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Effect of current density on the structure, composition and corrosion resistance of plasma electrolytic oxidation coatings on Mg–Li alloy
Journal of Alloys and Compounds 541 (2012) 380–391
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Effect of current density on the structure, composition and corrosion resistance of plasma electrolytic oxidation coatings on Mg–Li alloy Zhijun Li, Yi Yuan ⇑, Xiaoyan Jing Key Laboratory of Superlight Materials and Surface Technology, College of Materials Science and Chemical Engineering, Harbin Engineering University, Harbin 150001, China
a r t i c l e
i n f o
Article history: Received 5 April 2012 Received in revised form 20 June 2012 Accepted 30 June 2012 Available online 9 July 2012 Keywords: Mg–Li alloy Plasma electrolytic oxidation Current density
a b s t r a c t The effect of current density on the oxidation process, morphology, composition and anti-corrosion properties of coatings are elucidated. X-ray photoelectron spectroscopy and X-ray diffraction analysis of coatings show that coatings prepared at different current densities are composed of MgO and c-Mg2SiO4 and a-Mg2SiO4 phase. The chemical composition of PEO coatings varies from surface to the interior of the oxide coating. The PEO coatings exhibit tunable thickness, composition ratio, and porosity by controlling the current density, which ultimately affects film morphology and anti-corrosion properties. The superior corrosion resistance of coating obtained at 5 A/dm2 is attributed to the compactness of the barrier layer and the highest MgO/Mg2SiO4 ratio. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Owing to the unique characteristics of high strength-to-weight ratio, high dimensional stability, high specific stiffness, good machining property, excellent magnetic screen and shock resistance ability, Mg–Li alloys have application prospects in the fields such as aeronautics, astronautics, weapons, electronics and automobiles [1–4]. However, due to the high chemical reactivity of magnesium and lithium, it is susceptible to localized pitting corrosion, which lead to corrosion occurs with destructive consequences for structural integrity of Mg–Li alloys, especially in aggressive environments. During the last two decades, much effort has been exerted in the development of different methods for improving corrosion resistance of Mg alloys, such as chemical conversion [5–6], electroless plating [7], self-assembled monolayer [8], immersion [9–10], not only for their fundamental scientific interests, but also for their technological applications. However, studies on the corrosion resistance of Mg–Li alloys are seldom reported [11–15]. Furthermore, some of the methods above are subjected to certain limitations, such as severe conditions, complex process and poor durability. Hence, it is specially promising to develop a simple and highly effective approach to delay the onset of corrosion on Mg–Li alloys. Plasma electrolytic oxidation (PEO), as a relatively new and environmentally friendly surface modification technology developed from traditional anodic oxidation, has been used to fabricate coatings in vivo on valve metals such as Al, Mg and Ti [16–18]. The compact PEO coatings that adhere firmly to
⇑ Corresponding author. Tel./ fax: +86 451 82531768 E-mail address: [email protected] (Y. Yuan). 0925-8388/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2012.06.139
the substrate impart excellence properties, such as high hardness, wear resistance, anti-corrosion and thermal stability [19,20]. The properties of PEO coatings are affected by many conditions including composition of electrolyte, temperature of electrolyte, nature of the alloy and electrical parameters, etc. Among which current density is a key determinant which exerts considerable influence upon surface morphology, microstructure and composition of PEO coatings [21–25]. The influence of current density upon the properties of oxide film formed on magnesium alloy via PEO technique has previously been studied [26–28]. In our previous work, we have scrutinized the influence of different electrolytes upon PEO coatings formed on the surface of Mg–Li alloys [29–31]. However, to the best of our knowledge, the effect of current density on the behavior of spark discharges during the PEO process on Mg– Li alloys has not been evaluated systematically. Herein, in this work, we present a systematic investigation on the evident influence of current density upon the microstructure, morphology, chemical and phase composition, corrosion resistance of PEO coatings via a joint analysis of SEM, XRD, XPS, potentiodynamic polarization and electrochemical impedance spectroscopy. The fabrication of compact plasma electrolytic oxidation coatings with excellent corrosion resistance by controlling appropriate electrical parameters is valuable for the widespread applications of Mg–Li alloys. 2. Experimental section 2.1. Materials and specimen preparation The cylinder samples (height: 16 mm, diameter: 15 mm) of Mg–Li alloy (5.6 wt.% Li, 3.37 wt.% Al, 1.68 wt.% Zn, 1.14 wt.% Ce and Mg balance) were used as substrate for the deposition of PEO coatings. Prior to PEO treatment, the specimens were ground and polished with 800, 1000, 2000 grit silicon carbide paper
Z. Li et al. / Journal of Alloys and Compounds 541 (2012) 380–391 to achieve a smooth surface, then ultrasonically cleaned in acetone and rinsed with ethanol, and finally dried in cool air. The alkaline silicate electrolyte was prepared from a solution of Na2SiO3 (10.0 g/L) in distilled water with an addition of NaOH (3.0 g/L) and triethanolamine (10 ml/L). A DC pulsed electrical source was employed to control the voltage, current density and other electrical parameters such as frequency and duty cycle. The sample of Mg–Li alloy and a stainless steel container were used as anode and cathode, respectively. A cooling system was used to keep the temperature of electrolyte at room temperature. The appropriate electrical parameters were as following: frequency: 2000 Hz, duty cycle: 15%. The PEO process was carried out in an alkaline silicate electrolyte for 8 min at the current density of 3, 5 and 8 A/dm2, respectively. After PEO treatment, the coated samples were rinsed thoroughly with distilled water and dried in cool air. Three samples were made at each conditions of current density to ensure the reliability of the experiments. 2.2. Coating characterizations The surface, cross-sectional morphologies and elemental compositions of PEO coatings were examined by JSM-6480A scanning electron microscopy (SEM)
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equipped with EDX and S4800 Field-emission scanning electron microscopy (Hitachi). The thickness of samples was measured with an eddy current coating thickness measurement gauge (TT230, Time Group Inc, China). The thickness data given were the average of ten measurements made at different locations. Micro-hardness of coatings was measured by means of a hardness tester (HXS-1000Z) at a load of 100 g with a loading duration of 15 s from the surface and the cross section of the coating, respectively. Before we performed the measurement on the coating surface, the outer porous layer of oxide coating was removed by SiC paper, and micro-hardness values of two measurements were obtained by averaging the results of 8 measurements. The phase composition of coatings was analyzed by X-ray diffraction (XRD, Philip X’Pert, Holland), using a Cu Ka radiation as the excitation source at a grazing angle of 1°. The measurements were performed with a continuous scanning mode at a rate of 3° min 1. The X-ray photoelectron spectroscopy (XPS) analyses were performed on a ESCALAB-MKII X-ray photoelectron spectrometer (VG Instruments, UK) using monochromatized Al Ka radiation (photon energy 1486.6 eV) as the excitation source and the binding energy of C1s (284.6 eV) as the reference. Xpspeak 4.1 software was used to analyze the data. For corrosion resistance evaluation of coatings, potentiodynamic polarization and EIS measurements were performed on Im6ex electrochemical workstation (Zahner Co., Ltd) with THALES 3.08 software package. All electrochemical measurements were conducted in 3.5 wt.% NaCl solution at room temperature using a conventional three-electrode cell with Mg–Li alloy or coated alloy as the working electrode, a saturated calomel electrode (SCE) as the reference electrode and a platinum plate as the counter electrode. For the potentiodynamic polarization test, scanning at a rate of 2.5 mV/s was from -1.8 to 1.0 V after an initial 10 min delay. For EIS test, the frequency ranged from 100 kHz to 100 mHz with an AC amplitude of 10 mV after 10 min immersion in the electrolyte. ZsimpWin 3.2 software was used for the data fitting of impedance spectra.
3. Results and discussion 3.1. Voltage-time curves for PEO process
Fig. 1. Voltage–time response for PEO processes of Mg–Li alloy.
Fig. 1 shows voltage–time response for PEO processes of Mg–Li alloy formed at different current densities. The instantaneous variation of voltage was recorded every 10 s during the first 3 min and every 30 s after 3 min. Gu et al. [32] fabricated ceramic coatings on
Fig. 2. XPS survey spectra of (a) Mg–Li alloy, (b) high-resolution of Li 1s and Mg 2p spectra of Mg–Li alloy, (c) O 1s spectrum of Mg–Li alloy.
