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Effect of the second phases on corrosion behavior of the Mg-Al-Zn alloys
Accepted Manuscript Corrosion mechanism was studied by corrosion morphology and the equivalent circuits. Effect of the second phases on corrosion behavior of the Mg-Al-Zn alloys Hui Feng, Shuhong Liu, Yong Du, Ting Lei, Rongchang Zeng, Tiechui Yuan PII:
S0925-8388(16)33568-X
DOI:
10.1016/j.jallcom.2016.11.100
Reference:
JALCOM 39596
To appear in:
Journal of Alloys and Compounds
Received Date: 13 October 2016 Revised Date:
7 November 2016
Accepted Date: 8 November 2016
Please cite this article as: H. Feng, S. Liu, Y. Du, T. Lei, R. Zeng, T. Yuan, Corrosion mechanism was studied by corrosion morphology and the equivalent circuits. Effect of the second phases on corrosion behavior of the Mg-Al-Zn alloys, Journal of Alloys and Compounds (2016), doi: 10.1016/ j.jallcom.2016.11.100. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Effect of the second phases on corrosion behavior of the Mg-Al-Zn alloys Hui Feng a, Shuhong Liua,*, Yong Dua, Ting Leia, Rongchang Zengb, Tiechui Yuana State Key Laboratory of Powder Metallurgy, Central South University, Changsha, Hunan 410083,
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a
China b
College of Materials Science and Engineering, Shandong University of Science and Technology,
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Qingdao, Shandong 266590, China *E-mail address: [email protected];
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Tel.: +86-731-88877300; Fax: +86073188710855.
Abstract
Magnesium alloys have superior mechanical property for industry applications as structural materials, but their poor corrosion resistance is still a bottleneck problem.
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Mg-Al-Zn alloys are one of the most extensively used Mg alloys. In order to study the effects of the second phases on corrosion property systematically, Mg-xAl-(15-x) Zn (x = 12.5, 5.6, 3.3, 1.0 wt.%) two-phase alloys containing a certain amount of the
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different second phases, two binary compounds γ, MgZn, and two ternary second phases Φ, q, were prepared based on the Mg-Al-Zn phase diagram. On this basis, corrosion property of the alloys in the 3.5 wt.% NaCl solution was studied by
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corrosion morphology observation and electrochemical tests. The role of the different second phases in the corrosion processes was investigated. It was revealed that the second phases precipitated both in the Mg matrix and along the grain boundaries and acted as micro-cathode, accelerating corrosion dissolution of the Mg substrate. The acceleration effect of the second phases is in the order of γ-Mg17Al12 > qMg44Zn41Al1 > Φ-Mg21(Zn,Al)17 > MgZn. The alloys were also investigated for their diversity in corrosion product characters. The corrosion mechanism was discussed terminally by equivalent circuit of the electrochemical impedance spectrum.
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ACCEPTED MANUSCRIPT Keywords: Mg-Al-Zn; Second phases; Corrosion property; Corrosion mechanism.
1. Introduction
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Magnesium alloys have become an important light-weighting structural materials in various fields, such as electronic, automotive and aircraft industries, due to their high strength/weight ratios, low density, electro-magnetic shielding character, good conductivity and easy application recycling [1-5]. In spite of these superior properties,
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magnesium alloys easily undergo galvanic corrosion in a corrosive environment, because of the high reactivity of Mg substrate [6-10] and no stable protective film
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formed on the alloy surfaces [11]. Poor corrosion resistance of magnesium alloys has been one of the major obstacles to their widespread applications as structural materials. Thus, understanding of the factors that influence corrosion performance and the corrosion mechanism is of great importance.
Mg-Al-Zn series magnesium alloys are one type of the most popular commercial
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Mg alloys combining good mechanical property, castability and workability, and have received extensive attentions. However, they exhibit poor corrosion resistance. Regarding for that variation of different composition and second phases in Mg alloys
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may imply an improvement approach to the better mechanical performance, high-temperature creep resistance and corrosion resistance, the Mg-Al-Zn alloys with different compositions have been studied during past decades. With respect to their
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corrosion property, the corrosion property of the Mg-Al-Zn alloys has been studied by changing content of the alloying elements [12-16] and improving their surface films [17-22] to get better corrosion performance. Nevertheless, for most Mg alloys, the second phases precipitate and can play an important role in the corrosion procedure of Mg substrate in various manner. Therefore, understanding of the mechanism of the corrosion procedure effected by the second phases is of great importance for alloy development and corrosion research. However, for the corrosion behavior of the Mg-Al-Zn Mg alloys, only two binary intermetallic compounds, MgZn and Mg17Al12, have been reported for their influence. Song et al. [23] and Lu et al. [24] revealed that 2
ACCEPTED MANUSCRIPT second phase MgZn, which continuously distributed in the microstructure of Mg-5Zn and Mg-3Zn alloys, acted as a micro-cathode during the corrosion process and the alloy with the largest percent content of MgZn phase exhibited worst corrosion resistance. A considerable investigations demonstrated the influence of the second
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phase Mg17Al12 on the corrosion behavior of magnesium alloys [25-31]. Li et al. [25] studied corrosion behavior of the single phase Mg17Al12 in sodium chloride solution. Lunder et al. [26] and Ko et al. [27] pointed out that the amount and distribution of the Mg17Al12 precipitates had an important influence on corrosion behavior of AZ91
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magnesium alloy. Namely, the continuous network distribution of the second phase Mg17Al12 acted as an effective barrier against corrosion in some degree and resulted in
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an increase in the corrosion resistance of AZ91D alloy. Zhao et al. [28] and Kim et al. [29] investigated the effect of morphology and grain size of the phase Mg17Al12 on corrosion performance of the Mg alloys respectively. Biodegradability of the β-Mg17Al12 phase was studied by Bob et al. [30] and Liu et al. [31]. In general, only limited reports on the effects of the second phases on corrosion resistance of Mg
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alloys were available in the literatures. In view of the reported phase diagram in Mg-rich corner of the Mg-Al-Zn system [32], there are several additional stable ternary second phases, such as q-Mg44Zn41Al1 and Φ-Mg21(Zn, Al)17. However, there
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is no any report regarding their effects on corrosion behavior of AZ type Mg alloys. Systematical investigation and comparison of the influence of the different second phases on the corrosion performance of the alloys are of importance to the
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development of the corrosion resistant Mg-Al-Zn alloys. In this work, several ternary Mg-Al-Zn alloys in the Mg-rich corner, which
contain a certain amount of the different second phases in each alloy , including two binary (Mg17Al12 and MgZn) and two ternary (q and Φ) phases, were prepared. Corrosion property of the magnesium alloys, including influence of the second phases on the corrosion behavior and the corrosion mechanism, were studied by corrosion morphology observation and electrochemical test.
