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Investigation of surface oxide film on magnesium lithium alloy
Journal of Alloys and Compounds 484 (2009) 585–590
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Investigation of surface oxide film on magnesium lithium alloy Yingwei Song ∗ , Dayong Shan, Rongshi Chen, En-Hou Han State Key Laboratory for Corrosion and Protection, Institute of Metal Research, Chinese Academy of Sciences, 62 Wencui Road, Shenyang 110016, China
a r t i c l e
i n f o
Article history: Received 20 March 2009 Received in revised form 29 April 2009 Accepted 29 April 2009 Available online 6 May 2009 Keywords: Thin films Corrosion Oxidation Electrochemical impedance spectroscopy
a b s t r a c t Magnesium and its alloys are very chemically active metals. Oxide film can be spontaneously formed on their surface in an ambient atmosphere. The oxide film on the surface of Mg–8Li alloy was characterized by X-ray photoelectron spectroscopy (XPS) analysis and electrochemical measurements. The results indicated that the surface oxide film formed in the air exhibited multiple layer structure, which consisted of the elements of Li, Mg and O. The concentrations of Mg decreased and Li increased from the bottom layer to top layer of the oxide film. Lithium oxides mostly enriched in the outer oxide layer. The more the potential deviated from the Ecorr , the more severe the oxide film was destroyed. Below the film breakdown potential, the surface oxide film can slow down the corrosion of Mg–8Li alloy. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Magnesium and its alloys are very chemically active metals. Oxide film can be spontaneously formed on their surface in the ambient atmosphere. The formation of oxide film is affected by many parameters including alloying elements, medium types, temperature and potential. As a result, there is a great difference at the composition and structure of surface oxide films on various magnesium alloys. The characteristics of surface oxide film have a significant effect on the corrosion behaviors of magnesium and its alloys. There are some researches concerning the surface oxide film on pure Mg and Mg alloys [1–4]. The oxide film formed in the air is thin (20–50 nm) to display amorphous structure, which consists of MgO and Mg(OH)2 . The oxide film formed in the humid air contained outer oxide layer and inner oxide layer. The outer oxide layer is similar to the whole oxide film formed in the air: thin, amorphous and approximate 20–40 nm. The inner oxide layer is a cellular-like hydrated layer as a result of the water molecule penetrating the outer oxide layer to react with the Mg substrate. The oxide film formed in aqueous solution shows a sheet outer oxide layer, which consists of Mg(OH)2 and a small amount of MgO. The inner oxide layer is similar to the whole oxide film formed in the humid air. In other environments containing Cl− , CO2 and SO2 [5–7], MgCl2 , MgCO3 and MgSO4 are also found in the surface oxide film. Generally, the surface oxide film consisting of MgO and Mg(OH)2 is loose and cannot provide enough protection to the Mg substrate
∗ Corresponding author. Tel.: +86 24 23915897; fax: +86 24 23894149. E-mail address: [email protected] (Y. Song). 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.04.137
in the corrosive service conditions. However, the alloying elements can influence the structure and chemical composition of the surface oxide film. Especially, the addition of passivating elements improved the protection performance of the oxide film greatly [8,9]. Among plenty of alloying elements, the effects of Al on the characteristics of surface oxide film were investigated deeply [10,11]. For example, the oxide film on AZ91 Mg alloy consisting of MgO, Al2 O3 , Mg(OH)2 and Al(OH)3 displays a multiple layer structure of oxides of Mg/oxides of Mg and Al/substrate. The thickness of oxide film is several decade nanometers, and it decreases with the increase of Al concentration. This result indicates that the alloying elements can affect the physical and chemical properties of the surface oxide film then further affect the corrosion dynamics of Mg alloys. However, the effects of other alloying elements on the characteristics of surface oxide film are still not clear. It is well known that the novel magnesium lithium alloys show super light weight and excellent formability [12–14]. But the metal lithium is more susceptible to be oxidized than the metal magnesium in the air. The effects of Li on the structure and composition of surface oxide film on magnesium lithium alloys belong to an unknown field. The effects of surface oxide film on the corrosion resistance of magnesium lithium alloys are still ambiguous. Thus, the aim of this paper is to investigate the structure, composition and protection performance of the surface oxide film on magnesium lithium alloys. 2. Material and methods The experimental material used for this investigation was Mg–8Li cast alloy whose nominal composition (wt%) was 8% Li and 92% Mg. The samples for XPS analysis and electrochemical measurements were ground to 2000 grit SiC paper, ultrasonically cleaned in acetone, dried in cold air. The metallographic sample was continued to finely polish using 1 m diamond paste, and then was etched with 10% nital etchant (nitric acid mixed in ethanol).
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The chemical composition of surface oxide film was probed using ESCALAB 250 X-ray photoelectron spectroscopy (XPS) with Mg K␣ radiation (1253.6 eV). The powder was 300 W. All energy values were corrected according to the adventitious C 1s signal, which was set at 284.60 eV. The data were analyzed with Xpspeak 4.1 software. Electrochemical tests were performed using EG&G potentiostat model 273 equipped with a model 5210 lock in amplifier. A classical three-electrode cell was used with platinum as counter electrode, saturated calomel electrode SCE (+0.242 V vs. SHE) as reference electrode, and the samples mounted using epoxy resin with an exposed area of 1 cm2 as working electrode. The initial retard of 300 s was set for all the electrochemical tests. The potentiodynamic curve was measured from −300 to +450 mV vs. OCP at a constant scan rate of 0.5 mV/s. Electrochemical impedance spectroscopy (EIS) was carried out at the frequency scope of 100 kHz to 10 mHz, and the perturbation amplitude was 5 mV. The same sample was applied with various anodic and cathodic potential during EIS testing to study the effects of potential bias on the protection performance of surface oxide film. The corrosive medium of 0.1 M NaCl solution was used for the electrochemical investigation.
