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lanthanum-based composite conversion coatings on AZ31 magnesium alloy
Applied Surface Science 257 (2011) 2838–2842
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Study of molybdenum/lanthanum-based composite conversion coatings on AZ31 magnesium alloy Lihui Yang a,b,∗ , Junqing Li a , Cunguo Lin b , Milin Zhang a , Jianhua Wu b a b
College of Materials Science and Chemical Engineering, Harbin Engineering University, Harbin 150001, China State Key Laboratory for Marine Corrosion and Protection, Luoyang Ship Material Research Institute, Qingdao 266071, China
a r t i c l e
i n f o
Article history: Received 30 June 2010 Received in revised form 13 October 2010 Accepted 15 October 2010 Available online 23 October 2010 Keywords: Magnesium alloy Molybdenum Lanthanum Conversion coating Corrosion resistance
a b s t r a c t The molybdenum/lanthanum-based (Mo/La) composite conversion coating on AZ31 magnesium alloy was investigated and the corrosion resistance was evaluated as well. The morphology, composition and corrosion resistance of the coating were studied by scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and potentiodynamic polarization analysis, respectively. The results revealed that the conversion coating consisted of spherical nodular particles, which was mainly composed of Mo, La, O and Mg. After conversion treatment the corrosion potential shifts about 500 mV positively, and the corrosion current density decreases two orders of magnitude. The corrosion resistance of AZ31 alloy is remarkably improved by Mo/La composite conversion coating. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Magnesium and its alloys are the lightest metallic construction materials. These alloys have unique characteristics of high strength, good thermal conductivity, good magnetic screen and shock resistance ability [1,2]. Therefore, magnesium alloys have great potential for various applications, including automobile and computer parts, aerospace components. However, magnesium alloys have been limited in practice mainly due to undesirable property—poor corrosion resistance [3]. Thus, it is necessary to perform suitable surface treatments to improve the corrosion resistance of Mg alloys. There are a number of surface treatments to protect magnesium alloys, such as conversion treatment, anodizing, electroplating/electroless, organic coatings, gas-phase deposition, silanes and sol–gel technique [4–12]. Among these various surface techniques, the chemical conversion treatment is well known as a process of relatively low cost and ease in operation. The conventional conversion coatings are based on chromium compounds that have been shown to be highly toxic carcinogens. Therefore, it is urgent to develop new environmental-friendly conversion treatments for Mg alloys. There have been developed many non-chromate conver-
∗ Corresponding author at: State Key Laboratory for Marine Corrosion and Protection, Luoyang Ship Material Research Institute, Qingdao 266071, China. Tel.: +86 532 85843220. E-mail address: [email protected] (L. Yang). 0169-4332/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2010.10.077
sion coatings, including phosphate [13], phosphate/permanganate [4], stannate [14], phytic acid [15], cerium and lanthanum based conversion coatings [16,17]. In these non-chromate conversion coatings, however, only phosphate/permanganate conversion coatings have corrosion resistance comparable to chromate treatments [4], the others are still under investigation. In our previous study, a novel and environmental-friendly method was introduced for protecting AZ31 Mg alloy by Mo/La composite conversion technique. The morphology, composition and corrosion resistance of the composite conversion coating were investigated. 2. Experimental procedure The substrate material used was AZ31 magnesium alloy with a size of 10 mm × 10 mm × 1 mm. Prior to formation of conversion coating, specimens were abraded with 1500# SiC paper to obtain an even surface, ultrasonically cleaned using acetone and washed with an alkaline detergent. Then samples were immersed in a bath containing sodium molybdate and lanthanum nitrate for conversion treatment. The samples were rinsed in flow distilled water between each step of the operation. The bath composition and all operation parameters for the pretreatment and conversion treatment are shown in Fig. 1. The surface observation was characterized by SEM (JSM-6480A, Japan Electronics). The beam voltage of SEM system is 20 kV. The composition and chemical state of the coating were examined by X-ray photoelectron spectroscopy (XPS, Physical Electronics, PHI
L. Yang et al. / Applied Surface Science 257 (2011) 2838–2842
2839
Mechanical abrading 1500#SiC paper
Ultrasonic Acetone cleaning 10 min; room temperature
Alkaline degreasing NaOH 10g/L; Na3PO4 12H2O
Mo/La Composite Conversion treatment La(NO3)3 6g/L; Na2MoO4 3g/L; pH=4.00; 25
Hot-air dried Fig. 1. Flow chart of surface treatment process for AZ31 magnesium alloy.
