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phosphate composite conversion coating on magnesium alloy surface for corrosion protection
Applied Surface Science 255 (2008) 1672–1680
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Molybdate/phosphate composite conversion coating on magnesium alloy surface for corrosion protection Zhiyi Yong, Jin Zhu, Cheng Qiu, Yali Liu * College of Chemistry and Chemical Engineering, Hunan University, Changsha 410082, China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 29 March 2007 Received in revised form 1 February 2008 Accepted 18 April 2008 Available online 3 May 2008
In this paper, a new conversion coating—molybdate/phosphate (Mo/P) coating on magnesium alloy was prepared and investigated by electrochemical impedance spectra (EIS), scanning electron microscope (SEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and salt-water immersion experiments, respectively. The results demonstrated that the Mo/P coating contained composite phases, which were consisted of metaphosphate as well as molybdate oxide with an ‘‘alveolate-crystallized’’ structure. The composite Mo/P conversion coating had better corrosion resistance performance than molybdate (Mo) coating, and even had almost comparable corrosion protection for Mg alloy to the traditional chromate-based coating. ß 2008 Published by Elsevier B.V.
Keywords: Magnesium alloy Molybdate Molybdate/phosphate Conversion coating
1. Introduction Magnesium and its alloys have advantages of low density and high strength/weight ratio and can be used as ideal structural materials for automotives, computers, mobile telephones, aerospaces and a number of other light-weight engineering applications [1,2]. However, their poor corrosion resistance limits their applications significantly [3]. Therefore, how to treat the surfaces of magnesium alloys is crucial to enhance the corrosion resistance and to expend their application fields. The conventional treatment method uses chromium compounds-based conversion coatings, which are obviously starving for environment-friendly substitutes to provide requisite protection of the magnesium alloy surfaces since chromate-containing materials are extremely toxic carcinogens. In fact, there have been developed quite several nonchromate conversion coatings for the chemical conversion processing of magnesium alloys, including alkaline stannate [5], cerium-based aqueous solution [6], rare earth metal [7], fluorozirconate [8], phosphate/permanganate [4], even pure phosphate based conversion coatings [10–12]. In these non-chromate conversion coatings, however, only phosphate/permanganatebased conversion coating is occasionally used to substitute
* Corresponding author. Tel.: +86 731 8821081; fax: +86 731 8713642. E-mail address: [email protected] (Y. Liu). 0169-4332/$ – see front matter ß 2008 Published by Elsevier B.V. doi:10.1016/j.apsusc.2008.04.095
chromate-based conversion coatings for low required corrosion resistance of magnesium alloys since it does not have as good protection against corrosion as the chromate containing coating, the others are still under investigation. In this paper, we tried to prepare molybdate/phosphate-based (Mo/P) conversion coatings for the corrosion resistance of magnesium alloys. Although molybdate (Mo) is widely used as an alternative for corrosion protection on zinc and steel substrates, it is very little reported as the conversion coatings of Mg alloy surfaces [13–15]. The objective of this research was to explore whether Mo/P conversion coating worked for the corrosion resistance of Mg alloy surfaces or not in comparison with Mo conversion coating and chromate-based coating. 2. Experimental 2.1. Sample preparation Mg alloy (AM60, wt%: Al 5.6–6.4, Zn 0.2, Mn 0.26–0.5, Fe 0.004, Si 0.05, Cu 0.008, Ni 0.001and other metal impurity 0.01) was used as received. The Mg alloy sample was cut into 1-mm-thick disks and 5-mm-thick rods with 10 mm and 20 mm diameters for microstructure characterizations and salt-water immersion experiments of conversion coatings, respectively. The rods with 10 mm diameter embedded in an epoxy resin holder were used as working electrodes for electrochemical measurements.
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2.2. Preparation of conversion coating on Mg alloy
3. Results and discussion
The surfaces of Mg alloy were abraded with emery papers of 600–1200 grit, followed by rinsed with deionized water, and then ultrasonically degreased in acetone for 3–5min. The Mo and Mo/P conversion coatings were formed on Mg alloy surfaces at 50 8C by simply immersing Mg alloy substrates into the conversion baths with the typical composition as shown in Table 1. The H3PO4 and HNO3 in the recipe were used to adjust the pH value of the conversion baths. For the sake of comparison in salt immersion test, chromatebased coating was also prepared by immersing Mg alloy disk into the chromate-based bath containing 200 g L 1K2Cr2O7 and 180 mL L 1HNO3 for 120 s at ambient temperature [4].
3.1. The preparation of Mo and Mo/P conversion coatings
2.3. Electrochemical measurements Electrochemical measurements were performed using a CHI606B electrochemical workstation supplied by Shanghai Chenhua Instruments Inc. in a standard three-electrode cell of 500 ml at ambient temperature. A naked or Mo or Mo/P coated Mg alloy electrode was used as the working electrode. The counter electrode was a semi-cylindrical platinum slice with a surface area of 10 cm2 and the reference electrode was a saturated calomel electrode (SCE) and all of the measured potentials were referred to this electrode. Anodic and cathodic polarization experiments were carried out at a scan rate of 0.5 mV/s in borate buffer solution. Electrochemical impedance spectra (EIS) were obtained at the open circuit potential (OCP) in the frequency range from 10 kHz to 10 mHz. Prior to working the electrodes deposited with conversion solution were immersed in working solution containing 0.93 g/l H3BO4 and 9.86 g/l Na2B4O7 at pH 9.2 for 2 h [12]. The potential (E)–time (T) curve was used to elucidate the growth process of the conversion coating forming on the magnesium substrate and were recorded with the increase of conversion time. The working electrode was immersed for 5 min in Mo/P conversion bath and the start potential of potentiodynamic scans is the corrosion potential.
3.1.1. Effect of the molar ratios of MoO42 /H2PO4 Polarization curves of the coatings obtained from the different molar ratios of MoO42 /H2PO4 were demonstrated in Fig. 1 and the corresponding corrosion potential and corrosive current were tabulated in Table 2. It could be seen that the molar ratio of MoO42 /H2PO4 had a significant impact on the corrosion behavior. The corrosion current, icorr, reduced by near 2 orders of magnitude (1.26 mA in 4:1 to 0.005 mA in 1:2) and the corrosion potential, Ecorr, rose to around 250 mV when the molar ratio of MoO42 /H2PO4 decreased from 4:1 to 1:2 (see curves a–d), and the cathodic branches showed a marked passivation region because of deposition of metaphosphate. But when the content of H2PO4 continued to increase, the corrosive current started to increase (see curves e and f). Fig. 2 further revealed the corresponding SEM micrographs evolution with decreasing molar ratios of MoO42 /H2PO4 . Obviously, the microstructure of the conversion coatings strongly depended on the molar ratios of MoO42 /H2PO4 . When scant H2PO4 was used (see Fig. 2a and b), the coatings displayed a ‘‘dryriverbed’’ microstructure with many cracks. When the ratio of MoO42 /H2PO4 reached 1:1 (see Fig. 2c and d), the coatings displayed an alveolate microstructure. When excessive H2PO4 was used (see Fig. 2e and f), the coatings began to display many cracks in the film. According to the polarization curves, an alveolate microstructure without cracks could be achieved only at the molar ratio of 1/2 for MoO42 /H2PO4 (see Fig. 2d), suggesting the bath with this molar ratio could provide the optimum
2.4. Surface morphology and chemistry Scanning electron microscope (Philips XL30, Japan) was operated at 26.0 kV to observe the surface morphology. X-ray diffraction (XRD) (D8 Advance, Bruker/Axs Company, Germany) was used to determine the phase structure of the conversion coatings on the alloy. The XRD measurements were carried out at 40 kV of tube voltage, 30 mA of tube current with a curve graphite crystal monochomator and a board focus copper source. The samples were scanned at 1–908of scanning range at 2u. XPS experiments were performed on PHI 5000C ESCA system (PE Company, America) using Al K( radiation (1486.6 eV) at power of 250 W. The pass energy was set 93.9 eV and the binding energies were calibrated by using contaminant carbon at B.E. = 284.6 eV. The data analysis was carried out by using the RBD Auger Scan 3.21 software connected with XPS instrument.