3. Results and discussion Fig. 2. XPS survey scanning of the surface oxide film on Mg–8Li alloy.
3.1. Microstructure of Mg–8Li alloy Fig. 1 shows the backscattered image of Mg–8Li alloy. The white irregular regions are the matrix phase named as ␣ phase, which is a solution of lithium in magnesium. The dark regions surrounding ␣ phase are named as  phase, which is a solution of magnesium in lithium. The magnesium and lithium are both very chemically active metals, which are easily oxidized in the ambient environment. Thus, it can make sure that there exists oxide film on the surface of Mg–8Li alloy. The chemical composition and protection performance of the surface oxide film need to be investigated in detail. 3.2. Chemical composition of the surface oxide film The chemical composition of surface oxide film was analyzed with XPS. Fig. 2 shows the XPS survey scanning plot. The surface oxide film consisted of O, Mg, Li and C elements. The H element cannot be detected by XPS. The high concentration of C in the film surface is common in XPS survey scanning due to the adventitious hydrocarbons from the environment. The XPS depth analysis of the surface oxide film is shown in Fig. 3. The XPS plots were analyzed in terms of the Handbook of X-ray Photoelectron Spectroscopy [15]. Because the binding energy of Mg2p is approaching to Li1s, two elements are depicted in one graph. The surface oxide film exhibited a multiple layer structure. The top layer was characterized with the sputtering time of 0 s. The plot of Mg2p + Li1s contained two peaks, corresponding to Mg(OH)2 and Li2 O, respectively. The Mg1s peak was not found. Thus, the top layer of oxide film was composed of Mg(OH)2 and Li2 O. It was obvious that the Li1s peak was stronger
Fig. 1. Backscattered image of Mg–8Li alloy.
than the Mg2p peak, which implied that the lithium oxides enriched at the top layer of oxide film. With increasing the sputtering time to 30 s, the Mg2p + Li1s plot still contained two peaks, corresponding to Mg(OH)2 and Li2 O, but the intensity of Mg2p peak increased. In the meantime, the Mg1s peak was detected, indicating the existence of MgO. Thus, the second layer of oxide film consisted of Mg(OH)2 , Li2 O and MgO. When the sputtering time reached 120 s, there was significant change to the two plots. All the peaks broadened. The Mg2p peak was divided into two narrow peaks of Mg and Mg(OH)2 . The Li1s peak was also characterized with two peaks of Li2 O and LiOH. The Mg1s peak can be described with Mg and MgO. Besides Mg(OH)2 , Li2 O and MgO, Mg and LiOH were also contained at the third layer of oxide film. With increasing the sputtering time to 660 s, the Mg2p peak became narrow and only contained the contribution of Mg. Moreover, Li was found based on the Li1s peak. The Mg1s peak still indicated the existence of MgO and Mg, but the concentration of MgO reduced. The metal Mg was the main component at the bottom layer of oxide film. In the case of XPS depth profile, the surface oxide film contained four layers as shown in Fig. 4. The top layer was the mixture of Mg(OH)2 and Li2 O; the second layer was the mixture of Mg(OH)2 , Li2 O and MgO; the third layer was the mixture of Mg(OH)2 , MgO, LiOH, Li2 O and Mg; the bottom layer was the mixture of MgO, Li2 O, Li and Mg. The peak intensity of Mg element decreased and Li element increased from the bottom layer to the top layer of oxide film, and the lithium oxides mainly enriched at the outer oxide layer. The thickness of surface oxide film was more than 100 nm in light of the calculation by the sputtering time, which was twice thicker than that of on the pure Mg [4]. The growth process of the surface oxide film on Mg–8Li alloy was analyzed as follows. The Mg and Li both show high chemical activity and are very easy to be oxidized into MgO and Li2 O in the air. Then some water molecule in the ambient atmosphere can bond with MgO and Li2 O to form Mg(OH)2 and LiOH, respectively. The thermodynamic functions (H0 ) of MgO and Mg(OH)2 are −601.24 and −924.16 kJ mol−1 , respectively. The very negative H0 values prove that the MgO and Mg(OH)2 can be spontaneously formed in the air and Mg(OH)2 is more stable than MgO. Similarly, the thermodynamic functions (H0 ) of Li2 O and LiOH are −598.73 and −484.d7 kJ mol−1 , respectively, indicating that Li2 O is more stable than LiOH. Thus, the elements of Mg and Li at the top layer of oxide film presented the most stable state of Mg(OH)2 and Li2 O. Because the water molecule in the ambient environment do not penetrate the whole oxide film to reach the Mg substrate, the hydroxides are not contained at the bottom of oxide film. As we know that Li is more susceptible to be oxidized than Mg. Thus, the oxide film on Mg–8Li alloy is thicker than on the pure Mg. But the Pilling/Bedworth ratio of Li2 O is 0.58 [16]. The lithium oxides are loose and cannot form
Y. Song et al. / Journal of Alloys and Compounds 484 (2009) 585–590
Fig. 3. XPS depth profiles of the surface oxide film on Mg–8Li alloy.