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aluminum alloy by plasma electrolytic oxidation method and investigated the evolvement of discharges aspect and surface morphology of ceramic coatings during PEO process. The voltage–time responses during PEO process can be characterized by four stages
Fig. 3. XPS survey spectra of PEO coatings produced at different current densities.
by analyzing the coating characteristics. On the basis of this literature, four stages can be identified in the voltage-time curves (Fig. 1) during the PEO process. In the first stage, the dissolution of substrate is accompanied by the formation of a thin barrier layer on the surface of Mg–Li alloy, the voltage increases linearly with the time and the voltage gradient is the maximum among the four stages. The dominate current passing through the film increases with the increase in film resistance during the growth of PEO coatings, thus the voltage increases continually to ensure the stability of current density. Then the PEO process enters into the second stage, and a lot of visible small microsparks occur over the whole surface. The voltage at which the first spark appears is defined as the breakdown voltage. For the PEO process in an alkaline silicate electrolyte, the breakage voltage is about 200, 230 and 260 V, respectively. Furthermore, the breakdown voltage of PEO coatings affects the porosity of coatings [33], with the increase in the breakdown voltage, an increase in the size of pores is observed in FE–SEM images shown in Fig. 7. After about 3 min treatment, the voltage slackens down in the second stage and the moving microsparks become larger and phonic, and then the process enters into the third stage. In this stage, the microsparks grow bigger in size and the spark density on the surface comes down significantly.
Fig. 4. High-resolution XPS spectra of Si 2p, O 1s and Mg 2p of PEO coatings formed at different current densities: (a, d, g) 3 A/dm2; (b, e, h) 5 A/dm2; (c, f, i) 8 A/dm2.
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The voltage of the alloy treated in the alkaline silicate electrolyte at the current density of 8 A/dm2 is the highest compare with other two coatings during the PEO process. In the fourth stage, the discharge becomes more steady, and the final voltage for PEO coating formed at 8 A/dm2 (521 V) is higher than those (468 and 484 V) of the other two coatings. The discharge sparks behavior of PEO coating prepared in alkaline silicate electrolyte is similar to our previous work [29,31]. We also observe strong discharge sparks occur at the current density of 8 A/dm2 on the coating surface which is in good agreement with the literature [27]. The average coating thickness for three coatings are 8 ± 1, 13 ± 1, and 22 ± 2 lm, respectively, and the average weight gain are 0.0088 ± 0.002, 0.0151 ± 0.003, and 0.0196 ± 0.018 g, respectively. Therefore, the average coating thickness and weight-gain increase as the applied current density increases. Yerokhin et al. [34] demonstrated that strong discharges would make the coating more porous and increase the coating thickness and roughness, elimination of strong discharge may facilitate the formation of layers with denser microstructure and less porosity. This implies that the current density could affect PEO process and subsequently make differences in the formation and characteristics of coatings.
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3.2. Chemical and phase compositions of PEO coatings X-ray photoelectron spectroscopy (XPS) was used to evaluate the surface compositions of Mg–Li alloy substrate and PEO coatings formed in an alkaline silicate electrolyte at different current densities (3, 5 and 8 A/dm2), respectively. As shown in Fig. 2a, Mg–Li alloy consists of Mg, Li and O elements. As shown in Fig. 2b, the peaks at 48.8 and 54.8 eV are closed related to Mg(OH)2 [35] and LiOH [36], respectively. The O 1s spectrum takes the form of Mg(OH)2 at 531.3 eV [35], as shown in Fig. 2c. These results suggest that oxide (Mg(OH)2 and LiOH) coating can be spontaneously formed on the surface of Mg–Li alloy in an ambient atmosphere due to the chemically active properties of Mg–Li alloy [37]. XPS survey spectra of PEO coatings formed at different current densities (Fig. 3) show the presence of Mg, O, Si and Na element on the surface of PEO coatings. High-resolution XPS spectra of Si 2p, O 1s and Mg 2p of PEO coatings formed at different current densities are presented in Fig. 4. It can be seen that Si 2p spectra are well fitted to c-Mg2SiO4 (102.6 eV) and a-Mg2SiO4 (101.6 eV) for coating formed at 3 A/dm2, c-Mg2SiO4 (102.5 eV) and a-Mg2SiO4 (101.8 eV) for coating formed at 5 A/dm2, c-Mg2SiO4 (102.9 eV)
Fig. 5. High-resolution XPS spectra of Si 2p, O 1s and Mg 2p of polished PEO coatings formed at different current densities: (a, d, g) 3 A/dm2; (b, e, h) 5 A/dm2; (c, f, i) 8 A/dm2.
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and a-Mg2SiO4 (101.5 eV) for coating formed at 8 A/dm2 as shown in Fig. 4a–c, respectively [38]. The amount of surface c-Mg2SiO4 was found to increase with increasing current density, whereas the amount of surface a-Mg2SiO4 decrease with increasing current density. Mg2SiO4 with spinel structure (c-Mg2SiO4) is a high pressure polymorph of forsterite (a-Mg2SiO4). The transformation from a-Mg2SiO4 phase into c-Mg2SiO4 phase occurs at high pressure and high temperature. The photoelectron peak of O 1s centers at 531.6, 531.7 and 531.7 eV is attributed to Mg2SiO4 and MgO for coatings obtained at 3, 5 and 8 A/dm2 (shown in Fig. 4d–f), respectively. The photoelectron peaks of Mg 2p takes the form of Mg2SiO4 (50.0 eV) and MgO (49.0 eV) for coating formed at 3 A/dm2, Mg2SiO4 (50.7 eV) and MgO (49.7 eV) for coating formed at 5 A/dm2, Mg2SiO4 (50.6 eV) and MgO (49.3 eV) for coating formed at 8 A/ dm2, as shown in Fig. 4g–i, respectively. A change in the ratio of Mg 2p peak intensity around 49.0 eV (MgO) and 50.0 eV (Mg2SiO4) is clearly observed. The XPS analysis of Si 2p and O 1s and Mg 2p of PEO coatings suggests the formation of MgO, a-Mg2SiO4 and c-Mg2SiO4 on the surface of Mg–Li alloy. To assess the amount of different substances created on the surface quantitatively, the relative percentage was calculated based on the ratios of peak areas. The ratio of MgO to Mg2SiO4 is calculated by fitting models to be 1.44, 1.94 and 0.47 for PEO coatings formed at 3, 5 and 8 A/dm2, respectively. The coating obtained at 5 A/dm2 has the highest MgO/Mg2SiO4 ratio, whereas the one obtained at 8 A/dm2 affords the lowest MgO/Mg2SiO4 ratio. Besides, we also probe the surface composition of polished PEO coatings without the outer porous layer formed at different current densities. Curve-fitted Mg 2p, Si 2p and O 1s spectra of polished PEO coatings are presented in Fig. 5. Mg 2p, Si 2p and O 1s signals could be assigned to MgO and a-Mg2SiO4 and c-Mg2SiO4, suggesting that the surface compositions of PEO coatings are similar with those of unpolished PEO coatings. Si 2p spectrum (Fig. 5a) takes the form of c-Mg2SiO4 (102.9 eV) and a-Mg2SiO4 (101.7 eV) for coating formed at 3 A/dm2, and the ratio of c-Mg2SiO4 to a-Mg2SiO4 is 0.59, which is higher than that (0.27) of c-Mg2SiO4 to a-Mg2SiO4 ob-
tained from the spectrum shown in Fig. 4a. For polished PEO coating formed at 5 and 8 A/dm2 (Fig. 5b and c), the amount of c-Mg2SiO4 on the surface of polished PEO coating formed at 5 and 8 A/dm2 is comparable to that of a-Mg2SiO4, while for unpolished PEO coatings, the amount of a-Mg2SiO4 is much lower than that of c-Mg2SiO4. Peaks of O 1s at 531.3, 531.6 and 531.6 eV are attributed to Mg2SiO4 and MgO for coatings obtained at 3, 5 and 8 A/dm2 (shown in Fig. 5d–f), respectively. Mg 2p peaks around 50 and 49 eV could be assigned to Mg2SiO4 and MgO for PEO coatings formed at 3, 5 and 8 A/dm2. MgO/Mg2SiO4 ratio is calculated to be 2.49, 1.06 and 1.17, respectively. The coating obtained at 3 A/dm2 has the highest MgO/Mg2SiO4 ratio, whereas the other two coatings obtained at 5 and 8 A/dm2 afford lower MgO/Mg2SiO4 ratio. On the basis of XPS analysis on the surface of unpolished PEO coating and inner barrier layer of PEO coating, we come to the conclusion that the chemical composition of PEO coating varies from surface to the interior of the oxide coating, and the ratio of MgO/ Mg2SiO4 can be controlled by adjusting the current density. XRD studies were further preformed to probe additional insight of Mg–Li alloy and PEO coatings formed at different current densities. According to the results of XRD patterns in Fig. 6, Mg–Li alloy is composed of Mg and intermetallic compound MgZn2, while PEO coatings are mainly composed of MgO, c-Mg2SiO4 and a-Mg2SiO4. The formation mechanisms of MgO and Mg2SiO4 are similar to those described in ref [39]. Besides, we also observed strong peaks corresponding to Mg–Li alloy substrate in the XRD patterns of coatings. This can be explained by two reasons, one is the penetration of X-ray into the substrate due to a porous structure of PEO coating and the other is that oxide coating is not thick enough depending on the chemical composition of coating. With the increasing of current density, the intensity of the characteristic diffraction peaks of crystalline MgO gets stronger and the content of crystalline MgO increases, while XPS analysis demonstrates that MgO reaches maximum value at current density of 5 A/dm2. Hence, it can be inferred that some of MgO on the PEO coating formed at 5 A/cm2 may takes the form of amorphous state. Wang et al. demonstrated that coat-
4000 Mg
α−Mg2SiO4
MgO
γ−Mg2SiO4
MgZn2
2000
d
0 5000
Intensity (cps)
2500
c
0 6000 3000
b
0 7500 5000 2500
a
0 20
30
40
50
60
70
80
2 θ (degree) Fig. 6. XRD patterns of Mg–Li alloy (a) and PEO coatings produced at different current densities (b) 3, (c) 5, (d) 8 A/dm2.
