1. Introduction
Magnesium alloys are of great interest because of their excellent physical properties of high specific strength, low density, good damping capacity, biocompatibility and castability, making them attractive materials for various industrial applications. A number of researchers have been trying to develop parts and components using Mg alloys across different industries, such as automotive, aerospace, medical implant and electronics [1–3]. However, susceptibility of Mg alloys to corrosion by high reactivity with oxygen and water has limited their wide use in atmospheric environments.
Corrosion resistance of Mg alloys is strongly dependent on the type and concentrations of anions present in the environments by changing the protective properties of surface films formed naturally. The surface films on Mg alloys could be porous or dense, thin or thick and stable or unstable chemically by which corrosion behavior and corrosion rate are significantly changed [4–8]. Anions can interact readily with Mg and can form a surface film or reaction products. Chloride ion (Cl−) ions are typical anions present in the atmospheric environment by which corrosion of Mg alloys is induced. Several studies [9–15] have reported that Cl− ions can penetrate through the surface films formed naturally on the Mg alloy surface and disrupt the surface film, thereby inducing its corrosion.
Sulfate ions (SO42−), although they are less aggressive than Cl− ions, can also influence the corrosion behavior of Mg alloys. Previous researches [16–21] have shown that sulfate ions can result in the formation of slightly protective films on Mg alloys. These films can be composed of magnesium sulfate (MgSO4) or other sulfate-containing compounds. Although SO42− ions can facilitate the formation of protective films, these films can be disrupted easily under certain conditions, such as high temperatures, acidic environments, or the presence of other aggressive ions like Cl− ions.
Phosphate ions (PO43−) were used to form chemical conversion coatings but still their roles in the formation of the surface films and corrosion are not clearly understood. Fluoride ions (F−) can form a stable surface film of MgF2 on Mg alloys by which corrosion is inhibited [22,23]. However, the surface film of MgF2 cannot protect the surface of Mg alloys in chloride ion-containing environment. To understand why the MgF2 film cannot protect Mg alloys, physical properties, such as porosity and thickness and chemical stability and composition of the surface films should be clearly characterized.
Electrochemical behavior of Mg alloys is crucially influenced by solution pH, that is concentration of hydrogen ion (H+) and hydroxide ion (OH−) in solution [24–27]. H+ ions can react with magnesium hydroxide film by equation (1), leading to the corrosion of magnesium by equation (2).
OH− ions can lead to the passivation of Mg alloys by the formation of protective surface films of Mg(OH)2 or complex hydroxide compounds. This layer acts as a barrier against diffusion of aggressive ions, thereby protecting the Mg alloy surface from corrosion to some extent in corrosive environment [28–30].
Some researchers [31,32] have performed studies on properties of the surface films formed on Mg alloys in acidic or alkaline solutions containing PO43− and F− ions, reporting that these anions can form a uniform layer of magnesium phosphate (Mg3(PO4)2) and magnesium fluoride (MgF2). However, their studies include combined effect of anions together with H+ or OH− ions, they could not provide the information on the contribution of anions only to the electrochemical behavior of Mg alloys.
In the present work, we excluded the contribution of H+ and OH− ions by choosing 0.1 M HCl, H2SO4, H3PO4, and HF solutions and adjusting pH to 6 and studied the effects of four different anions of Cl−, SO42−, PO43− and F− on the electrochemical behavior of AZ31 Mg alloy in aqueous solution. Open-circuit potential (OCP) transient, electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization experiment were conducted and the experimental results were discussed in view of the role of anions in the formation of porous and dense surface films and corrosion of AZ31 Mg alloy.
2. Experimental
2.1 Sample preparation
The commercial AZ31B Mg alloy plate (wt.%, Al 2.94, Zn 0.8, Mn 0.3, Si < 0.1, Fe < 0.005, Cu < 0.05, and Mg balance) was employed for this work. One side of the plate specimens was polished from 220 grit to 4000 grit SiC abrasive papers with 98% ethyl alcohol, and then masked with an area of 1 cm2 using a masking tape. A copper wire was connected to the sample for electrical connection, and the contact resistance was checked to maintain less than 0.5 mΩ.