5700 EICA), using an Al K␣ (1486.6 eV) monochromatic source. All spectra were corrected using the signal for C 1s at 284.62 eV as an internal reference. The spatial resolution of XPS is 3 m, while the energetic resolution is 0.1 eV. To evaluate the corrosion performance and possible behavior of the samples, electrochemical measurements were performed on an electrochemical analyzer (IM6ex, Zahner, Germany). Potentiodynamic polarization was conducted in neutral 3.5 wt.% NaCl aqueous solution at room temperature (r.t.). A standard three-compartment cell was used with a saturated calomel electrode (SCE) and a platinum electrode as a reference and counter electrode, respectively. All of the electrodes were cleaned in acetone agitated ultrasonically, rinsed in deionized water before the electrochemical tests. The coated samples were masked with epoxy resins so that only 1 cm2 area was exposed to the electrolyte. During the potentiodynamic sweep experiments, the samples were first immersed into electrolyte for 10 min to stabilize the OCP. The sweeping rate was 10 mV/s for all measurements. 3. Results and discussion
Fig. 2. SEM images of the La/Mo coating.
(Fig. 4(a)). The existence of carbon is common in XPS surface scan due to adventitious hydrocarbons from the environment. Except carbon, the Mo/La conversion coating was mainly composed of Mo, La, O and Mg. Fig. 4(b), (c) and (d) shows the XPS spectra for the single element of Mo, La and O, respectively. The detailed XPS results of surface element compositions of Mo/P conversion coating were shown in Table 1. The distribution of Mo oxidation states was estimated by the deconvolution of Mo 3d spectra (Fig. 4(b)). The Mo 3d spectra typically consisted of two envelopes, a consequence of spin–orbit (j–j)
3.1. Morphology and composition of Mo/La conversion coating 3.1.1. SEM observation A white, non-powdery and uniform coating was formed on the AZ31 magnesium alloy. The coating was connected tightly to the substrate. The morphologies of the Mo/La conversion coating were shown in Fig. 2. A thick and cracked layer with “dry-mud” morphology was observed on the surface of the composite treated samples (Fig. 2(a)). The crack of the surface is the result of water evaporation when the conversion coating is dried. And the Mo/La composite conversion coating displayed a homogeneous spherical nodular microstructure in Fig. 2(b). The conversion coating provides extremely high coverage with thickness of about 5–6 m observed from Fig. 3. The coating was connected tightly to the substrate. 3.1.2. XPS analysis XPS analysis was performed to evaluate the composition of the surface of the coating. Fig. 4 shows the XPS spectra for the conversion coating. XPS survey spectra detected Mo, La, O, Mg and C peaks
Fig. 3. Cross-section morphology of La/Mo coating.
L. Yang et al. / Applied Surface Science 257 (2011) 2838–2842
C1s
La4p
Intensity / a.u.
b
La4d Mo4s O2s Mo4p
OKLL
La3d3/2 La3d5/2
Intensity / a.u.
CKLL
Mg1s
a
Mo3p1/2 Mo3p3/2 MgKLL MgKLL C1s Mo3d
O1s
2840
1200
1000
800
600
400
200
0
240
236
232
228
Binding Energy/eV
Binding Energy/eV
c
d 3+
La
La3d5/2
La3d3/2
860
855
Intensity / a.u.
Intensity / a.u.
sat.
850
845
840
835
830
Binding Energy/eV
536
534
532
530
528
Binding Energy/eV
Fig. 4. XPS spectra of composite coating on the AZ31 Mg alloy (a) survey spectra (b) Mo 3d spectra (c) La 3d spectra (d) O 1s spectra.
