- Email: [email protected]
The corrosion behavior of AZ31 alloy in chloride and sulfate media – A comparative study through electrochemical investigations
ARTICLE IN PRESS
JID: JMAA
[m5+;December 28, 2018;15:54]
Available online at www.sciencedirect.com
Journal of Magnesium and Alloys xxx (xxxx) xxx www.elsevier.com/locate/jma
Full Length Article
The corrosion behavior of AZ31 alloy in chloride and sulfate media – A comparative study through electrochemical investigations M. Gururaj Acharya, A. Nityanansda Shetty∗ Department of Chemistry, National Institute of Technology Karnataka, Surathkal, Srinivasnagar, Mangalore, Karnataka 575 025, India Received 8 June 2018; received in revised form 6 September 2018; accepted 19 September 2018 Available online xxx
Abstract The magnesium alloys are considered to be the best structural materials, because of their advantageous weight to strength ratio. But, the limitation in their real field applications lies on the fact that magnesium alloys are highly susceptible for corrosion. The corrosion behaviour of AZ31 alloy was investigated by electrochemical methods in sodium chloride and sodium sulphate of different concentrations at different temperatures. The corrosion rate was monitored by potentiodynamic polarization technique and electrochemical impedance spectroscopy method. The surface morphology and surface composition of the freshly polished surface of the alloy was compared with that of the corroded surface by recording their SEM images and EDS, respectively. The results showed that the corrosion rate of AZ31 alloy increased with the increase in the temperature of the medium and also with the increase in the salt concentration of the medium. The activation parameters for the corrosion process were calculated and interpreted. © 2019 Published by Elsevier B.V. on behalf of Chongqing University. This is an open access article under the CC BY-NC-ND license. (http://creativecommons.org/licenses/by-nc-nd/4.0/) Peer review under responsibility of Chongqing University Keywords: AZ31 alloy; Polarization techniques; EIS; SEM.
1. Introduction In the last few years, magnesium alloys have been the materials of great interest in automobile industry. Magnesium, with a specific gravity of 1.74 is one of the lightest structural metal, extracted from its principal ores dolomite, magnesite and carnallite [1]. Magnesium alloys possess many excellent properties, such as light mass, high specific strength, high thermal conductivity, electromagnetic interference resistance and ability to be recycled [2–5]. But the poor corrosion resistance has limited their wider applications. Magnesium is highly susceptible for corrosion in most of the inorganic acidic and neutral solutions high [6–14]. The localized corrosion in Mg alloys is considered to be irregular, which spread laterally and cover the whole surface, without going deep inside. There are two main reasons for poor corrosion resistance; the internal galvanic corrosion by second phases
∗
Corresponding author. E-mail address: [email protected] (A.N. Shetty).
or impurities, and the poor stability of the hydroxide film formed above the surface of magnesium alloy [15]. One of the important alloying elements for Magnesium is aluminum [16]. Usually commercial AZ alloys contain less than 10 wt% aluminum, with zinc and manganese as secondary alloying elements. Mg-Al-Zn series alloys are majorly used for structural applications, particularly in automobile industries [17]. The alloying elements significantly influence the mechanical, physical and chemical properties of magnesium alloys. Aluminum significantly improves the tensile strength by the formation of an intermetallic β- phase (Mg17 Al12 ) [15]. The presence aluminum also increases the ambient compressive and fatigue strength and improves the castability of the alloy [18,19]. In AZ series alloys, the secondary alloying element, Zn solidifies as a sufficiently fine grain to meet most of the physical and chemical property requirements [20]. Manganese improves the corrosion resistance of magnesium alloys by removing iron and other heavy metal elements to avoid the formation of harmful intermetallic compounds. Also, manganese refines the grain size and improves the welding properties of magnesium alloys [21].