Table 1 Composition and concentration of different conversion baths for Mg alloy Bath
Constitutes and concentration
Bath Mo
Na2MoO4 30 g L 1 + Ca(NO3)2 4 g L +additive 1 g L 1 NaNO3 1 g L 1 + HNO3
Bath Mo/P
Bath Mo + H3PO4 + NaH2PO440 g L
1
1
+ Mn(Ac)2 6 g L
1
Fig. 1. Polarization curves of molybdate/phosphate coatings with different molar ratios of MoO42 /H2PO4 : (a) 4:1, (b) 2:1, (c) 1:1, (d) 1:2, (e) 1:5, (f) 1:8.
Table 2 The corrosion potential and corrosive current of the Mo/P coatings prepared with various molar ratios of MoO42 /H2PO4 corresponding to Fig.1 Curves
Ratio of MoO42 /H2PO4
a b c d e f
4:1 2:1 1:1 1:2 1:5 1:8
Ecorr (V) 1.24 1.12 1.03 0.98 1.08 1.17
log icorr(A) 5.9 6.8 7.3 8.3 6.9 6.7
icorr(mA) 1.26 0.158 0.05 0.005 0.126 0.199
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Fig. 2. SEM micrographs of the Mo/P coatings prepared with various molar ratios of MoO42 /H2PO4 : (a) 4:1, (b) 2:1, (c) 1:1, (d) 1:2, (e) 1:5, (f) 1:8.
corrosion resistance for Mg alloy, and was employed in the following studies on Mo/P coatings. 3.1.2. Effect of pH and treating time Potential (E)–time (T) curves can be used to monitor the formation of the conversion coating [17]. Fig. 3 illustrated the potential-time curves of the Mo/P coating formation from the bath Mo/P with different pH. It was found that all plots exhibited a similar trend, that is, the values of open circuit potential (OCP) decreased in initial several seconds and then increased until the OCP values leveled off. Nevertheless, the absolute OCP value was strongly related to the pH value of the conversion bath. The highest OCP value
of approximate 1010 mV occurred at pH 5.0 (see Fig. 3 e) after 300 s immersion. As for the cases at pH 4.0 and 6.0, around 1378 mV and 1335 mV OCP values were obtained, respectively, and the lowest potential of about 1780 mV was achieved at pH 3.0, which was closed to the OCP value of the naked electrode (approx. 1900 mV, obtained in borate solution at pH 9.2). The pH dependence of OCP values could be ascribed to the numbers of cathodic sites induced by the acidity of bath Mo/P. Herein, the precipitates of metal salts acted as the main cathodic sites. At pH 6.0, the acid-induced cathodic sites were not sufficient to completely cover the whole surface of Mg alloy substrate. On the contrary, at pH 4.0 or 3.0, the excessive cathodic sites led to form the inhomogeneous and disorder coating structure
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Fig. 3. The potential-time curves of the conversion coating at bath Mo/P 1/2 MoO42 /H2PO4 molar ratio at different pH: (a) for the naked electrode, (b) pH 3.0, (c) pH 4.0, (d) pH 6.0, (e) pH 5.0.
because of the over-speedy deposition. Only at pH 5.0, the homogeneous and integrated conversion coating could form, causing the highest OCP value. The initial decrease of the OCP was probably caused by the surface activation of substrate, dissolution of substrate in acidic medium accompanying with simultaneous hydrogen evolution, while the minimum OCP value should be associated with the finish of the initial surface activation and the increase of the potential values with the beginning of the coating deposition. The plateau suggested that the reactions at the interface reached the steady state and the Mo/P coating covered the entire surface as possible as it could. From Fig. 3, it could be seen the immersion time to reach the plateau increased with the decrease of pH, which
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was likely due to the rise of the activation of Mg alloy surface at low pH, increasing the hydrogen evolution, thus the initial film deposition was delayed. At high pH, the hydrogen evolution was weakening, causing a faster film deposition. This behavior was in agreement with the tungstate-phosphoric coating on Zn [16] and phosphate coating on steel [17]. Therefore, pH value strongly influenced both the quality of conversion coating and conversion speed. Fig. 4 demonstrated the SEM micrographs of the conversion coatings prepared at different immersion time in Mo/P solution and their corresponding locations on the dynamic potential-time curve. It was found that some dark nests were enchased at the surface after 10 s immersion (see Fig. 4a) while an alveolate microstructure could be observed after 30 s immersion (see Fig. 4b) and the honeycomb was becoming larger and larger with increasing immersion time (see Fig. 4c–e), and the brims of nests were developing gradually into highlighting grinding after 60 s immersion (see Fig. 4c and d). The potential jumped from 1320 mV to 1150 mV for immersion time increasing from 10 s to 30 s, and slowly increased after 60 s. Based on these experimental results, it could be supposed that the growth mechanism of Mo/P coating was a corrosion/ precipitation process as follows: metal ions from substrate, such as Mg2+, Al3+, etc. were first electrochemically etched from the Mg alloy substrate on micro-anodic area. Meanwhile, metal ions in conversion bath with positive charge, such as Ca2+, Mn2+ etc. (Ca and Mn come from the molybdate/phosphate conversion bath), and Mg2+, Al3+ tended to transfer to micro-cathode and further reacted with MoO42 or H2PO4 to form insoluble metaphosphate deposited on Mg alloy surface after a short incubation time (approximately 6 s, herein). The surface coverage increased quickly with the rapid growth of individual crystals on Mg alloy surface, resulting in a mixed protection on the surface of Mg alloy.
Fig. 4. The SEM micrographs of Mo/P coating with 1/2 MoO42 /H2PO4 molar ratio at different immersion time (a. 6 s; b. 30 s; c. 60 s; d. 150 s and e. 300 s, at 50 8C and pH 5.0) and their corresponding locations on the dynamic potential-time curve.
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Fig. 5. SEM micrographs of conversion coatings on Mg alloy samples for 5 min immersion (a) in bath-Mo and (b) bath-Mo/P with 1/2 MoO42 /H2PO4 molar ratio.
