Abstract
The corrosion behavior of electrodeposited Zn and Zn-Co coatings in 0.1 mol dm-3 NaCl aqueous solutions was investigated using scanning electron microscopy (SEM), energy dispersive X-ray (EDAX), galvanostatic electrochemical stripping (GES), electrochemical impedance spectroscopy (EIS) techniques and open circuit measurements (Eoc). All coatings were electrodeposited on mild steel. Using GES it was possible to identify Zn-Co intermetallic phases in the coatings with Co content higher than 1 at.%. The GES, SEM and EDX results revealed that the corrosion mechanism of the Zn-Co layers with high Co content is related to the appearance of cracks due to the dissolution of the gamma Zn-rich phases, while the corrosion of the Zn and Zn-1Co layers was characterized by dissolution of the coatings. The Zn and Zn-Co coatings were found to contain the same insoluble corrosion products, which act as a corrosion barrier that delays but does not prevent corrosion. The better corrosion resistance of the Zn-Co coatings obtained in the bath without additive, in comparison with Zn-1Co obtained in the bath containing additive is due to the high Co content which leads to an ennoblement of the coating. All Zn-Co electrodeposits showed greater corrosion resistance than the Zn electrodeposits. The Zn-18Co layer showed the best corrosion resistance, with a lifetime about three times longer than that of the Zn layer.
Highlights
There is growing interest in zinc coatings combined with elements of the iron-group metals (Fe, Ni and Co) on steel substrate thanks to results reported in the literature
The layer obtained at 30 mA cm-2 presented a higher Zn/Co ratio in the layer than the Zn2+/Co2+ in the plating solution, which corresponds to anomalous electrodeposition.[17]
These results are congruent with those reported in the literature by Kirilova et al.,[18] who reported on a study of the electrodeposition of Zn-Co from sulfate-chloride plating solutions
Summary
There is growing interest in zinc coatings combined with elements of the iron-group metals (Fe, Ni and Co) on steel substrate thanks to results reported in the literature. From a practical and technological standpoint, is that these coatings must still have a high Zn content to maintain the cathodic protection of the steel substrate while still remaining less active than just Zn coatings. A previous study showed that the corrosion mechanism of Zn-Fe coatings presented a good correlation between a long-term total immersion laboratory test in NaCl solution and a natural exposing test in marine atmosphere for 3 years.[1]
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