2. Experimental procedure 3
ACCEPTED MANUSCRIPT 2.1 Preparation of the alloys Based on the Mg-Al-Zn phase diagram [32], seven two-phase alloys with nominal compositions of Mg-xAl-(15-x)Zn ( x=12.5 #1, 5.6 #2, 3.3 #3, 1.0 #4, wt.%) containing about mole fraction of 19.5 % of the different second phases in each alloy
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were designed and prepared from original high purity materials of Magnesium, Zinc and Aluminum (purity, 99.99 wt.%) by induction melting. Pure aluminum in form of foil was used to pack Mg and Zn pieces to decrease their volatilization loss during
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melting process. Weight losses of the alloys after melting were less than 1 wt.%. The real chemical compositions of the alloys were determined by ICP (Inductive Coupled
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Plasma Emission Spectrometer). The real volume percents of the second phases in each alloy were determined as the statistic result of the low magnification EPMA images. The alloys were then sealed in the vacuumized quartz tubes and annealed at 300 oC for 20 days, followed by quenching in cold water. The quenched alloys were cut into 10mm x 20mm x 5 mm cuboid samples by wire-electrode cutting. The
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samples were ground with SiC papers progressively up to 3000 grit, polished using 0.5 mm diamond paste, ultrasonically cleaned in ethanol and finally dried in the air. For the electrochemical tests, the samples were sealed in epoxy leaving a 1 cm2
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polished surface exposed and linked with copper lines.
2.2 Metallographic and immersion experiments
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Phase identification of the annealed alloys was conducted by XRD (X-ray
diffraction, BRUKER, D8 Advance type) and EPMA (Electron probe demographic Analysis, HYPERPROBE, KXA-8530F type). The volume percent of the second phases in each alloy were determined by statistical analysis of EPMA results at low magnification. The surface corrosion morphology were observed using OM (Optical microscope, Lei ca DMLP type) and SEM (Scanning electron microscopy, Nova, nanoSEM-230 type) equipped with EDX (Energy dispersive X-ray spectroscopy). In order to observe role of the second phases in corrosion process, the polished samples were immersed in 3.5 wt.% NaCl solution for 10 minutes (4, 6, 10 minutes for alloy 4
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2.3 Electrochemical measurements After an initial delay of 5 minutes to ensure a stable system before each test, open
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circuit potential (OCP), potentiodynamic polarization curve, and electrochemical impedance spectrum (EIS) were determined, using an electrochemical workstation (CHI604E type) at room temperature in 3.5 wt.% NaCl solution with aeration. A classical three-electrode cell was used, with a saturated calomel electrode as the
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reference electrode, a platinum plate as the counter electrode and the samples sealed by epoxy with an exposed area of 1cm2 as the working electrode. Potentiodynamic
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polarization tests were performed at a scan rate of 0.5 mV/s in the range of ±200 mV vs. open circuit potential. The electrochemical parameters were measured by Tafel extrapolation from polarization curves using the CorrView software. The EIS measurements were carried out over a frequency ranging from 100 kHz to 10 mHz with a 5 mV amplitude signal at OCP. The EIS results were fitted using Zview 2.0
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software. The fitting errors of the parameters are less than 5%. Both potentiodynamic polarization curve and electrochemical impedance spectrum were conducted for at least three times to ensure the reproducibility of the results.
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3. Results and discussions
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3.1 Phase identification and corrosion morphology analyses Phase identification of the annealed alloys was conducted. Figs. 1 to 4, XRD
patterns (a) and EPMA images (b), show existence of the four second phases: γ-Mg17Al12,
-Mg21(Zn,Al)17, q-Mg44Zn41Al1 and MgZn. The crystal structure
parameters of the second phases are listed in Table 1. The XRD peaks of the thermally stable icosahedral quasicrystal phase q agree well with that reported by Takeuchi and Mizutani [33]. We can see from EPMA images that all the four second phases present an approximatively continuous distribution along the grain boundaries forming a net-like morphology and a discrete distribution as particles of different geometry 5
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, q and MgZn phases in the Mg substrate show mainly ball or irregular
particle forms. Volume ratio of the second phases versus the Mg substrates in each alloy is determined about 19.4±0.5%. The nominal composition, the determined
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chemical composition by ICP and the volume percent of the second phases in each alloys are listed in Table 2.
After phase identification, immersion tests were performed to observe the corrosion behaviors of the alloys in 3.5 wt.% NaCl solution. Immersion tests of 4 and 6 minutes
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for the alloy #1, and 10 minutes for the alloys #1 - #4 were carried out, and the corrosion morphology of the alloys was observed by OM and SEM/EDX. As shown
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in Fig. 5 for the alloy #1, corrosion attack preferred to occur near the phase γ with white corrosion products near the grain boundaries and around the intragranular precipitation particles after 4 minutes immersion (as shown in image a), and it was obviously corroded after immersion for 6minutes (as shown in image b) while the second phase γ maintained basically its original morphology. The corrosion
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morphology of the alloys, #1 - #4, after immersion for 10 minutes were shown in Figs. 6 (a-d) respectively with high magnification images appended on the upper right corners. As shown in the high magnification images of Figs. 6 (b-d), the white
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corrosion products (denoted by red cross) were observed dominatly nearby the second phases in the Mg substrate and on the grain boundaries, while no obvious corrosion
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dissolution of the second phases was detected, which indicated that the second phases had more positive electrode potential than the Mg substrate and accelerated corrosion of the nearby Mg matrixes. In the low magnification images of Figs. 6 (a-d), it is obvious in the Fig. 6 (a) that the alloy #1 containing γ phase was corroded most seriously with lots of corrosion products and obvious corrosion crack formed on the alloy surface, while weak corrosion attack and several corrosive pitting position can be observed on the surface of the alloy #2 as shown in Fig. 6 (b) and a lot of corrosion products near the second phase q were observed for the alloy #3 as shown in Fig. 6 (c). The lamellar shape γ in the Mg substrate of the alloy #1 has a higher surface energy
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ACCEPTED MANUSCRIPT than the other second phases of particle shape in the Mg matrixes, which may promote the corrosion of the Mg substrate near the interface between the γ phase and the Mg matrix. No obvious corrosion for the alloy #4 is observed in the low magnification image of Fig. 6 (d) besides of a few localized corrosion points on the
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surface. In conclusion, from Figs. 5 - 6, it can be deduced that the second phases in the Mg matrix and along the grain boundaries acted as micro-cathode accelerating corrosion dissolution of the nearby Mg substrate. The diversity of the second phases cause differentiation in corrosion degree of the alloys with a certain percent of the
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saturated (Mg) solution as the substrates. Therefore, the corrosion degree of the alloys is observed in the order, #1> #3 >#2 > #4, which implies that the influence of the
sequence, γ > q > Φ > MgZn.
