemical test and electrochemical impedance spectroscopy (EIS) measurement in 3.5 wt% NaCl solution and surface analysis. In immersion test, the corrosion potential of all Zn-Co coatings was higher than that of pure Zn coating. Corrosion current density for the 0.9 wt%Co and 5.0 wt%Co was lower than that for pure Zn coating. Dezincification which occurred in the corrosion process made a Co-rich barrier in the coating layer and only 0.9 wt%Co and 5.0 wt%Co had an effective Co-rich barrier. The 58.0 wt%Co exhibited the highest current density owing to the localized corrosion at large cracks. The 0.9 wt%Co and 5.0 wt%Co, with a more active corrosion potential, were galvanically corroded at cracks in the coating to protect the exposed substrate cathodically. Whereas on the 58.0 wt%Co, anodic dissolution and galvanic corrosion of the substrate were concentrated at coating defects by a large cathode(58.0 wt%Co coating) and anode(substrate) surface area ratio. EIS and EPMA results confirmed that cobalt enrichment, together with the zinc corrosion products, acts as a barrier layer reducing the total corrosion rate. |
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