ANODIZING AS A METHOD OF CONTROLLING THE CORROSION RATE OF DEGRADABLE AZ91D ALLOY IN PHYSIOLOGICAL SOLUTION Olandir V. Correa1, Mara C. L. de Oliveira2, Stela M. C. Fernandes3, Viviam S. M. Pereira4, Renato A. Antunes4 1 Centro de Ciência e Tecnologia de Materiais, Instituto de Pesquisas Energéticas e Nucleares, São Paulo (SP), Brasil 2 Electrocell Ind. Com. De Equip. Elétricos LTDA, Centro de Inovação, Empreendedorismo e Tecnologia (CIETEC), São Paulo, Brasil 3 Universidade Nove de Julho (UNINOVE), São Paulo, Brasil 4 Centro de Engenharia, Modelagem e Ciências Sociais Aplicadas, Universidade Federal do ABC, Santo André (SP), Brasil E-mail: ovcorrea@ipen.br Abstract. In this work anodizing was investigated as a method of controlling the corrosion rate of the AZ91D magnesium alloy. The anodizing process was conducted in a 3 M KOH solution with the addition of 1 M Na2SiO3 at room temperature. The treatment was performed at 4 V and 6 V for 1 h and 2 h. The corrosion resistance of the anodized specimens was evaluated by means of electrochemical impedance spectroscopy and potentiodynamic polarization curves in a 0.9 wt.% NaCl solution at 37 ºC. The anodizing conditions had a marked influence on the electrochemical response of the AZ91D alloy. The best corrosion resistance was obtained for the specimens anodized at 6 V for 1 h. Keywords: AZ91D, Anodizing, Corrosion 1. INTRODUCTION Corrosion is a highly undesirable process for traditional metallic biomaterials such as stainless steels, titanium alloys and cobalt-based alloys [Antunes and De Oliveira, 2009]. It can generate biocompatibility problems due to the leaching of metallic ions and lead to the structural collapse of prostheses when combined with cyclic stresses if corrosion-fatigue mechanisms are active [Hosseinalipour et al., 2010; James and Sire, 2010]. However, corrosion is a necessary phenomenon for magnesium-based biodegradable alloys [Witte et al., 2008]. These materials are being currently designed for temporary fixation devices [Zhang et al., 2009]. In this application, the material must sustain the mechanical loads during the period necessary for the healing of the bone fracture. Furthermore, it must also be gradually absorbed by the human body, eliminating the need for an additional surgery to remove the fixation device [Xin et al., 2011]. Corrosion plays an important role in this process. Magnesium-based alloys are candidate materials for this application due the intrinsic biocompatibility of magnesium [Jamesh et al., 2011]. The major concern is related to the high corrosion rate of magnesium within the human body that can lead to the formation of subcutaneous gas cavities and premature mechanical failure before the necessary healing period of the bone fracture [Witte et al., 2005]. In this regard, it is imperative to properly control the corrosion rate of degradable magnesium alloys. Anodizing, though widely employed for the corrosion protection of these materials in industrial applications [Ardelean et al., 2009; Zuleta et al., 2011], is hardly considered as a method of controlling the corrosion rate of magnesium-based biomaterials [Xue et al., 2011]. The aim of this work was to investigate the influence of different anodizing conditions on the corrosion behavior of the AZ91D magnesium alloy. An environmentally friendly anodizing solution consisting of KOH and Na2SiO3 was used. Different anodizing conditions were tested and the corresponding electrochemical response was evaluated. 2. MATERIAL AND METHODS 2.1 Material Die-cast alloy AZ91D was used in this work. The nominal chemical composition of the alloy is given in Table 1. Table 1. Nominal chemical composition of the AZ91D alloy. Element Al Mn Zn Si Fe Cu Mass (%) 8,30 – 0,15 0,35 – 0,10 0,005 0,030 0,002 9,70 mín. 1,00 máx. máx. máx. Ni Mg Bal. máx. 2.2 Preparation of specimens for the anodizing process The specimens were cut from the as-cast ingot in rectangular parts with approximately 1 cm2. Next, the specimens were electrically connected to a copper wire and then embedded in room temperature curing epoxy resin. Before anodizing, the electrode surfaces were prepared by mechanical polishing with progressively finer SiC paper up to 1200 grit size. 2.3 Anodizing process The anodizing process was conducted in a 3 M KOH solution with addition of 1 M Na2SiO3 at room temperature. A three-electrode cell configuration was used for the treatment with the AZ91D as the working electrode, a standard calomel electrode (SCE) as reference and a platinum wire as the counter-electrode. The anodic films were produced at two different conditions of constant potentials: 4 V and 6 V. The potential was controlled with a potentiostat/galvanostat (Autolab PGSTAT 100). The treatment was carried out for 1 h and 2 h for each condition of potential. 