The pretreatment process plays a vital role in the development of a robust protective layer on magnesium alloys. This work presents a novel insight into the pretreatment of magnesium alloy AZ91 in alkaline silicate solution by anodic oxidation in the presence of C7H5NaO3 (sodium salicylate) to enhance surface resistance and introduce a passive biolayer. The electrochemical corrosion behavior of protective layers prepared at different voltages was evaluated by electrochemical impedance spectroscopy (EIS) in SBF solution. The Nyquist impedance plots for anodized films, which consist of a mixture of magnesium silicate and salicylate, indicate a significant corrosion protection ability of the layers formed under optimized conditions; an increase of the real impedance from the value of ~2 kΩ.cm2 for the bare alloy surface to about 10–12 kΩ.cm2 for the anodized film can be observed. The results indicate that surface modification by anodizing in silicate electrolyte in the presence of sodium salicylate is an alternative pretreatment to improve the corrosion resistance of AZ91.
Development of biomedical magnesium implants, which have suitable mechanical properties and additionally safely degrade in vivo, opens an avenue for the fabrication of future biodegradable metallic implants. For many years, metallic materials including stainless steel, cobalt-chromium, and titanium alloys have been the primary biomaterials used for load-bearing applications. Considering the release of undesired metallic ions and potential allergic reactions when these metallic alloy implants come in contact with surrounding body tissues and fluids, magnesium alloys are being suggested as promising biodegradable materials for temporary implants, to overcome the limitations and long-term drawbacks of the conventional (permanent) metallic materials being used . In addition, the elastic modulus of Mg has been reported as 45 GPa , which is far closer to the elastic modulus of human bone, in comparison to that of unalloyed commercially titanium, stainless steel, cobalt, and platinum alloys, which lie between 103 and 107 GPa , , 193, 195–210, and 147 GPa respectively . Magnesium is a candidate for orthopedic applications and for bone tissue regeneration approaches . However, the excessive corrosion rate of Mg alloys in physiological environment and the release of hydrogen gas during degradation represents a current limitation for clinical application of Mg and its alloys . Therefore, reducing the corrosion rate of magnesium is the most appropriate strategy which can be achieved with the use of different approaches including alloying , , , , , , , , surface treatment/coating , , , , , , , , , , ,  and mechanical processing such as squeeze casting, hot rolling, equal-channel angular pressing (ECAP), cyclic extrusion and compression, and double extrusion , , , , . Surface treatment or deposition of a coating is considered as a viable approach to improve the corrosion resistance and to pride an appropriate surface for better bone bonding and cell growth on Mg and Mg alloys , . The various aspects of surface modification methods and protective coatings on Mg and its alloys have been reviewed , , .
The challenge is to develop appropriate coatings or protective layers that would delay the rate of corrosion attack of Mg being at the same time biocompatible. Organic-based coatings appear as one of the most interesting options, as they offer protection against corrosion together with other functions such as drug delivery or functionalisation ability involving organic bio-molecules , , , . For instance, corrosion tests performed in NaCl solution on Mg-6Zn substrates coated by polylactic glycolacid (PLGA) showed reduced degradation rate due to the presence of the biopolymer coating . Following the same study, Chen et al.  successfully prepared polycaprolactone (PCL) as well as polylactic acid (PLA) coatings on the surface of pure magnesium. Corrosion studies in SBF solution including static polarization and dynamic immersion tests showed slightly improved corrosion resistance due to the interaction between substrate and polymer coating. Recently, Degner et al.  deposited a corrosion-resistant biodegradable polymer film of polycaprolactone (PCL) in different concentrations by spin coating. In the same context, the corrosion behavior of dip-coated magnesium alloys with PCL and PEI has been evaluated by Wong et al.  and Conceicao et al. , respectively.
However, Xu and his co-authors  introduced a two-step coating approach adopting a preliminary PEO treatment of the alloy WE42 followed by dropping organic phases (cross-linked gelatin/PLGA-particles solution) onto the surface. There was no significant difference between the corrosion resistance after PEO treatment and after the entire two-step processes.