Table 1 The XPS results of surface element compositions of Mo/La conversion coatings. Atomic %
Mo
La
O
Mg
Mo/La
14.15
7.29
74.17
4.40
coupling. The Mo 3d5/2 –Mo 3d3/2 doublets were fitted so that each peak had the same Gaussian line shape and width (FWHM). The relative area ratios of spin–orbit doublet peaks are given by the ratio of their respective degeneracies (2j + 1). Therefore, for the Mo 3d5/2 –Mo 3d3/2 doublet the intensity ratio should be I (3d5/2 )/I (3d3/2 ) = 3/2. A splitting energy of ∼3.15 eV was used for the Mo 3d5/2 –Mo 3d3/2 doublet [18]. The doublets at binding energies at 232.4 eV and 235.55 eV (FWHM = 2.56) are consistent with Mo6+ state in molybdenum trioxide [19,20]. The La 3d spectra were depicted in Fig. 4(c). It is well-known that the La 3d states in the XPS spectra show two doublets and the energy peaks appearing on the high energy side of the La 3d5/2 and La 3d3/2 peaks are satellite peaks. Not only are the La 3d states split into two lines, La 3d5/2 and La 3d3/2 , because of a spin–orbit interaction, but also each line is split due to a transfer of an electron Table 2 Corrosion potential and corrosion current density values obtained from the electrochemical polarization curves. Sample
Ecorr (V)
Icorr (A cm2 )
Substrate Composite coating
−1.505 −1.002
1.514 × 10−4 8.596 × 10−6
from O2p to the empty 4f shell of La leading to the 3d9 4f1 final state [21]. The spectra showed the La 3d5/2 and La 3d3/2 ionization and the corresponding satellites. The doublets at binding energies at 835.00 eV and 851.70 eV can be attributed to La3+ , which is almost coincident with the reference value [22]. The La species are oxides, hydroxides or eventually a mixture. The O 1s spectra were depicted in Fig. 4(d). It is well known that peaks at ∼530 eV are due to oxides and the one at ∼532 eV is attributed to oxygen species dissolved in the metal or to adsorbed oxygen or OH− group [19]. Therefore the O 1s lines positioned at 530.4 eV and 529.8 eV are due to oxides networks, the O 1s peak at 531.8 eV is attributed either to oxygen dissolved in the metal or to adsorbed oxygen and OH− group. The 3 oxygen peaks indicate oxide, hydroxide and water or carbonates. It can be concluded that the conversion coating may be mainly composed of lanthanum oxides/hydroxides, molybdenum oxides, and magnesium carbonate/hydroxides.
3.2. Corrosion characteristics of the coatings Corrosion current density (Icorr ) and corrosion potential (Ecorr ) are frequently used to evaluate corrosion resistance of the conversion coating. For the polarization curves, the anodic curve was the important feature related to the corrosion resistance, while the cathode reaction corresponded to the evolution of the hydrogen [23]. Fig. 5 presents the potentiodynamic polarization curves in neutral 3.5% NaCl aqueous solution at r.t. for the AZ31 Mg alloy sub-
L. Yang et al. / Applied Surface Science 257 (2011) 2838–2842
0.01
2841
(a) 7.4
1E-3
(b) pH
I/Acm
1E-4
7.2
1E-5
1E-6
7.0
1E-7 0
10
20
30
40
50
60
70
Time / min -2.5
-2.0
-1.5
-1.0
-0.5
0.0
0.5 Fig. 7. The evolution curve of pH-time while the treated sample was immersed in the neutral NaCl solutions at r.t.
E/V vs.SCE Fig. 5. The potentiodynamic polarization curves of (a) substrate, (b) composite coating in 3.5 wt.% NaCl solutions at r.t.
