Studying Rare Earth Surface Treatment for Magnesium Using Scanning Electrochemical Microscopy

Rapid dissolution of magnesium alloys in biological solutions limits its successful application as a biodegradable implant where a controlled dissolution/corrosion of magnesium along with healing of tissue is required. Treatment of magnesium surface with rare earth conversion layers has been subject of several studies ^1–4 revealing its potential as a passivating layer. Given the biocompatibility properties of such surface treatments, two type of rare earth conversion layers based on Ce(NO₃)₃ and Pr(NO₃)₃ have been studied in this present work as surface treatment for AZ80X in simulated biological (buffered) solution. Conversion layers were formed by acid etching the Mg samples in 15mM HCl followed by immersion in 0.05 M Pr(NO₃)₃ or Ce(NO₃)₃solutions. Morphology and elemental composition of the conversion layers were characterized by SEM-EDXS.

Scanning electrochemical microscope (SECM) was used to investigate protective properties and degradation behaviour of Ce and Pr conversion layers on a local scale. Surface generation/tip collection (SG/TC) mode was used to study the hydrogen evolution rate and localization while AC mode was used to resolve the resistive/capacitive behaviour of conversion layer. Figure 1 represents the schematic SG/TC mode for probing the local hydrogen evolution. Self-healing properties of the conversion layer in presence of Ce³⁺ and Pr³⁺was also studied using SECM. Electrochemical impedance spectroscopy (EIS) was used to look at the changes in charge transfer and diffusion phenomena due to the formation of conversion layer and its degradation in the long term. Preliminary results have shown that Ce and Pr conversion layers improve corrosion resistance in the short term by producing an electrochemically stable surface. In the long term, however, conversion layer shows instability in simulated biological fluid and tends to degrade locally exposing the magnesium substrate to corrosive media.

1. A. Rudd, C. Breslin, and F. Mansfeld, Corrosion Science, 42, 275-288 (2000).

2. M. F. Montemor, a. M. Simões, and M. J. Carmezim, Applied Surface Science, 253, 6922-6931 (2007).

3. K. Brunelli, M. Dabalà, I. Calliari, and M. Magrini, Corrosion Science, 47, 989-1000 (2005).

4. M. Dabalà, K. Brunelli, E. Napolitani, and M. Magrini, Surface and Coatings Technology, 172, 227-232 (2003).

Figure 1 Schematic representation of SG/TC mode SECM for probing hydrogen evolution