Pd-In and Pd-Fe as New Types of Ni-Free Top Coatings for Decorative Applications

摘要

Nickel is considered to be the primary cause of allergic contact dermatites. As a consequence, there is now a tendency to remove nickel of all manufacts designed to come into direct and prolonged contact with the human skin. However, the peculiar physical and chemical properties of nickel; i.e. very good corrosion resistance, white color and lustrous shining, make its substitution a very challenging task, especially for all the items requiring high aesthetical quality and durability. At present, the most diffused top finishing treatment for jewelry and fashion accessories is the electrodeposition (plating) of a thin layer of nickel alloyed with noble metals such as gold, rhodium and, mainly, palladium over a nickel intermediate layer. The presence of precious metals on the top layer creates the required aesthetical characteristics and, more importantly the Ni-content available for leaching results reduced. However, these solutions do not solve the problem since nickel is still present. In the long run, even these applications will have to be substituted with properly Ni-free technologies. The aim of this work is to characterize the corrosion resistance of Ni-free coatings constituted of a bronze intermediate layer and Pd/In and Pd/Fe top-coatings. The corrosion resistance of these systems was evaluated by means of electrochemical techniques, i.e. electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization curves (PD). The results evidenced corrosion resistance of these coatings similar to those of the traditional Ni-bearing ones. Finally, comparative free corrosion tests were carried out in neutral saline environment and artificial sweat solution in order to evidence the capability of the new Pd/Fe and Pd/In coating to constitute an aesthetically valuable "Ni-free" technology suitable for jewelry and fashion applications. The results showed that these new coatings have promising characteristics.