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ing containing non-crystal MgO shows better corrosion resistance [40]. MgO reacts with water to form thermodynamically stable Mg(OH)2 in neutral NaCl solution [41], and much Mg(OH)2 inside the micropores and microcracks could serve as a barrier layer, thus retarding the deterioration of corrosion resistance of oxide coating. According to this point of view, the PEO coating formed at the current density of 5 A/dm2 is expected to exhibit higher corrosion resistance than the rest two coatings, which is in agreement with the anti-corrosion evaluation of the electrochemical test below. 3.3. Morphology characteristics of PEO coatings Surface morphologies and the microstructure details of PEO coatings formed at different current densities were examined by FE–SEM, as presented in Fig. 7. We observe that the surface of PEO coatings prepared in an alkaline silicate electrolyte have porous microstructures and some volcano top-like pores and cracks distribute disorderly on the coating surface. The porous layer has an open structure that permits the penetration of corrosive ions to the substrate of Mg–Li alloy and corrosion proceeds. Pore diameters of three PEO coatings are around 1, 2 and 6 lm, respectively. And the cracks are obviously observed, even going through the pores in coating obtained at 8 A/dm2. Liang et al. [27] studied the effect of current density on the formation of PEO coating on magnesium alloy in a silicate-based electrolyte; they found that the higher energy density led to increased sparking discharge intensity by the pulse energy, which contributes to the enlarged pore size and
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microcracks after quenching by the electrolyte. With the current density increase, the number of pores seems to decrease while the size of pores becomes larger, making a significant change in the surface morphologies of coatings, which is connected to the larger and long-lived spark discharges during the PEO process. Large sparks tend to produce larger craters on the surface, resulting in a coarse and porous coating. While lower current density tends to form fine and less porous microstructure. The surface morphology of the coating prepared at current density of 5 A/dm2 hold the characteristic of intact and homogeneous with less inherent defects including the micropores and microcracks in the microstructure. Energy dispersive spectra of the PEO coatings formed at different current densities are presented in Fig. 8a–c. The composition of PEO coating is mainly composed of Mg, Si, O and a trace of Al. According to the discharge analyses, the coating may grow in two directions, inward and outward to the substrate [19]. Mg and Al originating from the substrate are presumed to have entered the dispersed discharge channels to exist in the coating. The presence of Si and O suggests that the components of electrolyte are intensively incorporated into the plasma chemical oxidation reaction to form PEO coating. The content of Al in all coatings is nearly the same. Mg, O and Si contents are found reaches their maximum value at current density of 5 A/dm2, which also means more MgO and Mg2SiO4 are formed. The elemental mapping is performed to determine the elemental distribution of Mg, O, Si and Al in microstructures. It can be seen that Mg, O, Si and Al signals from sample distribute uniformly on the surface of
Fig. 7. Surface images of PEO coatings formed in an alkaline silicate electrolyte at different current densities ((a, b) 3 A/dm2; (c, d) 5 A/dm2; (e, f) 8 A/dm2).
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Fig. 8. EDX images of PEO coatings formed in an alkaline silicate electrolyte at different current densities (a) 3, (b) 5, (c) 8 A/dm2.
the coatings. It further indicates that both substrate and electrolyte take part in the reaction in discharge channels and contribute to the formation of coating. To further analyze the morphologies of PEO coatings, we perform studies on the cross-sectional of coatings formed in consequence of melting by electric discharges and instantaneous quenching by electrolyte, integrated firmly due to sintering effect, onto the Mg–Li alloy substrate, as shown in Fig. 9. It can be seen that PEO coating is composed of an inner barrier layer and an outer porous layer, the boundary between two layers is clear as shown in backscattered electron images (Fig. 9b, d, f). Fig. 9g, h, i presents the cross-sectional elemental scanning of PEO coatings formed at different current densities. PEO coatings are mainly composed of Mg, O and Si, and the content of Si in the outer porous layer is higher than that in the inner barrier layer, whereas the content of O, Mg and Na are relatively uniform throughout cross-section of oxide coatings. There are micropores and micro-cracks in the cross-section morphologies, but these micropores and microcracks are not connected each other or perforated through the whole oxide film. The micropores are formed by molten oxide and gas bubbles thrown out of discharge channels, while the microcracks result from the molten oxide rapid quenching by the electrolyte. The
coating obtained at the lower current density is more compact and relatively homogeneous; while coating produced at higher current density levels have a higher degree of defects. Compared with the inner layer, the out layer is more porous which would permit more corrosive medium adsorb into the PEO coating and reach the substrate, thus decreasing the corrosion resistance of the coating on Mg–Li alloy. The cross-sectional micrographs show the thickness of coatings fabricated at 3 and 5 A/dm2 are about 7 and 12 lm, respectively, while the thickness of coating formed at 8 A/ dm2 is around 21 m, which corroborates the data obtained from the eddy current coating thickness measurement gauge. This indicates that the coating grows faster at 8 A/dm2, and it is evident that higher current density is responsible for the higher degree of plasma chemical reactions, leading to a larger thickness of PEO coatings and higher level of defects. 3.4. Micro-hardness of PEO coatings Before evaluating the micro-hardness of samples, outer most rough layer of the coating were removed by SiC paper. Fig. 10a shows micro-hardness of Mg–Li alloy and PEO coatings formed in alkaline silicate electrolyte at different current densities. The average micro-hardness values of Mg–Li alloy and PEO coatings formed
Z. Li et al. / Journal of Alloys and Compounds 541 (2012) 380–391
387
Fig. 8 (continued)
at different current densities are 150 ± 3, 376 ± 12, 415 ± 14 and 581 ± 22 HV, respectively. The hardness of coating formed at low current density is lower than that of coatings formed at high current densities. In addition, distribution of micro-hardness of PEO coatings formed at 8 A/dm2 is shown in Fig. 10b, it is clear that the micro-hardness of coating increases from substrate to the inner barrier layer, then it decreases sharply in the outer porous layer, but still higher than that of Mg–Li alloy substrate. The non-uniform distribution of micro-hardness form the substrate to coating surface can be attributed to the non-uniform distribution of pores in the planes parallel to the substrate [28], and the inner barrier layer makes great contribution to the micro-hardness of PEO coating. The difference of micro-hardness is mainly due to the phase composition of PEO coatings. The micro-hardness of Mg2SiO4 is higher than that of MgO [42,43], and the content of Mg2SiO4 increase with the increasing current densities as shown in Fig. 6. Thus the microhardness results are in agreement with XRD results. 3.5. Anti-corrosion behavior of PEO coatings To assess the effect of current density on the anti-corrosion properties of coatings, we characterized Mg–Li alloy and PEO coatings formed in alkaline silicate electrolyte at different current den-
