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Corrosion behavior of Mg–5Al–xZn alloys in 3.5 wt.% NaCl solution
Accepted Manuscript Corrosion behavior of Mg-5Al-xZn alloys in 3.5 wt.% NaCl solution Nguyen Dang Nam, Motilal Mathesh PII: DOI: Reference:
S0925-8388(14)01593-X http://dx.doi.org/10.1016/j.jallcom.2014.07.014 JALCOM 31642
To appear in:
Journal of Alloys and Compounds
Received Date: Revised Date: Accepted Date:
28 November 2012 3 July 2014 3 July 2014
Please cite this article as: N.D. Nam, M. Mathesh, Corrosion behavior of Mg-5Al-xZn alloys in 3.5 wt.% NaCl solution, Journal of Alloys and Compounds (2014), doi: http://dx.doi.org/10.1016/j.jallcom.2014.07.014
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Corrosion behavior of Mg-5Al-xZn alloys in 3.5 wt.% NaCl solution Nguyen Dang Nam1,*, Motilal Mathesh2 1
Petroleum Department, Petrovietnam University, Ba Ria City, Ba Ria - Vung Tau Province 74000, Vietnam
2
School of Life and Environmental Sciences, Deakin University, Geelong Waurn Ponds Campus, Victoria 3220, Australia *Corresponding author Tel.: Tel.: +84 643 738 879, Fax: +84 643 733 579 E-mail address: [email protected]
ABSTRACT Five types of Mg-5Al alloys with different weight percentages of Zn ranging from 0 to 4 wt.% were examined using electrochemical techniques and surface analysis. The electrochemical results indicated that the Mg-5Al alloys containing Zn have a lower corrosion and hydrogen evolution rates than the Mg-5Al based specimens with a decrease of value being observed with the decrease in Zn content. Zn addition induced the precipitation of Mg-Al and Mg-Zn phases in the Mg matrix along with grain refinement and increased an interaction of Zn oxide with Mg and Al products serving as a corrosion barrier. Keywords: Mg alloys, Zinc, Microstructure, Grain refinement, Hydrogen evolution rate, Corrosion resistance
1
1. Introduction
Mg and its alloys are used as engineering materials in automotive, aerospace and electronic fields [1-3] that requires high specific toughness, high specific strength to weight ratio and lightweight properties [4]. Magnesium alloys have superior physical and mechanical properties which make them extremely attractive for applications in field which have requirement of lightweight materials. Research activities on magnesium alloys have increased significantly, which includes the development of: computational materials science and engineering approaches in alloy development together with thermodynamic and first-principles modeling; mechanistic understanding and development of creep-resistant casting alloys; mechanistic understanding and modeling of deformation, including mechanical twinning and dynamic recrystallization; and texture modification via alloying and processing [5]. However, as one of the most reactive metals, the poor intrinsic corrosion resistance of magnesium alloys has limited their widespread application. Improvements in the mechanical properties and corrosion resistance have led to greater interest of magnesium alloys for aerospace and special applications. There have been many attempts to improve the corrosion resistance of magnesium alloys by adding certain alloying elements [6-15], refining its microstructure [16-21], anti-corrosion coating [22-27], and control of the orientations [28,29]. At present, it has been successfully used to enhance the strength and ductility of wrought magnesium alloys produced via processes such as extrusion, rolling, forging, twin-roll strip casting and equal channel angular pressing [30-36]. Among these techniques, extrusion is very useful for its technical and economical advantages in the production of structural
2
components [37]. Several wrought magnesium alloys with stable secondary-phase particles have been developed using grain size refinement by hot extrusion method [3846]. In these processes, grain growth could be inhibited due to the dispersion or precipition of secondary-phase particles and an improved microstructure could be achieved by thermomechanical processes. A thinner film formation is featured by Al addition to Mg alloys which contains a mixture of MgO/Al2O3 and/or Mg(OH)2/Al(OH)3 ameliorating its protective behavior due to the presence of Al in the passive layer [47,48]. But, Al improves the corrosion resistance of Mg alloys only at higher concentrations. In recent years, Zn has been added to improve the mechanical properties [49-51]. The enhancement in the mechanical properties of these magnesium alloys is due to formation of a long periodic stacking structure [52-54]. It was reported that the addition of Zn element can effectively increase the strength and improve the plasticity. In addition, Zn has been showed to have a great potential for employment in manufacture of new generation biodegradable implants [45-57]. Zn addition also significantly refined the grain size of the extruded Mg-Mn alloy as well as enhanced the mechanical properties [58-60]. This reveals that Zn addition as an alloying element enhances the mechanical property and it is necessary to provide a base for the understanding of corrosion performance. In order to determine the corrosion behavior of Mg-5Al alloys in corrosive solutions, this work was carried out on, Mg-5Al, Mg-5Al-1Zn, Mg-5Al-2Zn, Mg-5Al-3Zn, Mg-5Al-4Zn alloys and conducted on the basics of electrochemical measurements and surface analysis.