3.2. The surface morphology and composition of treated Mg alloy 3.2.1. SEM analysis Fig. 5 presented the SEM micrographs of the Mg substrates after immersed in bath Mo and bath Mo/P for 5 min. The Mo coatings displayed network-like cracks structure, which was possibly caused by the release of hydrogen for the chemical reaction during the conversion treatment and/or the dehydration of the surface layer after treating in acidic medium. According to previous studies, a-phase (Mg), and b-phase (Mg17Al12), in Mg alloy could cause local corrosive cell effect, which accelerated the dissolving of the a-phase[4,9]. The precipitation of insoluble salts tended to proceed at or along the grain boundary and the precipitate was nobler than the a-phase [9]. On the contrary, the Mo/P coating showed a homogeneous alveolate structure. 3.2.2. XPS analysis XPS is usually employed to identify the specific electron binding energies of elements at the surface of conversion coating and the main reacting products [18,19]. Fig. 6 illustrated the survey spectra of the Mo and Mo/P conversion coatings obtained by XPS and Table 3 showed the detailed XPS results of surface element compositions of Mo and Mo/P conversion coatings 1/2 MoO42 / H2PO4 molar ratio. For the Mo coating, Mg, Al, Mo, O, Ca and N were detected as the major elements. But for the Mo/P coating, two more major elements, Mn and P were detected. The main element contents were calculated by the XPS peak areas and summarized in Table 2. Comparing with Mo coating, obvious reductions in Mg, Mo, Ca and N and visible increases in P, Mn and O were observed for Mo/P coating, further confirming that metaphosphate played a dominant role on the deposition of converting products. A more detailed XPS analyses of specific electron binding energies of elements were illustrated in Fig. 7. According to the data previously reported [20], the XPS peaks of each element should be resulted from the corresponding compounds. Thus, it could be supposed that MgAl2O4, MoO3, MgO, Al2O3 and a little of CaMoO4 probably constituted the Mo conversion coating, while Mex(PO4)y, MgAl2O4, MgO, Al2O3, MnOOH, MnO, CaMoO4 and MoO3 probably composed of the Mo/P conversion coating. 3.3. Phase analysis of conversion coatings Fig. 8 illustrated the XRD spectra of non-treated (as received) and conversion-coated Mg substrates (see the left of Fig. 8). The a-
Fig. 6. XPS spectra of (a) Mo coating and (b) Mo/P coatings with 1/2 MoO42 /H2PO4 molar ratio for 5 min immersion pretreatment.
Table 3 The XPS results of surface element compositions of Mo and Mo/P conversion coatings 1/2 MoO42 /H2PO4 molar ratio Atomic %
P
Mg
Al
Mo
O
Ca
Mn
N
Mo Mo/P
0.16 11.35
12.45 5.80
2.64 1.76
11.83 4.63
55.92 65.04
4.11 2.42
– 9.00
12.89 –
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Fig. 7. XPS intensities of Al2p, Ca2p, Mg2p, O1s, Mo3d, P2p, Mn2 of conversion coatings coated in bath Mo and bath Mo/P with 1/2 MoO42 /H2PO4 molar ratio for 5 min immersion pretreatment.
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Fig. 8. On the left XRD spectra for non-treated sample (as received) and the Mo and Mo/P conversion coatings on Mg alloy substrates and on the right an amplified XRD spectra at 2u in 10 30˚ for the Mo and Mo/P coating with 1/2 MoO42 /H2PO4 molar ratio for 5 min immersion pretreatment.
phase (Mg) and b-phase (Mg17Al12) were observed in both nontreated and treated substrates. But, at 2u 308, there were several ‘‘broad’’ peaks observed for the Mo conversion coating (see the two amplified curves on the right of Fig. 8). This was the marked characteristic of amorphous structure, in other words, the Mo conversion coating was amorphous. However, for the Mo/P conversion coating, a couple of aculeated peaks were seen, suggesting that the Mo/P conversion coating had crystal structure. Since these peaks were not the characteristic peaks of Mn3(PO4)2, Ca3(PO4)2, or other metal insoluble salts, we would assume that the Mo/P coating was composed of complicated phases, such as Mex(PO4)y, CaMoO4, MgAl2O4, MgO, Al2O3 and MnO. This Mo/P process seemed to be very similar to the phosphatizing process currently used in steel surface [17]. The difference was that the Mo/P coatings contained a lot of netlike oxides, such as MoO3, to provide additional corrosion protection for Mg substrate. 3.4. The corrosion performance of the conversion coatings 3.4.1. Anodic and cathodic polarization plots Fig. 9 compared the polarization plots of Mo and Mo/P conversion coatings. The corrosion potential value increased from 1200 mV for bath Mo to 980 mV for bath Mo/P, and the current density for bath Mo/P was two orders of magnitude lower than that for bath Mo. Moreover, the current increasing speed of anodic branch of bath Mo
was slower than that of its cathodic branch, which was just reverse and a passivation behavior was observed in the cathodic branch for Mo/P process. This indicated that the addition of phosphate significantly increased the potential and decreased the corrosive current, suggesting that the corrosion resistance of molydbate coating could be obviously improved by addition of the phosphate. 3.4.2. The EIS results Fig. 10 displayed the EIS plots of the naked electrode and the electrodes treated with Mo and Mo/P processes with different time, respectively. All plots were obtained at the corrosion potential immediately after the stabilization of the OCP in borate buffer solution with pH 9.2. These Nyquist plots exhibited outright two capacitive loops, one for high and intermediate frequencies and another for low frequencies. The impedance plots were interpreted by electrical equivalent circuit described in Fig. 11 [1], in which Rs, Rcf, Rp, Cdl and Cc were designated as solution resistances, conversion coating resistance, polarization resistance, capacitance of the double layer and capacitance of the conversion coating, respectively. Because of the Mg alloy substrate containing several kinds of metal elements, such as Mg, Al and Mn, the outer surface of alloy should have a layered structure being composed of metal oxide and the Nyquist plot of naked electrode should show two semicircles, which was validated by Fig. 10a, indicating a very anticipant agreement between the experiment and the advance assumption. So, the same
Table 4 The calculated parameters of equivalent circuit corresponding to Fig.10 Nyquist plots of naked electrode (a) and Mo coating (b) and Mo/P coating (c and d) with 1/2 MoO42 /H2PO4 molar ratio coated electrodes in borate buffer solution at different conversion pretreatment time
In bath Mo
In bath Mo/P
Fig. 9. Polarization curves of conversion coating formed in bath Mo and bath Mo/P with 1/2 MoO42 /H2PO4 molar ratio, respectively, at 50˚ C for 5 min immersion pretreatment.
