A combined composition and morphology study of electrodeposited Zn-Co and Zn-Co-Fe alloy coatings

The composition and morphology of electrodeposited Zn-Co and Zn-Co-Fe alloy coatings are studied by a variety of complementary analytical techniques. Morphology of the alloy deposits is shown to change significantly with Co content in the alloy coating. An increase in the Co content in the range of 0.7-9 wt.% Co in Zn-Co and Zn-Co-Fe alloys results in a change in grain shape from angular to nodular and a further increase up to 10 wt.% Co corresponds to a characteristic growth mode. In the range of 10-29 wt.% of Co, the deposit contains two types of grains, i.e. one with low Co content (5-7 wt.%) and another with higher Co content (i.e. 15-35 wt.%). Zn-Co and Zn-Co-Fe alloys with Co contents of or higher than 32 wt.% Co show a homogeneous structure, which can be considered to be nanocrystalline in nature. The presence of two or more phases is not desired in terms of enhanced local corrosion by (micro-)galvanic coupling of phases while the single phase or nanocrystalline coatings provide good corrosion protection properties.