sities by potentiodynamic polarization in the presence of 3.5 wt.% NaCl solution. In a typical polarization curve, lower corrosion current densities, positive corrosion potentials and higher polarization resistance correspond to lower corrosion rates and better corrosion resistance of the coating. The potentiodynamic polarization curves are shown in Fig. 11, and the related parameters including corrosion potentials (Ecorr), corrosion current densities (icorr) and polarization resistance (Rp) are listed in Table 1. In general, the cathodic polarization curves are assumed to represent the cathodic hydrogen evolution through water reduction, while the anodic polarization curves represent the dissolution or destruction behaviors of protective layer [5]. By combining Fig. 11 and Table 1, we notice that, compared with Mg–Li alloy, the corrosion potential shifts to the positive direction (about 257 and 272 mV), the corrosion current density decreases approximately two or three orders, and the polarization resistance increase about two orders of magnitude of PEO coating prepared in an alkaline silicate electrolyte at the current density of 3 and 8 A/dm2, respectively. The corrosion potential of PEO coating at the current density of 5 A/dm2 shifts 295 mV in a positive direction, compared with Mg–Li alloy. And the corrosion current density decreases from 6.46 10 4 to 5.56 10 7 A/cm2, the polarization resistance
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Fig. 8 (continued)
increases from 126 to 38938 X. As we know that the corrosion rate of coatings is generally determined by the corrosion current densities (icorr) and polarization resistance (Rp), it demonstrates that the corrosion resistance of Mg–Li alloy can be enhanced to a certain extent by oxide coating formed in an alkaline silicate electrolyte at the current density of 5 A/dm2, the excellent corrosion resistant properties of PEO coating are confirmed. For the coating is relatively uniform and comparatively compact, the corrosive medium (Cl ) is difficult to penetrate through the outer porous layer quickly and react with the inner barrier layer as well as the substrate, which leads to a significant decrease in corrosion current density, suggesting that for a better corrosion resistance the coating needs to be not only thicker, but should be free from defects as well. Besides, the compositions of PEO coatings also play important role on enhancing the anti-corrosion properties. Fig. 12a-1 and b-1 present bode plots of bare Mg–Li alloy. The behavior of the bare Mg–Li alloy exhibits a distinctly different behavior compared to that of PEO coated specimens. An equivalent circuit model is used to fit the EIS data (Fig. 12c-1). The existence of LF inductance on Mg alloys during the EIS measurement is attributed to the relaxation of adsorbed species such as Mg(OH) or
Mg(OH)2 caused by pitting corrosion [44]. However, due to the chemical reactivity of Li in Mg–Li alloys, the oxidation of Li compositions and the release of their oxidation products are probably the main factor influencing the formation of intensive LF inductance. The EIS data of PEO coatings in 3.5 wt.% NaCl solution are presented in Fig. 12. It can be clearly observed that there are only two time constants, which reflect the responses of porous layer and barrier layer of coatings. The main difference in the Bode plot of the PEO coatings obtained at different current density appears in the low frequency (LF) range of Bode plots, while LF range impedance corresponds to inner layer properties which contribute chiefly to the corrosion resistance of PEO coating. On comparing |Z| vs. frequency of PEO coatings, the corrosion resistance of the alloys increases more than two orders due to the formation of PEO coatings, which is in accordance with the results of polarization resistance listed in Table 1. Higher impedance values reveal higher corrosion protection, whereas lower values account for stronger corrosion activity. To more accurately explain our results in details and consider the similar physical structure of PEO coatings and their impedance response, the equivalent circuit is used to fit the impedance data
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Fig. 9. Cross-sectional, backscattered electron images and EDX line scan of the PEO coatings formed in an alkaline silicate electrolyte at different current densities (a, b, g) 3 A/dm2, (c, d, h) 5 A/dm2, and (e, f, i) 8 A/dm2.
Fig. 11. Potentiodynamic polarization curves of (a) Mg–Li alloy substrate and PEO coatings produced at different current densities (b) 3, (c) 5, (d) 8 A/dm2.
Fig. 10. (a) Micro-hardness of Mg–Li alloy and PEO coating after removing the outer porous layer, (b) Micro-hardness variation from the substrate to the coating surface.
(Fig. 12c-2). Fitted results based on EIS plots of bare Mg–Li alloy and PEO coatings are listed in Table 2. Clearly, the corrosion resistance of Mg–Li alloys improves greatly after PEO treatment, which is in agreement with the polarization test result. For PEO coatings produced at 3 A/dm2, Rb2 is just three quarters of the outer layer resistance Rpo, so the outer layer resistance of PEO coating is the main corrosion-resistant coating. However, the outer layer resistance Rpo obtained at 5 A/dm2 is less than one fifth of Rb2 for PEO coating. Since the resistance of porous layer (Rpo) is insignificant as compared to that of inner barrier layer, the inner layer adjoining the substrate is the main corrosion-resistant layer for PEO coatings. While at 8 A/dm2, the outer layer resistance Rpo is more than three
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Z. Li et al. / Journal of Alloys and Compounds 541 (2012) 380–391 Table 1 Electrochemical parameters related to potentiodynamic polarization curves.
(a)
Sample
Ecorr (V)
Mg–Li alloy a (3 A/dm2) b (5 A/dm2) c (8 A/dm2)
1.68 1.43 1.39 1.41
icorr (A/cm2)
Rp (X)
4
126 20600 38938 32000
6.46 10 3.14 10 5.56 10 6.48 10
6 7 7
Table 2 EIS fitted results. Sample
Rs1( cm2) Qb1(F/cm2) Rb1( cm2)
Mg–Li alloy 50.24 Sample
1.23 10
5
234.90
Rs2(cm2) Qpo(F/cm2) Rpo(cm2)
a (3 A/dm2) 44.81 b (5 A/dm2) 29.84 c (8 A/dm2) 43.40
5.43 10 2.14 10 1.51 10
7 7 7
6.57 103 5.20 103 4.80 103
L(H cm2)
RL( cm2)
206.70
73.60
Qb2(F/cm2)
Rb2(cm2)
9.35 10 2.55 10 4.41 10
6 6 6
4.95 103 2.96 104 7.24 103
(b) 4. Conclusions In summary, PEO coatings have been fabricated onto the surface of the Mg–Li alloy in an alkaline silicate electrolyte at different current densities. X-ray photoelectron spectroscopy and X-ray diffraction analysis of coatings show that coatings formed are composed of MgO and c-Mg2SiO4 and a-Mg2SiO4. The coating formed at 5 A/dm2 has the highest MgO/Mg2SiO4 ratio, whereas the one obtained at 8 A/dm2 affords the lowest MgO/Mg2SiO4 ratio. Current density has a significant influence on the surface morphology, composition, thickness, micro-hardness of PEO coatings. Oxide coating with highest MgO/Mg2SiO4 ratio obtained at 5 A/dm2 offers a superior corrosion resistance in EIS and polarization test. We believe that our work will provide useful clues for further improvement of anti-corrosion properties based on a rational control of electrical parameters.
(c)
Acknowledgements
(1)
Qb1
Rs1
We gratefully acknowledge financial support from Harbin Engineering University Basic Research Foundation (No. HEUFT 07055) and the Foundation for Youth Science and Technology innovation Talents of Harbin of China (No. 2008RFQXG036).
Rb1
L
(2)
RL
Qpo
Rs2
Qb2 Rpo Rb2
Fig. 12. Electrochemical impedance spectroscopy of Mg–Li alloy (1) and PEO coatings produced at different current densities (2) 3, (3) 5, (4) 8 A/dm2; (a) |Z| vs. frequency; (b) Phase angle vs. frequency; (c) Fitted equivalent circuits.
fifths of Rb2 for PEO coating, suggesting the inner layer resistance of PEO coating still plays an important role in corrosion protection. The results of the electrochemical impedance spectroscopy reveal that the coating produced at 5 A/dm2 exhibits the highest corrosion resistance among three coatings, which is attributed to the relatively smooth surface, highest MgO/Mg2SiO4 ratio and more compact barrier layer.