Naked electrode
Time (s)
Rcf (kV cm2)
Rp (kV cm2)
Cdl (F cm
15 60 120 240 300 480 60 120 240 300 360 480 –
1.53 2.00 2.28 2.21 4.09 4.44 8.75 25.96 56.72 97.76 727.28 510.90 0.03
0.71 3.73 5.13 6.44 9.23 7.95 261.98 561.20 698.64 943.31 3390.67 3119.07 0.12
4.18E-3 2.27E-3 7.65E-4 5.94E-4 1.63E-4 1.91E-4 3.91E-4 2.68E-5 8.23E-5 1.93E-5 2.49E-6 1.04E-6 8.45E-3
2
)
Rs (V cm2) 43.8 39.7 13.4 65.9 63.8 35.8 45.6 41.6 42.3 35.3 55 36.8 29
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Fig. 10. The Nyquist plots of naked electrode (a) and Mo coating (b) and Mo/P coating (c and d) with 1/2 MoO42 /H2PO4 molar ratio coated electrodes in borate buffer solution at different conversion pretreatment time.
actual corrosion area, it was easily concluded from the Cdl data that the corrosion area of Mg alloy reduced after treated in bath Mo as well as bath Mo/P. Moreover, the corrosion area in bath Mo/P decreased more than that in bath Mo, namely, the Mo/P conversion coating could provide better corrosion protection than the Mo conversion coating. This could be also confirmed based on their Rp values. The Mo/P conversion coating had much higher Rp than the Mo coating as well as phosphate coating on Mg alloy (740 Vcm2) [11,12]. These indicated that the composite Mo/P conversion coating was a new promising for the effective protection of Mg alloys. Fig. 11. The electrical equivalent circuit of conversion coatings used in CHI606B software.
electrical equivalent circuit could be used to fit the impedance spectra of naked electrode. The values of the circuit elements were calculated by CHI606B software and summarized in Table 4. The maximum polarization resistance values (Rp) and the minimum capacitance values (Cdl) were obtained at the immersion conversion pretreatment time of 5 min and 6 min for the Mo and Mo/P conversion coatings, respectively. Since the Cdl is related to the
3.4.3. Salt-water immersion experiments The Mg alloy samples coated with Mo/P coating, Mo coating and chromate coating were immersed into 3.5% NaCl aqueous solution at 30%. Table 4 presented the experimental results. From Table 5, it could be seen that the Mo conversion coating had the worst corrosion resistance performance and was wrecked only for 2 h in chlorine medium. From the piled corrosion products of the Mo coating, it was likely that the chlorine ions had reached the surface of the underlying substrate, suggesting that Mo coating could only afford a transitory corrosion protection for Mg alloys.
Table 5 Conversion coatings Mo coating Mo/P coating Chromate coating
Tb/h
Sc/St (72h)
Appearance before immersion
Appearance after 72 h immersion
2
4/5
Golden-brown
22 24
2/5 1/3
Uniform grey Uniform grey
A lot of red-brown corrosion products piled on the surface Wirelike corrosion on the surface Peeling speckles in middle surface
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However, Mo/P conversion coating had almost equal corrosion protection to chromate coating based on the values of Tb and Sc/St, which further indicated that the addition of phosphate in molybdate-based solution improved the corrosion protection for Mg alloy significantly. 4. Conclusion Based on this study, it could be concluded that the composite molybdate/phosphate (Mo/P) conversion coating had better corrosion resistance performance than simplex molybdate conversion or phosphating coating for Mg alloy surface. EIS results and SEM observation showed the molar ratios of MoO42 /H2PO4 and pH values of conversion solution had obvious influences on the corrosion resistance performance of conversion coatings. Scant or excessive H2PO4 caused a coating with ‘‘dry-mud’’ microstructure and tinpot corrosion resistance. Relative high or low pH value could not cause homogeneous and integrated conversion coating. XPS and XRD analyses indicated that the Mo/P coating possibly contained composite phases which were consisted of Mex(PO4)y, CaMoO4, MgAl2O4, MgO, Al2O3, MnO, as well as molybdate oxide with an ‘‘alveolate-crystallized’’ structure. Electrochemical measurements and salt water immersion experiments displayed that composite Mo/P conversion coating had better corrosion resistance performance than Mo coating, and even had almost comparable corrosion protection for Mg alloy to the traditional chromate pretreatment. Acknowledgement The authors would like to thank the support of the Natural Science Foundation of Hunan Provice, China (Grant no. 06B0073). References [1] J.E. Gray, B. Luan, Protective coatings on magnesium and its alloys – a critical review, J. Alloys Compd. 336 (2002) 88–113.
[2] B.L. Mordike, T. Ebert, Magnesium properties – applications-potential, Mater. Sci. Eng. A 302 (2001) 37–45. [3] C.G. Silva, A.N. Correia, P.D. Lima-Neto, I.C.P. Margarit, O.R. Mattos, Study of conversion coatings obtained from tungstate-phosphoric acid solutions, Corros. Sci. 47 (2005) 709–722. [4] K.Z. Chong, T.S. Shih, Conversion-coating treatment for magnesium alloys by a permanganate-phosphate solution, Mater. Chem. Phys. 80 (2003) 191– 200. [5] M.A. Gonzalez-nunez, C.A. Nunez-lopez, P. Skeldon, A non-chromate conversion coating for magnesium alloys and magnesium-based metal matrix composites, Corros. Sci. 37 (1995) 1763–1772. [6] Katya Brunelli, Manuele Dabala, Irene Calliari, et al., Effect of HCl pre-treatment on corrosion resistance of cerium-based conversion coatings on magnesium and magnesium alloys, Corros. Sci. 47 (2005) 989–1000. [7] X.W. Yu, C.N. Cao, Z.M. Yao, D.R. Zhou, Z.D. Yin, Corrosion behavior of rare earth metal (REM) conversion coatings on aluminum alloy LY12, Mater. Sci. Eng. A 284 (2000) 56–63. [8] M. Naoharu, Formation of corrosion resistant coating film on magnesium alloy member, JP9031664A2 (1997). [9] H. Umehara, S. Takaya, Terauchi Chrome-free surface treatments for magnesium alloy, Surf. Coat. Technol. 169–170 (2003) 666–669. [10] F. Takayuki, K. Isao, Pretreatment of magnesium or magnesium alloy material before coating, JP6330341A2 (1994). [11] L. Kouisni, M. Azzi, F. Dalard, S. Maximovitch, Phosphate coatings on magnesium alloy AM60 part 1: study of the formation and the growth of zinc phosphate films, Surf. Coat. Technol. 185 (2004) 58–67. [12] L. Kouisni, M. Azzi, F. Dalard, S. Maximovitch, Phosphate coatings on magnesium alloy AM60 Part 2: Electrochemical behavior in borate buffer solution, Surf. Coat. Technol. 192 (2005) 239–246. [13] A.A.O. Magalhaes, I.C.P. Margarit, O.R. Mattos, Molybdate conversion coatings on zinc surfaces, J. Electroanal. Chem. 572 (2004) 433–440. [14] H. Konno, K. Narumi, H. Habzaki, Molybdate/Al (&) composite films on steel and zinc-plated steel by chemical conversion, Corros. Sci. 44 (2002) 1889– 1900. [15] E. Almeida, T.C. Diamantino, M.O. Figueiredo, Oxidising alternative species to chromium VI in zinc galvanised steel, Surf. Coat. Technol. 106 (1998) 8–17. [16] C.G. da Silva, A.N. Correia, P.D. Lima-Neto, Study of conversion coatings obtained form tungstate-phosphoric acid solutions, Corros. Sci. 47 (2005) 709–722. [17] D.B. Freeman, Phosphating and Metal Pre-treatments, Woodhead-Faulkener: Bucks, 1986. [18] S. Feliu Jr., V. Barranco, XPS study of the surface chemistry of conventional hot-dip galvanized pure Zn, galvanneal and Zn-Al alloy coatings on steel, Acta Mater. 51 (2003) 5413–5424. [19] M.A. Arensa, J.J. de, Damborenea, Surface characterization of cerium layers on galvanized steel, Surf. Coat. Technol. 187 (2004) 320–325. [20] C.D. Wagner, D.E. Bickham, NIST X-Ray Photoelectron Spectroscopy Database NIST (1989).