2. Experimental
3
2.1. Specimen preparation Pure Mg (99.9 %) ingot was melted in a stainless steel crucible under the protection of gas mixture containing SF6 and CO2. The calculated amounts of 5 wt.% Al and 1, 2, 3, and 4 wt.% Zn were added to the Mg melt. After solidification, the ingots were subjected to homogenizing treatment at 400 ˚C for 14 h. The homogenized ingots were machined, which were used as raw materials for extrusion. The extrusion of billets was performed at 320 ˚C. In addition to this, extrusion ratio of 25:1 and speed of 0.15 mm/s. were applied. The chemical compositions of tested alloys were determined by Optical Emission Spectroscopy. Alloys with chemical compositions were 5.000 Al, 0.005 Si, 0.004 Fe, 0.003 Cu, 0.007 Ni, while the difference between measured and specified composition of Zn is imperceptible. The specimens for electrochemical tests were first cold-mounted on a mounting cup and then finished by grinding with 600-grit silicon carbide paper. 2.2. Electrochemical investigation methods All of the electrochemical experiments were performed at room temperature in 1000 ml of 0.6 M NaCl solution with aeration. The exposed area was 1 cm2. Potentiodynamic polarization tests were performed using an EG&G PAR 263A potentiostat for the DC measurements. A graphite counter electrode was used, with a saturated calomel electrode as the reference. Prior to the potentiodynamic polarization test, the samples were immersed in the solution for 1 h in order to stabilize the open-circuit potential. The potential of the electrodes was swept at a rate of 0.166 mV/s in the range from initial potential of -250 mV versus Ecorr to final potential of -1.3 VSCE. The electrochemical impedance spectroscopy (EIS) and corrosion potential measurements were conducted
4
using a IM6e system with a commercial software program for the AC measurements. The amplitude of the sinusoidal perturbation was 10 mV. The frequency range was from 100 kHz to 1 Hz. The hydrogen evolution rate of the alloys was investigated by immersion tests. The specimens, with dimensions of 10 mm × 10 mm × 2 mm, were prepared by grinding each side with 600-grid emery paper and degreasing the surfaces with ethanol prior to corrosion testing. The hydrogen evolution rate was used as an indicator of the corrosion rate which was monitored every 1 hour. Alloy specimens for hydrogen gas collection, to characterize the corrosion rate during solution immersion, were immersed in 1000 ml of 0.6 M NaCl solution with aeration. The hydrogen evolved during the corrosion experiment was collected in a burette above the corroding specimens. The overall magnesium corrosion reaction, Mg + H+ + H2O = Mg2+ + H2
(1)
shows that one molecule of hydrogen is evolved for each atom of corroded magnesium. 2.3. Surface analysis The crystal structure of the specimens was investigated by XRD using Cu Kα radiation. For the observation of the microstructure, optical microscope was used. The specimens were mechanically sanded with sand paper (#220, 600, 1200, 2000, and 4000) and then fine polished with 0.1 µm alumina powders. These specimens were then etched in a mixture of solution containing acetic acid (10 ml), picric acid (5 g), distilled water (10 ml) and ethanol (70 ml of 95% purity). To investigate the relationship between the electrochemical behavior and surface morphology, the specimens were examined by SEM after 6 hours of immersion test. The surface products were examined by X-ray photoelectron spectroscopy (XPS) after 1 h of the open-circuit potential.
5
3. Results and discussion
Fig. 1 compares the microstructures of the Mg-5Al alloys with different amounts of Zn content. The microstructures consisted of only the α-Mg matrix. The addition of Zn decreased the grain size of the Mg-5Al alloy and also showed appearance of an increased amount of new fine grains at the grain boundary due to the recrystallization. In general, the microstructure included primary α grains surrounded by new fine grains. The specimens had relatively smaller α-Mg grains with increasing Zn content which may influence the corrosion performance since uniform corrosion product is expected to act as a barrier. The phase compositions of the specimens were examined by XRD as shown in Fig. 2. There is no significant difference in the α-Mg peaks between the Mg5Al and Zn-containing specimens. The results also show the well-defined peaks of Mg and Mg17Al12 with an additional peak close to the reflections by Mg2Zn. The intensity of the Mg2Zn diffraction peaks increases with increasing Zn content due to an increase reaction between Mg and Zn. Fig. 3 (a) shows the hydrogen evolution rate and Fig. 3 (b) shows the corrosion rate of the alloys calculated by hydrogen evolution rate. All specimens exhibited an increase in hydrogen evolution rate with increasing immersion time during 7 h. The hydrogen evolution rate for Mg-5Al based alloy increased strongly with exposure time and were significantly larger than that of Zn-containing specimens. The hydrogen evolution volume of Mg-5Al-xZn alloys in Fig. 3 (a) can be ranked in a decreasing series as: Mg5Al > Mg-5Al-4Zn > Mg-5Al-3Zn > Mg-5Al-2Zn > Mg-5Al-1Zn. The result showed that the corrosion rate of the Zn-containing samples was stable during 7 h immersion time, while it increased quickly in case of Mg-5Al based alloy. The result suggested that
6
hydrogen evolution rate of a corroding magnesium alloy at its open current potential is equal to its corrosion rate and reflects to some degree the electrochemical activity of Mg alloy. In addition, hydrogen evolution rate allows studying the variation in corrosion rate with immersion time. Fig. 4 shows the polarization curves of the Mg-5Al alloys as a function of Zn content in 0.6 M NaCl solution. All alloys demonstrated active corrosion behavior, as the current density increased continuously with increasing potential. The corrosion current density increased with increase in Zn content. Table 1 lists the corrosion properties observed from potentiodynamic polarization. The corrosion rate was determined using the Tafel extrapolation method, based on Faraday’s law [61-63]: Corrosion rate (cm/y) =
3.16 × 10 7 × icorr × M z×F ×ρ
(2)
where icorr is the corrosion current density (A/cm 2), M is the molar mass of the metal (g/mole), z is the number of electrons transferred per metal atom, F is the Faraday’s constant, and ρ is the density of the metal (g/cm 3). Polarization resistance (Rp) value was obtained from eq (2) [64]: Rp =
β a βc
(3)
2.3 × icorr ( β a + β c )
where βa and βc are the anodic and cathodic Tafel slope, respectively. The corrosion current density of the alloys was calculated using Eq. (3) under the assumption that βa and βc are equal to 0.1 V/decade. Similarly, corrosion rate of the alloys was calculated from the EIS measurements by using Eq. (2). Fig. 5 (a) and (b) present the Nyquist and Bode plots after immersion for 1 h at Ecorr. The high spectra are used to detect the local surface defects, whereas the medium and
7