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Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Effect of current density on the structure, composition and corrosion resistance of plasma electrolytic oxidation coatings on Mg–Li alloy Zhijun Li, Yi Yuan ⇑, Xiaoyan Jing Key Laboratory of Superlight Materials and Surface Technology, College of Materials Science and Chemical Engineering, Harbin Engineering University, Harbin 150001, China
a r t i c l e
i n f o
Article history: Received 5 April 2012 Received in revised form 20 June 2012 Accepted 30 June 2012 Available online 9 July 2012 Keywords: Mg–Li alloy Plasma electrolytic oxidation Current density
a b s t r a c t The effect of current density on the oxidation process, morphology, composition and anti-corrosion properties of coatings are elucidated. X-ray photoelectron spectroscopy and X-ray diffraction analysis of coatings show that coatings prepared at different current densities are composed of MgO and c-Mg2SiO4 and a-Mg2SiO4 phase. The chemical composition of PEO coatings varies from surface to the interior of the oxide coating. The PEO coatings exhibit tunable thickness, composition ratio, and porosity by controlling the current density, which ultimately affects film morphology and anti-corrosion properties. The superior corrosion resistance of coating obtained at 5 A/dm2 is attributed to the compactness of the barrier layer and the highest MgO/Mg2SiO4 ratio. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Owing to the unique characteristics of high strength-to-weight ratio, high dimensional stability, high specific stiffness, good machining property, excellent magnetic screen and shock resistance ability, Mg–Li alloys have application prospects in the fields such as aeronautics, astronautics, weapons, electronics and automobiles [1–4]. However, due to the high chemical reactivity of magnesium and lithium, it is susceptible to localized pitting corrosion, which lead to corrosion occurs with destructive consequences for structural integrity of Mg–Li alloys, especially in aggressive environments. During the last two decades, much effort has been exerted in the development of different methods for improving corrosion resistance of Mg alloys, such as chemical conversion [5–6], electroless plating [7], self-assembled monolayer [8], immersion [9–10], not only for their fundamental scientific interests, but also for their technological applications. However, studies on the corrosion resistance of Mg–Li alloys are seldom reported [11–15]. Furthermore, some of the methods above are subjected to certain limitations, such as severe conditions, complex process and poor durability. Hence, it is specially promising to develop a simple and highly effective approach to delay the onset of corrosion on Mg–Li alloys. Plasma electrolytic oxidation (PEO), as a relatively new and environmentally friendly surface modification technology developed from traditional anodic oxidation, has been used to fabricate coatings in vivo on valve metals such as Al, Mg and Ti [16–18]. The compact PEO coatings that adhere firmly to
⇑ Corresponding author. Tel./ fax: +86 451 82531768 E-mail address: [email protected] (Y. Yuan). 0925-8388/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2012.06.139
the substrate impart excellence properties, such as high hardness, wear resistance, anti-corrosion and thermal stability [19,20]. The properties of PEO coatings are affected by many conditions including composition of electrolyte, temperature of electrolyte, nature of the alloy and electrical parameters, etc. Among which current density is a key determinant which exerts considerable influence upon surface morphology, microstructure and composition of PEO coatings [21–25]. The influence of current density upon the properties of oxide film formed on magnesium alloy via PEO technique has previously been studied [26–28]. In our previous work, we have scrutinized the influence of different electrolytes upon PEO coatings formed on the surface of Mg–Li alloys [29–31]. However, to the best of our knowledge, the effect of current density on the behavior of spark discharges during the PEO process on Mg– Li alloys has not been evaluated systematically. Herein, in this work, we present a systematic investigation on the evident influence of current density upon the microstructure, morphology, chemical and phase composition, corrosion resistance of PEO coatings via a joint analysis of SEM, XRD, XPS, potentiodynamic polarization and electrochemical impedance spectroscopy. The fabrication of compact plasma electrolytic oxidation coatings with excellent corrosion resistance by controlling appropriate electrical parameters is valuable for the widespread applications of Mg–Li alloys. 2. Experimental section 2.1. Materials and specimen preparation The cylinder samples (height: 16 mm, diameter: 15 mm) of Mg–Li alloy (5.6 wt.% Li, 3.37 wt.% Al, 1.68 wt.% Zn, 1.14 wt.% Ce and Mg balance) were used as substrate for the deposition of PEO coatings. Prior to PEO treatment, the specimens were ground and polished with 800, 1000, 2000 grit silicon carbide paper
Z. Li et al. / Journal of Alloys and Compounds 541 (2012) 380–391 to achieve a smooth surface, then ultrasonically cleaned in acetone and rinsed with ethanol, and finally dried in cool air. The alkaline silicate electrolyte was prepared from a solution of Na2SiO3 (10.0 g/L) in distilled water with an addition of NaOH (3.0 g/L) and triethanolamine (10 ml/L). A DC pulsed electrical source was employed to control the voltage, current density and other electrical parameters such as frequency and duty cycle. The sample of Mg–Li alloy and a stainless steel container were used as anode and cathode, respectively. A cooling system was used to keep the temperature of electrolyte at room temperature. The appropriate electrical parameters were as following: frequency: 2000 Hz, duty cycle: 15%. The PEO process was carried out in an alkaline silicate electrolyte for 8 min at the current density of 3, 5 and 8 A/dm2, respectively. After PEO treatment, the coated samples were rinsed thoroughly with distilled water and dried in cool air. Three samples were made at each conditions of current density to ensure the reliability of the experiments. 2.2. Coating characterizations The surface, cross-sectional morphologies and elemental compositions of PEO coatings were examined by JSM-6480A scanning electron microscopy (SEM)
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equipped with EDX and S4800 Field-emission scanning electron microscopy (Hitachi). The thickness of samples was measured with an eddy current coating thickness measurement gauge (TT230, Time Group Inc, China). The thickness data given were the average of ten measurements made at different locations. Micro-hardness of coatings was measured by means of a hardness tester (HXS-1000Z) at a load of 100 g with a loading duration of 15 s from the surface and the cross section of the coating, respectively. Before we performed the measurement on the coating surface, the outer porous layer of oxide coating was removed by SiC paper, and micro-hardness values of two measurements were obtained by averaging the results of 8 measurements. The phase composition of coatings was analyzed by X-ray diffraction (XRD, Philip X’Pert, Holland), using a Cu Ka radiation as the excitation source at a grazing angle of 1°. The measurements were performed with a continuous scanning mode at a rate of 3° min 1. The X-ray photoelectron spectroscopy (XPS) analyses were performed on a ESCALAB-MKII X-ray photoelectron spectrometer (VG Instruments, UK) using monochromatized Al Ka radiation (photon energy 1486.6 eV) as the excitation source and the binding energy of C1s (284.6 eV) as the reference. Xpspeak 4.1 software was used to analyze the data. For corrosion resistance evaluation of coatings, potentiodynamic polarization and EIS measurements were performed on Im6ex electrochemical workstation (Zahner Co., Ltd) with THALES 3.08 software package. All electrochemical measurements were conducted in 3.5 wt.% NaCl solution at room temperature using a conventional three-electrode cell with Mg–Li alloy or coated alloy as the working electrode, a saturated calomel electrode (SCE) as the reference electrode and a platinum plate as the counter electrode. For the potentiodynamic polarization test, scanning at a rate of 2.5 mV/s was from -1.8 to 1.0 V after an initial 10 min delay. For EIS test, the frequency ranged from 100 kHz to 100 mHz with an AC amplitude of 10 mV after 10 min immersion in the electrolyte. ZsimpWin 3.2 software was used for the data fitting of impedance spectra.
3. Results and discussion 3.1. Voltage-time curves for PEO process
Fig. 1. Voltage–time response for PEO processes of Mg–Li alloy.
Fig. 1 shows voltage–time response for PEO processes of Mg–Li alloy formed at different current densities. The instantaneous variation of voltage was recorded every 10 s during the first 3 min and every 30 s after 3 min. Gu et al. [32] fabricated ceramic coatings on
Fig. 2. XPS survey spectra of (a) Mg–Li alloy, (b) high-resolution of Li 1s and Mg 2p spectra of Mg–Li alloy, (c) O 1s spectrum of Mg–Li alloy.