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second phases accelerating the corrosion dissolution of the Mg substrates is in the
Besides, in order to understand the corrosion behavior ulteriorly, micro-structure of the corrosion products formed on the alloy surfaces after 10 minutes immersion were investigated by SEM/EDX with a magnification of 20000. A grey needle-like
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corrosion product layer formed obviously on the surfaces of the alloys #1 and #3, shown in Fig. 7 (a) and (c) respectively, with the white flocculent products on the top layers. However, they are very loose and porous. The density of the grey product film
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on the alloy #3 surface are larger than that of the alloy #1. The needle-like products were not obviously observed on the alloy #2 and #4 surfaces, as shown in Fig.7 (b) and (d), surfaces. What’s more, the white corrosion products on the alloy #4 were
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more compact than that formed on the other alloy surfaces. As shown in Fig.7 (e), the corrosion products determined by EDX contain about 31 at.% Mg, 57 at.% O and 11 at.% C element except for H which can’t be detected by EDX. The EDX result is an average composition of the white and grey corrosion products because both the two corrosion products are too porous to be selected accurately. As was reported, the white corrosion products should be Mg(OH)2 [7] and the formation reaction of the grey corrosion products can be described as follows: 2Mg2+ + 2OH- +
CO 2 − 3
→
Mg2(OH)2CO3 [14]. 7
ACCEPTED MANUSCRIPT 3.2 Electrochemical analyses Potentiodynamic polarization curves, as shown in Fig. 8, were determined using 3.5 wt.% NaCl solution as electrolyte. The corrosion potential (Ecorr) and the corrosion current density (icorr) of the alloys were get from extrapolation method of the Tafel
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curves, and listed in Table 3. We can see that corrosion potential of the two-phase alloys increases in the sequence of #1 ((Mg) + γ) < #3 ((Mg) + q) < #2 ((Mg) + Φ) < #4 ((Mg) + MgZn). The more positive the corrosion potential is, the higher corrosion threshold the alloy has to be corroded. Corrosion rate were presented by icorr and
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increases in the order of #4 ((Mg) + MgZn) < #2 ((Mg) + Φ) < #3 ((Mg) + q) < #1 ((Mg) + γ), which is consistent with the conclusion from corrosion morphology
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analysis of the immersion tests. No passivation phenomenon was observed from the polarization curves corresponding to the loose and porous corrosion products observed in immersion tests.
EIS is a widely used tool to monitor the corrosion process. In this work, the corrosion characteristics of the four Mg-Al-Zn alloys in 3.5 wt. % NaCl solution were
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investigated with the EIS measurements at open circuit potential of the alloys. As shown in Fig. 9 (a), the dimension of the Nyquist plots increases in the sequence #1 < #3 < #2 < #4, corresponding to an increasing impedance trend of the alloys in the
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Bode plots of frequency versus impedance in Fig. 9 (b). The large capacity loops of the Nyquist plots at high frequency describe the features of the double electric layer at
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the interfaces between the alloy surfaces and electrolyte solution while the small medium frequency capacity loops present the capacity of the corrosion products. The low frequency induction loops indicate corrosive pitting and destruction of the layers of the corrosion products. The Nyquist plots of the alloys #1 and #3 consist of a large high frequency capacitance loop, a small medium frequency capacity loop and one low frequency inductance loop, which correspond to Fig. 9 (c) as the high frequency wave crests, the medium frequency wave crests and the wave troughs at low frequency. The existence of medium frequency capacity loop but small dimension of the total Nyquist plots for
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ACCEPTED MANUSCRIPT the alloys #1 and #3 may be attributed to a lot of the loose corrosion products formed due to rapid Mg dissolution accelerated by the second phases γ and q respectively. It is indicated that the corrosion processes of the alloys #1 and #3 include three steps, the corrosion of the Mg substrate, forming and breaking down of the corrosion
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product layers. In contrast, the Nyquist plots of the alloys #2 and #4 do not exhibit a medium frequency capacity loop, which indicates that little corrosion products formed on the two alloy surfaces and the high frequency capacity and low frequency inductance loops imply respectively corrosion attack of Mg substrate and the
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corrosive pitting near the second phases or localized corrosion at defect positions as observed in the immersion tests. The dimension of the Nyquist plots describes the
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corrosion resistance, in the order of #1 < #3 < #2 < #4, which is reciprocal to the relationship of corrosion rate of the alloys. In addition, the phase angle is usually used to evaluate difficulty of charge transfer in corrosion reaction process. As shown in Fig. 9 (c), the deviation of phase angle from 90° (an ideal capacitor) indicates that the charge transfer is facile relatively during the corrosion process. The phase angles of
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the high frequency capacity resistance crests decrease in the sequence, #4 > #2 > #3 > #1, which indicates that the corrosion rate of the Mg substrates increases from #4, #2, #3 to #1. The EIS results imply that acceleration effect of the second phases is in the
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order γ > q > Φ > MgZn. All the EIS results coincide with the immersion tests and polarization curve analyses.
In addition, in order to obtain corrosion mechanism from the EIS measurements,
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the EIS spectrums were fitted using the equivalent circuit elements. The corresponding fitting equivalent circuit for the alloys #1 and #3 are shown in Fig. 10 (a) while that for the alloys #2 and #4 are given in Fig. 10 (b). R1 represents the electrolyte solution resistance, and R2 denotes the charge transfer resistance. Constant phase elements CPE1 and CPE2 are used in place of a capacitor to compensate the non-homogeneity deviation from an ideal capacity. A CPE element is defined by two parameters, CPE-T and CPE-P. When CPE-P is equal to 1, CPE is identical to an ideal capacitor. CPE1 represents the electric double layer capacity at high frequency. CPE2 represents the medium frequency capacity of the corrosion products formed on the 9
ACCEPTED MANUSCRIPT alloy surfaces. R3 represents the resistance of the corrosion products. Inductance element L is used to describe the low frequency inductance loop, caused by corrosive pitting, deffect-induced corrosion and failure of the films of corrosion products. R4 represents the inductance resistances of the inductance element. The fitted parameters
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are listed in Table 4. The fitting errors are less than 5%. R2 value of the charge transfer resistance reflects dissolution rate of Mg substrate. The bigger the R2 value is, the smaller corrosion rate of the Mg matrix. The R2 increases according to #1 < #3 < #2 < #4, which implies accelerating dissolution rate of Mg substrate from #4, #2, #3
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to #1, corresponding to immersion tests and potentiodynamic polarization analyses. The adopted equivalent circuits can simulate and explain the general corrosion
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process.
Generally, the corrosion mechanisms described by the equivalent circuits can be explained as follows. When the Mg alloys were immersed in NaCl solution, the electric double layers formed at the interface between the alloy surfaces and electrolyte solution. Then, Mg substrate, especially the regions near the cathodic
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second phases and the surface defects was corroded firstly as a result of micro-cathode accelerating role of the second phases and corrosion condition of higher surface energy of the defect regions. Usually, the more positive electrode
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potential and the higher surface energy of a second phase, the greater effect of accelerating corrosion of the Mg substrate. Although a corrosion barrier inhibiting corrosion attack extending along grain boundary in some degree was reported [27] for
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the net-like continuous distribution of the second phase Mg17Al12. The Mg17Al12 in the Mg substrate mainly exhibits an obvious lamellar shape with a higher surface energy than other second phases q, Φ and MgZn, which may also cause higher corrosion reaction activity and corrosion rate of the nearby Mg matrix. Then, the different amounts of corrosion products formed for the different alloys due to different dissolution rates of the Mg matrix accelerated by the different second phases. The forming of enough corrosion products on the alloy surfaces, such as alloys #1 and #3, induced a capacity resistance character of the corrosion product layer. However, the corrosion products of Mg-based oxide, hydroxide, or carbonate formed on the surface 10
ACCEPTED MANUSCRIPT of the Mg alloys are loose and porous, as observed in this work for alloys #1 and #3. The chloride ions in NaCl solution can penetrate the loose Mg-based corrosion product layers by micro-pores, which caused persistent dissolution of the Mg substrate and corrosive pitting. When the corrosion current was large enough,
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corrosive holes extended and the product film desquamated. Corrosive pitting and fail of the product layer can lead to an induction resistance character in the Nyquist plots. Besides, the formation of hydroxide of Mg caused the removal of OH- and the decrease of PH value, which accelerated the dissolution of the Mg matrixes,
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especially when corrosive pitting occurred. The corrosion mechanisms are supported by the corrosion behaviors determined by corrosion morphology observation and
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electrochemical tests in this work.