low frequency spectra detect the processes within the corrosion product and at the metal/corrosion product interface, respectively. The impedance spectrum of the Mg5Al-xZn alloy exhibits a capacitive loop in the high frequency range and an inductive loop in the low frequency range. The aperture of impedances and impedance modulus |Z| of the Zn-containing alloys are greater than that of the Mg-5Al based alloy which increase with a decrease of Zn content. These results confirmed the increase in impedance gained via the addition of Zn due to the formation of a corrosion product layer. The electrochemical response to the impedance tests for these materials was best simulated with the equivalent circuits as shown in Fig. 5 (c) [65], in which Rs represents the solution resistance between working electrode and reference electrode. Rc represents resistance of corrosion product layer, Cc represents capacitance of corrosion product layer, CPE represents the constant phase element for the double layer capacitance, Rct represents the charge transfer resistance during electrochemical reaction. A constantphase element representing a shift from an ideal capacitor was used instead of the capacitance itself, for simplicity. The impedance of a phase element is defined as: ZCPE = [C(jω)n]-1
(4)
where C is capacitance; j is the current; ω is the frequency and -1 = n = 1. The value of n seems to be associated with the non-uniform distribution of current as a result of roughness and surface defects. The n value of a CPE indicates: capacitance, Warburg impedance, resistance and an inductance when n=1, 0.5, 0, -1 respectively. In the present study, n was consistently maintained near 0.8, as a result of the deviation from ideal dielectric behavior. The low frequency inductance loop is described with Rdiff (inductance resistance) and L (inductance). In this case, the polarization resistance, Rp, is calculated from the equivalent circuit in Fig. 4 (c) as shown in the equation:
8
Rp = Rs + Rct + Rdiff
(5)
The fitting results are presented in Table 2. It indicates that resistances of Zn-containing specimens are much higher than that of Mg-5Al based alloy which increases with decrease of Zn content. Fig. 6 compares the average corrosion rate for alloys obtained from hydrogen evolution rate measurement and electrochemical methods. The corrosion rate in the case of the electrochemical measurements can be inferred from the corrosion current density, based on Faraday’s law. In addition, the hydrogen evolution volume rate, VH (ml/cm2.d) can be related to the corrosion rate according to the equation [66]: PH (cm/y) = 0.2279 VH
(6)
The average corrosion rate of the alloys using the above methods was ranked in the following order: Mg-5Al > Mg-5Al-4Zn > Mg-5Al-3Zn > Mg-5Al-2Zn > Mg-5Al-1Zn. It indicated that Zn-containing Mg-5Al alloy have lower corrosion rates than Mg-5Al based alloy, and the corrosion rate decreases with decreasing Zn content. In summary, the electrochemical and hydrogen evolution rate measurements showed that the corrosion resistance of Mg-5Al alloy can be improved significantly by Zn addition. Fig. 7 shows SEM images of the surface morphology after the hydrogen evolution test for 7 h immersion at Ecorr. A severe corrosion was observed on the Mg-5Al based alloy due to large number of hydrogen bubbles, while less corrosion was observed in case of Zn containing specimens, especially for Mg-5Al-1Zn. Fig. 8 shows XPS results of the specimen surfaces after exposing the alloys to the 0.6 M NaCl solution for 1 h at room temperature. This figure shows that the peaks of Mg, Al, Cl, Zn and O exist. Oxygen KVV correspond to peak in the region near 1100 eV. The narrow XPS spectra for Mg 2p, Al 2p, Zn 2p, Cl 2p, and O 1s regions are shown in
9
Figs. 8 (b, c, d, e and f), respectively. The spectra of Mg 2s, 2p and Al 2p corresponds to MgO/Mg(OH)2 and Al2O3/Al(OH)3 on the surface of the alloys. When specimens are immersed in 0.6 M NaCl solution, the corrosion attack on magnesium is severe due to easy penetration of the oxide/hydroxide products by Cl‾ ions and the formation of a basic chloride salt (MgCl2), which is readily accommodated in the layered structure of MgO/Mg(OH)2 and Al2O3/Al(OH)3. The O 1s spectra were composed of two peaks corresponding to the signals from oxygen in the oxide at 530.05 eV and oxygen in the hydroxyl groups at 531.70 eV. In addition, a small Cl peak was observed in the Mg-5Al1Zn specimen. This figure also shows the appearance of Al 2p and O 1s peaks which are enriched with decreasing Zn content, while lesser intensity Mg peak was obtained in the case of Mg-5Al alloy. The binding energy of Zn 2p was approximately 1020 eV as shown in Fig. 8 (f). The presence of ZnO on the surface of the Zn-containing alloys decreased the corrosion rate due to a decrease in the hydrogen evolution rate. Compared to the Zn 2p oxide peaks in Fig. 8 (f), the alloy contribution for the 1 wt.% Zncontaining alloy has higher peaks than the peaks from other alloys. The results indicated that the enriched Mg, Al and Zn products played an important role in improving the corrosion products of magnesium alloys, thereby providing better corrosion protection -
which inhibited the adsorption of Cl ions.
4. Conclusions
The addition of Zn caused a decrease in grain size of the α-Mg solid solution phase. The corrosion and hydrogen evolution rates of Zn-containing alloys were lower than that of Mg-5Al based alloy which increased with increasing Zn content. EIS showed
10
that the semicircle was less depressed in case of Zn-containing specimens. In addition, the total resistance also decreased with increasing Zn content. Zn addition to Mg-5Al alloys facilitates the formation and interaction of Mg, Al, and Zn oxides on the alloy surface. In addition, the amount of chloride in the surface products decreases with decreasing Zn content, indicating a more adherent corrosion products on the Mg-5Al1Zn alloy surface. Acknowledgement The authors are grateful for the support of Vietnam Oil and Gas Group, PetroVietnam University and the National Foundation for Sciences and Technology Development (NAFOSTED 2014).
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Figure captions Fig. 1. Optical microscope of (a) Mg-5Al, (b) Mg-5Al-1Zn, (c) Mg-5Al-2Zn, (d) Mg5Al-3Zn, and (e) Mg-5Al-4Zn alloys. Fig. 2. XRD patterns of (a) Mg-5Al, (b) Mg-5Al-1Zn, (c) Mg-5Al-2Zn, (d) Mg-5Al3Zn, and (e) Mg-5Al-4Zn alloys: a; α-Mg, b; Mg17Al12, and c; Mg2Zn. Fig. 3. (a) Hydrogen evolution and (b) Corrosion rate of the Mg-5Al-xZn samples immersed in 3.5 wt.% NaCl at room temperature as function of immersion time. Fig. 4. Potentiodynamic polarization curves of Mg-5Al as a function of Zn content. Fig. 5. Impedance spectra on (a) Nyquist plots and (b) Bode plots and (c) equivalent circuit for fitting the EIS data. Fig. 6. Average the corrosion rate calculated using potentiodynamic polarization, EIS, and the hydrogen evolution measurements in 3.5 wt.% NaCl at room temperature. Fig. 7. SEM images of the specimens tested after 6 h immersion in 3.5 wt.% NaCl at room temperature: (a) Mg-5Al, (b) Mg-5Al-1Zn, (c) Mg-5Al-2Zn, (d) Mg-5Al-3Zn, and (e) Mg-5Al-4Zn alloys. Fig. 8. XPS peak analysis for the surface products of the Mg-5Al-xZn alloys: (a) survey scan spectra and narrow scan spectra of (b) Mg, (c) Al, (d) Zn, (e) Cl, and (f) O.