strate and Mo/La conversion coating. The electrochemical corrosion data could be obtained based on the curves of potentiodynamic polarization in Fig. 5, which are shown in Table 2. From these curves, it can be seen that the corrosion potential Ecorr of the AZ31 Mg alloy substrate and Mo/La conversion coating are −1.505 VSCE and −1.002 VSCE , respectively. There is a wide region the currents almost keep stable. The stable currents are most probably due to diffusion-controlled process, which implied the conversion coating act as a barrier to protect the substrate. The corrosion potential Ecorr of the coated sample shifts positively about 500 mV compared with that of the substrate. The corrosion current density (Icorr ) decreased about two orders of magnitude compared with that of substrate. These results demonstrated the corrosion resistance of AZ31 Mg substrate had been improved through the Mo/La composite conversion treatment. The change of open circuit potential (OCP) can be used to monitor the chemical stability and corrosion process of specimens during immersion [24]. The evolution of the OCP for bare alloy and La/Mo composite conversion coating in 3.5 wt.% NaCl aqueous solution was shown in Fig. 6. The OCP of the bare alloy increased from an initial value of −1.650 VSCE to a stable value of −1.581 VSCE after 4–5 min of immersion, possibly due to dissolution of the substrate followed by precipitation of magnesium hydroxide layer. The OCP of the La/Mo conversion coating decreased rapidly from the initial potential of −1.060 VSCE to a stable value of −1.322 VSCE after
6–7 min of immersion. The relative stable stage of OCP illustrate that there were slow and regular reaction through the conversion coating. A more positive OCP implied a better resistance to the corrosive environment due to a higher equilibrium potential, which make the oxidation process more difficult. The evolution of pH value while the treated sample was immersed in the neutral NaCl solutions was shown in Fig. 7. The results showed that the pH value kept almost stable at the initial stage and finally increased with immersion time. It can be concluded that the conversion coating provided good corrosion protection in the initial several hours. The increase of pH value in later stage may be attributed to the dissolution of Mg and the release of hydrogen. Since the pH value increased, the Mg ions could deposited with OH− . It might cause the rate of the pH value increased more and more slowly. 4. Conclusions In this study, molybdenum/lanthanum-based composite coating was investigated on AZ31 magnesium alloy by simple immersion method. A homogeneous, non-powdery and uniform coating was formed on the AZ31 magnesium alloy. It was found that the coating particles were mainly composed of spherical nodular structure. XPS analysis confirmed that the composite coating was mainly composed of elements O, Mo, La, and Mg. The coating provided good corrosion protection on the AZ31 Mg alloy proved by electrochemical measurements. Acknowledgments
-1.0
(a) Bare alloy (b)Conversion coating
-1.1 -1.2
E / V SCE
(b) -1.3
The authors gratefully acknowledge the financial support of the National Natural Science Foundation of China (No. 50701012) and the Development Program of China (863 Project No. 2006AA03Z511). References
-1.4 -1.5
(a)
-1.6 -1.7 0
100
200
300
400
500
600
Time / sec Fig. 6. The curves of open circuit potential-time for AZ31 Mg alloy without and with La/Mo conversion coating in 3.5 wt.% NaCl solutions at r.t.
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Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Study of molybdenum/lanthanum-based composite conversion coatings on AZ31 magnesium alloy Lihui Yang a,b,∗ , Junqing Li a , Cunguo Lin b , Milin Zhang a , Jianhua Wu b a b
College of Materials Science and Chemical Engineering, Harbin Engineering University, Harbin 150001, China State Key Laboratory for Marine Corrosion and Protection, Luoyang Ship Material Research Institute, Qingdao 266071, China
a r t i c l e
i n f o
Article history: Received 30 June 2010 Received in revised form 13 October 2010 Accepted 15 October 2010 Available online 23 October 2010 Keywords: Magnesium alloy Molybdenum Lanthanum Conversion coating Corrosion resistance
a b s t r a c t The molybdenum/lanthanum-based (Mo/La) composite conversion coating on AZ31 magnesium alloy was investigated and the corrosion resistance was evaluated as well. The morphology, composition and corrosion resistance of the coating were studied by scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and potentiodynamic polarization analysis, respectively. The results revealed that the conversion coating consisted of spherical nodular particles, which was mainly composed of Mo, La, O and Mg. After conversion treatment the corrosion potential shifts about 500 mV positively, and the corrosion current density decreases two orders of magnitude. The corrosion resistance of AZ31 alloy is remarkably improved by Mo/La composite conversion coating. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Magnesium and its alloys are the lightest metallic construction materials. These alloys have unique characteristics of high strength, good thermal conductivity, good magnetic screen and shock resistance ability [1,2]. Therefore, magnesium alloys have great potential for various applications, including automobile and computer parts, aerospace components. However, magnesium alloys have been limited in practice mainly due to undesirable property—poor corrosion resistance [3]. Thus, it is necessary to perform suitable surface treatments to improve the corrosion resistance of Mg alloys. There are a number of surface treatments to protect magnesium alloys, such as conversion treatment, anodizing, electroplating/electroless, organic coatings, gas-phase deposition, silanes and sol–gel technique [4–12]. Among these various surface techniques, the chemical conversion treatment is well known as a process of relatively low cost and ease in operation. The conventional conversion coatings are based on chromium compounds that have been shown to be highly toxic carcinogens. Therefore, it is urgent to develop new environmental-friendly conversion treatments for Mg alloys. There have been developed many non-chromate conver-