4. Conclusions
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Based on the Mg-Al-Zn phase diagram [32], the two-phase Mg alloys of Mg-Al-Zn system with a certain percent of the different second phases were designed and prepared and analyzed by XRD, EPMA, OM, SEM/EDX, and electrochemical tests. Phase identification, corrosion morphology of different corrosion stage, corrosion
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product characters, corrosion potential, corrosion rate, electrochemical impedance and corrosion mechanism were investigated. Effects of different precipitated phases on the
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corrosion behaviors of the alloys and corrosion mechanisms were discussed. The main conclusions are summarized as follows: (1) The second phases of the Mg alloys precipitate both along the grain boundaries and in the Mg matrix. The second phases act as cathodic phases, which accelerate the corrosion dissolution of Mg substrate. The acceleration effects of the different second phases on corrosion rate of the Mg alloys are revealed as: γ > q > Φ > MgZn. (2) Corrosion potential of the two-phase alloys increases in the sequence of #1 ((Mg) + γ) < #3 ((Mg) + q) < #2 ((Mg) + Φ) < #4 ((Mg) + MgZn). Corrosion rate of the alloys increase in the order #4 ((Mg) + MgZn) < #2 ((Mg) + Φ) < #3 ((Mg) + q) < #1 11
ACCEPTED MANUSCRIPT ((Mg) + γ), which are reciprocal to their total electrochemical impedance modulus. (3) The corrosion products of Mg-based oxide, hydroxide, or carbonate formed on the surface of the Mg alloys are loose and porous. The alloys with different second phases exhibit different corrosion product characters.
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(4) The shape of the second phases may influence the corrosion reaction rate of nearby Mg matrix. The larger surface energy caused by the shape of the second phase, the higher corrosion reactivity and corrosion rate of the nearby Mg substrate.
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Acknowledgements
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Thanks for the financial support from National Nature Science Foundation of China (Nos. 51531009 and 51671219) and the support by State Key Lab of powder metallurgy, Central South University.
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[26] O. Lunder, J.E. Lein, T.Kr. Aune, and K. Nisancioglu, The role of Mg17Al12 phase in the corrosion of Mg alloy AZ91, Corros. 45 (9) (1989) 741-748.
[27] Y.J. Ko, C.D. Yim, J.D. Lim, and K.S. Shin, Effect of Mg17Al12 precipitate on corrosion behavior of AZ91D Magnesium alloy, Mater. Sci. Forum 419-422 (2003) 851-856. [28] M.C. Zhao, M. Liu, G. Song, and A. Atrens, Influence of the β-phase morphology on corrosion of the Mg alloy AZ91, Corros. Sci. 50 (2008) 1939-1953. [29] H.S. Kim and W.J. Kim, Enhanced corrosion resistance of ultrafine-grained AZ61 alloy containing very fine particles of Mg17Al12 phase, Corros. Sci. 75 14
ACCEPTED MANUSCRIPT (2013) 228-238. [30] M. Bobby Kannan, E. Koc, and M. Unal, Biodegradability of β-Mg17Al12 phase in simulated body fluid, Mater. Lett. 82 (2012) 54-56. [31] C. Liu, H.Z. Yang, P. Wan, K.H. Wang, L.L. Tan, and K. Yang, Study on
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biodegradation of the second phase Mg17Al12 in Mg-Al-Zn alloys: in vitro experiment and thermodynamic calculation, Mater. Sci. Eng. C 35 (2014) 1-7.
[32] P. Liang, T. Tarfa, J.A. Robinson, et al., Experimental investigation and thermodynamic calculation of the Al-Mg-Zn system, Thermochim. Acta 314
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(1998) 87-110.
[33] T. Takeuchi and U. Mizutani, Electronic structure, electron transport properties
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and relative stability of icosahedral quasicrystals and their 1/1 and 2/1 approximants in the Al-Mg-Zn alloy system, Phys. Rev. B 52 (13) (1995)
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Tables and Figures captions Table 1 Crystallographic data of the second phases.
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Table 2 The nominal and determined chemical compositions of the alloys and the volume percents of the second phases in each alloy.
Table 3 Critical parameters determined from the potentiodynamic curves of the of the
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Mg-Al-Zn alloys.
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Table 4 Fitting results of the EIS of the Mg-Al-Zn alloys.
Fig. 1 (a) XRD pattern and (b) EPMA image show the coexistence of the second phase γ and the (Mg) substrate phase in the alloy #1.
phase
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Fig. 2 (a) XRD pattern and (b) EPMA image show the coexistence of the second and the (Mg) substrate phase in the alloy #2.
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Fig. 3 (a) XRD pattern and (b) EPMA image show the coexistence of the second
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phase q and the (Mg) substrate phase in the alloy #3.
Fig. 4 (a) XRD pattern and (b) EPMA image show the coexistence of the second phase MgZn and the (Mg) substrate phase in the alloy #4.
Fig. 5 SEM images on corrosion morphology of the alloy #1 after immersion in 3.5 wt.% NaCl solution for (a) 4 minutes and (b) 6 minutes.
Fig. 6 SEM images on the corrosion morphology of the alloys, (a) #1, (b) #2, (c) #3 and (d) #4 respectively, after 10 minutes immersion in 3.5 wt.% NaCl solution. 16
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Fig. 7 Morphology of the corrosion products formed on the surface of the alloys, (a) #1; (b) #2; (c) #3; (d) #4, after 10 minutes immersion in 3.5wt.% NaCl solution; (e) EDX analysis of the
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corrosion products on the alloy surfaces.
Fig. 8 Potentiodynamic polarization curves of the Mg-Al-Zn alloys.
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Fig. 9 (a) Nyquist plots of the alloys; (b) Bode plots of impedance vs frequency for the alloys; (c) Bode plots of phase angle vs frequency for the alloys
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EIS for the alloys #2 and #4.
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Fig.10 (a) Equivalent circuit of EIS for the alloys #1 and #3; (b) Equivalent circuit of
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Table 1 Crystallographic data of the second phases. Space group
I43m
Mg17Al12 (γ)
cI58
Mg21(Zn,Al)17 (Φ)
oP152
Pbcm
Mg44Zn41Al1 (q)
Quasicrystalline
Icosahedron
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Lattice parameters [pm]
Mg21(Zn,Al)17
_
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Prototype
aP96
P1
Mg1Zn1
Ref.
