Applications of scanning electrochemical microscopy (SECM) for local characterization of AZ31 surface during corrosion in a buffered media
Introduction
Magnesium and its alloys have gained increasing attraction in recent years as biodegradable, biocompatible materials with appropriate mechanical properties [1], [2]. However, rapid degradation of magnesium in the form of localized corrosion in biological environments is a serious drawback that can significantly reduce the in vivo service life [3]. For example it has been shown that the stent loses its mechanical integrity and load bearing characteristics much quicker when the corrosion is highly localized in contrast to a uniformly corroding specimen [4]. Therefore, it is crucial to develop techniques to study corrosion profiles in biological systems. Also, despite the advancements in understanding the corrosion behavior of Mg and its alloys in simple chloride containing media, there is still essential need for better understanding of corrosion mechanism in physiological environments [5].
Local corrosion behavior of magnesium alloys has been studied using scanning techniques such as localized EIS [6], [7], [8], scanning vibrating electrode technique [9], [10] and scanning Kelvin probe [11], [12]. Among the methods used to enable local probing of microscopic processes, scanning electrochemical microscopy (SECM) has the unique capability to recognize active/passive regions and allows for surface characterization with a resolution in the micrometer range or below [13]. Electrochemical data may be collected in three main modes; namely amperometric, potentiometric and AC modes. The amperometric and potentiometric modes have been used to study the passive layers [14], [15] and organic films [16], [17], [18], [19] on metal and defects and pitting [20], [21], [22] as well as sensing local pH on a corroding metal [23]. The more recently introduced AC mode has proven to be useful for studying the insulating/conducting domains as well as changes in electrolyte composition near a corroding metal surface [24], [25], [26]. The AC-SECM signal is most responsive to the changes in solution resistance at high frequencies, while at lower frequencies the capacitive/resistive behavior of the surface and the electrical double layer are the major contributions to the impedance [24], [27].
A particular area of interest in recent years has been bio-corrosion and associated electrochemical changes on the surface of Mg alloys in buffered solutions. However, a survey of the literature returns a limited number of published studies on characterizing Mg alloy surfaces using local probing techniques. Despite the unique capabilities of SECM in this area, it has not thus far been used for studying the corrosion of Mg in such environments. In this paper SECM is utilized to characterize the local corrosion processes, in real time, on the surface of AZ31 Mg alloy in simulated biological fluid (SBF). The aim of this work is to explore the use of SECM in studying the corrosion of magnesium with an emphasis on its applications in biological systems.
Section snippets
Materials and sample preparation
Simulated biological fluid (SBF) was prepared using analytical grade reagents consisting of 5.403 g/l NaCl, 0.504 g/l NaHCO3, 0.426 g/l NaCO3, 0.225 g/l KCl, 0.230 g/l K2HPO4·3H2O, 0.311 g/l MgCl2.6H2O, 0.8 g/l NaOH, 0.293 g/l CaCl2, 0.072 g/l Na2SO4 and 17.892 g/l HEPES (4-(2-Hydroxyethyl)piperazine-1-ethanesulfonic acid – C8H18N2O4S) as buffer agent. The pH was adjusted to 7.40 ± 0.05 using 1 M NaOH (Sigma) solution. Ferrocenemethanol (FcMeOH) was analytical grade from Sigma. Mg alloy AZ31 (3% Al, 1% Zn
Results and discussion
In addition to its high spatial resolution, SECM is best known for its versatility and application in different modes such as feedback mode to differentiate conducting/insulating domains or in generation/collection mode to study interaction with electroactive species on the surface. In this paper the technique is used in feedback, surface generation/tip collection (SG/TC), AC and potentiometry modes to gain information about local corrosion of AZ31 in SBF.
Conclusion
Four modes of SECM operation (i.e. SG/TC, feedback, AC and potentiometry) were used for in situ characterization of Mg surface corrosion in a biological buffered solution. Combining the variables measured in different modes of SECM provides new insight into the complex corrosion mechanism of Mg in a buffered media. The H2 probing performed using SG/TC provides a direct measure of the spontaneous corrosion rate on a local scale. H2 evolution is sensed in an electro-oxidation reaction at the SECM
Acknowledgments
The authors thank the Australian Research Council (ARC) for continuing financial support. G.G.W. and M.F. thank the ARC for their Laureate Fellowships and SEM thanks the ARC for his QEII Fellowship. S.S.J. would like to thank Prof. R.M. Souto and Prof. W. Schuhmann for their valuable suggestions. The authors also thank Dr. Chee O Too for his valued contribution in editing this manuscript.
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