The history of Mg as biomaterial was summarised by Witte . During the recent years different reviews have been published focusing on several aspects: the challenge of application of Mg-alloys as biomaterials [2–4], manufacturing procedure and their influence on the corrosion stability , application of different techniques to produce protection layers and/or coatings onto the surface of Mg-bulk devices , summarising the influence of various alloying elements for Mg in order to change the mechanical as well as the electrochemical properties .
All different possibilities of modifying the properties of Mg and Mg-alloys, which are summarised in Figure 1, have to be tested not only considering the mechanical, but also the corrosion, respectively, their degradation properties. The latter ought to be adapted to the targeted implantation site.
In most of the publications the immersion test is performed. Walker et al. and Kirkland [8–10] were able to show that this is an easy and reliable way to obtain compatible data to in vivo results. Effectively when the time is measured up to the time point, where the specimens are completely dissolved. This type of measurement may be very time consuming. For this reason hydrogen evolution measurements are performed parallely in order to reduce the time, with respect to reaction (1).
The redox-reaction (1) consists of two single reaction steps: anodic – the oxidation reaction (2a) and cathodic – the reduction reaction (2b).
Despite the fact that weight loss measurements are easy to perform, no information about the kinetics of the reaction may be gathered. Another mismatch is the large amount of material and specimens for statistical evaluation of the measurements. The measured hydrogen volume does not correlate exactly to the mass loss of Mg, caused by absorption of hydrogen in the bulk .
With respect to reaction (2a) and (2b) where free electrons are presented, the electrochemical measurements are an alternative and/or an additional option for corrosion rate assessment.
In spite of the very negative standard-potential of Mg, E∅=−2.36 V vs. normal hydrogen electrode (NHE) electrochemical measurements in aqueous electrolytes are possible .
The aim of this paper is to review and to comment electrochemical techniques applied to assess the corrosion behaviour of Mg and Mg-alloys. These assessments are based on reviews by [3, 11, 13–19].
Therefore, this review starts with a description of electrochemical techniques, Figure 2, and comments in relation to their application on Mg and Mg-alloys. In a separate part, a summary about the technical possibilities, especially in relation to the variation of resolution, will be given.
Electrochemical measurement techniques
When electrochemical techniques are applied to assess data on corrosion or degradation rate, the initial step is EOCP vs. time (t) measurements.
Thereafter current (I) vs. potential (E) – curves are performed, checking the reaction under cathodic and anodic polarization. Apart of the set of parameters in Figure 1, the collection of parameters in Figure 3 also has influence on the values of exchange current density (i0) or the polarisation resistance (Rp) as a basis for calculation of the corrosion rate (CR).
It is more and more common to perform electrochemical impedance spectroscopy (EIS), which allows measuring Rp directly, and to try to get more reliable data on corrosion rate and kinetics of degradation reaction of Mg and Mg-alloys [18, 20–28].
Open circuit potential (OCP or EOCP)
The OCP measured over time can give a first idea about the stability of the metal-electrolyte interface and the delay of appearance of any visible reactions such as gas evolutions. The knowledge of this time frame is essential for planning time consuming measurements as EIS. An example is shown in Figure 4. It is obvious that EOCP is a function of time, depending on the concentration of the free metal ions, or the relation of galvanic coupling areas .
As for pure corrosion processes, where metal ions are liberated to create a “classical” electrochemical double layer, EOCP is depending on the [Mez+] concentration in the vicinity of the surface. It can be calculated using the Nernst’ equation (3):
When the concentration of [Mez+] is 1 mol in the vicinity of the surface, consequently the experimental values obtained for EOCP are E⊗, as shown in Figure 4 for Zn (zinc) and Fe (iron) in Ringer solution.
For Mg and Mg-alloys the situation is not as simple, because the published EOCP values vary between 1.3 and 1.9 V vs. SCE .
Therefore the position and variation of the measured potential can be explained by the mixed potential theory [11, 14, 29, 30].
Voltammetry or potentiodynamic polarisation (PDP)
(I) vs. (E) curves are obtained to get information about the electrochemical behaviour of metals in contact with electrolytes . As for Mg a special feature has to be considered: the negative difference effect (NDE), describing the fact, that at anodic polarization a higher hydrogen evolution takes place .
This fact makes an evaluation of (I) vs. (E) curves more complicated to rate the exchange current density i0 or corrosion current density icorr. The corrosion rate (CR) is calculated using the Faraday’s law, equation (4.1) based on experimentally obtained i0 values. Both the mass loss and the corrosion depth can be generated using equation (4.2) or (4.3). The requirement of the Tafel approach is that the influence of the counter reaction, anodic or cathodic, is negligible. This is fulfilled at potentials of ±120 mV away from Ecorr.
where i0=exchange current density, t=time (year), z=exchanged electrons, F=Faraday constant, M=atomic mass, ρ=density, l=CR in (mm/year), m=CR in (g/year).