17
(a)
(b)
18
(c)
(d)
19
(e) Fig. 1.
20
Intensity (Arb.units)
(e)
a a ba ab
c
a
a
a b
aa a a
a
(d) (c)
(b) (a)
20
30
40
50 60 2θ (Degree)
Fig. 2.
21
70
80
Mg-5Al Mg-5Al-1Zn Mg-5Al-2Zn Mg-5Al-3Zn Mg-5Al-4Zn
8
2
Volume of H2 (ml/cm )
10
6 4 2 0
0
1
2
3
4 5 Time (hr)
6
7
8
6
7
8
(a)
Corrosion Rate (cm/yr)
8
Mg-5Al Mg-5Al-1Zn Mg-5Al-2Zn Mg-5Al-3Zn Mg-5Al-4Zn
6
4
2
0
0
1
2
3
4 5 Time (hr)
(b) Fig. 3.
22
-1.2
Mg-5Al Mg-5Al-1Zn Mg-5Al-2Zn Mg-5Al-3Zn Mg-5Al-4Zn
Potential (VSCE)
-1.3 -1.4 -1.5 -1.6 -1.7 -1.8 10
-6
-5
10
-4
-3
10 10 2 Current Density (A/cm )
Fig. 4.
23
-2
10
10
-1
600
Mg-5Al Mg-5Al-1Zn Mg-5Al-2Zn Mg-5Al-3Zn Mg-5Al-4Zn
-Z" (Ω.cm 2)
400 200 0
-200 -400 0
200
400
600
800 1000 1200 1400 1600 2 Z' (Ω.cm )
10
2
80 Mg-5Al Mg-5Al-1Zn Mg-5Al-2Zn 60 Mg-5Al-3Zn Mg-5Al-4Zn 40 20
Phase Angle (Deg)
10
3
2
|Z| (Ω.cm )
(a)
0 10
1
-20 -1
10
10
0
10
1
2
3
10 10 Frequency (Hz)
(b)
24
10
4
10
5
10
6
Cc
Rs
CPE Rc L Rct Rdiff
(c) Fig. 5.
25
Average Corrosion Rate (cm/yr)
0.85 0.80 0.75 0.70 0.4 0.3 0.2 0.1
0Zn
1Zn 2Zn 3Zn Mg-5Al-xZn Alloys
Fig. 6.
26
4Zn
(a)
(b)
27
(c)
(d)
28
(e) Fig. 7.
29
Intensity (arb. units)
Mg-5Al Mg-5Al-1Zn Mg-5Al-2Zn Mg-5Al-3Zn Mg-5Al-4Zn
O1s
O KVV Zn2p
Mg2p Al2p Cl Mg2s 2p
0
200
400
600
800
1200
1000
Binding Energy (eV)
(a)
Mg2s
Intensity (arb. units)
Mg-5Al Mg-5Al-1Zn Mg-5Al-2Zn Mg-5Al-3Zn Mg-5Al-4Zn
78
80
82
84
86 88 90 92 Binding Energy (eV)
30
94
96
98
Mg-5Al-1Zn Mg-5Al-2Zn Mg-5Al-3Zn Mg-5Al-4Zn
Intensity (arb. units)
Mg2p
40
42
44
46 48 50 52 Binding Energy (eV)
54
56
58
(b)
Intensity (arb. units)
Mg-5Al Mg-5Al-1Zn Mg-5Al-2Zn Mg-5Al-3Zn Mg-5Al-4Zn
62
64
66
68
70 72 74 76 Binding Energy (eV)
(c)
31
78
80
82
Intensity (arb. units)
Mg-5Al-1Zn Mg-5Al-2Zn Mg-5Al-3Zn Mg-5Al-4Zn
1000
1010
1020 1030 1040 Binding Energy (eV)
1050
1060
(d)
Intensity (arb. units)
Mg-5Al Mg-5Al-1Zn Mg-5Al-2Zn Mg-5Al-3Zn Mg-5Al-4Zn
190
192
194
196 198 200 Binding Energy (eV)
(e)
32
202
204
206
Intensity (arb. units)
Mg-5Al Mg-5Al-1Zn Mg-5Al-2Zn Mg-5Al-3Zn Mg-5Al-4Zn
522
525
528 531 534 Binding Energy (eV)
(f) Fig. 8.
33
537
540
Table 1. Critical parameters from potentiodynamic polarization curves for Mg-5Al-xZn alloys in 3.5 wt.% NaCl as a function of Zn addition. icorr (µA/cm2 )
βa (V/decade)
-βc (V/decade)
-1.53 -1.54 -1.54 Average -1.54
367 370 366 368
0.051 0.025 0.023
0.199 0.170 0.160
-1.57
67
0.020
0.146
-1.54
65
0.018
0.134
-1.53
63
0.019
0.142
Average -1.55
65
-1.55
102
0.016
0.140
-1.57
124
0.019
0.126
-1.54
112
0.017
0.132
Average -1.55
113
-1.55
173
0.022
0.167
-1.55
163
0.015
0.155
-1.56
169
0.020
0.158
Average -1.55
168
-1.56
202
0.033
0.172
-1.57
172
0.035
0.186
-1.57
197
0.033
0.178
Average -1.56
190
Sample
Ecorr (VSCE)
#1
#2
#3
#4
#5
34
Table 2. Fitting results of EIS measurements (CP = corrosion product).
Specimen
Rs
CCP 2
CPE2
RCP 2
2
(Ω.cm ) (µF/cm ) (Ω.cm )
C
n
(µF/cm2)
(0~1)
Rct
Rdiff
L 2
2
(Ω.cm ) (H.cm )
(Ω.cm2)
#1
15.2
19.1
0.5
102.3
0.4293 131
260
66
#2
5.4
8.6
52.2
11.5
0.8937 903
3315
230
#3
5.5
10.9
32.2
32.5
0.8453 783
3121
148
#4
4.9
11.4
23.1
55.5
0.8239 695
3273
138
#5
5.2
14.0
10.4
61.0
0.8180 443
3107
108
35
Research highlights ► Zinc is found to be a good alloying element for corrosion resistance of Mg-5Al alloy in 3.5 wt.% NaCl solution. ► Zinc has an advantage of improving corrosion performance with decreasing zinc content. ► The benefits of the zinc in corrosion performance of Mg-5Al alloy is satisfactorily discussed and confirmed by electrochemical and surface analysis results.