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Molybdate/phosphate composite conversion coating on magnesium alloy surface for corrosion protection Zhiyi Yong, Jin Zhu, Cheng Qiu, Yali Liu * College of Chemistry and Chemical Engineering, Hunan University, Changsha 410082, China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 29 March 2007 Received in revised form 1 February 2008 Accepted 18 April 2008 Available online 3 May 2008
In this paper, a new conversion coating—molybdate/phosphate (Mo/P) coating on magnesium alloy was prepared and investigated by electrochemical impedance spectra (EIS), scanning electron microscope (SEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and salt-water immersion experiments, respectively. The results demonstrated that the Mo/P coating contained composite phases, which were consisted of metaphosphate as well as molybdate oxide with an ‘‘alveolate-crystallized’’ structure. The composite Mo/P conversion coating had better corrosion resistance performance than molybdate (Mo) coating, and even had almost comparable corrosion protection for Mg alloy to the traditional chromate-based coating. ß 2008 Published by Elsevier B.V.
Keywords: Magnesium alloy Molybdate Molybdate/phosphate Conversion coating
1. Introduction Magnesium and its alloys have advantages of low density and high strength/weight ratio and can be used as ideal structural materials for automotives, computers, mobile telephones, aerospaces and a number of other light-weight engineering applications [1,2]. However, their poor corrosion resistance limits their applications significantly [3]. Therefore, how to treat the surfaces of magnesium alloys is crucial to enhance the corrosion resistance and to expend their application fields. The conventional treatment method uses chromium compounds-based conversion coatings, which are obviously starving for environment-friendly substitutes to provide requisite protection of the magnesium alloy surfaces since chromate-containing materials are extremely toxic carcinogens. In fact, there have been developed quite several nonchromate conversion coatings for the chemical conversion processing of magnesium alloys, including alkaline stannate [5], cerium-based aqueous solution [6], rare earth metal [7], fluorozirconate [8], phosphate/permanganate [4], even pure phosphate based conversion coatings [10–12]. In these non-chromate conversion coatings, however, only phosphate/permanganatebased conversion coating is occasionally used to substitute
* Corresponding author. Tel.: +86 731 8821081; fax: +86 731 8713642. E-mail address: [email protected] (Y. Liu). 0169-4332/$ – see front matter ß 2008 Published by Elsevier B.V. doi:10.1016/j.apsusc.2008.04.095
chromate-based conversion coatings for low required corrosion resistance of magnesium alloys since it does not have as good protection against corrosion as the chromate containing coating, the others are still under investigation. In this paper, we tried to prepare molybdate/phosphate-based (Mo/P) conversion coatings for the corrosion resistance of magnesium alloys. Although molybdate (Mo) is widely used as an alternative for corrosion protection on zinc and steel substrates, it is very little reported as the conversion coatings of Mg alloy surfaces [13–15]. The objective of this research was to explore whether Mo/P conversion coating worked for the corrosion resistance of Mg alloy surfaces or not in comparison with Mo conversion coating and chromate-based coating. 2. Experimental 2.1. Sample preparation Mg alloy (AM60, wt%: Al 5.6–6.4, Zn 0.2, Mn 0.26–0.5, Fe 0.004, Si 0.05, Cu 0.008, Ni 0.001and other metal impurity 0.01) was used as received. The Mg alloy sample was cut into 1-mm-thick disks and 5-mm-thick rods with 10 mm and 20 mm diameters for microstructure characterizations and salt-water immersion experiments of conversion coatings, respectively. The rods with 10 mm diameter embedded in an epoxy resin holder were used as working electrodes for electrochemical measurements.
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2.2. Preparation of conversion coating on Mg alloy
3. Results and discussion
The surfaces of Mg alloy were abraded with emery papers of 600–1200 grit, followed by rinsed with deionized water, and then ultrasonically degreased in acetone for 3–5min. The Mo and Mo/P conversion coatings were formed on Mg alloy surfaces at 50 8C by simply immersing Mg alloy substrates into the conversion baths with the typical composition as shown in Table 1. The H3PO4 and HNO3 in the recipe were used to adjust the pH value of the conversion baths. For the sake of comparison in salt immersion test, chromatebased coating was also prepared by immersing Mg alloy disk into the chromate-based bath containing 200 g L 1K2Cr2O7 and 180 mL L 1HNO3 for 120 s at ambient temperature [4].
3.1. The preparation of Mo and Mo/P conversion coatings
2.3. Electrochemical measurements Electrochemical measurements were performed using a CHI606B electrochemical workstation supplied by Shanghai Chenhua Instruments Inc. in a standard three-electrode cell of 500 ml at ambient temperature. A naked or Mo or Mo/P coated Mg alloy electrode was used as the working electrode. The counter electrode was a semi-cylindrical platinum slice with a surface area of 10 cm2 and the reference electrode was a saturated calomel electrode (SCE) and all of the measured potentials were referred to this electrode. Anodic and cathodic polarization experiments were carried out at a scan rate of 0.5 mV/s in borate buffer solution. Electrochemical impedance spectra (EIS) were obtained at the open circuit potential (OCP) in the frequency range from 10 kHz to 10 mHz. Prior to working the electrodes deposited with conversion solution were immersed in working solution containing 0.93 g/l H3BO4 and 9.86 g/l Na2B4O7 at pH 9.2 for 2 h [12]. The potential (E)–time (T) curve was used to elucidate the growth process of the conversion coating forming on the magnesium substrate and were recorded with the increase of conversion time. The working electrode was immersed for 5 min in Mo/P conversion bath and the start potential of potentiodynamic scans is the corrosion potential.
3.1.1. Effect of the molar ratios of MoO42 /H2PO4 Polarization curves of the coatings obtained from the different molar ratios of MoO42 /H2PO4 were demonstrated in Fig. 1 and the corresponding corrosion potential and corrosive current were tabulated in Table 2. It could be seen that the molar ratio of MoO42 /H2PO4 had a significant impact on the corrosion behavior. The corrosion current, icorr, reduced by near 2 orders of magnitude (1.26 mA in 4:1 to 0.005 mA in 1:2) and the corrosion potential, Ecorr, rose to around 250 mV when the molar ratio of MoO42 /H2PO4 decreased from 4:1 to 1:2 (see curves a–d), and the cathodic branches showed a marked passivation region because of deposition of metaphosphate. But when the content of H2PO4 continued to increase, the corrosive current started to increase (see curves e and f). Fig. 2 further revealed the corresponding SEM micrographs evolution with decreasing molar ratios of MoO42 /H2PO4 . Obviously, the microstructure of the conversion coatings strongly depended on the molar ratios of MoO42 /H2PO4 . When scant H2PO4 was used (see Fig. 2a and b), the coatings displayed a ‘‘dryriverbed’’ microstructure with many cracks. When the ratio of MoO42 /H2PO4 reached 1:1 (see Fig. 2c and d), the coatings displayed an alveolate microstructure. When excessive H2PO4 was used (see Fig. 2e and f), the coatings began to display many cracks in the film. According to the polarization curves, an alveolate microstructure without cracks could be achieved only at the molar ratio of 1/2 for MoO42 /H2PO4 (see Fig. 2d), suggesting the bath with this molar ratio could provide the optimum
2.4. Surface morphology and chemistry Scanning electron microscope (Philips XL30, Japan) was operated at 26.0 kV to observe the surface morphology. X-ray diffraction (XRD) (D8 Advance, Bruker/Axs Company, Germany) was used to determine the phase structure of the conversion coatings on the alloy. The XRD measurements were carried out at 40 kV of tube voltage, 30 mA of tube current with a curve graphite crystal monochomator and a board focus copper source. The samples were scanned at 1–908of scanning range at 2u. XPS experiments were performed on PHI 5000C ESCA system (PE Company, America) using Al K( radiation (1486.6 eV) at power of 250 W. The pass energy was set 93.9 eV and the binding energies were calibrated by using contaminant carbon at B.E. = 284.6 eV. The data analysis was carried out by using the RBD Auger Scan 3.21 software connected with XPS instrument.