2.4 Electrochemical tests The corrosion resistance of the anodizing specimens was evaluated by means of electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization curves. All the tests were carried out using a potentiostat/galvanostat Autolab PGSTAT 100 with a frequency response analyzer (FRA) module for the impedance measurements, using the same configuration described for the anodizing process. The EIS spectra were obtained over the frequency range of 100 kHz to 10 mHz, with acquisition of 10 points per decade, at the open circuit potential (OCP), with an amplitude of the perturbation signal of 10 mV. Potentiodynamic polarization curves were obtained at a scanning rate of 1 mV.s -1 from the OCP up to 0 V. The potentials mentioned in this work are referred to the standard calomel electrode. The tests were performed in a 0.9 wt.% NaCl solution at 37 ºC up to 7 days of immersion. 2.5 Scanning electron microscopy (SEM) The surface morphology of the anodic films was observed by SEM, using a TM3000 Hitachi microscope. 3. RESULTS AND DISCUSSION 3.1 Anodizing process The anodizing process was conducted at 4 V and 6 V, during 1 h and 2 h. The corresponding chronoamperometric curves obtained at each condition are shown in Fig. 1.The evolution of the resulting electrical current with time can be promptly observed. 6.0E-02 4V 1h 4V 2h 6V 1h 6V 2h 5.0E-02 I (A.cm-2) 4.0E-02 3.0E-02 2.0E-02 1.0E-02 0.0E+00 0 1000 2000 3000 4000 5000 6000 7000 8000 Time (s) Figure 1. Chronoamperometric curves obtained for each anodizing condition. The anodic currents can result from both the anodic dissolution of the alloy and the formation of the anodic film [Fukuda and Matsumoto, 2004]. The presence of Na2SiO3 in the anodizing solution can give rise to a suppression of the anodic dissolution of the alloy by forming a denser and more uniform film. From Fig. 1 it is seen that the anodic currents decreased with increasing the potential from 4 V to 6 V. It is clear that the treatments conducted at 4 V yielded a very sharp increase of the anodic currents, while for those performed at 6 V the rate of current increase was much slower as well as the maximum currents reached during the treatment. In this regard, it is likely that the film growth is more homogeneous at 6 V which can be favorable to the corrosion resistance of the anodized alloy. 3.2 Electrochemical tests 3.2.1 Electrochemical impedance spectroscopy (EIS) Nyquist plots of the anodized AZ91D alloy are shown in Fig. 2. Results for the as-cast (non-anodized) specimen are also displayed for comparison. The specimens were immersed in a 0.9 wt.% NaCl solution at 37 ºC for 7 days. The plots in Fig. 2a were obtained after 1 day of immersion in the electrolyte. They consist of one capacitive loop spanning from the high frequency to the medium frequency domain while an inductive loop is clearly present in the low frequency region of the spectra. The capacitive loop in the high to medium frequency domain is frequently ascribed to charge transfer reactions [Wen et al., 2009]. The inductive loop in the low frequency region is commonly reported for Mg alloys [Chen et al., 2007; Alvarez-Lopez et al., 2010]. The inductive behavior is often explained by the presence of adsorbed surface species such as Mg(OH)2ads and Mg(OH)+ads [Song et al., 1998]. This behavior was observed for both the non-anodized and anodized specimens, independently of the anodizing condition. The diameter of the Nyquist plot along the real axis is associated with the charge transfer resistance of the material, thereby indicating its corrosion resistance [Turhan et al., 2009]. In this regard, the best results were obtained for the specimens anodized at 6 V for 1 h and at 4 V for 1 h. The specimen anodized at 6 V for 2 h presented a more flattened capacitive loop and consequently a smaller diameter of the Nyquist plot, but it is still better than the non-anodized specimen. The specimen anodized at 4 V for 2 h, in turn, presented the lowest impedance values, even in comparison with the non-treated material. The evolution of the electrochemical behavior after 7 days of immersion can be assessed by analyzing the results shown in Fig. 2b. The diameter of the Nyquist plots decreased for all the specimens, indicating a loss of corrosion resistance with time. This behavior was more pronounced for the specimen anodized at 4 V for 1 h and for the specimen anodized at 6 V for 2 h. The first one