https://doi.org/10.1016/j.jma.2018.09.003 2213-9567/© 2019 Published by Elsevier B.V. on behalf of Chongqing University. This is an open access article under the CC BY-NC-ND license. (http://creativecommons.org/licenses/by-nc-nd/4.0/) Peer review under responsibility of Chongqing University Please cite this article as: M.G. Acharya and A.N. Shetty, The corrosion behavior of AZ31 alloy in chloride and sulfate media – A comparative study through electrochemical investigations, Journal of Magnesium and Alloys, https:// doi.org/ 10.1016/ j.jma.2018.09.003
ARTICLE IN PRESS
JID: JMAA
2
[m5+;December 28, 2018;15:54]
M.G. Acharya and A.N. Shetty / Journal of Magnesium and Alloys xxx (xxxx) xxx Table 1 Composition of AZ31 alloy (% by weight). Chemical composition
Al
Zn
Mn
Si
Cu
Ni
Fe
Mg
(Mass fraction)
2.96
0.83
0.43
0.004
0.004
<0.001
0.002
Balance
However, there are many differing reports about the influence of aluminum on the corrosion resistance of magnesium alloy. Lunder and his coworkers reported that the corrosion resistance of magnesium alloy could be greatly improved when the aluminum content reaches 8% (mass fraction) [22]. The reports are also available, indicating that 5% Al in magnesium alloy improved the corrosion resistance significantly [23]. The studies carried out on the corrosion behavior of magnesium alloys with aluminum content of 9.0%−62.3% revealed that aluminum content from 9.6% to 23.4% was beneficial to improving the corrosion resistance of magnesium alloy [24]. AZ31 alloy is one of the best structural alloy used in aerospace and automobile industries, for its straight sheet like shape, which has been used as brackets and structural bones of aircrafts where they get exposed to different temperatures, aqueous medium such as acid rain, salt water, etc. When compared with other commercially available magnesium alloys, AZ31 alloy has excellent properties, such as ultra-low density, good energy absorption and excellent damping performance [5,25–27]. But, attention must be paid to its corrosion properties as it has low corrosion resistance. There are only a few reports in the literature, on the corrosion behavior of AZ31 magnesium alloy in dilute sodium chloride solutions and sodium sulfate solutions [28,29]. However, a detailed study on the effect of concentration of the media and temperature are not reported. This paper reports the detailed study on the corrosion behavior of AZ31 alloy in NaCl and Na2 SO4 media, by varying the concentrations of salts and also by varying the temperature of the corrosive media. A comparison on the corrosion behavior of the alloy in the two media are discussed herein.
2. Experimental 2.1. Material The specimen studied was magnesium alloy AZ31. The composition of the alloy sample is given in Table 1. The working electrode received as a sheet was cut and embedded in epoxy resin, exposing a definite open surface area of 0.69 cm2 . This coupon was polished as per standard metallographic practice, belt grinding, followed by polishing on emery paper of grade 600, 800, 1000, 1200, 1500, 2000; and finally on polishing wheel using legated alumina abrasive to obtain a mirror finish. The polished specimen was washed with double distilled water, degreased with acetone and dried before immersing in the sodium chloride medium.
2.2. Medium The electrolyte media of five concentrations 0.05 M, 0.1 M, 0.15 M, 0.2 M, 0.25 M were prepared by dissolving appropriate amounts of, analytical grade samples of sodium chloride salt and sodium sulfate salt, separately, in double distilled water. The corrosion studies were carried out at temperatures 30 °C, 35 °C, 40 °C, 45 °C and 50 °C in a calibrated thermostat. 2.3. Electrochemical measurements Electrochemical measurements were carried out using electrochemical work station, Gill AC having ACM instrument Version 5 software. The arrangement employed was a conventional three-electrode Pyrex glass cell with a platinum counter electrode, a saturated calomel electrode (SCE) as reference and the AZ31 alloy specimen as the working electrode. All the values of potential reported are referred to the SCE. The polarization studies were carried out immediately after the EIS studies on the same exposed electrode surface without any additional surface treatment. 2.3.1. Potentiodynamic polarization studies Well-polished AZ31 alloy specimen coupon was exposed to the corrosion medium and allowed to establish a steadystate open circuit potentials (OCP). The potentiodynamic current potential curves (Tafel curves) were recorded by polarizing the specimen to −250 mV cathodically and +250 mV anodically, relative to the OCP at a scan rate of 1 mV s−1 . 2.3.2. Electrochemical impedance spectroscopy (EIS) studies Impedance measurements were performed at open circuit potential (OCP) by the application of a periodic small amplitude (10 mV) ac voltage signal with a wide spectrum of frequency ranging from 100 kHz to 0.01 Hz. The impedance data were analysed using Nyquist plots. In all the above measurements, at least three similar results were considered and their average values have been reported. 2.4. Scanning electron microscopy (SEM) analysis The surface morphology of the fresh polished surface and the corroded surface of the AZ31 alloy, in the corrosive media were analyzed through the SEM images recorded using JEOL JSM-6380LA analytical scanning electron microscope. The surface composition of the fresh surface and corroded surface were determined from their corresponding EDX spectra.