36
S0925-8388(14)01593-X http://dx.doi.org/10.1016/j.jallcom.2014.07.014 JALCOM 31642
To appear in:
Journal of Alloys and Compounds
Received Date: Revised Date: Accepted Date:
28 November 2012 3 July 2014 3 July 2014
Please cite this article as: N.D. Nam, M. Mathesh, Corrosion behavior of Mg-5Al-xZn alloys in 3.5 wt.% NaCl solution, Journal of Alloys and Compounds (2014), doi: http://dx.doi.org/10.1016/j.jallcom.2014.07.014
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.
Corrosion behavior of Mg-5Al-xZn alloys in 3.5 wt.% NaCl solution Nguyen Dang Nam1,*, Motilal Mathesh2 1
Petroleum Department, Petrovietnam University, Ba Ria City, Ba Ria - Vung Tau Province 74000, Vietnam
2
School of Life and Environmental Sciences, Deakin University, Geelong Waurn Ponds Campus, Victoria 3220, Australia *Corresponding author Tel.: Tel.: +84 643 738 879, Fax: +84 643 733 579 E-mail address: [email protected]
ABSTRACT Five types of Mg-5Al alloys with different weight percentages of Zn ranging from 0 to 4 wt.% were examined using electrochemical techniques and surface analysis. The electrochemical results indicated that the Mg-5Al alloys containing Zn have a lower corrosion and hydrogen evolution rates than the Mg-5Al based specimens with a decrease of value being observed with the decrease in Zn content. Zn addition induced the precipitation of Mg-Al and Mg-Zn phases in the Mg matrix along with grain refinement and increased an interaction of Zn oxide with Mg and Al products serving as a corrosion barrier. Keywords: Mg alloys, Zinc, Microstructure, Grain refinement, Hydrogen evolution rate, Corrosion resistance
1
1. Introduction
Mg and its alloys are used as engineering materials in automotive, aerospace and electronic fields [1-3] that requires high specific toughness, high specific strength to weight ratio and lightweight properties [4]. Magnesium alloys have superior physical and mechanical properties which make them extremely attractive for applications in field which have requirement of lightweight materials. Research activities on magnesium alloys have increased significantly, which includes the development of: computational materials science and engineering approaches in alloy development together with thermodynamic and first-principles modeling; mechanistic understanding and development of creep-resistant casting alloys; mechanistic understanding and modeling of deformation, including mechanical twinning and dynamic recrystallization; and texture modification via alloying and processing [5]. However, as one of the most reactive metals, the poor intrinsic corrosion resistance of magnesium alloys has limited their widespread application. Improvements in the mechanical properties and corrosion resistance have led to greater interest of magnesium alloys for aerospace and special applications. There have been many attempts to improve the corrosion resistance of magnesium alloys by adding certain alloying elements [6-15], refining its microstructure [16-21], anti-corrosion coating [22-27], and control of the orientations [28,29]. At present, it has been successfully used to enhance the strength and ductility of wrought magnesium alloys produced via processes such as extrusion, rolling, forging, twin-roll strip casting and equal channel angular pressing [30-36]. Among these techniques, extrusion is very useful for its technical and economical advantages in the production of structural
2
components [37]. Several wrought magnesium alloys with stable secondary-phase particles have been developed using grain size refinement by hot extrusion method [3846]. In these processes, grain growth could be inhibited due to the dispersion or precipition of secondary-phase particles and an improved microstructure could be achieved by thermomechanical processes. A thinner film formation is featured by Al addition to Mg alloys which contains a mixture of MgO/Al2O3 and/or Mg(OH)2/Al(OH)3 ameliorating its protective behavior due to the presence of Al in the passive layer [47,48]. But, Al improves the corrosion resistance of Mg alloys only at higher concentrations. In recent years, Zn has been added to improve the mechanical properties [49-51]. The enhancement in the mechanical properties of these magnesium alloys is due to formation of a long periodic stacking structure [52-54]. It was reported that the addition of Zn element can effectively increase the strength and improve the plasticity. In addition, Zn has been showed to have a great potential for employment in manufacture of new generation biodegradable implants [45-57]. Zn addition also significantly refined the grain size of the extruded Mg-Mn alloy as well as enhanced the mechanical properties [58-60]. This reveals that Zn addition as an alloying element enhances the mechanical property and it is necessary to provide a base for the understanding of corrosion performance. In order to determine the corrosion behavior of Mg-5Al alloys in corrosive solutions, this work was carried out on, Mg-5Al, Mg-5Al-1Zn, Mg-5Al-2Zn, Mg-5Al-3Zn, Mg-5Al-4Zn alloys and conducted on the basics of electrochemical measurements and surface analysis.