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Fig. 4. Sketch map of the surface oxide film on Mg–8Li alloy. The top layer: mixture of Mg(OH)2 and Li2 O; The second layer: mixture of Mg(OH)2 , Li2 O and MgO; The third layer: mixture of Mg(OH)2 , MgO, LiOH, Li2 O and Mg; The bottom layer: mixture of MgO, Li2 O, Li and Mg.
a compact oxide film layer as aluminum oxides. So it is speculated that the protection performance of surface oxide film on Mg–8Li alloy is limited. 3.3. Protection performance of the surface oxide film The protection performance of surface oxide film on Mg–8Li alloy was investigated by polarization curve and EIS plots. Fig. 5 shows the polarization curve of Mg–8Li alloy in 0.1 M NaCl solution. It was found that the anodic side can be divided into three stages. At the stage I (A → B), the corrosion current density slowly increased with the promotion of anodic potential. The curve slope for this stage was 96 mV/decade according to the fitting results of CorrView software. When the potential was near to Ecorr + 65 mV, the anodic curve entered into the stage II (B → C): the corrosion current density quickly increased with the promotion of anodic potential compared with the stage I. The curve slope for this stage was 51.8 mV/decade. The low anodic curve slope implied the fast anodic dissolution rate. Thus, the change of curve slope can be attributed to that there was a protective oxide film on the Mg–8Li alloy surface to slow down the corrosion of substrate material at the stage I. Once the potential reached the film breakdown potential, the surface oxide film fractured and the Mg–8Li substrate was corroded quickly at the stage II. At the stage III (C → D): the corrosion current density nearly kept constant with the increase of anodic potential, which should result from the errors of testing system including the great change of sample surface state and the solution resistance due to the fast corrosion reaction [17]. The cathodic side of polarization curve conformed to the Tafel feature, which was driven by cathodic hydrogen evolution reaction. The corrosion current density gradually diminished when the cathodic potential moved toward Ecorr . It also implied that the
Fig. 5. Polarization curve of Mg–8Li alloy in 0.1 M NaCl solution.
hydrogen generation rate gradually reduced. But the effects of surface oxide film on the hydrogen evolution reaction were not found based on the cathodic curve. The EIS measurements with application of various potentials were performed to investigate the effects of potential bias on the protection performance of surface oxide film. The selected potential values are shown in Fig. 5. At the anodic side, two potential values near to the film breakdown potential, Ecorr + 50 mV (more negative than the film breakdown potential) and Ecorr + 80 mV (more positive than the film breakdown potential), were selected, and another more positive potential value of Ecorr + 100 mV was selected, too. At the cathodic side, the corresponding potential values in the weak polarization regions and strong polarization regions were selected as Ecorr − 50 mV and Ecorr − 120 mV, respectively. The EIS results are shown in Fig. 6. The electrochemical reactions of anodic dissolution and cathodic hydrogen evolution simultaneously occurred on the surface of Mg–8Li alloy at Ecorr . The Nyquist plot comprised three loops, one high frequency capacitive loop, one medium frequency capacitive loop and one short low frequency inductive loop. The high frequency capacitive loop was due to the contribution of electric double layer at the interface of substrate and solution [18]. The medium frequency capacitive loop originated from the diffusion through a porous solid film [19,20], which indicated that the surface oxide film on Mg–8Li alloy was loose. The low frequency inductive loop was attributed to the corrosion nucleation at the initiation stage of localized corrosion [18,21]. The inductive loop only presented at a short frequency range at Ecorr , implying that the Mg substrate did not suffer from severe attack and the oxide film can provide protection to some extent. When the applied anodic potential was more negative than the film breakdown potential, Ecorr + 50 mV, the electrochemical process mainly contained the anodic dissolution reaction. The shape of EIS plot was similar to that of at Ecorr , but its dimension shrank. The shrinkage of high frequency capacitive loop indicated the enhancement of anodic dissolution rate [18]. But the surface oxide film still exhibited protection performance and can continue to restrain the corrosion of Mg–8Li alloy. When the applied anodic potential was more positive than the film breakdown potential, Ecorr + 80 mV, the EIS plot changed greatly. Only one high frequency capacitive loop and one low frequency inductive loop were observed. The medium frequency capacitive loop disappeared and the low frequency inductive loop extended to a large frequency scale, which implied the severe damage of surface oxide film and the initiation of localized corrosion. The surface oxide film suffered severe attack above the film breakdown potential and cannot provide protection to the Mg–8Li substrate again.
Fig. 6. EIS plots of Mg–8Li alloy applied with various potentials in 0.1 M NaCl solution.
Y. Song et al. / Journal of Alloys and Compounds 484 (2009) 585–590
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Fig. 7. Equivalent circuits of Mg–8Li alloy applied with various potentials. (a) Ecorr and Ecorr + 50 mV; (b) Ecorr + 80 mV and Ecorr + 100 mV; (c) Ecorr − 50 mV and Ecorr − 120 mV.