∗ Corresponding author at: State Key Laboratory for Marine Corrosion and Protection, Luoyang Ship Material Research Institute, Qingdao 266071, China. Tel.: +86 532 85843220. E-mail address: [email protected] (L. Yang). 0169-4332/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2010.10.077
sion coatings, including phosphate [13], phosphate/permanganate [4], stannate [14], phytic acid [15], cerium and lanthanum based conversion coatings [16,17]. In these non-chromate conversion coatings, however, only phosphate/permanganate conversion coatings have corrosion resistance comparable to chromate treatments [4], the others are still under investigation. In our previous study, a novel and environmental-friendly method was introduced for protecting AZ31 Mg alloy by Mo/La composite conversion technique. The morphology, composition and corrosion resistance of the composite conversion coating were investigated. 2. Experimental procedure The substrate material used was AZ31 magnesium alloy with a size of 10 mm × 10 mm × 1 mm. Prior to formation of conversion coating, specimens were abraded with 1500# SiC paper to obtain an even surface, ultrasonically cleaned using acetone and washed with an alkaline detergent. Then samples were immersed in a bath containing sodium molybdate and lanthanum nitrate for conversion treatment. The samples were rinsed in flow distilled water between each step of the operation. The bath composition and all operation parameters for the pretreatment and conversion treatment are shown in Fig. 1. The surface observation was characterized by SEM (JSM-6480A, Japan Electronics). The beam voltage of SEM system is 20 kV. The composition and chemical state of the coating were examined by X-ray photoelectron spectroscopy (XPS, Physical Electronics, PHI
L. Yang et al. / Applied Surface Science 257 (2011) 2838–2842
2839
Mechanical abrading 1500#SiC paper
Ultrasonic Acetone cleaning 10 min; room temperature
Alkaline degreasing NaOH 10g/L; Na3PO4 12H2O
Mo/La Composite Conversion treatment La(NO3)3 6g/L; Na2MoO4 3g/L; pH=4.00; 25
Hot-air dried Fig. 1. Flow chart of surface treatment process for AZ31 magnesium alloy.
5700 EICA), using an Al K␣ (1486.6 eV) monochromatic source. All spectra were corrected using the signal for C 1s at 284.62 eV as an internal reference. The spatial resolution of XPS is 3 m, while the energetic resolution is 0.1 eV. To evaluate the corrosion performance and possible behavior of the samples, electrochemical measurements were performed on an electrochemical analyzer (IM6ex, Zahner, Germany). Potentiodynamic polarization was conducted in neutral 3.5 wt.% NaCl aqueous solution at room temperature (r.t.). A standard three-compartment cell was used with a saturated calomel electrode (SCE) and a platinum electrode as a reference and counter electrode, respectively. All of the electrodes were cleaned in acetone agitated ultrasonically, rinsed in deionized water before the electrochemical tests. The coated samples were masked with epoxy resins so that only 1 cm2 area was exposed to the electrolyte. During the potentiodynamic sweep experiments, the samples were first immersed into electrolyte for 10 min to stabilize the OCP. The sweeping rate was 10 mV/s for all measurements. 3. Results and discussion
Fig. 2. SEM images of the La/Mo coating.
(Fig. 4(a)). The existence of carbon is common in XPS surface scan due to adventitious hydrocarbons from the environment. Except carbon, the Mo/La conversion coating was mainly composed of Mo, La, O and Mg. Fig. 4(b), (c) and (d) shows the XPS spectra for the single element of Mo, La and O, respectively. The detailed XPS results of surface element compositions of Mo/P conversion coating were shown in Table 1. The distribution of Mo oxidation states was estimated by the deconvolution of Mo 3d spectra (Fig. 4(b)). The Mo 3d spectra typically consisted of two envelopes, a consequence of spin–orbit (j–j)
3.1. Morphology and composition of Mo/La conversion coating 3.1.1. SEM observation A white, non-powdery and uniform coating was formed on the AZ31 magnesium alloy. The coating was connected tightly to the substrate. The morphologies of the Mo/La conversion coating were shown in Fig. 2. A thick and cracked layer with “dry-mud” morphology was observed on the surface of the composite treated samples (Fig. 2(a)). The crack of the surface is the result of water evaporation when the conversion coating is dried. And the Mo/La composite conversion coating displayed a homogeneous spherical nodular microstructure in Fig. 2(b). The conversion coating provides extremely high coverage with thickness of about 5–6 m observed from Fig. 3. The coating was connected tightly to the substrate. 3.1.2. XPS analysis XPS analysis was performed to evaluate the composition of the surface of the coating. Fig. 4 shows the XPS spectra for the conversion coating. XPS survey spectra detected Mo, La, O, Mg and C peaks
Fig. 3. Cross-section morphology of La/Mo coating.