2. Experimental
3
2.1. Specimen preparation Pure Mg (99.9 %) ingot was melted in a stainless steel crucible under the protection of gas mixture containing SF6 and CO2. The calculated amounts of 5 wt.% Al and 1, 2, 3, and 4 wt.% Zn were added to the Mg melt. After solidification, the ingots were subjected to homogenizing treatment at 400 ˚C for 14 h. The homogenized ingots were machined, which were used as raw materials for extrusion. The extrusion of billets was performed at 320 ˚C. In addition to this, extrusion ratio of 25:1 and speed of 0.15 mm/s. were applied. The chemical compositions of tested alloys were determined by Optical Emission Spectroscopy. Alloys with chemical compositions were 5.000 Al, 0.005 Si, 0.004 Fe, 0.003 Cu, 0.007 Ni, while the difference between measured and specified composition of Zn is imperceptible. The specimens for electrochemical tests were first cold-mounted on a mounting cup and then finished by grinding with 600-grit silicon carbide paper. 2.2. Electrochemical investigation methods All of the electrochemical experiments were performed at room temperature in 1000 ml of 0.6 M NaCl solution with aeration. The exposed area was 1 cm2. Potentiodynamic polarization tests were performed using an EG&G PAR 263A potentiostat for the DC measurements. A graphite counter electrode was used, with a saturated calomel electrode as the reference. Prior to the potentiodynamic polarization test, the samples were immersed in the solution for 1 h in order to stabilize the open-circuit potential. The potential of the electrodes was swept at a rate of 0.166 mV/s in the range from initial potential of -250 mV versus Ecorr to final potential of -1.3 VSCE. The electrochemical impedance spectroscopy (EIS) and corrosion potential measurements were conducted
4
using a IM6e system with a commercial software program for the AC measurements. The amplitude of the sinusoidal perturbation was 10 mV. The frequency range was from 100 kHz to 1 Hz. The hydrogen evolution rate of the alloys was investigated by immersion tests. The specimens, with dimensions of 10 mm × 10 mm × 2 mm, were prepared by grinding each side with 600-grid emery paper and degreasing the surfaces with ethanol prior to corrosion testing. The hydrogen evolution rate was used as an indicator of the corrosion rate which was monitored every 1 hour. Alloy specimens for hydrogen gas collection, to characterize the corrosion rate during solution immersion, were immersed in 1000 ml of 0.6 M NaCl solution with aeration. The hydrogen evolved during the corrosion experiment was collected in a burette above the corroding specimens. The overall magnesium corrosion reaction, Mg + H+ + H2O = Mg2+ + H2
(1)
shows that one molecule of hydrogen is evolved for each atom of corroded magnesium. 2.3. Surface analysis The crystal structure of the specimens was investigated by XRD using Cu Kα radiation. For the observation of the microstructure, optical microscope was used. The specimens were mechanically sanded with sand paper (#220, 600, 1200, 2000, and 4000) and then fine polished with 0.1 µm alumina powders. These specimens were then etched in a mixture of solution containing acetic acid (10 ml), picric acid (5 g), distilled water (10 ml) and ethanol (70 ml of 95% purity). To investigate the relationship between the electrochemical behavior and surface morphology, the specimens were examined by SEM after 6 hours of immersion test. The surface products were examined by X-ray photoelectron spectroscopy (XPS) after 1 h of the open-circuit potential.
5
3. Results and discussion
Fig. 1 compares the microstructures of the Mg-5Al alloys with different amounts of Zn content. The microstructures consisted of only the α-Mg matrix. The addition of Zn decreased the grain size of the Mg-5Al alloy and also showed appearance of an increased amount of new fine grains at the grain boundary due to the recrystallization. In general, the microstructure included primary α grains surrounded by new fine grains. The specimens had relatively smaller α-Mg grains with increasing Zn content which may influence the corrosion performance since uniform corrosion product is expected to act as a barrier. The phase compositions of the specimens were examined by XRD as shown in Fig. 2. There is no significant difference in the α-Mg peaks between the Mg5Al and Zn-containing specimens. The results also show the well-defined peaks of Mg and Mg17Al12 with an additional peak close to the reflections by Mg2Zn. The intensity of the Mg2Zn diffraction peaks increases with increasing Zn content due to an increase reaction between Mg and Zn. Fig. 3 (a) shows the hydrogen evolution rate and Fig. 3 (b) shows the corrosion rate of the alloys calculated by hydrogen evolution rate. All specimens exhibited an increase in hydrogen evolution rate with increasing immersion time during 7 h. The hydrogen evolution rate for Mg-5Al based alloy increased strongly with exposure time and were significantly larger than that of Zn-containing specimens. The hydrogen evolution volume of Mg-5Al-xZn alloys in Fig. 3 (a) can be ranked in a decreasing series as: Mg5Al > Mg-5Al-4Zn > Mg-5Al-3Zn > Mg-5Al-2Zn > Mg-5Al-1Zn. The result showed that the corrosion rate of the Zn-containing samples was stable during 7 h immersion time, while it increased quickly in case of Mg-5Al based alloy. The result suggested that
6
hydrogen evolution rate of a corroding magnesium alloy at its open current potential is equal to its corrosion rate and reflects to some degree the electrochemical activity of Mg alloy. In addition, hydrogen evolution rate allows studying the variation in corrosion rate with immersion time. Fig. 4 shows the polarization curves of the Mg-5Al alloys as a function of Zn content in 0.6 M NaCl solution. All alloys demonstrated active corrosion behavior, as the current density increased continuously with increasing potential. The corrosion current density increased with increase in Zn content. Table 1 lists the corrosion properties observed from potentiodynamic polarization. The corrosion rate was determined using the Tafel extrapolation method, based on Faraday’s law [61-63]: Corrosion rate (cm/y) =
3.16 × 10 7 × icorr × M z×F ×ρ
(2)
where icorr is the corrosion current density (A/cm 2), M is the molar mass of the metal (g/mole), z is the number of electrons transferred per metal atom, F is the Faraday’s constant, and ρ is the density of the metal (g/cm 3). Polarization resistance (Rp) value was obtained from eq (2) [64]: Rp =
β a βc
(3)
2.3 × icorr ( β a + β c )
where βa and βc are the anodic and cathodic Tafel slope, respectively. The corrosion current density of the alloys was calculated using Eq. (3) under the assumption that βa and βc are equal to 0.1 V/decade. Similarly, corrosion rate of the alloys was calculated from the EIS measurements by using Eq. (2). Fig. 5 (a) and (b) present the Nyquist and Bode plots after immersion for 1 h at Ecorr. The high spectra are used to detect the local surface defects, whereas the medium and
7