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Pearson symbol
a= 105492
[32]
a = 897.9 b = 1698.8 c =1934 _
[32]
a = 106600 b = 106600 c = 171600
[32]
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Second phase
[33]
Table 2 The nominal and determined chemical compositions of the alloys and the volume percents of the second phases in each alloy.
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#1 (Mg+γ) #2 (Mg+Φ) #3 (Mg+q) #4 (Mg+MgZn)
Nominal composition (wt.%) Mg-12.5Al-2.5Zn Mg-5.6Al-9.4Zn Mg-3.3Al-11.7Zn Mg-1.0Al-14.0Zn
Real composition (ICP) (wt.%) Mg-12.1Al-2.3Zn Mg-5.9Al-9.2Zn Mg-3.7Al-12.5Zn Mg-0.8Al-14.4Zn
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Alloys
Volume percent of the second phases 19.3 19.5 19.1 19.7
Table 3 Critical parameters determined from the potentiodynamic curves of the of the Mg-Al-Zn alloys.
Alloys #1 #2 #3 #4
Ecorr (VSCE) icorr (A cm-2) -1.525 -1.511 -1.518 -1.488
6.971E-3 1.175E-3 1.860E-3 7.300E-4
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R1
CPE1 2
R2
CPE2 2
R3
L 2
R4 2
2
CPE1-T(F/cm )
CPE-P
(Ωcm )
CPE2-T(F/cm )
CPE-P
(Ω cm )
(H cm )
(Ωcm )
#1
5.302
4.835E-4
0.771
3.016
2.802E-1
0.798
0.887
105.2
12.39
#2
5.118
1.865E-5
0.987
23.45
-
-
-
17.09
26.24
#3
5.081
1.846E-4
0.871
19.52
1.536E-2
0.998
0.58
147.5
84.53
#4
4.986
3.393E-5
0.949
27.18
-
-
2
-
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2
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(Ω cm )
50.46
37.06
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Highlights Two ternary and two binary second phases were confirmed stable at 300
.
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The roles of the second phases in corrosion processes were investigated. Corrosion property effected by different second phases were compared.
Corrosion mechanism was studied by corrosion morphology and the
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equivalent circuits.
S0925-8388(16)33568-X
DOI:
10.1016/j.jallcom.2016.11.100
Reference:
JALCOM 39596
To appear in:
Journal of Alloys and Compounds
Received Date: 13 October 2016 Revised Date:
7 November 2016
Accepted Date: 8 November 2016
Please cite this article as: H. Feng, S. Liu, Y. Du, T. Lei, R. Zeng, T. Yuan, Corrosion mechanism was studied by corrosion morphology and the equivalent circuits. Effect of the second phases on corrosion behavior of the Mg-Al-Zn alloys, Journal of Alloys and Compounds (2016), doi: 10.1016/ j.jallcom.2016.11.100. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Effect of the second phases on corrosion behavior of the Mg-Al-Zn alloys Hui Feng a, Shuhong Liua,*, Yong Dua, Ting Leia, Rongchang Zengb, Tiechui Yuana State Key Laboratory of Powder Metallurgy, Central South University, Changsha, Hunan 410083,
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a
China b
College of Materials Science and Engineering, Shandong University of Science and Technology,
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Qingdao, Shandong 266590, China *E-mail address: [email protected];
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Tel.: +86-731-88877300; Fax: +86073188710855.
Abstract
Magnesium alloys have superior mechanical property for industry applications as structural materials, but their poor corrosion resistance is still a bottleneck problem.
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Mg-Al-Zn alloys are one of the most extensively used Mg alloys. In order to study the effects of the second phases on corrosion property systematically, Mg-xAl-(15-x) Zn (x = 12.5, 5.6, 3.3, 1.0 wt.%) two-phase alloys containing a certain amount of the
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different second phases, two binary compounds γ, MgZn, and two ternary second phases Φ, q, were prepared based on the Mg-Al-Zn phase diagram. On this basis, corrosion property of the alloys in the 3.5 wt.% NaCl solution was studied by
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corrosion morphology observation and electrochemical tests. The role of the different second phases in the corrosion processes was investigated. It was revealed that the second phases precipitated both in the Mg matrix and along the grain boundaries and acted as micro-cathode, accelerating corrosion dissolution of the Mg substrate. The acceleration effect of the second phases is in the order of γ-Mg17Al12 > qMg44Zn41Al1 > Φ-Mg21(Zn,Al)17 > MgZn. The alloys were also investigated for their diversity in corrosion product characters. The corrosion mechanism was discussed terminally by equivalent circuit of the electrochemical impedance spectrum.
1
ACCEPTED MANUSCRIPT Keywords: Mg-Al-Zn; Second phases; Corrosion property; Corrosion mechanism.
1. Introduction
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Magnesium alloys have become an important light-weighting structural materials in various fields, such as electronic, automotive and aircraft industries, due to their high strength/weight ratios, low density, electro-magnetic shielding character, good conductivity and easy application recycling [1-5]. In spite of these superior properties,
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magnesium alloys easily undergo galvanic corrosion in a corrosive environment, because of the high reactivity of Mg substrate [6-10] and no stable protective film
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formed on the alloy surfaces [11]. Poor corrosion resistance of magnesium alloys has been one of the major obstacles to their widespread applications as structural materials. Thus, understanding of the factors that influence corrosion performance and the corrosion mechanism is of great importance.
Mg-Al-Zn series magnesium alloys are one type of the most popular commercial
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Mg alloys combining good mechanical property, castability and workability, and have received extensive attentions. However, they exhibit poor corrosion resistance. Regarding for that variation of different composition and second phases in Mg alloys
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may imply an improvement approach to the better mechanical performance, high-temperature creep resistance and corrosion resistance, the Mg-Al-Zn alloys with different compositions have been studied during past decades. With respect to their
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corrosion property, the corrosion property of the Mg-Al-Zn alloys has been studied by changing content of the alloying elements [12-16] and improving their surface films [17-22] to get better corrosion performance. Nevertheless, for most Mg alloys, the second phases precipitate and can play an important role in the corrosion procedure of Mg substrate in various manner. Therefore, understanding of the mechanism of the corrosion procedure effected by the second phases is of great importance for alloy development and corrosion research. However, for the corrosion behavior of the Mg-Al-Zn Mg alloys, only two binary intermetallic compounds, MgZn and Mg17Al12, have been reported for their influence. Song et al. [23] and Lu et al. [24] revealed that 2
ACCEPTED MANUSCRIPT second phase MgZn, which continuously distributed in the microstructure of Mg-5Zn and Mg-3Zn alloys, acted as a micro-cathode during the corrosion process and the alloy with the largest percent content of MgZn phase exhibited worst corrosion resistance. A considerable investigations demonstrated the influence of the second
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phase Mg17Al12 on the corrosion behavior of magnesium alloys [25-31]. Li et al. [25] studied corrosion behavior of the single phase Mg17Al12 in sodium chloride solution. Lunder et al. [26] and Ko et al. [27] pointed out that the amount and distribution of the Mg17Al12 precipitates had an important influence on corrosion behavior of AZ91
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magnesium alloy. Namely, the continuous network distribution of the second phase Mg17Al12 acted as an effective barrier against corrosion in some degree and resulted in
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an increase in the corrosion resistance of AZ91D alloy. Zhao et al. [28] and Kim et al. [29] investigated the effect of morphology and grain size of the phase Mg17Al12 on corrosion performance of the Mg alloys respectively. Biodegradability of the β-Mg17Al12 phase was studied by Bob et al. [30] and Liu et al. [31]. In general, only limited reports on the effects of the second phases on corrosion resistance of Mg
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alloys were available in the literatures. In view of the reported phase diagram in Mg-rich corner of the Mg-Al-Zn system [32], there are several additional stable ternary second phases, such as q-Mg44Zn41Al1 and Φ-Mg21(Zn, Al)17. However, there
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is no any report regarding their effects on corrosion behavior of AZ type Mg alloys. Systematical investigation and comparison of the influence of the different second phases on the corrosion performance of the alloys are of importance to the
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development of the corrosion resistant Mg-Al-Zn alloys. In this work, several ternary Mg-Al-Zn alloys in the Mg-rich corner, which
contain a certain amount of the different second phases in each alloy , including two binary (Mg17Al12 and MgZn) and two ternary (q and Φ) phases, were prepared. Corrosion property of the magnesium alloys, including influence of the second phases on the corrosion behavior and the corrosion mechanism, were studied by corrosion morphology observation and electrochemical test.