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aluminum alloy by plasma electrolytic oxidation method and investigated the evolvement of discharges aspect and surface morphology of ceramic coatings during PEO process. The voltage–time responses during PEO process can be characterized by four stages
Fig. 3. XPS survey spectra of PEO coatings produced at different current densities.
by analyzing the coating characteristics. On the basis of this literature, four stages can be identified in the voltage-time curves (Fig. 1) during the PEO process. In the first stage, the dissolution of substrate is accompanied by the formation of a thin barrier layer on the surface of Mg–Li alloy, the voltage increases linearly with the time and the voltage gradient is the maximum among the four stages. The dominate current passing through the film increases with the increase in film resistance during the growth of PEO coatings, thus the voltage increases continually to ensure the stability of current density. Then the PEO process enters into the second stage, and a lot of visible small microsparks occur over the whole surface. The voltage at which the first spark appears is defined as the breakdown voltage. For the PEO process in an alkaline silicate electrolyte, the breakage voltage is about 200, 230 and 260 V, respectively. Furthermore, the breakdown voltage of PEO coatings affects the porosity of coatings [33], with the increase in the breakdown voltage, an increase in the size of pores is observed in FE–SEM images shown in Fig. 7. After about 3 min treatment, the voltage slackens down in the second stage and the moving microsparks become larger and phonic, and then the process enters into the third stage. In this stage, the microsparks grow bigger in size and the spark density on the surface comes down significantly.
Fig. 4. High-resolution XPS spectra of Si 2p, O 1s and Mg 2p of PEO coatings formed at different current densities: (a, d, g) 3 A/dm2; (b, e, h) 5 A/dm2; (c, f, i) 8 A/dm2.
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The voltage of the alloy treated in the alkaline silicate electrolyte at the current density of 8 A/dm2 is the highest compare with other two coatings during the PEO process. In the fourth stage, the discharge becomes more steady, and the final voltage for PEO coating formed at 8 A/dm2 (521 V) is higher than those (468 and 484 V) of the other two coatings. The discharge sparks behavior of PEO coating prepared in alkaline silicate electrolyte is similar to our previous work [29,31]. We also observe strong discharge sparks occur at the current density of 8 A/dm2 on the coating surface which is in good agreement with the literature [27]. The average coating thickness for three coatings are 8 ± 1, 13 ± 1, and 22 ± 2 lm, respectively, and the average weight gain are 0.0088 ± 0.002, 0.0151 ± 0.003, and 0.0196 ± 0.018 g, respectively. Therefore, the average coating thickness and weight-gain increase as the applied current density increases. Yerokhin et al. [34] demonstrated that strong discharges would make the coating more porous and increase the coating thickness and roughness, elimination of strong discharge may facilitate the formation of layers with denser microstructure and less porosity. This implies that the current density could affect PEO process and subsequently make differences in the formation and characteristics of coatings.
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3.2. Chemical and phase compositions of PEO coatings X-ray photoelectron spectroscopy (XPS) was used to evaluate the surface compositions of Mg–Li alloy substrate and PEO coatings formed in an alkaline silicate electrolyte at different current densities (3, 5 and 8 A/dm2), respectively. As shown in Fig. 2a, Mg–Li alloy consists of Mg, Li and O elements. As shown in Fig. 2b, the peaks at 48.8 and 54.8 eV are closed related to Mg(OH)2 [35] and LiOH [36], respectively. The O 1s spectrum takes the form of Mg(OH)2 at 531.3 eV [35], as shown in Fig. 2c. These results suggest that oxide (Mg(OH)2 and LiOH) coating can be spontaneously formed on the surface of Mg–Li alloy in an ambient atmosphere due to the chemically active properties of Mg–Li alloy [37]. XPS survey spectra of PEO coatings formed at different current densities (Fig. 3) show the presence of Mg, O, Si and Na element on the surface of PEO coatings. High-resolution XPS spectra of Si 2p, O 1s and Mg 2p of PEO coatings formed at different current densities are presented in Fig. 4. It can be seen that Si 2p spectra are well fitted to c-Mg2SiO4 (102.6 eV) and a-Mg2SiO4 (101.6 eV) for coating formed at 3 A/dm2, c-Mg2SiO4 (102.5 eV) and a-Mg2SiO4 (101.8 eV) for coating formed at 5 A/dm2, c-Mg2SiO4 (102.9 eV)
Fig. 5. High-resolution XPS spectra of Si 2p, O 1s and Mg 2p of polished PEO coatings formed at different current densities: (a, d, g) 3 A/dm2; (b, e, h) 5 A/dm2; (c, f, i) 8 A/dm2.
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and a-Mg2SiO4 (101.5 eV) for coating formed at 8 A/dm2 as shown in Fig. 4a–c, respectively [38]. The amount of surface c-Mg2SiO4 was found to increase with increasing current density, whereas the amount of surface a-Mg2SiO4 decrease with increasing current density. Mg2SiO4 with spinel structure (c-Mg2SiO4) is a high pressure polymorph of forsterite (a-Mg2SiO4). The transformation from a-Mg2SiO4 phase into c-Mg2SiO4 phase occurs at high pressure and high temperature. The photoelectron peak of O 1s centers at 531.6, 531.7 and 531.7 eV is attributed to Mg2SiO4 and MgO for coatings obtained at 3, 5 and 8 A/dm2 (shown in Fig. 4d–f), respectively. The photoelectron peaks of Mg 2p takes the form of Mg2SiO4 (50.0 eV) and MgO (49.0 eV) for coating formed at 3 A/dm2, Mg2SiO4 (50.7 eV) and MgO (49.7 eV) for coating formed at 5 A/dm2, Mg2SiO4 (50.6 eV) and MgO (49.3 eV) for coating formed at 8 A/ dm2, as shown in Fig. 4g–i, respectively. A change in the ratio of Mg 2p peak intensity around 49.0 eV (MgO) and 50.0 eV (Mg2SiO4) is clearly observed. The XPS analysis of Si 2p and O 1s and Mg 2p of PEO coatings suggests the formation of MgO, a-Mg2SiO4 and c-Mg2SiO4 on the surface of Mg–Li alloy. To assess the amount of different substances created on the surface quantitatively, the relative percentage was calculated based on the ratios of peak areas. The ratio of MgO to Mg2SiO4 is calculated by fitting models to be 1.44, 1.94 and 0.47 for PEO coatings formed at 3, 5 and 8 A/dm2, respectively. The coating obtained at 5 A/dm2 has the highest MgO/Mg2SiO4 ratio, whereas the one obtained at 8 A/dm2 affords the lowest MgO/Mg2SiO4 ratio. Besides, we also probe the surface composition of polished PEO coatings without the outer porous layer formed at different current densities. Curve-fitted Mg 2p, Si 2p and O 1s spectra of polished PEO coatings are presented in Fig. 5. Mg 2p, Si 2p and O 1s signals could be assigned to MgO and a-Mg2SiO4 and c-Mg2SiO4, suggesting that the surface compositions of PEO coatings are similar with those of unpolished PEO coatings. Si 2p spectrum (Fig. 5a) takes the form of c-Mg2SiO4 (102.9 eV) and a-Mg2SiO4 (101.7 eV) for coating formed at 3 A/dm2, and the ratio of c-Mg2SiO4 to a-Mg2SiO4 is 0.59, which is higher than that (0.27) of c-Mg2SiO4 to a-Mg2SiO4 ob-
tained from the spectrum shown in Fig. 4a. For polished PEO coating formed at 5 and 8 A/dm2 (Fig. 5b and c), the amount of c-Mg2SiO4 on the surface of polished PEO coating formed at 5 and 8 A/dm2 is comparable to that of a-Mg2SiO4, while for unpolished PEO coatings, the amount of a-Mg2SiO4 is much lower than that of c-Mg2SiO4. Peaks of O 1s at 531.3, 531.6 and 531.6 eV are attributed to Mg2SiO4 and MgO for coatings obtained at 3, 5 and 8 A/dm2 (shown in Fig. 5d–f), respectively. Mg 2p peaks around 50 and 49 eV could be assigned to Mg2SiO4 and MgO for PEO coatings formed at 3, 5 and 8 A/dm2. MgO/Mg2SiO4 ratio is calculated to be 2.49, 1.06 and 1.17, respectively. The coating obtained at 3 A/dm2 has the highest MgO/Mg2SiO4 ratio, whereas the other two coatings obtained at 5 and 8 A/dm2 afford lower MgO/Mg2SiO4 ratio. On the basis of XPS analysis on the surface of unpolished PEO coating and inner barrier layer of PEO coating, we come to the conclusion that the chemical composition of PEO coating varies from surface to the interior of the oxide coating, and the ratio of MgO/ Mg2SiO4 can be controlled by adjusting the current density. XRD studies were further preformed to probe additional insight of Mg–Li alloy and PEO coatings formed at different current densities. According to the results of XRD patterns in Fig. 6, Mg–Li alloy is composed of Mg and intermetallic compound MgZn2, while PEO coatings are mainly composed of MgO, c-Mg2SiO4 and a-Mg2SiO4. The formation mechanisms of MgO and Mg2SiO4 are similar to those described in ref [39]. Besides, we also observed strong peaks corresponding to Mg–Li alloy substrate in the XRD patterns of coatings. This can be explained by two reasons, one is the penetration of X-ray into the substrate due to a porous structure of PEO coating and the other is that oxide coating is not thick enough depending on the chemical composition of coating. With the increasing of current density, the intensity of the characteristic diffraction peaks of crystalline MgO gets stronger and the content of crystalline MgO increases, while XPS analysis demonstrates that MgO reaches maximum value at current density of 5 A/dm2. Hence, it can be inferred that some of MgO on the PEO coating formed at 5 A/cm2 may takes the form of amorphous state. Wang et al. demonstrated that coat-
4000 Mg
α−Mg2SiO4
MgO
γ−Mg2SiO4
MgZn2
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d
0 5000
Intensity (cps)
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2 θ (degree) Fig. 6. XRD patterns of Mg–Li alloy (a) and PEO coatings produced at different current densities (b) 3, (c) 5, (d) 8 A/dm2.
