Quantifying the Role of Transition Metal (Re)Plating in the Cathodic Activation of Corroding Magnesium

Abstract

Magnesium (Mg) and magnesium-based alloys as biodegradable implants form a distinct pole of attraction for the scientific world due to their advantages over conventional biodegradable metallic materials. Regardless, though, of the promising properties that magnesium offers, its extensive use in vivo is restricted with the main limiting factor to be its high rate of degradation. This paper examines the effect that the transition metals, which have been intentionally or unintentionally added as traces within the metal bulk composition, have on the corrosion properties of commercially high purity magnesium. Furthermore, a series of experiments provide an indication regarding the ability of the selected transition metals to accelerate corrosion through the process of (re)plating aiming to gain an insight on the Mg corrosion activation described in recent publications. Consequently, the current research presents the kinetics involved behind the role of the transition metals in the cathodic activation of corroding magnesium. During the experimental procedure, high purity magnesium samples containing <80ppm Fe were used, while the kinetics of the effect of Fe2+, Cu2+, Zn2+ and Mn2+ transition metal ions were studied as these are the most common traces of addition alloying or impurities found within the bulk composition of magnesium and magnesium alloys. The transition metals were either added into a corrosive electrolyte of 5% aqueous NaCl at near neutral pH at concentrations ranging from 10-6M to 8·10-3M or they were injected in solid form directly on the immediate vicinity of the corroding surface at a weight range of 5mg to 50mg. Following this step was the volumetrically recording of the evolution of hydrogen gas over a constant period of time in order to follow the kinetics of magnesium corrosion. The results indicate that the presence of transition metal ions within the solution leads to transition metal (re)plating and to an increase of the magnesium corrosion rate. Furthermore, by systematically varying the transition metal ion concentration it was possible to determine the relative efficiencies of the selected metal cations. During the experiment, it was also observed that the metal (re)plating process and the efficiency of the cathodic activation were limited by the formation of insoluble transition metal (hydr)oxide precipitates and the time-dependent hydrolysis. It was possible, though, to minimize the negative effect of hydrolysis by introducing the metal cations directly on the magnesium surface as it may be assumed then that both the diffusion path length and the time period during which hydrolysis can initiate are shorter.