Effect of Micro-Arc Oxidation Voltage on the Morphology and Electrochemical Properties of AZ31B Magnesium Alloy

Magnesium and its alloys are recognized as ultimate suited and lightest metallic material for structural and industrial applications because of its novel properties as high strength to weight ratio, recyclability, high thermal conductivity and good machinability [1]. Because of their conventional applications magnesium and its alloys stand out in automobile industry, electronic devices and in biodegradable implants as they are nontoxic and biodegradable in body environment [2,3]. Due to high reactivity of magnesium, it is susceptible to undergo corrosion reaction and develops an unstable oxide layer hence unable to provide protection to its surface [4].

As a result of interaction with dissimilar metals acquiring electrochemical potential in ionic path, magnesium acts as an anode at high potential as it is highly active metal and promote galvanic corrosion [5,6]. Intergranular corrosion (IGC) at grain boundaries is another form of corrosion in contrast to pitting because it is crucial to identify surface pits due to corrosion products [7,8]. Acute localized dissolution to metal matrix takes place due to these corrosive pits i.e., pitting is very crucial in orthopedic applications as it enhances corrosion and propagates cracks [9]. Evolving hydrogen due to corrosion and causes balloon effect when absorbed in vivo condition [10]. There are various approaches to enhance the biocompatibility and corrosion resistance i.e., adding different alloying elements, developing biocompatible, protective and corrosion resistance coatings [11]. Surface modification is a well-known technique to carry out different challenges associated with metallic implants in the highly corrosive environment i.e., human body and for enhancement of implants related to corrosion behavior [12]. Utilization of biomedical coatings [13], Micro-arc oxidation coating (MAO) / Plasma electrolytic oxidation (PEO) [14,15], calcium phosphate coatings [16], organic coatings [17], oxide layer development, surface texturing and ion beam processing [18]. Inorganic coatings offer surprising resistant properties, still they have limitations i.e., high temperatures for densification [19,20]. Densification of organ- ic and inorganic coatings usually develops at low temperature (300 °C) which permits the achievement of glassy structures without crystallization and altering the microstructure of magnesium alloys [21,22]. Micro-arc oxidation (MAO) has been generally utilized to develop strong and porous coatings on magnesium and its alloys and then widely used as improved corrosion resistant implants. MAO is a surface engineering method that shows the capacity to improve the adhesion of conclusive top-coat [23,24]. MAO coatings on magnesium have tri-layer structure. The top layer consists of deep pores and cavities having a permeable nature, middle layer is less porous layer, and the bottom layer is a thin obstruction layer. To attain high bond strength and enhance mechanical properties, these pores are advantageous as they release the residual stresses. Distribution of pores, pore density and pore structure are the major aspects that determine the capacity of coating to resist corrosion [25-27]. During MAO of magnesium alloys the nature and concentration of the alloying elements, applied voltage or current, type of electrolyte and microstructures determine the behavior of anodization. At various anodizing current regions and voltages different active-passive states are developed i.e., micro-arcing or sparking is often seen at higher voltages (above 50 V) and known as plasma electrolytic oxidation (PEO) or micro arc oxidation (MAO) [28-30]. MAO is considered as a substitute method for surface treatment because for magnesium alloys traditional anodization becomes uncertain as it did not lower the risk of premature fatigue failure which is not acceptable for engineering practices as fatigue strength plays an important role for load-bearing applications [31]. Number of studies shows that MAO on magnesium alloys concentrated on elevating anti-corrosion properties. Chen et al. applied polymer coatings over MAO treated AZ31 alloy then used nickel by electroless means and they proclaimed enhanced corrosion behavior [32]. Gnedenkov et al. utilized polytetrafluoroethylene over MAO layers in a scattered manner and reported decreased corrosion rate for magnesium alloys [33]. Simancas et al. analyzed different pretreatments with respect to protection against corrosion and proposed MAO for application of top-coat [34].

The aim of this research is to perform surface modification to protect magnesium and its alloys from corrosion in human body environments and finding the biocompatibility and effectiveness of Micro Arc Oxidation (MAO) at various electrolyte and voltages with further coating of fatty acids like stearic acid over Magnesium AZ31B alloy. The corrosion protection of these modified surfaces can be assessed through electrochemical techniques i.e., open circuit potential (OCP) and cyclic polarization (CP) potentiodynamic in simulated body fluid and temperature at different voltages. Scanning electron microscopy (SEM) and Energy-dispersive X-ray spectroscopy (EDX) will provide morphological characterization and elemental composition of materials, respectively.

Material and Surface Preparation

The chemical composition of AZ31B magnesium alloy’s is illustrated in Table 1. The sample of dimension 20×10×5 mm2 was cut from the cylindrical rod of AZ3B and ground with SiC papers upto 3500 grit papers. The samples were then degreased with 70 vo.% solution of ethanol, then cleaned in de-ionized water by rinsing it time and stored in ethanol solution.

Table 1:Chemical composition of AZ31B magnesium alloy (wt.%).

Micro-Arc Oxidation Process

The micro-arc oxidation electrolytic bath was prepared in de-ionized water by using the analytical grade chemicals of Merck with the composition illustrated in Table 2. The pH and conductivity of the electrolytic bath was 11.5 and 2.1 mS/cm respectively. The stainless steel 316L container was used as a cathode and high voltage was applied through specially designed DC power supply. The schematic diagram is shown in Figure 1.

Table 2:Composition of electrolytic bath of Micro Arc Oxidation.