presented impedance values that are only slightly higher than those of the non-anodized specimen. The charge transfer resistance of the second one, in turn, is smaller than that of the non-anodized material as denoted by the comparatively small radius of its capacitive loop. The specimen anodized at 6 V for 1 h presented the more stable behavior, maintaining a high diameter of the Nyquist plot even after 7 days of immersion, being more corrosion resistant than the non-anodized material and the specimens anodized according to the other experimental conditions. As observed after 1 day of immersion, the lowest corrosion resistance is associated with the specimen anodized at 4 V for 2 h. Potentiodynamic polarization curves of the anodized and as-cast specimens of the AZ91D alloy after immersion in a 0.9 wt.% NaCl solution at 37 ºC for 7 days are shown in Fig. 3. The corresponding electrochemical parameters are found in Table 2. 1600 Non-anodized 4V 1h 4V 2h 6V 1h 6V 2h a) -Z'' (W.cm2) 1200 800 400 0 -400 1200 800 400 0 1600 Z' (W.cm2) 1500 b) Non-anodized 4V 1h 4V 2h 6V 1h 6V 2h 1200 2 -Z'' (W.cm ) 900 600 300 0 0 -300 300 600 900 1200 1500 Z' (W.cm2) Figure 2. Nyquist plots of the anodized and as-cast specimens of the AZ91D alloy after immersion in a 0.9 wt.% NaCl solution at 37 ºC for: a) 1 day; b) 7 days. 0.4 Non-anodized 4V 1h 4V 2h 6V 1h 6V 2h 0 E (V) -0.4 -0.8 -1.2 -1.6 -2 1.0E-09 1.0E-07 1.0E-05 -2 1.0E-03 1.0E-01 I (A.cm ) Figure 3. Potentiodynamic polarization curves of the anodized and as-cast specimens of the AZ91D alloy after immersion in a 0.9 wt.% NaCl solution at 37 ºC for 7 days. Table 2. Electrochemical parameters determined from the potentiodynamic polarization curves shown in Fig. 3. Specimen Electrochemical Non- parameters 4V 1h 4V 2h 6V 1h 6V 2h Ecorr (V) -1.25 -1.38 -1.23 -1.30 -1.24 Icorr( μA/cm²) 0.11 4.67 0.12 0.56 0.18 anodized The results shown in Table 2 evidenced that the corrosion potential (Ecorr) of the specimens anodized at 4 V for 1 h and at 6 V for 1 h were only slightly different from that of the non-anodized material. The corrosion current density (Icorr) values of these specimens, in turn, are lower than that of the non-anodized material, indicating that the corrosion resistance was increased after the anodizing process at these conditions. Conversely, the corrosion potentials of the specimens anodized at 4 V for 2 h and at 6 V for 2 h are less noble than that of the non-anodized material, suggesting that they are less stable and more active in the electrolyte. In the same way, the Icorr values of these specimens are higher than that of the non-anodized material, indicating their relatively low corrosion resistance. These results support those obtained from the EIS measurements. The best anodizing condition was for the treatment conducted at 6 V for 1 h, followed by that conducted at 4 V for 1 h. It is also noteworthy from the polarization curves that the as-cast material and the specimens anodized at 6 V for 1h and at 4 V for 1h present a self-passivation characteristic while the specimens anodized at 4 V for 2h and at 6 V for 2h presented active dissolution. The specimen anodized at 6 V for 1h presented the highest breakdown potential (-0.84 V) and the widest passive region, confirming the more protective character of the anodic film grown at this condition. 3.3 SEM micrographs SEM micrographs of the as-anodized specimens are shown in Fig. 4. The anodic films formed at 6 V were more uniform than those formed at 4 V. The films formed at 4 V displays several cracks and holes while those formed at 6 V are less defective. This result confirms the indication of the chronoamperometric curves shown in Fig. 1 where the steep increase of the anodic currents observed for the films grown at 4 V were hypothesized to favor the formation of a less homogeneous layer on the surface of the AZ91D alloy. It is noteworthy that the film formed at 6 V for 1 h is more uniform and less defective than those grown at the other anodizing conditions. This explains the superior stability of this condition in comparison with the other films as indicated by the EIS plots shown in Fig. 2. Figure 4. SEM micrographs of the as-anodized specimens. 4. CONCLUSIONS Anodizing can be used to control the corrosion rate of the AZ91D alloy in physiological solution. The effectiveness of the corrosion protection imparted by the anodic film is strongly related to the experimental conditions of the anodizing process. Both the potential and time of treatment affect the corrosion behavior of the anodized specimens. 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