low frequency spectra detect the processes within the corrosion product and at the metal/corrosion product interface, respectively. The impedance spectrum of the Mg5Al-xZn alloy exhibits a capacitive loop in the high frequency range and an inductive loop in the low frequency range. The aperture of impedances and impedance modulus |Z| of the Zn-containing alloys are greater than that of the Mg-5Al based alloy which increase with a decrease of Zn content. These results confirmed the increase in impedance gained via the addition of Zn due to the formation of a corrosion product layer. The electrochemical response to the impedance tests for these materials was best simulated with the equivalent circuits as shown in Fig. 5 (c) [65], in which Rs represents the solution resistance between working electrode and reference electrode. Rc represents resistance of corrosion product layer, Cc represents capacitance of corrosion product layer, CPE represents the constant phase element for the double layer capacitance, Rct represents the charge transfer resistance during electrochemical reaction. A constantphase element representing a shift from an ideal capacitor was used instead of the capacitance itself, for simplicity. The impedance of a phase element is defined as: ZCPE = [C(jω)n]-1
(4)
where C is capacitance; j is the current; ω is the frequency and -1 = n = 1. The value of n seems to be associated with the non-uniform distribution of current as a result of roughness and surface defects. The n value of a CPE indicates: capacitance, Warburg impedance, resistance and an inductance when n=1, 0.5, 0, -1 respectively. In the present study, n was consistently maintained near 0.8, as a result of the deviation from ideal dielectric behavior. The low frequency inductance loop is described with Rdiff (inductance resistance) and L (inductance). In this case, the polarization resistance, Rp, is calculated from the equivalent circuit in Fig. 4 (c) as shown in the equation:
8
Rp = Rs + Rct + Rdiff
(5)
The fitting results are presented in Table 2. It indicates that resistances of Zn-containing specimens are much higher than that of Mg-5Al based alloy which increases with decrease of Zn content. Fig. 6 compares the average corrosion rate for alloys obtained from hydrogen evolution rate measurement and electrochemical methods. The corrosion rate in the case of the electrochemical measurements can be inferred from the corrosion current density, based on Faraday’s law. In addition, the hydrogen evolution volume rate, VH (ml/cm2.d) can be related to the corrosion rate according to the equation [66]: PH (cm/y) = 0.2279 VH
(6)
The average corrosion rate of the alloys using the above methods was ranked in the following order: Mg-5Al > Mg-5Al-4Zn > Mg-5Al-3Zn > Mg-5Al-2Zn > Mg-5Al-1Zn. It indicated that Zn-containing Mg-5Al alloy have lower corrosion rates than Mg-5Al based alloy, and the corrosion rate decreases with decreasing Zn content. In summary, the electrochemical and hydrogen evolution rate measurements showed that the corrosion resistance of Mg-5Al alloy can be improved significantly by Zn addition. Fig. 7 shows SEM images of the surface morphology after the hydrogen evolution test for 7 h immersion at Ecorr. A severe corrosion was observed on the Mg-5Al based alloy due to large number of hydrogen bubbles, while less corrosion was observed in case of Zn containing specimens, especially for Mg-5Al-1Zn. Fig. 8 shows XPS results of the specimen surfaces after exposing the alloys to the 0.6 M NaCl solution for 1 h at room temperature. This figure shows that the peaks of Mg, Al, Cl, Zn and O exist. Oxygen KVV correspond to peak in the region near 1100 eV. The narrow XPS spectra for Mg 2p, Al 2p, Zn 2p, Cl 2p, and O 1s regions are shown in
9
Figs. 8 (b, c, d, e and f), respectively. The spectra of Mg 2s, 2p and Al 2p corresponds to MgO/Mg(OH)2 and Al2O3/Al(OH)3 on the surface of the alloys. When specimens are immersed in 0.6 M NaCl solution, the corrosion attack on magnesium is severe due to easy penetration of the oxide/hydroxide products by Cl‾ ions and the formation of a basic chloride salt (MgCl2), which is readily accommodated in the layered structure of MgO/Mg(OH)2 and Al2O3/Al(OH)3. The O 1s spectra were composed of two peaks corresponding to the signals from oxygen in the oxide at 530.05 eV and oxygen in the hydroxyl groups at 531.70 eV. In addition, a small Cl peak was observed in the Mg-5Al1Zn specimen. This figure also shows the appearance of Al 2p and O 1s peaks which are enriched with decreasing Zn content, while lesser intensity Mg peak was obtained in the case of Mg-5Al alloy. The binding energy of Zn 2p was approximately 1020 eV as shown in Fig. 8 (f). The presence of ZnO on the surface of the Zn-containing alloys decreased the corrosion rate due to a decrease in the hydrogen evolution rate. Compared to the Zn 2p oxide peaks in Fig. 8 (f), the alloy contribution for the 1 wt.% Zncontaining alloy has higher peaks than the peaks from other alloys. The results indicated that the enriched Mg, Al and Zn products played an important role in improving the corrosion products of magnesium alloys, thereby providing better corrosion protection -
which inhibited the adsorption of Cl ions.
4. Conclusions
The addition of Zn caused a decrease in grain size of the α-Mg solid solution phase. The corrosion and hydrogen evolution rates of Zn-containing alloys were lower than that of Mg-5Al based alloy which increased with increasing Zn content. EIS showed
10
that the semicircle was less depressed in case of Zn-containing specimens. In addition, the total resistance also decreased with increasing Zn content. Zn addition to Mg-5Al alloys facilitates the formation and interaction of Mg, Al, and Zn oxides on the alloy surface. In addition, the amount of chloride in the surface products decreases with decreasing Zn content, indicating a more adherent corrosion products on the Mg-5Al1Zn alloy surface. Acknowledgement The authors are grateful for the support of Vietnam Oil and Gas Group, PetroVietnam University and the National Foundation for Sciences and Technology Development (NAFOSTED 2014).
11
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Figure captions Fig. 1. Optical microscope of (a) Mg-5Al, (b) Mg-5Al-1Zn, (c) Mg-5Al-2Zn, (d) Mg5Al-3Zn, and (e) Mg-5Al-4Zn alloys. Fig. 2. XRD patterns of (a) Mg-5Al, (b) Mg-5Al-1Zn, (c) Mg-5Al-2Zn, (d) Mg-5Al3Zn, and (e) Mg-5Al-4Zn alloys: a; α-Mg, b; Mg17Al12, and c; Mg2Zn. Fig. 3. (a) Hydrogen evolution and (b) Corrosion rate of the Mg-5Al-xZn samples immersed in 3.5 wt.% NaCl at room temperature as function of immersion time. Fig. 4. Potentiodynamic polarization curves of Mg-5Al as a function of Zn content. Fig. 5. Impedance spectra on (a) Nyquist plots and (b) Bode plots and (c) equivalent circuit for fitting the EIS data. Fig. 6. Average the corrosion rate calculated using potentiodynamic polarization, EIS, and the hydrogen evolution measurements in 3.5 wt.% NaCl at room temperature. Fig. 7. SEM images of the specimens tested after 6 h immersion in 3.5 wt.% NaCl at room temperature: (a) Mg-5Al, (b) Mg-5Al-1Zn, (c) Mg-5Al-2Zn, (d) Mg-5Al-3Zn, and (e) Mg-5Al-4Zn alloys. Fig. 8. XPS peak analysis for the surface products of the Mg-5Al-xZn alloys: (a) survey scan spectra and narrow scan spectra of (b) Mg, (c) Al, (d) Zn, (e) Cl, and (f) O.