* Sodium dodecyl sulfate (SDS)

Three voltages i.e., 125 V, 200 V and 250 V were applied on the surface of anode (AZ31B sample) for 10 minutes with continued air purging. The MAO samples were then cleaned in de-ionized water to remove residual salts several time and then stored in ethanol solution.

Characterization of MAO coating

The MAO samples were analyzed by stereomicroscope to check the smoothness or uniformity of the surface modification. The Scanning Electron Microscopy (SEM) and Elemental Dispersive Spectroscopy (EDS) for the morphology of MAO sample. The Open Circuit Potential (OCP) and Cyclic Polarization (CP) measurement to find the electrochemical behavior in simulated body fluid Ringer’s Lactate solution at 37±1 ℃. The hydrogen evolution behavior of the MAO samples was investigated by Potentio-dynamic Polarization (PDP) Test in simulated body fluid and to evaluate the corrosion rate. To find the pitting susceptibility in the MAO sample Cyclic Polarization (CP) was conducted.

Stereo microscopy

Figure 2 shows the stereomicrograph images before and after micro-arc oxidation (MAO) treatment on AZ31B magnesium alloy surface. The results show that MAO significantly change the morphology and topography of the magnesium alloy. As the voltages during the MAO changes, the uniformity as well as topography also changes. As in Figure 2 (b), when MAO was performed at 125 V a thin partial layer formed with precipitations on the surface of AZ31B sample. When the voltage was further increased to 200 V, a uniform layer thick layer was formed with constant immersion/ exposure time. When the voltage increases to 250 V, the thick but non-uniform layer formed on the AZ31B sample with clusters of precipitates formed on the surface. The layers formed on the three samples were porous in nature and it was observed that porosity increases with the increase in the applied voltage.

Scanning electron microscope

Figure 3 shows the SEM images after MAO treatment, on the AZ31B magnesium alloy with different applied voltage for constant immersion time. During MAO, many chimps and porosity formed on the surface of the oxide coating. The porosity is of different nature depending on the applied voltage. Due to the high voltage during MAO process, the sparking occurs on the AZ31B samples which formed sintered oxide coating with micro-pores. Those micro- probes entrapped the oxygen bubbles formed due to external air purging in the MAO electrolytic bath. Those entrapped oxygen bubbles increase the porosity in the MAO oxide coatings [35]. The mechanism of the MAO process explained with the help of schematic diagram in Figure 4.

When the applied voltage was 125 V, non-homogenous oxide layer was developed as shown in Figure 4a. Partial oxide coating developed on the AZ31B sample, maximum surface area was unaffected. This is because for the certain composition of the electrolytic bath this was the low applied voltage. The oxide coating developed has pores and clusters of precipitation. The thickness of the oxide was ~ 6 to 8 μm. When the applied voltage increases to 200 V, the oxide coating develop has uniformity in it which support the fact that with the increase in the voltage homogeneity was developed on the AZ31B samples and also the oxide layer thickness increases to ~ 15 to 17 μm. With further increase in the applied voltage increases, the coating thickness of oxide layer increases to ~ 18 to 22 μm but again oxide layer become non-uniform. The cluster formed were large in size which decrease the contact with the surface.

Electrochemical Properties

Open circuit potential (OCP): Figure 5 shows the free potential or open circuit potential (OCP) of bare, and MAO treated AZ31B in the simulated body fluid i.e., Ringer’s Lactate. The bare AZ31B has an OCP value of – 1.560 V vs Ag/AgCl, the potential remains stable after one hour in the Ringer’s Lactate solution which shows that no side reaction or film formation on the bare sample during the testing. On MAO sample developed at 125 V applied voltage, the potential shifted towards more negative reduction value of – 1.592 V vs Ag/AgCl, which shows that due to MAO treatment the surface become more active and susceptible to more dissolution.

At 200 V, the potential shifted towards less negative reduction potential value of about – 1.539 V vs Ag/AgCl which was due to the uniform oxide formation. While at 250 V, the potential again shifted towards negative reduction potential value of about – 1.557 V vs Ag/AgCl.

Cyclic Polarization: Figure 6 shows the cyclic polarization curves of AZ31B before and after MAO treatment in Ringer Lactate solution. The kinetic parameters calculated from the Tafel fitting on the forward scan of the polarization curve are illustrated in Table 3. β_a is the anodic slope, β_c is the cathodic slope, i_corr is the corrosion current density and E_corr is the corrosion potential. The increase in the β_a value reflects that less or slow kinetics of dissolution or anodic reaction. The β_a value of the bare AZ31B is 85.60 mV/decade which increases to 89.60 mV/decade after MAO at 125 V. With the increase in the MAO applied voltage to 200 V, the β_a value further increases to is 96.60 mV/decade which confirm that due to the MAO coating the dissolution or anodic reaction decreases. The decrease in the β_c value reflects that more or fast kinetics of deposition or cathodic reaction. After the MAO process, the AZ31B samples shows decreases in the β_c value confirms the fast reduction reaction on the surface.

Table 3:Kinetic parameters of bare and MAO treated AZ31B magnesium alloy.

• With the increase in the voltage in the optimized electrolyte, the oxide layer thickness increases up to ~22 μm thickness.
• At high voltage of 250 V clusters formation of the ceramic oxide layer and porosity increases.
• The high voltage of 250 V shows improved corrosion resistance as compared to bare AZ31B substrate.

This research was funded by University of the Punjab, Lahore, Pakistan, grant number (2019-2121/TR).

No conflict of interest.

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