Table 1 Composition and concentration of different conversion baths for Mg alloy Bath
Constitutes and concentration
Bath Mo
Na2MoO4 30 g L 1 + Ca(NO3)2 4 g L +additive 1 g L 1 NaNO3 1 g L 1 + HNO3
Bath Mo/P
Bath Mo + H3PO4 + NaH2PO440 g L
1
1
+ Mn(Ac)2 6 g L
1
Fig. 1. Polarization curves of molybdate/phosphate coatings with different molar ratios of MoO42 /H2PO4 : (a) 4:1, (b) 2:1, (c) 1:1, (d) 1:2, (e) 1:5, (f) 1:8.
Table 2 The corrosion potential and corrosive current of the Mo/P coatings prepared with various molar ratios of MoO42 /H2PO4 corresponding to Fig.1 Curves
Ratio of MoO42 /H2PO4
a b c d e f
4:1 2:1 1:1 1:2 1:5 1:8
Ecorr (V) 1.24 1.12 1.03 0.98 1.08 1.17
log icorr(A) 5.9 6.8 7.3 8.3 6.9 6.7
icorr(mA) 1.26 0.158 0.05 0.005 0.126 0.199
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Fig. 2. SEM micrographs of the Mo/P coatings prepared with various molar ratios of MoO42 /H2PO4 : (a) 4:1, (b) 2:1, (c) 1:1, (d) 1:2, (e) 1:5, (f) 1:8.
corrosion resistance for Mg alloy, and was employed in the following studies on Mo/P coatings. 3.1.2. Effect of pH and treating time Potential (E)–time (T) curves can be used to monitor the formation of the conversion coating [17]. Fig. 3 illustrated the potential-time curves of the Mo/P coating formation from the bath Mo/P with different pH. It was found that all plots exhibited a similar trend, that is, the values of open circuit potential (OCP) decreased in initial several seconds and then increased until the OCP values leveled off. Nevertheless, the absolute OCP value was strongly related to the pH value of the conversion bath. The highest OCP value
of approximate 1010 mV occurred at pH 5.0 (see Fig. 3 e) after 300 s immersion. As for the cases at pH 4.0 and 6.0, around 1378 mV and 1335 mV OCP values were obtained, respectively, and the lowest potential of about 1780 mV was achieved at pH 3.0, which was closed to the OCP value of the naked electrode (approx. 1900 mV, obtained in borate solution at pH 9.2). The pH dependence of OCP values could be ascribed to the numbers of cathodic sites induced by the acidity of bath Mo/P. Herein, the precipitates of metal salts acted as the main cathodic sites. At pH 6.0, the acid-induced cathodic sites were not sufficient to completely cover the whole surface of Mg alloy substrate. On the contrary, at pH 4.0 or 3.0, the excessive cathodic sites led to form the inhomogeneous and disorder coating structure
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Fig. 3. The potential-time curves of the conversion coating at bath Mo/P 1/2 MoO42 /H2PO4 molar ratio at different pH: (a) for the naked electrode, (b) pH 3.0, (c) pH 4.0, (d) pH 6.0, (e) pH 5.0.
because of the over-speedy deposition. Only at pH 5.0, the homogeneous and integrated conversion coating could form, causing the highest OCP value. The initial decrease of the OCP was probably caused by the surface activation of substrate, dissolution of substrate in acidic medium accompanying with simultaneous hydrogen evolution, while the minimum OCP value should be associated with the finish of the initial surface activation and the increase of the potential values with the beginning of the coating deposition. The plateau suggested that the reactions at the interface reached the steady state and the Mo/P coating covered the entire surface as possible as it could. From Fig. 3, it could be seen the immersion time to reach the plateau increased with the decrease of pH, which
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was likely due to the rise of the activation of Mg alloy surface at low pH, increasing the hydrogen evolution, thus the initial film deposition was delayed. At high pH, the hydrogen evolution was weakening, causing a faster film deposition. This behavior was in agreement with the tungstate-phosphoric coating on Zn [16] and phosphate coating on steel [17]. Therefore, pH value strongly influenced both the quality of conversion coating and conversion speed. Fig. 4 demonstrated the SEM micrographs of the conversion coatings prepared at different immersion time in Mo/P solution and their corresponding locations on the dynamic potential-time curve. It was found that some dark nests were enchased at the surface after 10 s immersion (see Fig. 4a) while an alveolate microstructure could be observed after 30 s immersion (see Fig. 4b) and the honeycomb was becoming larger and larger with increasing immersion time (see Fig. 4c–e), and the brims of nests were developing gradually into highlighting grinding after 60 s immersion (see Fig. 4c and d). The potential jumped from 1320 mV to 1150 mV for immersion time increasing from 10 s to 30 s, and slowly increased after 60 s. Based on these experimental results, it could be supposed that the growth mechanism of Mo/P coating was a corrosion/ precipitation process as follows: metal ions from substrate, such as Mg2+, Al3+, etc. were first electrochemically etched from the Mg alloy substrate on micro-anodic area. Meanwhile, metal ions in conversion bath with positive charge, such as Ca2+, Mn2+ etc. (Ca and Mn come from the molybdate/phosphate conversion bath), and Mg2+, Al3+ tended to transfer to micro-cathode and further reacted with MoO42 or H2PO4 to form insoluble metaphosphate deposited on Mg alloy surface after a short incubation time (approximately 6 s, herein). The surface coverage increased quickly with the rapid growth of individual crystals on Mg alloy surface, resulting in a mixed protection on the surface of Mg alloy.
Fig. 4. The SEM micrographs of Mo/P coating with 1/2 MoO42 /H2PO4 molar ratio at different immersion time (a. 6 s; b. 30 s; c. 60 s; d. 150 s and e. 300 s, at 50 8C and pH 5.0) and their corresponding locations on the dynamic potential-time curve.
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Fig. 5. SEM micrographs of conversion coatings on Mg alloy samples for 5 min immersion (a) in bath-Mo and (b) bath-Mo/P with 1/2 MoO42 /H2PO4 molar ratio.