Table 1 EIS fitting results of Mg–8Li alloy applied with various potentials. mV
Rs ( cm2 )
Rt ( cm2 )
Y0 (−1 cm−2 s−n × 10−6 )
n
100 80 50 0 −50 −120
31.14 30.09 30.06 37.22 32.45 30.49
125.9 159.8 510.3 788.1 477.3 311.1
98.18 65.51 37.56 33.34 41.92 47.81
0.8163 0.8262 0.8689 0.8733 0.8623 0.8780
After increasing the potential to Ecorr + 100 mV, the dimension of EIS plot slightly shrank but the shape did not change compared with Ecorr + 80 mV. When the applied potential was Ecorr − 50 mV, the Nyquist plot contained one high frequency capacitive loop and one low frequency capacitive loop. The dimension of low frequency capacitive loop was very small. After negatively moving the potential to Ecorr − 120 mV, the shape of EIS plot was similar and only the dimension shrank to some extent. The hydrogen evolution reaction was the main electrochemical process in the cathodic regions. The enhancement of cathodic potential (toward more negative direction) accelerated the hydrogen evolution rate due to the decrease of charge transfer resistance at the electric double layer. Thus, the high frequency capacitive loop shrank in the more negative cathodic regions. In order to investigate the EIS plots deeply, the equivalent circuit for Ecorr and Ecorr + 50 mV is shown in Fig. 7a. Rs was the solution resistance. Rt referred to the charge transfer resistance and CPEdl described the electric double layer capacity. The constant phase element (CPEdl ) was used to compensate for the non-homogeneity in the system [22]. The CPEdl was defined by two values, Y0 and n. If n was equal to 1, CPEdl was identical to a capacitor. Rt in parallel with CPEdl was used to describe the high frequency capacitance loop. The film resistance Rf in parallel with the film capacity Cf characterized the medium frequency capacitance loop, exhibiting the feature of surface oxide film. Inductive resistance RL in series with inductance L described the low frequency inductance loop. When the applied anodic potentials were more positive than the film breakdown potential, Ecorr + 80 mV and Ecorr + 100 mV, the plots can be explained by the equivalent circuit as shown in Fig. 7b. Because the medium frequency capacitive loop disappeared, Rf and Cf were not used in Fig. 7b. The EIS plots with application of cathodic potential are equivalent to Fig. 7c. Two capacitive loops were depicted by two sets of elements of CPEdl //Rt and Rf //Cf , respectively. The RL and L did not present because the anodic dissolution reaction was restrained in the cathodic potential regions. All the EIS data were fitted with ZSimpWin 3.20 software based on the equivalent circuits in Fig. 7. The fitting results are listed in Table 1. In the anodic regions, the Rt values reduced with the increase of potential bias, indicating the enhancement of anodic dissolution reaction. In the cathodic regions, the Rt values also reduced with the increase of potential bias, implying the promotion of hydrogen evolution reaction rate. The CPEdl values showed the reverse tendency in contrast with Rt . The change of CPEdl and Rt with the applied potentials proved that the surface oxide film showed the optimum protective ability to the Mg–8Li alloy substrate when the potential was
Rf ( cm2 )
212.0 228.9 96.1 59.8
Cf (F cm−2 )
523.8 409 2816 9038
RL ( cm2 )
L (H cm−2 )
27.15 33.42 735.2 1072
341.4 411 11950 21240
near to Ecorr . The more the applied potential deviated from the Ecorr , the more severe the surface oxide film was damaged. In the anodic potential regions of Ecorr to Ecorr + 50 mV, the decrease of Rt values was due to the dissolution reaction of surface oxide film. Correspondingly, the protective ability of surface oxide film disappeared after the potential reached the film breakdown potential, resulting in the extinction of medium frequency capacitive loop. In the negative cathodic potential regions, the intense hydrogen evolution reaction was possible to destroy the integrity of surface oxide film, corresponding to deteriorate its protection performance. The polarization curve and EIS plots both proved that there was oxide film on the surface of Mg–8Li alloy. But the oxide film was loose and can only show protection performance to some extent near to Ecorr . In a word, a thick but loose surface oxide film was formed on Mg–8Li alloy. The oxide film exhibited a multiple layer structure and lithium oxides enriched at the outer oxide layer. The protection performance of surface oxide film was limited. 4. Conclusions The oxide film was spontaneously formed on the surface of Mg–8Li alloy in the ambient atmosphere. It included four layers: the top layer was the mixture of Mg(OH)2 and Li2 O; the second layer was the mixture of Mg(OH)2 , Li2 O and MgO; the third layer was the mixture of Mg(OH)2 , MgO, LiOH, Li2 O and Mg; the bottom layer was the mixture of MgO, Li2 O, Li and Mg. Lithium oxides enriched at the outer oxide layer, resulting in the loose oxide film on Mg–8Li alloy surface. Thus, the protection performance of the surface oxide film was limited. The film breakdown potential was about Ecorr + 65 mV in 0.1 M NaCl solution. Below the film breakdown potential, the surface film can slow down the corrosion of substrate material. Above the film breakdown potential, the surface film fractured and Mg–8Li alloy suffered severe attack. In the cathodic potential regions, the intense hydrogen evolution reaction destroyed the integrity of surface oxide film. The surface oxide film can only provide limited protection to the Mg–8Li substrate. Acknowledgements This work was supported by the National Basic Research Program of China (no. 2007CB613705) and the Technology R&D Program of Chongqing (no. CSTC, 2007AC4073).
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References [1] M. Santamaria, F.D. Quarto, S. Zanna, P. Marcus, Electrochim. Acta 53 (2007) 1314. [2] S.J. Splinter, N.S. McIntyre, P.A.W. van der Heide, T. Do, Surf. Sci. 317 (1994) 194. [3] N. Hara, Y. Kobayashi, D. Kagaya, N. Akao, Corros. Sci. 49 (2007) 166. [4] J.H. Nordlien, S. Ono, N. Masuko, K. Nisancioglu, J. Electrochem. Soc. 142 (1995) 3320. [5] M. Jonsson, D. Persson, D. Thierry, Corros. Sci. 49 (2007) 1540. [6] G. Song, A. Atrens, D. St John, J. Nairn, Y. Li, Corros. Sci. 39 (1997) 855. [7] R. Lindstrom, L.G. Johansson, G.E. Thompson, P. Skeldon, J.E. Svensson, Corros. Sci. 46 (2004) 1141. [8] X.H. Zhou, Y.W. Huang, Z.L. Wei, Q.R. Chen, F.X. Gan, Corros. Sci. 48 (2006) 4223. [9] F. Rosalbino, E. Angelini, S. De Negri, A. Saccone, S. Delfino, Intermetallics 14 (2006) 1487. [10] F. Czerwinski, Acta Mater. 50 (2002) 2639. [11] J.H. Nordlien, K. Nisancioglu, S. Ono, N. Masuko, J. Electrochem. Soc. 144 (1997) 461.