2.2 Electrochemical measurements and morphological characterization of the surface films
The electrochemical behavior of the AZ31 Mg alloy was studied via open-circuit potential (OCP), electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization tests (PDP) using electrochemical workstation (Biologic VMP3 multichannel potentiostat). Thee-electrode system including a working electrode, counter electrode (Pt mesh) and reference electrode (Ag/AgCl electrode) was used for the electrochemical experiments. OCP was measured for 30 min and then EIS and PDP measurements were conducted continuously. EIS was carried out at OCP in a frequency range from 105 to 10−1 Hz with the signal amplitude of 10 mV and the PDP was conducted from −0.25 to 0.5 VAg/AgCl at a scan rate of 1 mV/s. EIS and PDP data were fitted and analyzed by EC-Lab software. Electrochemical test was repeated at least three times for checking its reproducibility. The electrolytes used for testing were 0.1 M HCl, 0.1 M H2SO4, 0.1 M H3PO4 and 0.1 M HF solutions in which solution pH was adjusted to 6 by adding NaOH solution. The solution temperature was maintained to be 20±0.5°C. Surface morphologies of the films formed on AZ31 Mg alloy by immersion for 30 min in neutral aqueous solutions containing four different anions, were observed by field emission scanning electron microscopy (FE-SEM, JEOL JSM-7800F).
3. Results and Discussion
Open-circuit potential (OCP) transients of the AZ31 Mg alloys in neutral aqueous solutions containing four different anions of Cl−, SO42−, PO43− and F−, were recorded for 30 minutes and the results are presented in Fig. 1. The OCP of AZ31 Mg alloy increased rapidly during the first 100 seconds of immersion and then exhibited a slow increase up to 30 min, irrespective of type of anions. The rapid OCP increases in the early stage can be explained by initial fast formation of a surface layer through electrochemical reactions between metallic elements and anions together with water. This surface layer may act as a barrier layer, thereby reducing the dissolution reactions of the alloy surface and stabilizing the OCP. The slowly increased OCP after about 100 s of immersion time, suggests a continuous growth of the surface layer. This suggests that the surface films formed in neutral aqueous solutions containing Cl−, SO42−, PO43− and F− ions are not perfectly dense so that electrochemical reactions can occur continuously through the surface film. The porous nature of surface films on AZ31 Mg alloy is still not so clear so it should be studied in more detail in view of the effect of alloying elements of aluminum and zinc and impurities.
The OCP value of AZ31 Mg alloy after immersion for 30 minutes differed with type of anions in neutral aqueous solution. The OCP value decreased in the order of F− (−1.51 V Ag/AgCl) > Cl− (−1.54 V Ag/AgCl) > SO42− (−1.57 V Ag/AgCl) > PO42− (−1.64 V Ag/AgCl). The highest OCP value in F- containing solution reveals the formation of relatively dense and protective surface films, which could be mainly MgF2. The lowest OCP value in PO43− containing solution represents that the surface films formed are more porous and less protective. The OCP value obtained in Cl− containing solution showed higher value than those formed in SO42− and PO43−-containing solutions which suggests the formation of relatively stable surface layer before corrosion initiation. Relatively lower OCP value in SO42−-containing solution than those in F− and Cl−- containing neutral solutions may suggest the formation more porous and less protective films.
Fig. 2 shows Nyquist plots obtained from the AZ31 Mg alloy specimens at open-circuit potential in neutral aqueous solutions containing various anions after immersion for 30 minutes. The Nyquist plots are characterized by two capacitive semicircles. In addition, one more inductive loop together with two capacitive semicircles was observed only in the neutral solution containing phosphate ion (see the inset in Fig. 2).
Two capacitive semicircles can be fitted by using an equivalent circuit of Fig. 3(a), consisting of solution resistance (Rs), charge transfer resistance (Rct), double layer capacitance (Cdl) at the metal/film interface beneath the porous films, resistance of porous film (Rpf) and capacitance of dense films (Cdf). A constant phase element (CPE) was used for accurate fitting of non-ideal capacitance. Two capacitive semicircles and one inductive loop obtained in the neutral aqueous solution containing phosphate ion were fitted using an equivalent circuit of Fig. 3(b).
The values obtained by fitting of EIS data are presented in Table 1. The solution resistance (Rs) was obtained to decrease in the order of PO43− > F− > Cl− > SO42−. The high resistances of solution containing PO43− and F− ions are ascribed to high dissociation constants of H3PO4 and HF in water.
The high frequency capacitive semicircle seems to arise from capacitance of dense films (Cdf) and resistance of porous film (Rpf) because of relatively low RC time constant. The capacitance of dense films was obtained to decrease in the order of PO43− > SO42− ≈ Cl− > F−, while the resistance of porous film decreased in the order of F− > Cl− > SO42− > PO43−.