L. Yang et al. / Applied Surface Science 257 (2011) 2838–2842
C1s
La4p
Intensity / a.u.
b
La4d Mo4s O2s Mo4p
OKLL
La3d3/2 La3d5/2
Intensity / a.u.
CKLL
Mg1s
a
Mo3p1/2 Mo3p3/2 MgKLL MgKLL C1s Mo3d
O1s
2840
1200
1000
800
600
400
200
0
240
236
232
228
Binding Energy/eV
Binding Energy/eV
c
d 3+
La
La3d5/2
La3d3/2
860
855
Intensity / a.u.
Intensity / a.u.
sat.
850
845
840
835
830
Binding Energy/eV
536
534
532
530
528
Binding Energy/eV
Fig. 4. XPS spectra of composite coating on the AZ31 Mg alloy (a) survey spectra (b) Mo 3d spectra (c) La 3d spectra (d) O 1s spectra.
Table 1 The XPS results of surface element compositions of Mo/La conversion coatings. Atomic %
Mo
La
O
Mg
Mo/La
14.15
7.29
74.17
4.40
coupling. The Mo 3d5/2 –Mo 3d3/2 doublets were fitted so that each peak had the same Gaussian line shape and width (FWHM). The relative area ratios of spin–orbit doublet peaks are given by the ratio of their respective degeneracies (2j + 1). Therefore, for the Mo 3d5/2 –Mo 3d3/2 doublet the intensity ratio should be I (3d5/2 )/I (3d3/2 ) = 3/2. A splitting energy of ∼3.15 eV was used for the Mo 3d5/2 –Mo 3d3/2 doublet [18]. The doublets at binding energies at 232.4 eV and 235.55 eV (FWHM = 2.56) are consistent with Mo6+ state in molybdenum trioxide [19,20]. The La 3d spectra were depicted in Fig. 4(c). It is well-known that the La 3d states in the XPS spectra show two doublets and the energy peaks appearing on the high energy side of the La 3d5/2 and La 3d3/2 peaks are satellite peaks. Not only are the La 3d states split into two lines, La 3d5/2 and La 3d3/2 , because of a spin–orbit interaction, but also each line is split due to a transfer of an electron Table 2 Corrosion potential and corrosion current density values obtained from the electrochemical polarization curves. Sample
Ecorr (V)
Icorr (A cm2 )
Substrate Composite coating
−1.505 −1.002
1.514 × 10−4 8.596 × 10−6
from O2p to the empty 4f shell of La leading to the 3d9 4f1 final state [21]. The spectra showed the La 3d5/2 and La 3d3/2 ionization and the corresponding satellites. The doublets at binding energies at 835.00 eV and 851.70 eV can be attributed to La3+ , which is almost coincident with the reference value [22]. The La species are oxides, hydroxides or eventually a mixture. The O 1s spectra were depicted in Fig. 4(d). It is well known that peaks at ∼530 eV are due to oxides and the one at ∼532 eV is attributed to oxygen species dissolved in the metal or to adsorbed oxygen or OH− group [19]. Therefore the O 1s lines positioned at 530.4 eV and 529.8 eV are due to oxides networks, the O 1s peak at 531.8 eV is attributed either to oxygen dissolved in the metal or to adsorbed oxygen and OH− group. The 3 oxygen peaks indicate oxide, hydroxide and water or carbonates. It can be concluded that the conversion coating may be mainly composed of lanthanum oxides/hydroxides, molybdenum oxides, and magnesium carbonate/hydroxides.