[12] H. Takuda, T. Enami, K. Kubota, N. Hatta, J. Mater. Process. Technol. 101 (2000) 281. [13] T.C. Chang, J.Y. Wang, C.L. Chu, S. Lee, Mater. Lett. 60 (2006) 3272. [14] J.M. Song, T.X. Wen, J.Y. Wang, Scripta Mater. 56 (2007) 529. [15] J.F. Moulder, W.F. Stickle, P.E. Sobol, K.D. Bomben, Handbook of X-ray Photoelectron Spectroscopy, Physical Electronics Inc., Minnesota, 1995. [16] M.S. Li, High Temperature Oxide of Metal, Metallurgical and Industrial Press, Beijing, 2001. [17] C.N. Cao, Principle of Corrosion Electrochemistry, Chemical Industry Press, Beijing, 2004. [18] J.Q. Zhang, C.N. Cao, Introduction of Electrochemical Impedance Spectroscopy, Science Press, Beijing, 2002. [19] G. Baril, N. Pebere, Corros. Sci. 43 (2001) 471. [20] M. Anik, G. Celikten, Corros. Sci. 49 (2007) 1878. [21] Y.W. Song, D.Y. Shan, R.S. Chen, E.H. Han, Corros. Sci. 51 (2009) 1087. [22] Y.S. Huang, X.T. Zeng, X.F. Hu, Electrochim. Acta 49 (2004) 4313.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Investigation of surface oxide film on magnesium lithium alloy Yingwei Song ∗ , Dayong Shan, Rongshi Chen, En-Hou Han State Key Laboratory for Corrosion and Protection, Institute of Metal Research, Chinese Academy of Sciences, 62 Wencui Road, Shenyang 110016, China
a r t i c l e
i n f o
Article history: Received 20 March 2009 Received in revised form 29 April 2009 Accepted 29 April 2009 Available online 6 May 2009 Keywords: Thin films Corrosion Oxidation Electrochemical impedance spectroscopy
a b s t r a c t Magnesium and its alloys are very chemically active metals. Oxide film can be spontaneously formed on their surface in an ambient atmosphere. The oxide film on the surface of Mg–8Li alloy was characterized by X-ray photoelectron spectroscopy (XPS) analysis and electrochemical measurements. The results indicated that the surface oxide film formed in the air exhibited multiple layer structure, which consisted of the elements of Li, Mg and O. The concentrations of Mg decreased and Li increased from the bottom layer to top layer of the oxide film. Lithium oxides mostly enriched in the outer oxide layer. The more the potential deviated from the Ecorr , the more severe the oxide film was destroyed. Below the film breakdown potential, the surface oxide film can slow down the corrosion of Mg–8Li alloy. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Magnesium and its alloys are very chemically active metals. Oxide film can be spontaneously formed on their surface in the ambient atmosphere. The formation of oxide film is affected by many parameters including alloying elements, medium types, temperature and potential. As a result, there is a great difference at the composition and structure of surface oxide films on various magnesium alloys. The characteristics of surface oxide film have a significant effect on the corrosion behaviors of magnesium and its alloys. There are some researches concerning the surface oxide film on pure Mg and Mg alloys [1–4]. The oxide film formed in the air is thin (20–50 nm) to display amorphous structure, which consists of MgO and Mg(OH)2 . The oxide film formed in the humid air contained outer oxide layer and inner oxide layer. The outer oxide layer is similar to the whole oxide film formed in the air: thin, amorphous and approximate 20–40 nm. The inner oxide layer is a cellular-like hydrated layer as a result of the water molecule penetrating the outer oxide layer to react with the Mg substrate. The oxide film formed in aqueous solution shows a sheet outer oxide layer, which consists of Mg(OH)2 and a small amount of MgO. The inner oxide layer is similar to the whole oxide film formed in the humid air. In other environments containing Cl− , CO2 and SO2 [5–7], MgCl2 , MgCO3 and MgSO4 are also found in the surface oxide film. Generally, the surface oxide film consisting of MgO and Mg(OH)2 is loose and cannot provide enough protection to the Mg substrate
∗ Corresponding author. Tel.: +86 24 23915897; fax: +86 24 23894149. E-mail address: [email protected] (Y. Song). 0925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.04.137
in the corrosive service conditions. However, the alloying elements can influence the structure and chemical composition of the surface oxide film. Especially, the addition of passivating elements improved the protection performance of the oxide film greatly [8,9]. Among plenty of alloying elements, the effects of Al on the characteristics of surface oxide film were investigated deeply [10,11]. For example, the oxide film on AZ91 Mg alloy consisting of MgO, Al2 O3 , Mg(OH)2 and Al(OH)3 displays a multiple layer structure of oxides of Mg/oxides of Mg and Al/substrate. The thickness of oxide film is several decade nanometers, and it decreases with the increase of Al concentration. This result indicates that the alloying elements can affect the physical and chemical properties of the surface oxide film then further affect the corrosion dynamics of Mg alloys. However, the effects of other alloying elements on the characteristics of surface oxide film are still not clear. It is well known that the novel magnesium lithium alloys show super light weight and excellent formability [12–14]. But the metal lithium is more susceptible to be oxidized than the metal magnesium in the air. The effects of Li on the structure and composition of surface oxide film on magnesium lithium alloys belong to an unknown field. The effects of surface oxide film on the corrosion resistance of magnesium lithium alloys are still ambiguous. Thus, the aim of this paper is to investigate the structure, composition and protection performance of the surface oxide film on magnesium lithium alloys. 2. Material and methods The experimental material used for this investigation was Mg–8Li cast alloy whose nominal composition (wt%) was 8% Li and 92% Mg. The samples for XPS analysis and electrochemical measurements were ground to 2000 grit SiC paper, ultrasonically cleaned in acetone, dried in cold air. The metallographic sample was continued to finely polish using 1 m diamond paste, and then was etched with 10% nital etchant (nitric acid mixed in ethanol).
