Abstract
A comparative study has been carried out on AZ31 and AZ91 magnesium alloys in order to understand the electrochemical behavior in both alkaline and chloride containing solutions. The open circuit potential (OCP) was examined in 1 M NaOH and 3.5 mass % NaCl solutions. AZ31 magnesium alloy shows several potential drops throughout the immersion in 1 M NaOH solution, though AZ91 does not show this phenomenon. The specimens were anodized at a constant potential of 3 V for 30 minutes at 298 K in 1 M NaOH solution. The anticorrosion behavior of the anodized specimens was better than those of nonanodized specimens. The anodized AZ91 has better corrosion resistance compared to nonanodized specimen and anodized AZ31 magnesium alloy.
1. Introduction
The importance of magnesium alloys has increased significantly in various industries due to the high strength/weight ratio, high dimensional stability, good machining, and ability to be recycled [1]. Unfortunately, magnesium has also some inadequate characteristics that have delayed its wide scale use in many applications. Poor corrosion resistance is one of the major problems that prevent the widespread of magnesium alloys in outdoor applications due to high chemical and electrochemical activity compared with other structural metals such as steels and Al alloys. There are two primary reasons for the poor corrosion resistance of magnesium alloys; the first reason is the internal galvanic corrosion by second phases or impurities; the second reason is that the hydroxide film on magnesium is much less stable than passive films that form on metals such as aluminum alloys and stainless steels [2]. Magnesium is resistant to corrosion in alkaline environments; Mg forms passivating crystalline film of magnesium hydroxide on the surface of magnesium alloys. This film is very stable in pure alkaline aqueous solutions with [3]. Dilute alkali solutions show negligible corrosive effects at temperatures up to the boiling point [4]. To improve the corrosion resistance, further surface treatment is needed in order to protect magnesium alloys against corrosion and to achieve the resistance necessary for many applications. Chromate bath was traditionally applied in spite of being not friendly to the environment. It is toxic to human and difficult to recycle. Several surface treatment processes were performed in order to achieve a good corrosion resistance such as electrochemical plating, chemical conversion, anodizing, thermal spraying, chemical and physical vapor deposition, and plasma polymerization [5]. In our previous work, we investigated the anodizing of AZ31 magnesium alloy in alkaline solution at various parameters [6–8]. In the present work, a comparative study has been carried out on AZ31 and AZ91 magnesium alloys in order to understand the electrochemical behavior in different solutions. The open circuit potential (OCP) was measured in 1 M NaOH and 3.5 mass % NaCl solutions. The specimens were anodized at a constant potential of 3 V for 30 minutes at 298 K. The anticorrosion behaviors were evaluated using the anodic polarization curves and the electrochemical impedance spectroscopy (EIS).
2. Experimental Procedure
2.1. Specimens Preparation
Commercially available AZ91 and AZ31 magnesium alloys were used as the substrate; the chemical composition of the alloys is listed in Table 1. The surface of the alloy was polished up to # 2000 emery paper followed by 0.05 m alumina powders. The specimens were carefully cleaned with water, rinsed with acetone, and dried under air. All of the experiment specimens were mounted using polytetrafluoroethylene (PTFE) resin tape, leaving 1 cm2 surface area.
2.2. The Open-Circuit Potential (OCP)
Electrochemical measurements were carried out using a conventional electrochemical cell equipped with three electrodes. Magnesium alloy specimen, platinum, and Ag/AgCl sat. KCl were served as working, counter, and reference electrodes, respectively. This electrochemical cell was used also in all electrochemical measurements in this research. The OCP of both alloys was observed in 3.5 mass % NaCl and 1 M NaOH solutions without deaeration for 30 minutes. The solution was agitated using a magnetic stirrer throughout the treatments.
2.3. Anodic Polarization Test
The potentiodynamic polarization tests were carried out using a Solartron 1285 Potentiostat (Solartron Analytical, Farnborough, UK), controlled with the CorrWare software (Scribner Associates, Inc.). The anodic polarization curves were measured in 17 mM NaCl and 0.1 M Na2SO4 solution at 298 K with a scanning rate of 1 mV/s.
