Electrodeposition and corrosion properties of zn-co and zn-co-fe alloy coatings

Cadmium (Cd) has been extensively used as an excellent corrosion protective coating for steel components in aerospace, automotive, electrical and fasteners industries. However, Cd is banned due to its toxic nature and strict environmental regulations. In this study, the electrodeposition mechanism and kinetics, coating morphology and corrosion resistance of alternative, electrodeposited Zinc-Cobalt (Zn-Co) and Zinc-Cobalt-Iron (Zn- Co-Fe) alloys have been investigated. Coatings with relatively high amounts of Co are very difficult to achieve due to anomalism associated with their deposition and are therefore not much reported so far. In this research Zn-Co and Zn-Co-Fe alloys with varying amount of Co (2 to 40 wt-%) and Fe (up to 1 wt-%) are electrodeposited and the effects of variation of process parameter settings (i.e. cathodic polarization, current density, temperature and electrolyte composition) on the electrodeposition mechanism and kinetics are investigated. The microstructure of the alloy coatings changed significantly with the variation in Co content in the deposits. The barrier and sacrificial corrosion protection provided by the coatings were investigated with a variety of electrochemical techniques and industrial accelerated tests. It was found that the sacrificial properties and the protection range decreases with increasing Co content in the alloy. For the highest Co content in the alloy, the coating may become more noble to steel and loses its sacrificial protection. The barrier resistance of the coatings increases with the increase of Co content in the alloy coating. Both Zn-Co and Zn-Co-Fe alloys with high Co content (> 32 wt-% Co) showed excellent barrier properties. An intermediate region of compositions can be distinguished in which the coatings would provide a good combination of sacrificial and barrier resistance properties and also a reasonable protection range. © 2010 by Nova Science Publishers, Inc. All rights reserved.