Corrosion behavior in saline solution of Pulsed Electrodeposited Nanocomposite Zn-Ni/CeO₂ coatings

Abstract

Zn-Ni coatings are widely used as sacrificial coatings in automotive applications. It becomes important to still improve their durability to limite the release of heavy metals towards the environment and increase their functional properties.

Incorporation of ceria particles permits to enhance physico-chemical properties of the electrodeposited coatings, but the particles distribution through the coating thickness cannot be ensured by conventional dc electrodeposition process. The reverse pulse plating electrodeposition process was then used, leading to better dispersion of nanoparticles. Even if the impact of the particles on the corrosion resistance is not so obvious, the tribological resistance is strongly improved probably due to modification in the plastic deformation of the Zn-Ni matrix.

Introduction

Electrodeposited (ED) Zn-Ni alloys¹ are widely used in automotive industry and actually in aeronautic application for the replacement of cadmium coatings. Electrodeposition process using acidic chloride baths are described as advantageous and it is characterized by a very good corrosion behavior^2-6, probably due to the presence of a first quite pure nickel interlayer between the substrate and the Zn-Ni layer^7.

Watkins shows that zinc nickel coatings have a better durability in corrosive media, as evidenced by lower mass losses⁸. Recent evolutions of the European environmental regulations are now focused on the nocivity and toxicity of cobalt and nickel salts. Nickel compounds are still under examination, and two factors have to be taken into considerations. The first one concerns the deposition process, involving polluting bath. This issue is now quite resolved because most systems are equipped with specific treatments of effluents. The second factor is related to the coating itself, that could release nickel compounds due to corrosion. So the durability of the coating has to be drastically improved to decrease the metallic ions release towards the environment^9,10. One of the solution is to develop metal matrix composite coatings¹¹ and/or the innovative pulse current deposition^12,13. Metal composite coatings consist of a dispersion of particles from different size and nature into a metallic matrix" (TiO₂¹⁴,SiC¹⁵,Al₂O₃^13,16,17, Fe₂O₃¹⁸ or carbon nanotubes CNTs¹⁹). Since the incorporation might be intergranular²⁰ [20], a solution to incorporate a large amount of nanoparticles is to get a metallic matrix characterized by a fine-grained microstructure. Pulsed current plating is suitable to achieve this objective^21,22,23. To improve the dispersion of particles, use of ultrasounds from a sonoprobe could be an efficient way17. These two options are often related to improve both the amount of particles and their dispersion, which would result in an important enhancement of the coatings properties (hardness, tribological properties, corrosion behavior...). Objective of this work is to present a critical analysis of the effect of the deposition mode (dc vs pulsed reverse) on the functional properties of Zn-Ni coatings. We have chosen as the coating reference a ED zinc-nickel matrix reinforced by ceria nanoparticles (NPs). Indeed, very little works are devoted to the zinc-nickel-ceria composite system contrary to tthe zinc-ceria system20,24,25.