3.2. The surface morphology and composition of treated Mg alloy 3.2.1. SEM analysis Fig. 5 presented the SEM micrographs of the Mg substrates after immersed in bath Mo and bath Mo/P for 5 min. The Mo coatings displayed network-like cracks structure, which was possibly caused by the release of hydrogen for the chemical reaction during the conversion treatment and/or the dehydration of the surface layer after treating in acidic medium. According to previous studies, a-phase (Mg), and b-phase (Mg17Al12), in Mg alloy could cause local corrosive cell effect, which accelerated the dissolving of the a-phase[4,9]. The precipitation of insoluble salts tended to proceed at or along the grain boundary and the precipitate was nobler than the a-phase [9]. On the contrary, the Mo/P coating showed a homogeneous alveolate structure. 3.2.2. XPS analysis XPS is usually employed to identify the specific electron binding energies of elements at the surface of conversion coating and the main reacting products [18,19]. Fig. 6 illustrated the survey spectra of the Mo and Mo/P conversion coatings obtained by XPS and Table 3 showed the detailed XPS results of surface element compositions of Mo and Mo/P conversion coatings 1/2 MoO42 / H2PO4 molar ratio. For the Mo coating, Mg, Al, Mo, O, Ca and N were detected as the major elements. But for the Mo/P coating, two more major elements, Mn and P were detected. The main element contents were calculated by the XPS peak areas and summarized in Table 2. Comparing with Mo coating, obvious reductions in Mg, Mo, Ca and N and visible increases in P, Mn and O were observed for Mo/P coating, further confirming that metaphosphate played a dominant role on the deposition of converting products. A more detailed XPS analyses of specific electron binding energies of elements were illustrated in Fig. 7. According to the data previously reported [20], the XPS peaks of each element should be resulted from the corresponding compounds. Thus, it could be supposed that MgAl2O4, MoO3, MgO, Al2O3 and a little of CaMoO4 probably constituted the Mo conversion coating, while Mex(PO4)y, MgAl2O4, MgO, Al2O3, MnOOH, MnO, CaMoO4 and MoO3 probably composed of the Mo/P conversion coating. 3.3. Phase analysis of conversion coatings Fig. 8 illustrated the XRD spectra of non-treated (as received) and conversion-coated Mg substrates (see the left of Fig. 8). The a-
Fig. 6. XPS spectra of (a) Mo coating and (b) Mo/P coatings with 1/2 MoO42 /H2PO4 molar ratio for 5 min immersion pretreatment.
Table 3 The XPS results of surface element compositions of Mo and Mo/P conversion coatings 1/2 MoO42 /H2PO4 molar ratio Atomic %
P
Mg
Al
Mo
O
Ca
Mn
N
Mo Mo/P
0.16 11.35
12.45 5.80
2.64 1.76
11.83 4.63
55.92 65.04
4.11 2.42
– 9.00
12.89 –
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Fig. 7. XPS intensities of Al2p, Ca2p, Mg2p, O1s, Mo3d, P2p, Mn2 of conversion coatings coated in bath Mo and bath Mo/P with 1/2 MoO42 /H2PO4 molar ratio for 5 min immersion pretreatment.
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Fig. 8. On the left XRD spectra for non-treated sample (as received) and the Mo and Mo/P conversion coatings on Mg alloy substrates and on the right an amplified XRD spectra at 2u in 10 30˚ for the Mo and Mo/P coating with 1/2 MoO42 /H2PO4 molar ratio for 5 min immersion pretreatment.
phase (Mg) and b-phase (Mg17Al12) were observed in both nontreated and treated substrates. But, at 2u 308, there were several ‘‘broad’’ peaks observed for the Mo conversion coating (see the two amplified curves on the right of Fig. 8). This was the marked characteristic of amorphous structure, in other words, the Mo conversion coating was amorphous. However, for the Mo/P conversion coating, a couple of aculeated peaks were seen, suggesting that the Mo/P conversion coating had crystal structure. Since these peaks were not the characteristic peaks of Mn3(PO4)2, Ca3(PO4)2, or other metal insoluble salts, we would assume that the Mo/P coating was composed of complicated phases, such as Mex(PO4)y, CaMoO4, MgAl2O4, MgO, Al2O3 and MnO. This Mo/P process seemed to be very similar to the phosphatizing process currently used in steel surface [17]. The difference was that the Mo/P coatings contained a lot of netlike oxides, such as MoO3, to provide additional corrosion protection for Mg substrate. 3.4. The corrosion performance of the conversion coatings 3.4.1. Anodic and cathodic polarization plots Fig. 9 compared the polarization plots of Mo and Mo/P conversion coatings. The corrosion potential value increased from 1200 mV for bath Mo to 980 mV for bath Mo/P, and the current density for bath Mo/P was two orders of magnitude lower than that for bath Mo. Moreover, the current increasing speed of anodic branch of bath Mo
was slower than that of its cathodic branch, which was just reverse and a passivation behavior was observed in the cathodic branch for Mo/P process. This indicated that the addition of phosphate significantly increased the potential and decreased the corrosive current, suggesting that the corrosion resistance of molydbate coating could be obviously improved by addition of the phosphate. 3.4.2. The EIS results Fig. 10 displayed the EIS plots of the naked electrode and the electrodes treated with Mo and Mo/P processes with different time, respectively. All plots were obtained at the corrosion potential immediately after the stabilization of the OCP in borate buffer solution with pH 9.2. These Nyquist plots exhibited outright two capacitive loops, one for high and intermediate frequencies and another for low frequencies. The impedance plots were interpreted by electrical equivalent circuit described in Fig. 11 [1], in which Rs, Rcf, Rp, Cdl and Cc were designated as solution resistances, conversion coating resistance, polarization resistance, capacitance of the double layer and capacitance of the conversion coating, respectively. Because of the Mg alloy substrate containing several kinds of metal elements, such as Mg, Al and Mn, the outer surface of alloy should have a layered structure being composed of metal oxide and the Nyquist plot of naked electrode should show two semicircles, which was validated by Fig. 10a, indicating a very anticipant agreement between the experiment and the advance assumption. So, the same
Table 4 The calculated parameters of equivalent circuit corresponding to Fig.10 Nyquist plots of naked electrode (a) and Mo coating (b) and Mo/P coating (c and d) with 1/2 MoO42 /H2PO4 molar ratio coated electrodes in borate buffer solution at different conversion pretreatment time
In bath Mo
In bath Mo/P
Fig. 9. Polarization curves of conversion coating formed in bath Mo and bath Mo/P with 1/2 MoO42 /H2PO4 molar ratio, respectively, at 50˚ C for 5 min immersion pretreatment.