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ing containing non-crystal MgO shows better corrosion resistance [40]. MgO reacts with water to form thermodynamically stable Mg(OH)2 in neutral NaCl solution [41], and much Mg(OH)2 inside the micropores and microcracks could serve as a barrier layer, thus retarding the deterioration of corrosion resistance of oxide coating. According to this point of view, the PEO coating formed at the current density of 5 A/dm2 is expected to exhibit higher corrosion resistance than the rest two coatings, which is in agreement with the anti-corrosion evaluation of the electrochemical test below. 3.3. Morphology characteristics of PEO coatings Surface morphologies and the microstructure details of PEO coatings formed at different current densities were examined by FE–SEM, as presented in Fig. 7. We observe that the surface of PEO coatings prepared in an alkaline silicate electrolyte have porous microstructures and some volcano top-like pores and cracks distribute disorderly on the coating surface. The porous layer has an open structure that permits the penetration of corrosive ions to the substrate of Mg–Li alloy and corrosion proceeds. Pore diameters of three PEO coatings are around 1, 2 and 6 lm, respectively. And the cracks are obviously observed, even going through the pores in coating obtained at 8 A/dm2. Liang et al. [27] studied the effect of current density on the formation of PEO coating on magnesium alloy in a silicate-based electrolyte; they found that the higher energy density led to increased sparking discharge intensity by the pulse energy, which contributes to the enlarged pore size and
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microcracks after quenching by the electrolyte. With the current density increase, the number of pores seems to decrease while the size of pores becomes larger, making a significant change in the surface morphologies of coatings, which is connected to the larger and long-lived spark discharges during the PEO process. Large sparks tend to produce larger craters on the surface, resulting in a coarse and porous coating. While lower current density tends to form fine and less porous microstructure. The surface morphology of the coating prepared at current density of 5 A/dm2 hold the characteristic of intact and homogeneous with less inherent defects including the micropores and microcracks in the microstructure. Energy dispersive spectra of the PEO coatings formed at different current densities are presented in Fig. 8a–c. The composition of PEO coating is mainly composed of Mg, Si, O and a trace of Al. According to the discharge analyses, the coating may grow in two directions, inward and outward to the substrate [19]. Mg and Al originating from the substrate are presumed to have entered the dispersed discharge channels to exist in the coating. The presence of Si and O suggests that the components of electrolyte are intensively incorporated into the plasma chemical oxidation reaction to form PEO coating. The content of Al in all coatings is nearly the same. Mg, O and Si contents are found reaches their maximum value at current density of 5 A/dm2, which also means more MgO and Mg2SiO4 are formed. The elemental mapping is performed to determine the elemental distribution of Mg, O, Si and Al in microstructures. It can be seen that Mg, O, Si and Al signals from sample distribute uniformly on the surface of
Fig. 7. Surface images of PEO coatings formed in an alkaline silicate electrolyte at different current densities ((a, b) 3 A/dm2; (c, d) 5 A/dm2; (e, f) 8 A/dm2).
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Fig. 8. EDX images of PEO coatings formed in an alkaline silicate electrolyte at different current densities (a) 3, (b) 5, (c) 8 A/dm2.
the coatings. It further indicates that both substrate and electrolyte take part in the reaction in discharge channels and contribute to the formation of coating. To further analyze the morphologies of PEO coatings, we perform studies on the cross-sectional of coatings formed in consequence of melting by electric discharges and instantaneous quenching by electrolyte, integrated firmly due to sintering effect, onto the Mg–Li alloy substrate, as shown in Fig. 9. It can be seen that PEO coating is composed of an inner barrier layer and an outer porous layer, the boundary between two layers is clear as shown in backscattered electron images (Fig. 9b, d, f). Fig. 9g, h, i presents the cross-sectional elemental scanning of PEO coatings formed at different current densities. PEO coatings are mainly composed of Mg, O and Si, and the content of Si in the outer porous layer is higher than that in the inner barrier layer, whereas the content of O, Mg and Na are relatively uniform throughout cross-section of oxide coatings. There are micropores and micro-cracks in the cross-section morphologies, but these micropores and microcracks are not connected each other or perforated through the whole oxide film. The micropores are formed by molten oxide and gas bubbles thrown out of discharge channels, while the microcracks result from the molten oxide rapid quenching by the electrolyte. The
coating obtained at the lower current density is more compact and relatively homogeneous; while coating produced at higher current density levels have a higher degree of defects. Compared with the inner layer, the out layer is more porous which would permit more corrosive medium adsorb into the PEO coating and reach the substrate, thus decreasing the corrosion resistance of the coating on Mg–Li alloy. The cross-sectional micrographs show the thickness of coatings fabricated at 3 and 5 A/dm2 are about 7 and 12 lm, respectively, while the thickness of coating formed at 8 A/ dm2 is around 21 m, which corroborates the data obtained from the eddy current coating thickness measurement gauge. This indicates that the coating grows faster at 8 A/dm2, and it is evident that higher current density is responsible for the higher degree of plasma chemical reactions, leading to a larger thickness of PEO coatings and higher level of defects. 3.4. Micro-hardness of PEO coatings Before evaluating the micro-hardness of samples, outer most rough layer of the coating were removed by SiC paper. Fig. 10a shows micro-hardness of Mg–Li alloy and PEO coatings formed in alkaline silicate electrolyte at different current densities. The average micro-hardness values of Mg–Li alloy and PEO coatings formed
Z. Li et al. / Journal of Alloys and Compounds 541 (2012) 380–391
387
Fig. 8 (continued)
at different current densities are 150 ± 3, 376 ± 12, 415 ± 14 and 581 ± 22 HV, respectively. The hardness of coating formed at low current density is lower than that of coatings formed at high current densities. In addition, distribution of micro-hardness of PEO coatings formed at 8 A/dm2 is shown in Fig. 10b, it is clear that the micro-hardness of coating increases from substrate to the inner barrier layer, then it decreases sharply in the outer porous layer, but still higher than that of Mg–Li alloy substrate. The non-uniform distribution of micro-hardness form the substrate to coating surface can be attributed to the non-uniform distribution of pores in the planes parallel to the substrate [28], and the inner barrier layer makes great contribution to the micro-hardness of PEO coating. The difference of micro-hardness is mainly due to the phase composition of PEO coatings. The micro-hardness of Mg2SiO4 is higher than that of MgO [42,43], and the content of Mg2SiO4 increase with the increasing current densities as shown in Fig. 6. Thus the microhardness results are in agreement with XRD results. 3.5. Anti-corrosion behavior of PEO coatings To assess the effect of current density on the anti-corrosion properties of coatings, we characterized Mg–Li alloy and PEO coatings formed in alkaline silicate electrolyte at different current den-