The low frequency capacitive semicircle stems from combination of electric double layer capacitance (Cdl) and charge transfer resistance (Rct). Cdl was obtained to decrease in the order of SO42− PO43− > Cl− > F− and Rct decreased in the order of F− > Cl− > SO42− > PO43−. It is noted that Rct and Rpf are extremely low in the solution containing PO43−, comparing with the other three anions-containing solutions. These indicate that the AZ31 Mg alloy surface is largely covered with very thin porous films in the PO43−-containing neutral aqueous solution.
It should be also pointed out that 1.2~4.5 mF cm−2 of Cdl is much higher than Cdf by about three orders. In general, 1~40 μF of Cdl has been reported for Mg alloy [33–35]. In this work, three orders higher Cdl can be attributed to extremely larger surface area of porous layer than that of relatively dense layer on the AZ31 Mg alloy surface.
In addition, capacitance of a dense film (Cdf) of 4~41 μF cm−2 is similar with those of barrier type anodic films, which are 2~15 μF cm−2 on the surfaces of pure aluminum [36,37]. Considering that the anodic film on pure aluminum covers the whole surface but the dense film is formed only partly on the AZ31 Mg alloy surface, the similar Cdl suggests that the thickness of dense film on the AZ31 Mg alloy surface is much thinner than that of the anodic film on pure aluminum.
The n constant, which is the exponent in the Constant Phase Element (CPE), was employed in the equivalent circuits in Fig. 3 to describe non-ideal behavior of real capacitance in electrochemical systems. The nf, constant of CPEf, appeared to be in a range of 0.866 to 0.984, and the highest value was obtained in a F−-containing solution, and the lowest value appeared in a PO43−-containing solution. This means that F− ion-induced film is more uniform, while PO43− ion causes relatively non-uniform film formation on AZ31 Mg alloy surface.
The ndl of CPEdl presented a range of 0.679 to 0.947. The lowest ndl value of 0.679 was obtained in a Cl−-containing solution. This indicates that porous films formed on AZ31 Mg alloy surface by corrosion reaction in a Cl−-containing solution are very non-uniform, which could be related with the occurrence of irregular formation of corrosion products locally.
The morphologies of the surface films formed for 30 min on the AZ31 Mg alloy surface during immersion in the neutral aqueous solutions containing four different anions, were observed by FESEM and their typical surface images are given in Fig. 4. In the solution containing chloride ions, flake-type surface films with 200~300 nm length were formed over the entire surface (Fig. 4(a)). In the SO42−-containing neutral solution, needle-type surface films 100~200 nm length were observed (Fig. 4(b)). In the F−-containing solution, needle-type surface films were also observed and the size of needles became smaller and density of needles was much higher than the films formed in SO42−-containing solution. Interestingly, in PO43−-containing neutral aqueous solution, highly porous surface films were formed partly together with dense films over the AZ31 Mg alloy surface. Numerous nanopores were observed on the highly porous films (Fig. 4(e)). However, the surface film around the highly porous film did not show any pores.
The surface films formed naturally on the AZ31 Mg alloy surface are illustrated schematically in Fig. 5 as a layer with different thicknesses and coverages of porous and dense layers, depending on the type of anions in neutral aqueous solution. The thicknesses and coverages of porous and dense films were deduced from the surface morphologies in FE-SEM image (Fig. 4) and capacitance and resistance values obtained from EIS data (Table 1). A thinner layer with high coverage of dense film could result in high Cdf. A thicker layer and low coverage of porous film on the AZ31 Mg alloy surface can increase Rpf. On the other hand, Cdl and Rct are influenced by the area of liquid solution beneath the porous films. The effect of dielectric properties of the dense film on Cdf was not considered in this work. This model can explain the nature of the surface films formed on the AZ31 Mg alloy surface by four different anions of Cl−, SO42−, PO43− and F−, excluding the effect of OH− ions in aqueous solution.
If the area of highly porous films is higher than that of dense films (as shown Fig. 4(c)), which was obtained in PO43−-containing solution, it may facilitate electrochemical reactions at the porous film/metal interface, leading to lowered Rct values and a significantly increased Cdl (Table 1). The extremely low Rpf and Rct values in PO43-containing solution would results from not only high coverage of highly porous films, as illustrated in Fig. 5(c). High Cdf in PO43−-containing solution may stem from the extremely low thickness and/or high dielectric constant of dense films.
If the thickness of porous films is higher than that of dense films, which was obtained in F− -containing solution, it leads to increased Rpf and Rct and a significantly decreased Cdl (Table 1), as depicted in Fig. 5(d). The extremely high Rpf value could be explained by combination of reduced area and increased thickness of porous films, as illustrated in Fig. 5(d). The very low capacitance of dense film (Cdf) obtained in F−- containing solution in Table 1 would result from increased thickness and large area of the dense films, as demonstrated in Fig. 5(d).