2. Experimental procedure 3
ACCEPTED MANUSCRIPT 2.1 Preparation of the alloys Based on the Mg-Al-Zn phase diagram [32], seven two-phase alloys with nominal compositions of Mg-xAl-(15-x)Zn ( x=12.5 #1, 5.6 #2, 3.3 #3, 1.0 #4, wt.%) containing about mole fraction of 19.5 % of the different second phases in each alloy
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were designed and prepared from original high purity materials of Magnesium, Zinc and Aluminum (purity, 99.99 wt.%) by induction melting. Pure aluminum in form of foil was used to pack Mg and Zn pieces to decrease their volatilization loss during
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melting process. Weight losses of the alloys after melting were less than 1 wt.%. The real chemical compositions of the alloys were determined by ICP (Inductive Coupled
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Plasma Emission Spectrometer). The real volume percents of the second phases in each alloy were determined as the statistic result of the low magnification EPMA images. The alloys were then sealed in the vacuumized quartz tubes and annealed at 300 oC for 20 days, followed by quenching in cold water. The quenched alloys were cut into 10mm x 20mm x 5 mm cuboid samples by wire-electrode cutting. The
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samples were ground with SiC papers progressively up to 3000 grit, polished using 0.5 mm diamond paste, ultrasonically cleaned in ethanol and finally dried in the air. For the electrochemical tests, the samples were sealed in epoxy leaving a 1 cm2
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polished surface exposed and linked with copper lines.
2.2 Metallographic and immersion experiments
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Phase identification of the annealed alloys was conducted by XRD (X-ray
diffraction, BRUKER, D8 Advance type) and EPMA (Electron probe demographic Analysis, HYPERPROBE, KXA-8530F type). The volume percent of the second phases in each alloy were determined by statistical analysis of EPMA results at low magnification. The surface corrosion morphology were observed using OM (Optical microscope, Lei ca DMLP type) and SEM (Scanning electron microscopy, Nova, nanoSEM-230 type) equipped with EDX (Energy dispersive X-ray spectroscopy). In order to observe role of the second phases in corrosion process, the polished samples were immersed in 3.5 wt.% NaCl solution for 10 minutes (4, 6, 10 minutes for alloy 4
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2.3 Electrochemical measurements After an initial delay of 5 minutes to ensure a stable system before each test, open
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circuit potential (OCP), potentiodynamic polarization curve, and electrochemical impedance spectrum (EIS) were determined, using an electrochemical workstation (CHI604E type) at room temperature in 3.5 wt.% NaCl solution with aeration. A classical three-electrode cell was used, with a saturated calomel electrode as the
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reference electrode, a platinum plate as the counter electrode and the samples sealed by epoxy with an exposed area of 1cm2 as the working electrode. Potentiodynamic
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polarization tests were performed at a scan rate of 0.5 mV/s in the range of ±200 mV vs. open circuit potential. The electrochemical parameters were measured by Tafel extrapolation from polarization curves using the CorrView software. The EIS measurements were carried out over a frequency ranging from 100 kHz to 10 mHz with a 5 mV amplitude signal at OCP. The EIS results were fitted using Zview 2.0
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software. The fitting errors of the parameters are less than 5%. Both potentiodynamic polarization curve and electrochemical impedance spectrum were conducted for at least three times to ensure the reproducibility of the results.
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3. Results and discussions
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3.1 Phase identification and corrosion morphology analyses Phase identification of the annealed alloys was conducted. Figs. 1 to 4, XRD
patterns (a) and EPMA images (b), show existence of the four second phases: γ-Mg17Al12,
-Mg21(Zn,Al)17, q-Mg44Zn41Al1 and MgZn. The crystal structure
parameters of the second phases are listed in Table 1. The XRD peaks of the thermally stable icosahedral quasicrystal phase q agree well with that reported by Takeuchi and Mizutani [33]. We can see from EPMA images that all the four second phases present an approximatively continuous distribution along the grain boundaries forming a net-like morphology and a discrete distribution as particles of different geometry 5
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, q and MgZn phases in the Mg substrate show mainly ball or irregular
particle forms. Volume ratio of the second phases versus the Mg substrates in each alloy is determined about 19.4±0.5%. The nominal composition, the determined
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chemical composition by ICP and the volume percent of the second phases in each alloys are listed in Table 2.
After phase identification, immersion tests were performed to observe the corrosion behaviors of the alloys in 3.5 wt.% NaCl solution. Immersion tests of 4 and 6 minutes
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for the alloy #1, and 10 minutes for the alloys #1 - #4 were carried out, and the corrosion morphology of the alloys was observed by OM and SEM/EDX. As shown
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in Fig. 5 for the alloy #1, corrosion attack preferred to occur near the phase γ with white corrosion products near the grain boundaries and around the intragranular precipitation particles after 4 minutes immersion (as shown in image a), and it was obviously corroded after immersion for 6minutes (as shown in image b) while the second phase γ maintained basically its original morphology. The corrosion
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morphology of the alloys, #1 - #4, after immersion for 10 minutes were shown in Figs. 6 (a-d) respectively with high magnification images appended on the upper right corners. As shown in the high magnification images of Figs. 6 (b-d), the white
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corrosion products (denoted by red cross) were observed dominatly nearby the second phases in the Mg substrate and on the grain boundaries, while no obvious corrosion
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dissolution of the second phases was detected, which indicated that the second phases had more positive electrode potential than the Mg substrate and accelerated corrosion of the nearby Mg matrixes. In the low magnification images of Figs. 6 (a-d), it is obvious in the Fig. 6 (a) that the alloy #1 containing γ phase was corroded most seriously with lots of corrosion products and obvious corrosion crack formed on the alloy surface, while weak corrosion attack and several corrosive pitting position can be observed on the surface of the alloy #2 as shown in Fig. 6 (b) and a lot of corrosion products near the second phase q were observed for the alloy #3 as shown in Fig. 6 (c). The lamellar shape γ in the Mg substrate of the alloy #1 has a higher surface energy
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surface. In conclusion, from Figs. 5 - 6, it can be deduced that the second phases in the Mg matrix and along the grain boundaries acted as micro-cathode accelerating corrosion dissolution of the nearby Mg substrate. The diversity of the second phases cause differentiation in corrosion degree of the alloys with a certain percent of the
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saturated (Mg) solution as the substrates. Therefore, the corrosion degree of the alloys is observed in the order, #1> #3 >#2 > #4, which implies that the influence of the
sequence, γ > q > Φ > MgZn.