The irregularity of the shape of (I) vs. (E) curves, found in various publications [20, 22, 31–36], depend on the variation of hydrogen evolution between immediately strong and delayed. The creation of MgH2 [30, 32, 37], appropriate to reaction (5), is an additional complication for the experimental estimation of icorr and consequently for CR.
The scheme in Figure 5 shows one (I) vs. (E) curve of Mg in Ringer solution performed at a scan rate of 10 mV/s using the mini cell system (MCS) set up.
Two Ecorr values are in the focus, Efwcorr of the forward scan and Ebwcorr of the backward scan, after crossing the threshold potential of −1.2 V vs. SCE. The shape of the forward scan is not symmetrical. At E=−1.38 V vs. SCE the visible hydrogen evolution sets in. The backward scan is symmetrical. The Ebwcorr is shifted cathodically in comparison to Efwcorr.
Three different approaches are drawn in the (I) vs. (E) curve in order to evaluate icorr.
Sketches A and B follow the intention of Kirkland et al. , and demonstrate the variability of the icorr values obtained by extrapolation.
Sketch C is dealing with the back scan, after visible evolution of hydrogen, causing an increase of electrochemically active surface. Figure 6 shows an explanation for this fact considering the mixed potential theory and the possible reactions. In the equations (4.1) to (4.3) z has to be changed from 2 to 4 for calculation of CR.
Another factor which has strong influence on these results has not clearly been stated neither: the real surface, used for calculation of the current density. Many pictures in the publication show covered area, cracks and pits [11, 38]. Here it can be assumed that only a part of the geometric area is responsible for the current flow. This effect is clearly observable when surface covering or protecting layers onto Mg or Mg-alloys are investigated [9, 21, 26, 32, 39, 40]. This is also pointed out by Atrens et al. as “partially protective surface film mechanism” (PPSFM) .
Subsequently it seems to be necessary to recommend that in all papers, dealing with electrochemical measurements on Mg or Mg-alloys, a clear and explicit description of the pre- and random conditions of the measurements has to be made, especially focussing on the description of the evaluation of icorr and the applied software and their models. This is mandatory to improve the reliability of electrochemical data related to the prediction of corrosion rate and to raise acceptance of these methods.
Electrochemical impedance spectroscopy (EIS)
An alternative or additional technique for the assessment of corrosion processes at metallic surfaces in contact with aqueous electrolytes is the EIS. It is the analysis of the response of a time dependent disturbed equilibrium. EIS is able to determine a number of fundamental parameters related to electrochemical kinetics [25, 29]. Concerning that, various processes at the metal-electrolyte interface show a more or less discrete response to the oscillating distortion of the equilibrium. Depending on the frequency (f) and the amplitude (ΔE) of their displacement, different processes and reactions at and in the surface layer can be distinguished. The amplitude can be chosen between 5 and 20 mV, superimposed on Ecorr, respectively, EI=0. The range of frequencies lies between 100 kHz and 1 mHz. However, it has to be considered that the lower the frequencies the longer the measurement time will be. The measurement results are presented by Nyquist- or Bode plot, as seen in Figure 7.
Based on a description of the interface using of equivalent circuits, capacities and resistances are maintained. The polarisation resistance (Rp) is one of the values of interest which can be used in order to calculate the corrosion current, due to equation (6) and afterwards the corrosion rate.
The quality of the result will be determined by the quality of the model chosen for fitting, as , have shown.
In summary the short description of some examples shows a broad and essential field of further investigations. The EIS technique is about to be applied routinely. Nevertheless there are some remarks necessary:
Appliance of equivalent circuits without respect to physico-chemical processes at the interface metal-electrolyte is not advisable;
Rp-polarisation resistant as one result is typically not applied for calculation of corrosion rate, but it should be. The requirements therefore are an excellent fit of the chosen equivalent circuit to the experimental data and a good description of the interface situation.
Experimental set ups
In most of experiments published, specimens with 1 cm2 of active measurement areas are used. Obviously, a lot of material is necessary. It is easy to handle and well accepted in the corrosion research community. The advantage is that hydrogen evolution measurements and electrochemical experiments can be performed at the same time. In such cases the electrochemical signals contain a summary of various processes and an explicit description where and how the presentation of the degradation reaction is not possible. Apparently crack corrosion at the interface to the polymer embedding material takes place, which may lead to mistakes caused by corrosion attacks at the sites of back contacts, as described by Atrens et al. .