2.4. Electrochemical Impedance Spectroscopy
Electrochemical impedance spectroscopy of the immersed specimens was measured in 17 mM NaCl and 0.1 M Na2SO4 solution at 298 K using a Solarton 1287 electrochemical interface and a Solarton 1260 frequency response analyzer (Solartron Analytical, Farnborough, UK), with a frequency range from 100,000 to 0.01 Hz. controlled with Z-Plot software (Scribner Associates, Inc.).
2.5. Anodizing
The specimens were anodized in 1 M NaOH alkaline solution at a constant potential of 3 V for 30 minutes at 298 K. After the treatment, the specimens were carefully rinsed using distilled water and dried under air before analysis.
2.6. Morphology and Structure of Anodic Film
The morphology and microstructure of the anodic films were observed with a Hitachi S-800 scanning electron microscope (SEM). The crystalline phases were identified with X-ray diffraction (XRD).
3. Results and Discussion
3.1. Influence of Material Structure on the Electrochemical Behaviors
The microstructure of AZ91 magnesium alloy shows a nearly continuous network of grain boundary phase (Mg17Al12). However, AZ31 contains a few amounts of phase as shown in Figure 1. The phase was more stable in NaCl and was more inert to corrosion. The free corrosion potential of the -phase is more positive than the free corrosion potential of the -phase in sodium chloride solutions. The phase corrodes due to its very negative free corrosion potential and there is the tendency for the corrosion rate of the -phase to be accelerated by microgalvanic coupling between the -phase and the -phase [9–11]. The phase mainly served as a galvanic cathode and accelerated the corrosion process of the matrix if the volume fraction of -phase was small; however, for a high volume fraction, the phase might act as an anodic barrier to inhibit the overall corrosion of the alloy [12].
Figure 2 shows the OCP of AZ31 and AZ91 magnesium alloys in 1 M NaOH solution for 30 minutes. The OCP of AZ31 shifts toward noble potential values, showing several potential drops throughout the immersion. The occurrence of the potential drops suggests that several cycles of dissolution/repassivation processes take place during the formation of magnesium hydroxide film. The variation of the OCP could be caused by different factors, such as the variation of surface pH or the temperature. However, such a large potential drop can hardly be attributed to naturally occurring small fluctuations of the temperature or pH. Moreover, the potential drop from its highest value (passive region) to almost its initial value (active region) indicates that the potential variation mainly results from dissolution of the passive film and a subsequent exposure of a bare metal surface [13]. On the other hand, the OCP of AZ91 shifts toward noble potential values with no notable potential drops was observed throughout the immersion, indicating that the passive film formed on AZ91 is more protective and strongly adhered to the substrate.
The corrosion mechanism of magnesium alloys in aqueous environments generally proceeds by electrochemical reaction with water to produce hydrogen gas and magnesium hydroxide [14]. Magnesium dissolution takes place immediately after immersion in the solution Hydrogen evolution is associated with Mg dissolution: The pH increases due to production of , which favors the formation of Mg hydroxide film by the precipitation reaction:
The OCP of AZ31 and AZ91 magnesium alloys was performed in 3.5 mass % NaCl solutions for 30 minutes as shown in Figure 3. The higher NaCl concentration was chosen in order to accelerate the corrosion rate of the test specimens. The formation of the surface films or the covered corrosion products on the material retarded the further corrosion of AZ31 and AZ91 magnesium alloys in dilute solutions [15, 16]. At the beginning of the immersion, the OCP of both alloys shifts toward noble potential due to the passivation of the film possibly because the formation of the corrosion product film could act as an effective barrier to further oxidation. After 85 seconds of immersion, the OCP of AZ31 magnesium alloy decreased to reach the initial potential value due to the pitting corrosion on the surface. The OCP of AZ91 magnesium alloy continued to increase, possibly because of the relatively large amount of Mg17Al12, which reduces the activity of the pure Mg in 3.5 mass % NaCl. The corrosion product film covered the whole surface of AZ31 magnesium alloy due to easily corrosion of phase, which is the main constituent of AZ31 magnesium alloy. Moreover, the surface contains wide microcracks with porous structure. The passivation film on AZ91 seems to be compact and the corrosion product film does not cover the entire surface as shown in Figure 4.