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Y. Song et al. / Journal of Alloys and Compounds 484 (2009) 585–590
The chemical composition of surface oxide film was probed using ESCALAB 250 X-ray photoelectron spectroscopy (XPS) with Mg K␣ radiation (1253.6 eV). The powder was 300 W. All energy values were corrected according to the adventitious C 1s signal, which was set at 284.60 eV. The data were analyzed with Xpspeak 4.1 software. Electrochemical tests were performed using EG&G potentiostat model 273 equipped with a model 5210 lock in amplifier. A classical three-electrode cell was used with platinum as counter electrode, saturated calomel electrode SCE (+0.242 V vs. SHE) as reference electrode, and the samples mounted using epoxy resin with an exposed area of 1 cm2 as working electrode. The initial retard of 300 s was set for all the electrochemical tests. The potentiodynamic curve was measured from −300 to +450 mV vs. OCP at a constant scan rate of 0.5 mV/s. Electrochemical impedance spectroscopy (EIS) was carried out at the frequency scope of 100 kHz to 10 mHz, and the perturbation amplitude was 5 mV. The same sample was applied with various anodic and cathodic potential during EIS testing to study the effects of potential bias on the protection performance of surface oxide film. The corrosive medium of 0.1 M NaCl solution was used for the electrochemical investigation.
3. Results and discussion Fig. 2. XPS survey scanning of the surface oxide film on Mg–8Li alloy.
3.1. Microstructure of Mg–8Li alloy Fig. 1 shows the backscattered image of Mg–8Li alloy. The white irregular regions are the matrix phase named as ␣ phase, which is a solution of lithium in magnesium. The dark regions surrounding ␣ phase are named as  phase, which is a solution of magnesium in lithium. The magnesium and lithium are both very chemically active metals, which are easily oxidized in the ambient environment. Thus, it can make sure that there exists oxide film on the surface of Mg–8Li alloy. The chemical composition and protection performance of the surface oxide film need to be investigated in detail. 3.2. Chemical composition of the surface oxide film The chemical composition of surface oxide film was analyzed with XPS. Fig. 2 shows the XPS survey scanning plot. The surface oxide film consisted of O, Mg, Li and C elements. The H element cannot be detected by XPS. The high concentration of C in the film surface is common in XPS survey scanning due to the adventitious hydrocarbons from the environment. The XPS depth analysis of the surface oxide film is shown in Fig. 3. The XPS plots were analyzed in terms of the Handbook of X-ray Photoelectron Spectroscopy [15]. Because the binding energy of Mg2p is approaching to Li1s, two elements are depicted in one graph. The surface oxide film exhibited a multiple layer structure. The top layer was characterized with the sputtering time of 0 s. The plot of Mg2p + Li1s contained two peaks, corresponding to Mg(OH)2 and Li2 O, respectively. The Mg1s peak was not found. Thus, the top layer of oxide film was composed of Mg(OH)2 and Li2 O. It was obvious that the Li1s peak was stronger
Fig. 1. Backscattered image of Mg–8Li alloy.
than the Mg2p peak, which implied that the lithium oxides enriched at the top layer of oxide film. With increasing the sputtering time to 30 s, the Mg2p + Li1s plot still contained two peaks, corresponding to Mg(OH)2 and Li2 O, but the intensity of Mg2p peak increased. In the meantime, the Mg1s peak was detected, indicating the existence of MgO. Thus, the second layer of oxide film consisted of Mg(OH)2 , Li2 O and MgO. When the sputtering time reached 120 s, there was significant change to the two plots. All the peaks broadened. The Mg2p peak was divided into two narrow peaks of Mg and Mg(OH)2 . The Li1s peak was also characterized with two peaks of Li2 O and LiOH. The Mg1s peak can be described with Mg and MgO. Besides Mg(OH)2 , Li2 O and MgO, Mg and LiOH were also contained at the third layer of oxide film. With increasing the sputtering time to 660 s, the Mg2p peak became narrow and only contained the contribution of Mg. Moreover, Li was found based on the Li1s peak. The Mg1s peak still indicated the existence of MgO and Mg, but the concentration of MgO reduced. The metal Mg was the main component at the bottom layer of oxide film. In the case of XPS depth profile, the surface oxide film contained four layers as shown in Fig. 4. The top layer was the mixture of Mg(OH)2 and Li2 O; the second layer was the mixture of Mg(OH)2 , Li2 O and MgO; the third layer was the mixture of Mg(OH)2 , MgO, LiOH, Li2 O and Mg; the bottom layer was the mixture of MgO, Li2 O, Li and Mg. The peak intensity of Mg element decreased and Li element increased from the bottom layer to the top layer of oxide film, and the lithium oxides mainly enriched at the outer oxide layer. The thickness of surface oxide film was more than 100 nm in light of the calculation by the sputtering time, which was twice thicker than that of on the pure Mg [4]. The growth process of the surface oxide film on Mg–8Li alloy was analyzed as follows. The Mg and Li both show high chemical activity and are very easy to be oxidized into MgO and Li2 O in the air. Then some water molecule in the ambient atmosphere can bond with MgO and Li2 O to form Mg(OH)2 and LiOH, respectively. The thermodynamic functions (H0 ) of MgO and Mg(OH)2 are −601.24 and −924.16 kJ mol−1 , respectively. The very negative H0 values prove that the MgO and Mg(OH)2 can be spontaneously formed in the air and Mg(OH)2 is more stable than MgO. Similarly, the thermodynamic functions (H0 ) of Li2 O and LiOH are −598.73 and −484.d7 kJ mol−1 , respectively, indicating that Li2 O is more stable than LiOH. Thus, the elements of Mg and Li at the top layer of oxide film presented the most stable state of Mg(OH)2 and Li2 O. Because the water molecule in the ambient environment do not penetrate the whole oxide film to reach the Mg substrate, the hydroxides are not contained at the bottom of oxide film. As we know that Li is more susceptible to be oxidized than Mg. Thus, the oxide film on Mg–8Li alloy is thicker than on the pure Mg. But the Pilling/Bedworth ratio of Li2 O is 0.58 [16]. The lithium oxides are loose and cannot form
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Fig. 3. XPS depth profiles of the surface oxide film on Mg–8Li alloy.