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second phases accelerating the corrosion dissolution of the Mg substrates is in the
Besides, in order to understand the corrosion behavior ulteriorly, micro-structure of the corrosion products formed on the alloy surfaces after 10 minutes immersion were investigated by SEM/EDX with a magnification of 20000. A grey needle-like
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corrosion product layer formed obviously on the surfaces of the alloys #1 and #3, shown in Fig. 7 (a) and (c) respectively, with the white flocculent products on the top layers. However, they are very loose and porous. The density of the grey product film
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on the alloy #3 surface are larger than that of the alloy #1. The needle-like products were not obviously observed on the alloy #2 and #4 surfaces, as shown in Fig.7 (b) and (d), surfaces. What’s more, the white corrosion products on the alloy #4 were
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more compact than that formed on the other alloy surfaces. As shown in Fig.7 (e), the corrosion products determined by EDX contain about 31 at.% Mg, 57 at.% O and 11 at.% C element except for H which can’t be detected by EDX. The EDX result is an average composition of the white and grey corrosion products because both the two corrosion products are too porous to be selected accurately. As was reported, the white corrosion products should be Mg(OH)2 [7] and the formation reaction of the grey corrosion products can be described as follows: 2Mg2+ + 2OH- +
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curves, and listed in Table 3. We can see that corrosion potential of the two-phase alloys increases in the sequence of #1 ((Mg) + γ) < #3 ((Mg) + q) < #2 ((Mg) + Φ) < #4 ((Mg) + MgZn). The more positive the corrosion potential is, the higher corrosion threshold the alloy has to be corroded. Corrosion rate were presented by icorr and
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increases in the order of #4 ((Mg) + MgZn) < #2 ((Mg) + Φ) < #3 ((Mg) + q) < #1 ((Mg) + γ), which is consistent with the conclusion from corrosion morphology
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analysis of the immersion tests. No passivation phenomenon was observed from the polarization curves corresponding to the loose and porous corrosion products observed in immersion tests.
EIS is a widely used tool to monitor the corrosion process. In this work, the corrosion characteristics of the four Mg-Al-Zn alloys in 3.5 wt. % NaCl solution were
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investigated with the EIS measurements at open circuit potential of the alloys. As shown in Fig. 9 (a), the dimension of the Nyquist plots increases in the sequence #1 < #3 < #2 < #4, corresponding to an increasing impedance trend of the alloys in the
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Bode plots of frequency versus impedance in Fig. 9 (b). The large capacity loops of the Nyquist plots at high frequency describe the features of the double electric layer at
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the interfaces between the alloy surfaces and electrolyte solution while the small medium frequency capacity loops present the capacity of the corrosion products. The low frequency induction loops indicate corrosive pitting and destruction of the layers of the corrosion products. The Nyquist plots of the alloys #1 and #3 consist of a large high frequency capacitance loop, a small medium frequency capacity loop and one low frequency inductance loop, which correspond to Fig. 9 (c) as the high frequency wave crests, the medium frequency wave crests and the wave troughs at low frequency. The existence of medium frequency capacity loop but small dimension of the total Nyquist plots for
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product layers. In contrast, the Nyquist plots of the alloys #2 and #4 do not exhibit a medium frequency capacity loop, which indicates that little corrosion products formed on the two alloy surfaces and the high frequency capacity and low frequency inductance loops imply respectively corrosion attack of Mg substrate and the
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corrosive pitting near the second phases or localized corrosion at defect positions as observed in the immersion tests. The dimension of the Nyquist plots describes the
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corrosion resistance, in the order of #1 < #3 < #2 < #4, which is reciprocal to the relationship of corrosion rate of the alloys. In addition, the phase angle is usually used to evaluate difficulty of charge transfer in corrosion reaction process. As shown in Fig. 9 (c), the deviation of phase angle from 90° (an ideal capacitor) indicates that the charge transfer is facile relatively during the corrosion process. The phase angles of
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the high frequency capacity resistance crests decrease in the sequence, #4 > #2 > #3 > #1, which indicates that the corrosion rate of the Mg substrates increases from #4, #2, #3 to #1. The EIS results imply that acceleration effect of the second phases is in the
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order γ > q > Φ > MgZn. All the EIS results coincide with the immersion tests and polarization curve analyses.