Considering the two aspects of appropriate protective layer and biocompatibility, the present work presents a preliminary study on the pretreatment/coating of magnesium alloy AZ91 in alkaline silicate solution by anodic oxidation in the presence of sodium salicylate to introduce a protective coating with uniform coverage for the application of magnesium alloys in biodegradable implants. It is also of special interest to determine the role of salicylate in improving the corrosion behavior of Mg alloys and its reaction with silicates and magnesium alloys. Salicylates are compounds derived from salicylic acid, which has been used as a common painkiller and to reduce fever and inflammation for centuries. Clinical studies have shown the anti-aging benefits of sodium salicylate in human skin . Salicylate and its derivatives ,  have been regarded as one of the most interesting pharmacologic compounds with many undisclosed therapeutic properties. Sodium salicylate (NaSAL) could also represent a powerful antidote to be used against paraquat (PQ) poisonings. Moreover, the presence of the salicylate molecule in many commercial drugs , and its complexation ability with metallic ions and metal surfaces  is an interesting topic which deserves a deep research. The usage of salicylate as passivating agent for copper alloys has already been discussed , . In a study on the role of salicylate ions in the anodic behavior of copper, Cascalheira et al.  observed the formation of Cu(II)-salicylate film at the electrode surface which contributes to its passivation. They also determined the oxidation peak of salicylate at around 1V. For the Mg alloy AZ91, it has been demonstrated that release of salicylate as dopant in polypyrrole coatings can function as inhibitor for the Mg alloy . As limited work has been published regarding the surface pretreatment/coating of Mg alloys by organic molecules, it is interesting to explore the use of salicylate as passivating agent that leads to the delay of corrosion attack for Mg. The use of salicylate in the present work can only be fully justified if high quality corrosion-resistant coatings can be achieved, which might be able to compete with current organic or bioactive materials for biomedical applications.
As-received Mg alloy samples (AZ91) were used as substrates. They were cut into 20×20 mm2 pieces and used as deposition substrates during surface treatment. The AZ91 substrate was ground with silicon carbide paper (Grade P 1200), rinsed, cleaned with ethanol by ultrasonication, and dried in nitrogen at room temperature. Pretreatment of AZ91D was performed in an electrolyte of sodium hydroxide, sodium silicate and sodium salicylate (a 200 mL solution containing 0.25 M NaOH, 0.1 M Na2SiO3 and 0.026 M C7H5NaO3) and carried out at various voltages. Figure 1 illustrates the set-up experiment for the anodic spark oxidation. The corrosion studies on coated and uncoated samples were performed using electrochemical impedance spectroscopy (EIS) technique (Autolab PGSTAT 30). The electrochemical cell consisted of three electrode set-up, in which the particular Mg alloy samples act as working electrode. A platinum sheet was used as counter electrode and a Ag/AgCl with 3 M KCl was used as reference electrode. As electrolyte a physiological simulated body solution (SBF) was prepared according to the recipe of SBF5 by Müller and Müller . The pH of the solution was maintained at 7.4, and the temperature was kept at 37°C using a water bath. Open circuit potential (OCP) was monitored continuously for 30 min until the steady state was reached. During EIS experiments, a sinusoidal perturbation of ±10 mV for 10 points per decade in a frequency range between 100 kHz and 10 mHz was used. The surfaces of coatings were characterized using optical microscopy. Infrared spectrum of the anodized film on AZ91 MG alloy was performed with a Bruker Fourier Transform Infrared spectrometer (FTIR). The range from 4000 to 400 cm−1 was scanned.
Results and discussion
There is an increasing need for assessing the corrosion resistance of coated magnesium alloys under physiological environment. Anodization presents one possible coating route, and has been previously explored also for Mg alloys , , , , , , . In the present study, anodization using a new type of electrolyte was investigated. It should be pointed out that a pretreatment coating or a pretreatment followed by another bioactive coating can effectively prevent corrosion and provide a passive layer between the metal and the physiological environment. The coating bath for anodizing is a novel alkaline electrolyte (NaOH+Na2SiO3)+C7H5NaO3 (organic material). Figure 2 displays the deposition current as a function of deposition time at different voltages. At start, the deposition current decreased drastically in a short time after applying 4 and 10V, and then reached a steady level as the deposition progressed. Experimentally, anodizing can be performed either under voltage or current controlled conditions. Under voltage control, the current drops with treatment time as the insulating oxide film is growing. However, the situation was completely reversed at 5 and 7 V; this may be due to the reaction between the Mg substrate and the electrolyte that leads to the formation of a steady protective layer on the Mg substrate.
The pretreatment leads to a modification of the macro-structural morphology of the Mg alloy as shown in Figure 3. It is clear that pretreatments changed the macroscopic color of the surface and almost covered the whole substrate of the magnesium alloy.