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Fig. 4. Sketch map of the surface oxide film on Mg–8Li alloy. The top layer: mixture of Mg(OH)2 and Li2 O; The second layer: mixture of Mg(OH)2 , Li2 O and MgO; The third layer: mixture of Mg(OH)2 , MgO, LiOH, Li2 O and Mg; The bottom layer: mixture of MgO, Li2 O, Li and Mg.
a compact oxide film layer as aluminum oxides. So it is speculated that the protection performance of surface oxide film on Mg–8Li alloy is limited. 3.3. Protection performance of the surface oxide film The protection performance of surface oxide film on Mg–8Li alloy was investigated by polarization curve and EIS plots. Fig. 5 shows the polarization curve of Mg–8Li alloy in 0.1 M NaCl solution. It was found that the anodic side can be divided into three stages. At the stage I (A → B), the corrosion current density slowly increased with the promotion of anodic potential. The curve slope for this stage was 96 mV/decade according to the fitting results of CorrView software. When the potential was near to Ecorr + 65 mV, the anodic curve entered into the stage II (B → C): the corrosion current density quickly increased with the promotion of anodic potential compared with the stage I. The curve slope for this stage was 51.8 mV/decade. The low anodic curve slope implied the fast anodic dissolution rate. Thus, the change of curve slope can be attributed to that there was a protective oxide film on the Mg–8Li alloy surface to slow down the corrosion of substrate material at the stage I. Once the potential reached the film breakdown potential, the surface oxide film fractured and the Mg–8Li substrate was corroded quickly at the stage II. At the stage III (C → D): the corrosion current density nearly kept constant with the increase of anodic potential, which should result from the errors of testing system including the great change of sample surface state and the solution resistance due to the fast corrosion reaction [17]. The cathodic side of polarization curve conformed to the Tafel feature, which was driven by cathodic hydrogen evolution reaction. The corrosion current density gradually diminished when the cathodic potential moved toward Ecorr . It also implied that the
Fig. 5. Polarization curve of Mg–8Li alloy in 0.1 M NaCl solution.
hydrogen generation rate gradually reduced. But the effects of surface oxide film on the hydrogen evolution reaction were not found based on the cathodic curve. The EIS measurements with application of various potentials were performed to investigate the effects of potential bias on the protection performance of surface oxide film. The selected potential values are shown in Fig. 5. At the anodic side, two potential values near to the film breakdown potential, Ecorr + 50 mV (more negative than the film breakdown potential) and Ecorr + 80 mV (more positive than the film breakdown potential), were selected, and another more positive potential value of Ecorr + 100 mV was selected, too. At the cathodic side, the corresponding potential values in the weak polarization regions and strong polarization regions were selected as Ecorr − 50 mV and Ecorr − 120 mV, respectively. The EIS results are shown in Fig. 6. The electrochemical reactions of anodic dissolution and cathodic hydrogen evolution simultaneously occurred on the surface of Mg–8Li alloy at Ecorr . The Nyquist plot comprised three loops, one high frequency capacitive loop, one medium frequency capacitive loop and one short low frequency inductive loop. The high frequency capacitive loop was due to the contribution of electric double layer at the interface of substrate and solution [18]. The medium frequency capacitive loop originated from the diffusion through a porous solid film [19,20], which indicated that the surface oxide film on Mg–8Li alloy was loose. The low frequency inductive loop was attributed to the corrosion nucleation at the initiation stage of localized corrosion [18,21]. The inductive loop only presented at a short frequency range at Ecorr , implying that the Mg substrate did not suffer from severe attack and the oxide film can provide protection to some extent. When the applied anodic potential was more negative than the film breakdown potential, Ecorr + 50 mV, the electrochemical process mainly contained the anodic dissolution reaction. The shape of EIS plot was similar to that of at Ecorr , but its dimension shrank. The shrinkage of high frequency capacitive loop indicated the enhancement of anodic dissolution rate [18]. But the surface oxide film still exhibited protection performance and can continue to restrain the corrosion of Mg–8Li alloy. When the applied anodic potential was more positive than the film breakdown potential, Ecorr + 80 mV, the EIS plot changed greatly. Only one high frequency capacitive loop and one low frequency inductive loop were observed. The medium frequency capacitive loop disappeared and the low frequency inductive loop extended to a large frequency scale, which implied the severe damage of surface oxide film and the initiation of localized corrosion. The surface oxide film suffered severe attack above the film breakdown potential and cannot provide protection to the Mg–8Li substrate again.
Fig. 6. EIS plots of Mg–8Li alloy applied with various potentials in 0.1 M NaCl solution.