3.2. Corrosion characteristics of the coatings Corrosion current density (Icorr ) and corrosion potential (Ecorr ) are frequently used to evaluate corrosion resistance of the conversion coating. For the polarization curves, the anodic curve was the important feature related to the corrosion resistance, while the cathode reaction corresponded to the evolution of the hydrogen [23]. Fig. 5 presents the potentiodynamic polarization curves in neutral 3.5% NaCl aqueous solution at r.t. for the AZ31 Mg alloy sub-
L. Yang et al. / Applied Surface Science 257 (2011) 2838–2842
0.01
2841
(a) 7.4
1E-3
(b) pH
I/Acm
1E-4
7.2
1E-5
1E-6
7.0
1E-7 0
10
20
30
40
50
60
70
Time / min -2.5
-2.0
-1.5
-1.0
-0.5
0.0
0.5 Fig. 7. The evolution curve of pH-time while the treated sample was immersed in the neutral NaCl solutions at r.t.
E/V vs.SCE Fig. 5. The potentiodynamic polarization curves of (a) substrate, (b) composite coating in 3.5 wt.% NaCl solutions at r.t.
strate and Mo/La conversion coating. The electrochemical corrosion data could be obtained based on the curves of potentiodynamic polarization in Fig. 5, which are shown in Table 2. From these curves, it can be seen that the corrosion potential Ecorr of the AZ31 Mg alloy substrate and Mo/La conversion coating are −1.505 VSCE and −1.002 VSCE , respectively. There is a wide region the currents almost keep stable. The stable currents are most probably due to diffusion-controlled process, which implied the conversion coating act as a barrier to protect the substrate. The corrosion potential Ecorr of the coated sample shifts positively about 500 mV compared with that of the substrate. The corrosion current density (Icorr ) decreased about two orders of magnitude compared with that of substrate. These results demonstrated the corrosion resistance of AZ31 Mg substrate had been improved through the Mo/La composite conversion treatment. The change of open circuit potential (OCP) can be used to monitor the chemical stability and corrosion process of specimens during immersion [24]. The evolution of the OCP for bare alloy and La/Mo composite conversion coating in 3.5 wt.% NaCl aqueous solution was shown in Fig. 6. The OCP of the bare alloy increased from an initial value of −1.650 VSCE to a stable value of −1.581 VSCE after 4–5 min of immersion, possibly due to dissolution of the substrate followed by precipitation of magnesium hydroxide layer. The OCP of the La/Mo conversion coating decreased rapidly from the initial potential of −1.060 VSCE to a stable value of −1.322 VSCE after
6–7 min of immersion. The relative stable stage of OCP illustrate that there were slow and regular reaction through the conversion coating. A more positive OCP implied a better resistance to the corrosive environment due to a higher equilibrium potential, which make the oxidation process more difficult. The evolution of pH value while the treated sample was immersed in the neutral NaCl solutions was shown in Fig. 7. The results showed that the pH value kept almost stable at the initial stage and finally increased with immersion time. It can be concluded that the conversion coating provided good corrosion protection in the initial several hours. The increase of pH value in later stage may be attributed to the dissolution of Mg and the release of hydrogen. Since the pH value increased, the Mg ions could deposited with OH− . It might cause the rate of the pH value increased more and more slowly. 4. Conclusions In this study, molybdenum/lanthanum-based composite coating was investigated on AZ31 magnesium alloy by simple immersion method. A homogeneous, non-powdery and uniform coating was formed on the AZ31 magnesium alloy. It was found that the coating particles were mainly composed of spherical nodular structure. XPS analysis confirmed that the composite coating was mainly composed of elements O, Mo, La, and Mg. The coating provided good corrosion protection on the AZ31 Mg alloy proved by electrochemical measurements. Acknowledgments
-1.0
(a) Bare alloy (b)Conversion coating
-1.1 -1.2
E / V SCE
(b) -1.3
The authors gratefully acknowledge the financial support of the National Natural Science Foundation of China (No. 50701012) and the Development Program of China (863 Project No. 2006AA03Z511). References
-1.4 -1.5
(a)
-1.6 -1.7 0
100
200
300
400
500
600
Time / sec Fig. 6. The curves of open circuit potential-time for AZ31 Mg alloy without and with La/Mo conversion coating in 3.5 wt.% NaCl solutions at r.t.
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