sities by potentiodynamic polarization in the presence of 3.5 wt.% NaCl solution. In a typical polarization curve, lower corrosion current densities, positive corrosion potentials and higher polarization resistance correspond to lower corrosion rates and better corrosion resistance of the coating. The potentiodynamic polarization curves are shown in Fig. 11, and the related parameters including corrosion potentials (Ecorr), corrosion current densities (icorr) and polarization resistance (Rp) are listed in Table 1. In general, the cathodic polarization curves are assumed to represent the cathodic hydrogen evolution through water reduction, while the anodic polarization curves represent the dissolution or destruction behaviors of protective layer [5]. By combining Fig. 11 and Table 1, we notice that, compared with Mg–Li alloy, the corrosion potential shifts to the positive direction (about 257 and 272 mV), the corrosion current density decreases approximately two or three orders, and the polarization resistance increase about two orders of magnitude of PEO coating prepared in an alkaline silicate electrolyte at the current density of 3 and 8 A/dm2, respectively. The corrosion potential of PEO coating at the current density of 5 A/dm2 shifts 295 mV in a positive direction, compared with Mg–Li alloy. And the corrosion current density decreases from 6.46 10 4 to 5.56 10 7 A/cm2, the polarization resistance
388
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Fig. 8 (continued)
increases from 126 to 38938 X. As we know that the corrosion rate of coatings is generally determined by the corrosion current densities (icorr) and polarization resistance (Rp), it demonstrates that the corrosion resistance of Mg–Li alloy can be enhanced to a certain extent by oxide coating formed in an alkaline silicate electrolyte at the current density of 5 A/dm2, the excellent corrosion resistant properties of PEO coating are confirmed. For the coating is relatively uniform and comparatively compact, the corrosive medium (Cl ) is difficult to penetrate through the outer porous layer quickly and react with the inner barrier layer as well as the substrate, which leads to a significant decrease in corrosion current density, suggesting that for a better corrosion resistance the coating needs to be not only thicker, but should be free from defects as well. Besides, the compositions of PEO coatings also play important role on enhancing the anti-corrosion properties. Fig. 12a-1 and b-1 present bode plots of bare Mg–Li alloy. The behavior of the bare Mg–Li alloy exhibits a distinctly different behavior compared to that of PEO coated specimens. An equivalent circuit model is used to fit the EIS data (Fig. 12c-1). The existence of LF inductance on Mg alloys during the EIS measurement is attributed to the relaxation of adsorbed species such as Mg(OH) or
Mg(OH)2 caused by pitting corrosion [44]. However, due to the chemical reactivity of Li in Mg–Li alloys, the oxidation of Li compositions and the release of their oxidation products are probably the main factor influencing the formation of intensive LF inductance. The EIS data of PEO coatings in 3.5 wt.% NaCl solution are presented in Fig. 12. It can be clearly observed that there are only two time constants, which reflect the responses of porous layer and barrier layer of coatings. The main difference in the Bode plot of the PEO coatings obtained at different current density appears in the low frequency (LF) range of Bode plots, while LF range impedance corresponds to inner layer properties which contribute chiefly to the corrosion resistance of PEO coating. On comparing |Z| vs. frequency of PEO coatings, the corrosion resistance of the alloys increases more than two orders due to the formation of PEO coatings, which is in accordance with the results of polarization resistance listed in Table 1. Higher impedance values reveal higher corrosion protection, whereas lower values account for stronger corrosion activity. To more accurately explain our results in details and consider the similar physical structure of PEO coatings and their impedance response, the equivalent circuit is used to fit the impedance data
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Fig. 9. Cross-sectional, backscattered electron images and EDX line scan of the PEO coatings formed in an alkaline silicate electrolyte at different current densities (a, b, g) 3 A/dm2, (c, d, h) 5 A/dm2, and (e, f, i) 8 A/dm2.
Fig. 11. Potentiodynamic polarization curves of (a) Mg–Li alloy substrate and PEO coatings produced at different current densities (b) 3, (c) 5, (d) 8 A/dm2.
Fig. 10. (a) Micro-hardness of Mg–Li alloy and PEO coating after removing the outer porous layer, (b) Micro-hardness variation from the substrate to the coating surface.
(Fig. 12c-2). Fitted results based on EIS plots of bare Mg–Li alloy and PEO coatings are listed in Table 2. Clearly, the corrosion resistance of Mg–Li alloys improves greatly after PEO treatment, which is in agreement with the polarization test result. For PEO coatings produced at 3 A/dm2, Rb2 is just three quarters of the outer layer resistance Rpo, so the outer layer resistance of PEO coating is the main corrosion-resistant coating. However, the outer layer resistance Rpo obtained at 5 A/dm2 is less than one fifth of Rb2 for PEO coating. Since the resistance of porous layer (Rpo) is insignificant as compared to that of inner barrier layer, the inner layer adjoining the substrate is the main corrosion-resistant layer for PEO coatings. While at 8 A/dm2, the outer layer resistance Rpo is more than three
390
Z. Li et al. / Journal of Alloys and Compounds 541 (2012) 380–391 Table 1 Electrochemical parameters related to potentiodynamic polarization curves.
(a)
Sample
Ecorr (V)
Mg–Li alloy a (3 A/dm2) b (5 A/dm2) c (8 A/dm2)
1.68 1.43 1.39 1.41
icorr (A/cm2)
Rp (X)
4
126 20600 38938 32000
6.46 10 3.14 10 5.56 10 6.48 10
6 7 7
Table 2 EIS fitted results. Sample
Rs1( cm2) Qb1(F/cm2) Rb1( cm2)
Mg–Li alloy 50.24 Sample
1.23 10
5
234.90
Rs2(cm2) Qpo(F/cm2) Rpo(cm2)
a (3 A/dm2) 44.81 b (5 A/dm2) 29.84 c (8 A/dm2) 43.40
5.43 10 2.14 10 1.51 10
7 7 7
6.57 103 5.20 103 4.80 103
L(H cm2)
RL( cm2)
206.70
73.60
Qb2(F/cm2)
Rb2(cm2)
9.35 10 2.55 10 4.41 10
6 6 6
4.95 103 2.96 104 7.24 103
(b) 4. Conclusions In summary, PEO coatings have been fabricated onto the surface of the Mg–Li alloy in an alkaline silicate electrolyte at different current densities. X-ray photoelectron spectroscopy and X-ray diffraction analysis of coatings show that coatings formed are composed of MgO and c-Mg2SiO4 and a-Mg2SiO4. The coating formed at 5 A/dm2 has the highest MgO/Mg2SiO4 ratio, whereas the one obtained at 8 A/dm2 affords the lowest MgO/Mg2SiO4 ratio. Current density has a significant influence on the surface morphology, composition, thickness, micro-hardness of PEO coatings. Oxide coating with highest MgO/Mg2SiO4 ratio obtained at 5 A/dm2 offers a superior corrosion resistance in EIS and polarization test. We believe that our work will provide useful clues for further improvement of anti-corrosion properties based on a rational control of electrical parameters.
(c)
Acknowledgements
(1)
Qb1
Rs1
We gratefully acknowledge financial support from Harbin Engineering University Basic Research Foundation (No. HEUFT 07055) and the Foundation for Youth Science and Technology innovation Talents of Harbin of China (No. 2008RFQXG036).
Rb1
L
(2)
RL
Qpo
Rs2
Qb2 Rpo Rb2
Fig. 12. Electrochemical impedance spectroscopy of Mg–Li alloy (1) and PEO coatings produced at different current densities (2) 3, (3) 5, (4) 8 A/dm2; (a) |Z| vs. frequency; (b) Phase angle vs. frequency; (c) Fitted equivalent circuits.
fifths of Rb2 for PEO coating, suggesting the inner layer resistance of PEO coating still plays an important role in corrosion protection. The results of the electrochemical impedance spectroscopy reveal that the coating produced at 5 A/dm2 exhibits the highest corrosion resistance among three coatings, which is attributed to the relatively smooth surface, highest MgO/Mg2SiO4 ratio and more compact barrier layer.
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