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Fig. 7. Equivalent circuits of Mg–8Li alloy applied with various potentials. (a) Ecorr and Ecorr + 50 mV; (b) Ecorr + 80 mV and Ecorr + 100 mV; (c) Ecorr − 50 mV and Ecorr − 120 mV.
Table 1 EIS fitting results of Mg–8Li alloy applied with various potentials. mV
Rs ( cm2 )
Rt ( cm2 )
Y0 (−1 cm−2 s−n × 10−6 )
n
100 80 50 0 −50 −120
31.14 30.09 30.06 37.22 32.45 30.49
125.9 159.8 510.3 788.1 477.3 311.1
98.18 65.51 37.56 33.34 41.92 47.81
0.8163 0.8262 0.8689 0.8733 0.8623 0.8780
After increasing the potential to Ecorr + 100 mV, the dimension of EIS plot slightly shrank but the shape did not change compared with Ecorr + 80 mV. When the applied potential was Ecorr − 50 mV, the Nyquist plot contained one high frequency capacitive loop and one low frequency capacitive loop. The dimension of low frequency capacitive loop was very small. After negatively moving the potential to Ecorr − 120 mV, the shape of EIS plot was similar and only the dimension shrank to some extent. The hydrogen evolution reaction was the main electrochemical process in the cathodic regions. The enhancement of cathodic potential (toward more negative direction) accelerated the hydrogen evolution rate due to the decrease of charge transfer resistance at the electric double layer. Thus, the high frequency capacitive loop shrank in the more negative cathodic regions. In order to investigate the EIS plots deeply, the equivalent circuit for Ecorr and Ecorr + 50 mV is shown in Fig. 7a. Rs was the solution resistance. Rt referred to the charge transfer resistance and CPEdl described the electric double layer capacity. The constant phase element (CPEdl ) was used to compensate for the non-homogeneity in the system [22]. The CPEdl was defined by two values, Y0 and n. If n was equal to 1, CPEdl was identical to a capacitor. Rt in parallel with CPEdl was used to describe the high frequency capacitance loop. The film resistance Rf in parallel with the film capacity Cf characterized the medium frequency capacitance loop, exhibiting the feature of surface oxide film. Inductive resistance RL in series with inductance L described the low frequency inductance loop. When the applied anodic potentials were more positive than the film breakdown potential, Ecorr + 80 mV and Ecorr + 100 mV, the plots can be explained by the equivalent circuit as shown in Fig. 7b. Because the medium frequency capacitive loop disappeared, Rf and Cf were not used in Fig. 7b. The EIS plots with application of cathodic potential are equivalent to Fig. 7c. Two capacitive loops were depicted by two sets of elements of CPEdl //Rt and Rf //Cf , respectively. The RL and L did not present because the anodic dissolution reaction was restrained in the cathodic potential regions. All the EIS data were fitted with ZSimpWin 3.20 software based on the equivalent circuits in Fig. 7. The fitting results are listed in Table 1. In the anodic regions, the Rt values reduced with the increase of potential bias, indicating the enhancement of anodic dissolution reaction. In the cathodic regions, the Rt values also reduced with the increase of potential bias, implying the promotion of hydrogen evolution reaction rate. The CPEdl values showed the reverse tendency in contrast with Rt . The change of CPEdl and Rt with the applied potentials proved that the surface oxide film showed the optimum protective ability to the Mg–8Li alloy substrate when the potential was
Rf ( cm2 )
212.0 228.9 96.1 59.8
Cf (F cm−2 )
523.8 409 2816 9038
RL ( cm2 )
L (H cm−2 )
27.15 33.42 735.2 1072
341.4 411 11950 21240
near to Ecorr . The more the applied potential deviated from the Ecorr , the more severe the surface oxide film was damaged. In the anodic potential regions of Ecorr to Ecorr + 50 mV, the decrease of Rt values was due to the dissolution reaction of surface oxide film. Correspondingly, the protective ability of surface oxide film disappeared after the potential reached the film breakdown potential, resulting in the extinction of medium frequency capacitive loop. In the negative cathodic potential regions, the intense hydrogen evolution reaction was possible to destroy the integrity of surface oxide film, corresponding to deteriorate its protection performance. The polarization curve and EIS plots both proved that there was oxide film on the surface of Mg–8Li alloy. But the oxide film was loose and can only show protection performance to some extent near to Ecorr . In a word, a thick but loose surface oxide film was formed on Mg–8Li alloy. The oxide film exhibited a multiple layer structure and lithium oxides enriched at the outer oxide layer. The protection performance of surface oxide film was limited. 4. Conclusions The oxide film was spontaneously formed on the surface of Mg–8Li alloy in the ambient atmosphere. It included four layers: the top layer was the mixture of Mg(OH)2 and Li2 O; the second layer was the mixture of Mg(OH)2 , Li2 O and MgO; the third layer was the mixture of Mg(OH)2 , MgO, LiOH, Li2 O and Mg; the bottom layer was the mixture of MgO, Li2 O, Li and Mg. Lithium oxides enriched at the outer oxide layer, resulting in the loose oxide film on Mg–8Li alloy surface. Thus, the protection performance of the surface oxide film was limited. The film breakdown potential was about Ecorr + 65 mV in 0.1 M NaCl solution. Below the film breakdown potential, the surface film can slow down the corrosion of substrate material. Above the film breakdown potential, the surface film fractured and Mg–8Li alloy suffered severe attack. In the cathodic potential regions, the intense hydrogen evolution reaction destroyed the integrity of surface oxide film. The surface oxide film can only provide limited protection to the Mg–8Li substrate. Acknowledgements This work was supported by the National Basic Research Program of China (no. 2007CB613705) and the Technology R&D Program of Chongqing (no. CSTC, 2007AC4073).
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