3.2. The Anticorrosion Property of Nontreated Alloys
The anticorrosion behavior of both alloys was examined using potentiodynamic anodic polarization technique. The pitting potential of AZ91 magnesium alloy was the noblest compared with AZ31 magnesium alloy as shown in Figure 5. The corrosion resistance of AZ31 and AZ91 can be further confirmed using electrochemical impedance spectroscopy (EIS). The EIS results of AZ91 and AZ31 magnesium alloys are shown in Figure 6. The diameter of a capacitive loop in the Nyquist plane represents the polarization resistance of the test specimens. A greater polarization resistance normally means a lower corrosion rate. The Nyquist plot (Figure 6(a)) shows that both alloys have a single capacitive loop at all frequencies, which mean that the corrosion rate is different with the same mechanism. The experiment carried out for 4 hours; the diameter of a capacitive loop in the Nyquist plane for AZ91 is much larger than that of AZ31 magnesium alloy, which signifies that the corrosion rate of AZ91 is much lower than AZ31 magnesium alloy. A Bode plot which is a variation on the frequency-response curve was shown in Figure 6(b). AZ91 has higher impedance value in comparison to AZ31 magnesium alloy, indicating that AZ91 has better anticorrosion behavior in comparison to AZ31 magnesium alloy.
(a)
(b)
3.3. Anodizing in 1 M NaOH Solution
The anodic film formed on AZ31 magnesium alloy in I M NaOH solution at 3 V had an effective corrosion resistance [7]. Figure 7 shows the current density-time transient during anodizing of AZ91 and AZ31 magnesium alloys at 3 V in 1 M NaOH solution at . At the first period of anodizing, AZ31 magnesium alloy is protected by a thin magnesium hydroxide film that offers little resistance to the current. The current density decreased to its lowest value after 5 seconds of anodizing time and then increased sharply due to magnesium dissolution reaction. The current density reached its maximum value, 0.3 A/cm2, after 140 seconds of the anodizing time. The current was nearly stable with further increasing in the anodizing time because of the stationary dissolution condition and film formation. On the other hand, the current density of AZ91 magnesium alloy reached its maximum value, 0.15 A/cm2, after 260 seconds of the anodizing time, and then the current density decreased due to the formation of magnesium hydroxide film. After 1000 seconds treatment the current density reached the stable value. The low current density of AZ91 was attributed to the formation of a high impedance anodic film on the surface.
The microstructures of anodic films on AZ91 and AZ31 magnesium alloys are shown in Figure 8(a). AZ31 has a granular rough surface with some microcracks. However, AZ91 has a smooth surface with pores structure. The diameters of the pore are from several micrometers to more than 20 m; it seems that the pores are located only on the outer part of the coating. The thickness of the anodic films on AZ91 and AZ31 was 10 m and 5.0 m, respectively, as shown in Figure 8(b).
The X-ray diffraction patterns of anodic films formed in 1 M NaOH solution are shown in Figure 9. Mg and peaks were observed in both anodized surfaces. However, phase (Mg17Al12) was observed only in the anodic film on AZ91 magnesium alloy. MgO was also considering as a main constituent of the anodic film on AZ91 magnesium alloy; the following well-known two equations show the formation mechanism of MgO:
Figure 10 shows the anticorrosion behavior of both alloys after anodizing in 1 M NaOH solution. The pitting potential of AZ31 and AZ91 magnesium alloys with and without anodizing is shown in Table 2. The anodized AZ91 has the best anticorrosion behavior compared to Anodized AZ31 and those of nonanodized specimen; this may be because of the formation of MgO and the relatively large amount of phase on AZ91 surface and also the film formed on AZ91 specimen was almost two times as thick as the one formed on AZ31 magnesium alloy.