In SO42−-containing solution, the Rpf and Rct values were higher than those obtained in a PO43−-containing solution, and a relatively low value of Cdf, and a similar value of Cdl to that in a PO43−-containing solution were obtained (Table 1). Thus, it seems reasonable to infer that the porous film in the SO42−-containing solution has a less porous structure compared to the highly porous film in the PO43−-containing neutral solution, as illustrated schematically in Fig. 5(b). The flake-like surface films might have a highly porous structure and it is believed that they do not significantly affects the electrochemical impedance.
In Cl−-containing solution, highly porous films of flake-like films were formed together with less porous films on the AZ31 Mg alloy surface, as illustrated in Fig. 5(a). Cdf obtained in Cl−-containing solution is similar to that in SO42−-containing solution, suggesting similar thickness and coverage of dense films. Higher Rpf in Cl−-containing solution than that in SO42−-containing solution, suggests that porous films are covered large area in Cl−-containing solution than in SO42−-containing solution.
Fig. 6 and Table 2 present potentiodynamic polarization curves of AZ31 Mg alloy in neutral aqueous solutions containing various anions and parameter values obtained by Tafel fitting of the polarization curves, respectively. The polarization curves revealed that corrosion current density is strongly dependent on the type of anions present in neutral aqueous solution, decreasing in the order of PO43− > SO42− > Cl− ≥ F−. This is in good accordance with the order of resistance of porous films and charge transfer resistance obtained from EIS data (Table 1).
Anodic behavior of AZ31 Mg alloy in the potentiodynamic polarization curves is strongly dependent on the type anions present in neutral aqueous solution, as can be seen in Fig. 6. In the solutions containing Cl− and SO42− ions, a sudden increase in current density was observed as the potential increased at about −1.3 VAg/AgCl for chloride ions and −1.4 VAg/AgCl for sulfate ions, which suggests that slightly protective films are formed initially at open-circuit potential and they are broken with increasing anodic potential in solutions containing Cl− and SO42− ions.
However, in the solutions containing PO43− and F− ions, the sudden increase in anodic current density did not appear as the potential increased. Instead, continuous increase in anodic current density was obtained in PO43−-containing solution and a slight decrease in anodic current density appeared around −1.2 VAg/AgCl in F−-containing solution as the potential increased. These indicate that fluoride ions can form a protective passive film, which is related with the highest corrosion potential in Fig. 1 and high impedance in Fig. 2 but porous film is formed in PO43−-containing solution which might be related with the lowest corrosion potential in Fig. 1 and extremely low impedance in Fig. 2.
4. Conclusions
In this work, electrochemical behavior of AZ31 Mg alloy was investigated in neutral aqueous solutions containing various anions of Cl−, SO42−, PO43− and F− ions.
Open-circuit potential (OCP) of AZ31 Mg alloy measured during immersion for 30 min in neutral aqueous solutions was observed to be strongly dependent on type of anions, decreasing in the order of F− > Cl− > SO42− > PO43− ions.
The OCP increased rapidly during the first 100 seconds of immersion and then exhibited a slow increase up to 30 min, irrespective of type of anions in the neutral aqueous solutions, suggesting initial fast growth and then slow growth of the surface films on AZ31 Mg alloy. The continuous increase of OCP with time reveals that the surface films formed in neutral aqueous solutions containing Cl−, SO42−, PO43− and F− ions are not perfectly dense and electrochemical reactions can occur continuously through the surface films.
A simplified model was suggested for the formation of porous and dense films on the AZ31 Mg alloy surface based on the values of capacitance of dense film (Cdf), resistance of porous film (Rpf) and double layer capacitance (Cdl) and charge transfer resistance (Rct) beneath the porous films obtained from EIS data. This model reveals the natures of the surface films formed on the AZ31 Mg alloy surface by four different anions of Cl−, SO42−, PO43− and F−.
Morphological observation of the surface films by FESEM revealed that flake-type porous films were formed in Cl− ions-containing solution, and needle-type porous films were formed in neutral aqueous solutions containing SO42− and F− ions. In PO43− ion-containing solution, highly porous films were formed partly together with thin and dense films on the AZ31 Mg alloy surface.
Corrosion current density of the AZ31 Mg alloy appeared to decrease in the order of PO43− > SO42− > Cl− ≥ F−. This result is consistent with the model of porous and dense film formation, where relatively dense films are mainly formed in fluoride ion-containing solution, showing the lowest corrosion current density while highly porous films with wide coverage are formed in phosphate ion-containing solution, resulting in the highest corrosion current density.
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