17
(a)
(b)
18
(c)
(d)
19
(e) Fig. 1.
20
Intensity (Arb.units)
(e)
a a ba ab
c
a
a
a b
aa a a
a
(d) (c)
(b) (a)
20
30
40
50 60 2θ (Degree)
Fig. 2.
21
70
80
Mg-5Al Mg-5Al-1Zn Mg-5Al-2Zn Mg-5Al-3Zn Mg-5Al-4Zn
8
2
Volume of H2 (ml/cm )
10
6 4 2 0
0
1
2
3
4 5 Time (hr)
6
7
8
6
7
8
(a)
Corrosion Rate (cm/yr)
8
Mg-5Al Mg-5Al-1Zn Mg-5Al-2Zn Mg-5Al-3Zn Mg-5Al-4Zn
6
4
2
0
0
1
2
3
4 5 Time (hr)
(b) Fig. 3.
22
-1.2
Mg-5Al Mg-5Al-1Zn Mg-5Al-2Zn Mg-5Al-3Zn Mg-5Al-4Zn
Potential (VSCE)
-1.3 -1.4 -1.5 -1.6 -1.7 -1.8 10
-6
-5
10
-4
-3
10 10 2 Current Density (A/cm )
Fig. 4.
23
-2
10
10
-1
600
Mg-5Al Mg-5Al-1Zn Mg-5Al-2Zn Mg-5Al-3Zn Mg-5Al-4Zn
-Z" (Ω.cm 2)
400 200 0
-200 -400 0
200
400
600
800 1000 1200 1400 1600 2 Z' (Ω.cm )
10
2
80 Mg-5Al Mg-5Al-1Zn Mg-5Al-2Zn 60 Mg-5Al-3Zn Mg-5Al-4Zn 40 20
Phase Angle (Deg)
10
3
2
|Z| (Ω.cm )
(a)
0 10
1
-20 -1
10
10
0
10
1
2
3
10 10 Frequency (Hz)
(b)
24
10
4
10
5
10
6
Cc
Rs
CPE Rc L Rct Rdiff
(c) Fig. 5.
25
Average Corrosion Rate (cm/yr)
0.85 0.80 0.75 0.70 0.4 0.3 0.2 0.1
0Zn
1Zn 2Zn 3Zn Mg-5Al-xZn Alloys
Fig. 6.
26
4Zn
(a)
(b)
27
(c)
(d)
28
(e) Fig. 7.
29
Intensity (arb. units)
Mg-5Al Mg-5Al-1Zn Mg-5Al-2Zn Mg-5Al-3Zn Mg-5Al-4Zn
O1s
O KVV Zn2p
Mg2p Al2p Cl Mg2s 2p
0
200
400
600
800
1200
1000
Binding Energy (eV)
(a)
Mg2s
Intensity (arb. units)
Mg-5Al Mg-5Al-1Zn Mg-5Al-2Zn Mg-5Al-3Zn Mg-5Al-4Zn
78
80
82
84
86 88 90 92 Binding Energy (eV)
30
94
96
98
Mg-5Al-1Zn Mg-5Al-2Zn Mg-5Al-3Zn Mg-5Al-4Zn
Intensity (arb. units)
Mg2p
40
42
44
46 48 50 52 Binding Energy (eV)
54
56
58
(b)
Intensity (arb. units)
Mg-5Al Mg-5Al-1Zn Mg-5Al-2Zn Mg-5Al-3Zn Mg-5Al-4Zn
62
64
66
68
70 72 74 76 Binding Energy (eV)
(c)
31
78
80
82
Intensity (arb. units)
Mg-5Al-1Zn Mg-5Al-2Zn Mg-5Al-3Zn Mg-5Al-4Zn
1000
1010
1020 1030 1040 Binding Energy (eV)
1050
1060
(d)
Intensity (arb. units)
Mg-5Al Mg-5Al-1Zn Mg-5Al-2Zn Mg-5Al-3Zn Mg-5Al-4Zn
190
192
194
196 198 200 Binding Energy (eV)
(e)
32
202
204
206
Intensity (arb. units)
Mg-5Al Mg-5Al-1Zn Mg-5Al-2Zn Mg-5Al-3Zn Mg-5Al-4Zn
522
525
528 531 534 Binding Energy (eV)
(f) Fig. 8.
33
537
540
Table 1. Critical parameters from potentiodynamic polarization curves for Mg-5Al-xZn alloys in 3.5 wt.% NaCl as a function of Zn addition. icorr (µA/cm2 )
βa (V/decade)
-βc (V/decade)
-1.53 -1.54 -1.54 Average -1.54
367 370 366 368
0.051 0.025 0.023
0.199 0.170 0.160
-1.57
67
0.020
0.146
-1.54
65
0.018
0.134
-1.53
63
0.019
0.142
Average -1.55
65
-1.55
102
0.016
0.140
-1.57
124
0.019
0.126
-1.54
112
0.017
0.132
Average -1.55
113
-1.55
173
0.022
0.167
-1.55
163
0.015
0.155
-1.56
169
0.020
0.158
Average -1.55
168
-1.56
202
0.033
0.172
-1.57
172
0.035
0.186
-1.57
197
0.033
0.178
Average -1.56
190
Sample
Ecorr (VSCE)
#1
#2
#3
#4
#5
34
Table 2. Fitting results of EIS measurements (CP = corrosion product).
Specimen
Rs
CCP 2
CPE2
RCP 2
2
(Ω.cm ) (µF/cm ) (Ω.cm )
C
n
(µF/cm2)
(0~1)
Rct
Rdiff
L 2
2
(Ω.cm ) (H.cm )
(Ω.cm2)
#1
15.2
19.1
0.5
102.3
0.4293 131
260
66
#2
5.4
8.6
52.2
11.5
0.8937 903
3315
230
#3
5.5
10.9
32.2
32.5
0.8453 783
3121
148
#4
4.9
11.4
23.1
55.5
0.8239 695
3273
138
#5
5.2
14.0
10.4
61.0
0.8180 443
3107
108
35
Research highlights ► Zinc is found to be a good alloying element for corrosion resistance of Mg-5Al alloy in 3.5 wt.% NaCl solution. ► Zinc has an advantage of improving corrosion performance with decreasing zinc content. ► The benefits of the zinc in corrosion performance of Mg-5Al alloy is satisfactorily discussed and confirmed by electrochemical and surface analysis results.
36