Figure 4 demonstrates the Nyquist plots which depict the imaginary impedance, which is indicative of the capacitive and inductive character, vs. the real impedance. It is apparent from these plots that anodization at 4 and 7 Volt leads to coatings with the best corrosion resistance in physiological solution, reaching an impedance value up to 10 and 12 kΩ. The existence of a pseudoinductive loop which appears as scattered points at low frequency can be attributed to relaxation processes corresponding to nucleation of corrosion pits . However, the EIS spectrum of the layer formed by anodizing at 7V for 5 min shows the best corrosion resistance without any sign of the inductive loop, additionally indicating a good protection performance. From the comparison of results at different voltages and pretreatment durations up to 10 min with and without sodium salicylate, it can be concluded that the pretreatment in the presence of salicylate passivates the alloy surface in the course of anodizing, and thus increases its resistance and leads to the formation of an effective barrier against corrosion of the alloy. The lowest impedance values at 10 V in comparison to the other applied voltages might be due to the deterioration of the coating layers or the presence of defects, possibly generated during interactions of the silicate/hydroxide/salicylate solutions with magnesium alloy at this relatively high voltage.
The anodized film formed over AZ91 Mg alloy was analyzed using IR spectroscopy, Figure 5, and the obtained spectrum confirmed the presence of both magnesium silicate and salicylate. The band at 1650 cm−1 is attributed to the characteristic stretching frequency of C=O in Mg-salicylate salt. The broad band at around 1400 cm−1 is due to C-O stretching in Mg-salicylate . The peak around 856 cm−1 is due to MgO. Magnesium silicate shows a strong vibrational band at 1020 cm−1 which is due to the Si-O-Si symmetrical and asymmetrical stretching vibrations . The sharp band at around 3700 is attributed to the O-H stretching . However, the band at 2360 and the striations between 1450 and 1600 cm−1 might be associated with the bending vibration of Si-O, stretching vibration of Mg-O, and bending vibration of Si-O-Mg.
In order to compare the current results with previous studies in the literature, we consider the real impedance at which the imaginary part vanishes for the capacitive part to be the charge transfer resistance (Rt), and regard it as a measure of corrosion resistance . For example, Ye et al.  found that the Rt value of bioactive glass–ceramic coated AZ31 Mg substrate has a value of 2.51 kΩ.cm2 in comparison to the uncoated alloy (0.8 kΩ.cm2). Some authors  showed that a 5 min MAO coated bio-compatible magnesium alloy had better corrosion resistance (≤0.477 kΩ.cm2) after immersion in SBF solution for different durations up to 7 days. On the other hand, Wang et al.  have suggested that dicalcium phosphate hydrate-polycaprolactone (DCPD-PCL) composite coatings are a promising choice in comparison to DCPD alone to protect the Mg-Zn alloy. The real impedance (Zre) of the DCPD-PCL coated Mg-Zn alloy reached about 3000 Ω, approximately 4 times as large as that of the DCPD coated sample. Earlier research  has investigated the biodegradable behavior of HA coating on AZ91D magnesium alloy. The EIS plot showed that the capacitance loop diameter of the HA coating was larger than that of the magnesium alloy substrate but still the real impedance was less than 3000 Ω. In prior studies, Chiu et al.  demonstrated that the EIS of fluoride conversion coating on biodegradable Mg implants showed a polarization resistance of 5.2 kΩ.cm2.
Therefore, the coating presented here represents an effective and innovative way to improve the corrosion resistance of magnesium alloys using salicylate as electrolyte. The coated AZ91 substrates exhibited the best corrosion resistance during electrochemical tests in SBF, which was demonstrated by the larger value of the real impedance in comparison to data from the literature dealing with one-step, two-step and even layer-by-layer coatings.
The preliminary measurements presented in this paper indicate the potential of the one step pre-treatment/coating route of anodic oxidation in the presence of sodium salicylate to enhance the corrosion resistance of magnesium alloys. To the authors’ knowledge, the corrosion behavior of the anodized films over AZ91 in the presence of salicylate salt has not been studied before. Further experiments are required, however, to assess quantitatively and biologically the biocorrosion rate and mechanisms of the Mg alloy (AZ91).
Anodizing in the presence of a mixture of silicate electrolyte and sodium salicylate generates a protective coating film on AZ91 magnesium substrates. The effect of the coating was analyzed using optical microscopy and EIS. The improved corrosion resistance of anodized films is attributed to the structure modification with formation of both magnesium silicate and salicylate layer which form a coating that acts as an effective barrier to SBF. FTIR confirmed the presence of both magnesium silicate and salicylate as robust films formed on the magnesium alloy.
A. Francis is grateful to the Alexander von Humboldt foundation and DAAD for support in the form of research stays in 2014 and 2011. A. Francis extends his gratitude to all the colleagues at the Institute of Biomaterials and at the Institute of Surface Science and Corrosion at the University of Erlangen-Nuremberg.
Conflict of interest: Authors state no conflict of interest.
Materials and methods
Informed consent: Informed consent has been obtained from all individuals included in this study.
Ethical approval: The research related to human use has been complied with all the relevant national regulations, institutional policies and in accordance the tenets of the Helsinki Declaration, and has been approved by the authors’ institutional review board or equivalent committee.
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