Miniaturised cell set up
Disadvantages occurring using classical cell set ups may be avoided by using miniaturised electrochemical cells set ups , such as the MCS [30, 34, 41] or miniaturized sensors .
The miniaturization of the contact area permits a control of contacts. At the same time the resolution will be also improved, as schematically summarized in Figure 8.
Miniaturization of measurement devices provides another advantage of simplification of external influences which are migration and diffusion of reactants to and reaction products from the interface . However, the reduction of contact area is accompanied proportionally by experimental current which claims more focus on the analysis equipment.
The final step in order to increase the resolution of electrochemical probes is the application of Ultra Micro Electrodes (UME) in SECM, which is discussed in the next chapter.
Scanning electro-chemical microscopy (SECM)
This technique was introduced by A. Bard in the late 1980s of the last century [44, 45]. SECM uses a mobile UME to investigate the activity and/or topography of an interface locally at a high resolution . Depending on the probe mode, as well in the amperometric as the potentiometric mode, different information from the interface can be obtained. SECM uses a similar set up as atomic force microscopy (AFM) to drive the UME, and potentiostat and frequency analyser plugged to it in order to receive electrochemical data . Using this setup, measurements inside of the electrochemical double layer can be performed, and a 3D description of the interface layer on a metal surface in contact with an electrolyte can be obtained. Thus, information about the inhomogeneity of the potential distribution, the current flow over active sites or the migration of reaction products may be gathered. Figure 9 shows a schematic description of setup and measurement techniques applicable using SECM. The dimensions of the UME range between 1 and 25 μm. Usually Pt or Ir wires are used for this purpose. They are fixed in glass tubes with a diameter of <1 mm [19, 46]. Depending on the problem in focus, various techniques have been developed. All well known electrochemical techniques are applicable: from cyclic voltammetry up to impedance spectroscopy, called “localy resolved EIS”.
Some interesting papers have been published during the recent years, demonstrating the impact of this technique for a detailed assessment of the Mg-electrolyte interface. Furthermore they broadened our skills on the kinetics, pH shift and the circumstances under which the hydrogen evolution takes place. One of the broad and controversially discussed questions is the change of pH during the degradation of Mg, as well as increases and the fact of no changes of pH during the corrosion of Mg and Mg-alloys.  have shown over a Mg strip in 10 mM NaCl solution,that the pH measured with a UME (diameter of 175 μm) in a constant height mode at a distance of 80 μm away from the interface and moved along a distance of 1 mm diversifies between 8.5 and 5.5. The shift to lower pH was detected at surface areas where hydrogen evolution took place.
Similar results were published by Karavai et al. . At a distance of 100 μm away from the surface of Mg-alloy AZ31 in 0.05 M NaCl solution, they found that the pH around the pits reached values up to 11.3, but outside the pH decreased down to 5.
Using a Pt-probe in a potentiometric mode  revealed that the pH is increasing from 7.6 up to 11.9 when the probe was moved vertically down to the surface.
The discovering and understanding of the hydrogen evolution at the surface of Mg and Mg-alloys is also very interesting. Both Tefashe et al. , Jamali et al.  as well as Izquierdo et al. , showed first interesting results in this field. In Figure 9 principles of measurement are presented, more effort should be done to broaden these measurements.
The results published using SECM measurement done at Mg and Mg-alloys have demonstrated the possibility of a discrete, highly locally resolved analysis of interfaces due to assessing the pH variance, the hydrogen evolution and the distribution of active and passive sites along the surface.
Electrochemical techniques have a great potential to obtain reliable CR data of Mg and Mg-alloys. The future task will be to develop standardised measurement protocols for these purposes.
Advantages of electrochemical measurements are:
the possibility of measuring under nearly real conditions
to assess the interface processes under nearly real conditions
to reduce the number of specimens for measurements
to measure using various resolutions
to improve the reproducibility
to improve the resolution.
The techniques mentioned above should be combined, and the results should be compared with results from independent investigations, as seen in Figure 10, in order to improve and qualify the results of electrochemical measurements.
One option may be the reduction of the measurement area, combined with an improvement of resolution. In order to prevent contact problems at clamps, the MCS setup or miniaturized sensors ought to be chosen. Both, MCS and UME (SECM) are able to render information about degradation process kinetics.
Another task is to find reliable models to fit the impedance data much more perfectly . Therefore the application of computer simulation may have a big input .
At least the application of flow cell set up , which is not discussed in this paper, will improve the knowledge about the degradation behaviour of degradable metallic biomedical devices under near real conditions.
The common goal should be to improve the protocols for electrochemical measurements to get more reliable and comparable data from screening tests during the phase of development of new alloys and later for process and quality control.
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