In addition, in order to obtain corrosion mechanism from the EIS measurements,
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the EIS spectrums were fitted using the equivalent circuit elements. The corresponding fitting equivalent circuit for the alloys #1 and #3 are shown in Fig. 10 (a) while that for the alloys #2 and #4 are given in Fig. 10 (b). R1 represents the electrolyte solution resistance, and R2 denotes the charge transfer resistance. Constant phase elements CPE1 and CPE2 are used in place of a capacitor to compensate the non-homogeneity deviation from an ideal capacity. A CPE element is defined by two parameters, CPE-T and CPE-P. When CPE-P is equal to 1, CPE is identical to an ideal capacitor. CPE1 represents the electric double layer capacity at high frequency. CPE2 represents the medium frequency capacity of the corrosion products formed on the 9
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are listed in Table 4. The fitting errors are less than 5%. R2 value of the charge transfer resistance reflects dissolution rate of Mg substrate. The bigger the R2 value is, the smaller corrosion rate of the Mg matrix. The R2 increases according to #1 < #3 < #2 < #4, which implies accelerating dissolution rate of Mg substrate from #4, #2, #3
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to #1, corresponding to immersion tests and potentiodynamic polarization analyses. The adopted equivalent circuits can simulate and explain the general corrosion
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Generally, the corrosion mechanisms described by the equivalent circuits can be explained as follows. When the Mg alloys were immersed in NaCl solution, the electric double layers formed at the interface between the alloy surfaces and electrolyte solution. Then, Mg substrate, especially the regions near the cathodic
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second phases and the surface defects was corroded firstly as a result of micro-cathode accelerating role of the second phases and corrosion condition of higher surface energy of the defect regions. Usually, the more positive electrode
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potential and the higher surface energy of a second phase, the greater effect of accelerating corrosion of the Mg substrate. Although a corrosion barrier inhibiting corrosion attack extending along grain boundary in some degree was reported [27] for
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the net-like continuous distribution of the second phase Mg17Al12. The Mg17Al12 in the Mg substrate mainly exhibits an obvious lamellar shape with a higher surface energy than other second phases q, Φ and MgZn, which may also cause higher corrosion reaction activity and corrosion rate of the nearby Mg matrix. Then, the different amounts of corrosion products formed for the different alloys due to different dissolution rates of the Mg matrix accelerated by the different second phases. The forming of enough corrosion products on the alloy surfaces, such as alloys #1 and #3, induced a capacity resistance character of the corrosion product layer. However, the corrosion products of Mg-based oxide, hydroxide, or carbonate formed on the surface 10
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corrosive holes extended and the product film desquamated. Corrosive pitting and fail of the product layer can lead to an induction resistance character in the Nyquist plots. Besides, the formation of hydroxide of Mg caused the removal of OH- and the decrease of PH value, which accelerated the dissolution of the Mg matrixes,
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4. Conclusions
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Based on the Mg-Al-Zn phase diagram [32], the two-phase Mg alloys of Mg-Al-Zn system with a certain percent of the different second phases were designed and prepared and analyzed by XRD, EPMA, OM, SEM/EDX, and electrochemical tests. Phase identification, corrosion morphology of different corrosion stage, corrosion
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product characters, corrosion potential, corrosion rate, electrochemical impedance and corrosion mechanism were investigated. Effects of different precipitated phases on the
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corrosion behaviors of the alloys and corrosion mechanisms were discussed. The main conclusions are summarized as follows: (1) The second phases of the Mg alloys precipitate both along the grain boundaries and in the Mg matrix. The second phases act as cathodic phases, which accelerate the corrosion dissolution of Mg substrate. The acceleration effects of the different second phases on corrosion rate of the Mg alloys are revealed as: γ > q > Φ > MgZn. (2) Corrosion potential of the two-phase alloys increases in the sequence of #1 ((Mg) + γ) < #3 ((Mg) + q) < #2 ((Mg) + Φ) < #4 ((Mg) + MgZn). Corrosion rate of the alloys increase in the order #4 ((Mg) + MgZn) < #2 ((Mg) + Φ) < #3 ((Mg) + q) < #1 11
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(4) The shape of the second phases may influence the corrosion reaction rate of nearby Mg matrix. The larger surface energy caused by the shape of the second phase, the higher corrosion reactivity and corrosion rate of the nearby Mg substrate.
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Acknowledgements
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Thanks for the financial support from National Nature Science Foundation of China (Nos. 51531009 and 51671219) and the support by State Key Lab of powder metallurgy, Central South University.
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Tables and Figures captions Table 1 Crystallographic data of the second phases.
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Table 2 The nominal and determined chemical compositions of the alloys and the volume percents of the second phases in each alloy.
Table 3 Critical parameters determined from the potentiodynamic curves of the of the
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Mg-Al-Zn alloys.
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Table 4 Fitting results of the EIS of the Mg-Al-Zn alloys.
Fig. 1 (a) XRD pattern and (b) EPMA image show the coexistence of the second phase γ and the (Mg) substrate phase in the alloy #1.
phase
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Fig. 2 (a) XRD pattern and (b) EPMA image show the coexistence of the second and the (Mg) substrate phase in the alloy #2.
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Fig. 3 (a) XRD pattern and (b) EPMA image show the coexistence of the second
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phase q and the (Mg) substrate phase in the alloy #3.
Fig. 4 (a) XRD pattern and (b) EPMA image show the coexistence of the second phase MgZn and the (Mg) substrate phase in the alloy #4.
Fig. 5 SEM images on corrosion morphology of the alloy #1 after immersion in 3.5 wt.% NaCl solution for (a) 4 minutes and (b) 6 minutes.
Fig. 6 SEM images on the corrosion morphology of the alloys, (a) #1, (b) #2, (c) #3 and (d) #4 respectively, after 10 minutes immersion in 3.5 wt.% NaCl solution. 16
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Fig. 7 Morphology of the corrosion products formed on the surface of the alloys, (a) #1; (b) #2; (c) #3; (d) #4, after 10 minutes immersion in 3.5wt.% NaCl solution; (e) EDX analysis of the
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corrosion products on the alloy surfaces.
Fig. 8 Potentiodynamic polarization curves of the Mg-Al-Zn alloys.
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Fig. 9 (a) Nyquist plots of the alloys; (b) Bode plots of impedance vs frequency for the alloys; (c) Bode plots of phase angle vs frequency for the alloys
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EIS for the alloys #2 and #4.
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Fig.10 (a) Equivalent circuit of EIS for the alloys #1 and #3; (b) Equivalent circuit of
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Table 1 Crystallographic data of the second phases. Space group
I43m
Mg17Al12 (γ)
cI58
Mg21(Zn,Al)17 (Φ)
oP152
Pbcm
Mg44Zn41Al1 (q)
Quasicrystalline
Icosahedron
α-Mn
Lattice parameters [pm]
Mg21(Zn,Al)17
_
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Prototype
aP96
P1
Mg1Zn1
Ref.
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Pearson symbol
a= 105492
[32]
a = 897.9 b = 1698.8 c =1934 _
[32]
a = 106600 b = 106600 c = 171600
[32]
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Second phase
[33]
Table 2 The nominal and determined chemical compositions of the alloys and the volume percents of the second phases in each alloy.
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#1 (Mg+γ) #2 (Mg+Φ) #3 (Mg+q) #4 (Mg+MgZn)
Nominal composition (wt.%) Mg-12.5Al-2.5Zn Mg-5.6Al-9.4Zn Mg-3.3Al-11.7Zn Mg-1.0Al-14.0Zn
Real composition (ICP) (wt.%) Mg-12.1Al-2.3Zn Mg-5.9Al-9.2Zn Mg-3.7Al-12.5Zn Mg-0.8Al-14.4Zn
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Volume percent of the second phases 19.3 19.5 19.1 19.7
Table 3 Critical parameters determined from the potentiodynamic curves of the of the Mg-Al-Zn alloys.
Alloys #1 #2 #3 #4
Ecorr (VSCE) icorr (A cm-2) -1.525 -1.511 -1.518 -1.488
6.971E-3 1.175E-3 1.860E-3 7.300E-4
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R1
CPE1 2
R2
CPE2 2
R3
L 2
R4 2
2
CPE1-T(F/cm )
CPE-P
(Ωcm )
CPE2-T(F/cm )
CPE-P
(Ω cm )
(H cm )
(Ωcm )
#1
5.302
4.835E-4
0.771
3.016
2.802E-1
0.798
0.887
105.2
12.39
#2
5.118
1.865E-5
0.987
23.45
-
-
-
17.09
26.24
#3
5.081
1.846E-4
0.871
19.52
1.536E-2
0.998
0.58
147.5
84.53
#4
4.986
3.393E-5
0.949
27.18
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2
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50.46
37.06
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Highlights Two ternary and two binary second phases were confirmed stable at 300
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The roles of the second phases in corrosion processes were investigated. Corrosion property effected by different second phases were compared.
Corrosion mechanism was studied by corrosion morphology and the
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equivalent circuits.