Naked electrode
Time (s)
Rcf (kV cm2)
Rp (kV cm2)
Cdl (F cm
15 60 120 240 300 480 60 120 240 300 360 480 –
1.53 2.00 2.28 2.21 4.09 4.44 8.75 25.96 56.72 97.76 727.28 510.90 0.03
0.71 3.73 5.13 6.44 9.23 7.95 261.98 561.20 698.64 943.31 3390.67 3119.07 0.12
4.18E-3 2.27E-3 7.65E-4 5.94E-4 1.63E-4 1.91E-4 3.91E-4 2.68E-5 8.23E-5 1.93E-5 2.49E-6 1.04E-6 8.45E-3
2
)
Rs (V cm2) 43.8 39.7 13.4 65.9 63.8 35.8 45.6 41.6 42.3 35.3 55 36.8 29
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Fig. 10. The Nyquist plots of naked electrode (a) and Mo coating (b) and Mo/P coating (c and d) with 1/2 MoO42 /H2PO4 molar ratio coated electrodes in borate buffer solution at different conversion pretreatment time.
actual corrosion area, it was easily concluded from the Cdl data that the corrosion area of Mg alloy reduced after treated in bath Mo as well as bath Mo/P. Moreover, the corrosion area in bath Mo/P decreased more than that in bath Mo, namely, the Mo/P conversion coating could provide better corrosion protection than the Mo conversion coating. This could be also confirmed based on their Rp values. The Mo/P conversion coating had much higher Rp than the Mo coating as well as phosphate coating on Mg alloy (740 Vcm2) [11,12]. These indicated that the composite Mo/P conversion coating was a new promising for the effective protection of Mg alloys. Fig. 11. The electrical equivalent circuit of conversion coatings used in CHI606B software.
electrical equivalent circuit could be used to fit the impedance spectra of naked electrode. The values of the circuit elements were calculated by CHI606B software and summarized in Table 4. The maximum polarization resistance values (Rp) and the minimum capacitance values (Cdl) were obtained at the immersion conversion pretreatment time of 5 min and 6 min for the Mo and Mo/P conversion coatings, respectively. Since the Cdl is related to the
3.4.3. Salt-water immersion experiments The Mg alloy samples coated with Mo/P coating, Mo coating and chromate coating were immersed into 3.5% NaCl aqueous solution at 30%. Table 4 presented the experimental results. From Table 5, it could be seen that the Mo conversion coating had the worst corrosion resistance performance and was wrecked only for 2 h in chlorine medium. From the piled corrosion products of the Mo coating, it was likely that the chlorine ions had reached the surface of the underlying substrate, suggesting that Mo coating could only afford a transitory corrosion protection for Mg alloys.
Table 5 Conversion coatings Mo coating Mo/P coating Chromate coating
Tb/h
Sc/St (72h)
Appearance before immersion
Appearance after 72 h immersion
2
4/5
Golden-brown
22 24
2/5 1/3
Uniform grey Uniform grey
A lot of red-brown corrosion products piled on the surface Wirelike corrosion on the surface Peeling speckles in middle surface
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However, Mo/P conversion coating had almost equal corrosion protection to chromate coating based on the values of Tb and Sc/St, which further indicated that the addition of phosphate in molybdate-based solution improved the corrosion protection for Mg alloy significantly. 4. Conclusion Based on this study, it could be concluded that the composite molybdate/phosphate (Mo/P) conversion coating had better corrosion resistance performance than simplex molybdate conversion or phosphating coating for Mg alloy surface. EIS results and SEM observation showed the molar ratios of MoO42 /H2PO4 and pH values of conversion solution had obvious influences on the corrosion resistance performance of conversion coatings. Scant or excessive H2PO4 caused a coating with ‘‘dry-mud’’ microstructure and tinpot corrosion resistance. Relative high or low pH value could not cause homogeneous and integrated conversion coating. XPS and XRD analyses indicated that the Mo/P coating possibly contained composite phases which were consisted of Mex(PO4)y, CaMoO4, MgAl2O4, MgO, Al2O3, MnO, as well as molybdate oxide with an ‘‘alveolate-crystallized’’ structure. Electrochemical measurements and salt water immersion experiments displayed that composite Mo/P conversion coating had better corrosion resistance performance than Mo coating, and even had almost comparable corrosion protection for Mg alloy to the traditional chromate pretreatment. Acknowledgement The authors would like to thank the support of the Natural Science Foundation of Hunan Provice, China (Grant no. 06B0073). References [1] J.E. Gray, B. Luan, Protective coatings on magnesium and its alloys – a critical review, J. Alloys Compd. 336 (2002) 88–113.
[2] B.L. Mordike, T. Ebert, Magnesium properties – applications-potential, Mater. Sci. Eng. A 302 (2001) 37–45. [3] C.G. Silva, A.N. Correia, P.D. Lima-Neto, I.C.P. Margarit, O.R. Mattos, Study of conversion coatings obtained from tungstate-phosphoric acid solutions, Corros. Sci. 47 (2005) 709–722. [4] K.Z. Chong, T.S. Shih, Conversion-coating treatment for magnesium alloys by a permanganate-phosphate solution, Mater. Chem. Phys. 80 (2003) 191– 200. [5] M.A. Gonzalez-nunez, C.A. Nunez-lopez, P. Skeldon, A non-chromate conversion coating for magnesium alloys and magnesium-based metal matrix composites, Corros. Sci. 37 (1995) 1763–1772. [6] Katya Brunelli, Manuele Dabala, Irene Calliari, et al., Effect of HCl pre-treatment on corrosion resistance of cerium-based conversion coatings on magnesium and magnesium alloys, Corros. Sci. 47 (2005) 989–1000. [7] X.W. Yu, C.N. Cao, Z.M. Yao, D.R. Zhou, Z.D. Yin, Corrosion behavior of rare earth metal (REM) conversion coatings on aluminum alloy LY12, Mater. Sci. Eng. A 284 (2000) 56–63. [8] M. Naoharu, Formation of corrosion resistant coating film on magnesium alloy member, JP9031664A2 (1997). [9] H. Umehara, S. Takaya, Terauchi Chrome-free surface treatments for magnesium alloy, Surf. Coat. Technol. 169–170 (2003) 666–669. [10] F. Takayuki, K. Isao, Pretreatment of magnesium or magnesium alloy material before coating, JP6330341A2 (1994). [11] L. Kouisni, M. Azzi, F. Dalard, S. Maximovitch, Phosphate coatings on magnesium alloy AM60 part 1: study of the formation and the growth of zinc phosphate films, Surf. Coat. Technol. 185 (2004) 58–67. [12] L. Kouisni, M. Azzi, F. Dalard, S. Maximovitch, Phosphate coatings on magnesium alloy AM60 Part 2: Electrochemical behavior in borate buffer solution, Surf. Coat. Technol. 192 (2005) 239–246. [13] A.A.O. Magalhaes, I.C.P. Margarit, O.R. Mattos, Molybdate conversion coatings on zinc surfaces, J. Electroanal. Chem. 572 (2004) 433–440. [14] H. Konno, K. Narumi, H. Habzaki, Molybdate/Al (&) composite films on steel and zinc-plated steel by chemical conversion, Corros. Sci. 44 (2002) 1889– 1900. [15] E. Almeida, T.C. Diamantino, M.O. Figueiredo, Oxidising alternative species to chromium VI in zinc galvanised steel, Surf. Coat. Technol. 106 (1998) 8–17. [16] C.G. da Silva, A.N. Correia, P.D. Lima-Neto, Study of conversion coatings obtained form tungstate-phosphoric acid solutions, Corros. Sci. 47 (2005) 709–722. [17] D.B. Freeman, Phosphating and Metal Pre-treatments, Woodhead-Faulkener: Bucks, 1986. [18] S. Feliu Jr., V. Barranco, XPS study of the surface chemistry of conventional hot-dip galvanized pure Zn, galvanneal and Zn-Al alloy coatings on steel, Acta Mater. 51 (2003) 5413–5424. [19] M.A. Arensa, J.J. de, Damborenea, Surface characterization of cerium layers on galvanized steel, Surf. Coat. Technol. 187 (2004) 320–325. [20] C.D. Wagner, D.E. Bickham, NIST X-Ray Photoelectron Spectroscopy Database NIST (1989).