Please cite this article as: M.G. Acharya and A.N. Shetty, The corrosion behavior of AZ31 alloy in chloride and sulfate media – A comparative study through electrochemical investigations, Journal of Magnesium and Alloys, https:// doi.org/ 10.1016/ j.jma.2018.09.003
ARTICLE IN PRESS
JID: JMAA
[m5+;December 28, 2018;15:54]
M.G. Acharya and A.N. Shetty / Journal of Magnesium and Alloys xxx (xxxx) xxx
3
Fig. 1. Potentiodynamic polarization curves for the corrosion of AZ31 alloy in NaCl solutions of different concentrations at 50 °C.
3. Results and discussions 3.1. Potentiodynamic polarization measurements The corrosion behavior of AZ31 alloy was investigated sodium chloride and sodium sulfate media of different concentrations, at different temperatures by potentiodynamic polarization method. The potentiodynamic polarization curves for the corrosion of AZ31 alloy in sodium chloride media of different concentrations are presented in Fig. 1. The corresponding curves in sodium sulfate media of different concentrations at 40 °C presented in Fig. 2. Similar curves were obtained at other temperatures also. It can be observed from Figs. 1 and 2 that the polarization curves are shifted to the higher current density region, indicating an increase in the corrosion rate with the increase in chloride and sulfate concentration in the corrosion media. The anodic polarization curves represent the anodic oxidation of the magnesium alloy, while the cathodic curves represent the hydrogen evolution through reduction of water at the cathode. The anodic curves show the inflection points, characterized by two different slopes, indicating a kinetic barrier effect, possibly due to the deposition of a surface film of magnesium hydroxide, followed by its dissolution at higher anodic potential [30,31]. It is also observed from Figs. 1 and 2 that there is no significant change in overall shapes of the Tafel branches with varying salt concentration in the corrosion media, indicating that medium concentration only alters the rate, without altering the mechanism of corrosion reaction [30,31].
The corrosion current density (icorr ) was deduced by extrapolating the cathodic branch of the polarization curves to the OCP, as the anodic curves do not possess distinct Tafel regions. The corrosion rate (υ corr ) was calculated using the following equation [32]: υcorr (mmy−1 ) =
K × icorr × E W ρ
(1)
where, the constant, K = 0.00327, defines the unit of corrosion rate (mm y−1 ), icorr is the corrosion current density in μA cm−2 , ρ is the density of the corroding material, 1.84 g m−3 , EW is the equivalent weight of the alloy calculated using equation shown below: 1 EW = ni × f i
(2)
wi
where fi is the weight fraction of the ith element in the alloy, wi is the atomic weight of the ith element in the alloy and ni is the valence of the ith element of the alloy. The potentiodynamic polarization parameters, including corrosion potential (Ecorr ), corrosion current (icorr ), cathodic slopes (bc ) and corrosion rate (υ) are summarized in Tables 2 and 3 for the corrosion of AZ31 alloy in NaCl and Na2 SO4 media, respectively. It is evident from the data in Tables 2 and 3, that the corrosion rate increases with the increase in the concentration of chloride / sulfate in the corrosion medium. These ions are corrosive towards magnesium and its alloys because of their tendency to cause surface film breakdown by the disso-
Please cite this article as: M.G. Acharya and A.N. Shetty, The corrosion behavior of AZ31 alloy in chloride and sulfate media – A comparative study through electrochemical investigations, Journal of Magnesium and Alloys, https:// doi.org/ 10.1016/ j.jma.2018.09.003
ARTICLE IN PRESS
JID: JMAA
4
[m5+;December 28, 2018;15:54]
M.G. Acharya and A.N. Shetty / Journal of Magnesium and Alloys xxx (xxxx) xxx
Fig. 2. Potentiodynamic polarization curves for the corrosion of AZ31 alloy in Na2 SO4 solutions of different concentrations at 40 °C.
Table 2 Electrochemical polarization parameters for the corrosion of AZ31 alloy in NaCl medium with different temperatures. Concentration (mol dm−3 )
Temperature (°C)
Ecorr (mV/ SCE)
icorr (μA cm−2 )
β c (mV dec−1 )
ʋcorr (mm y−1 )
Rp (ohm cm2 )
0.05
30 35 40 45 50
−1514 −1490 −1500 −1503 −1493
41.11 87.10 96.09 125.20 145.39
99.7 117.1 119.1 114.2 130.3
0.89 1.89 2.08 2.72 3.16
870.9 860.5 615.4 612.6 584.8
0.1
30 35 40 45 50
−1483 −1489 −1502 −1516 −1490
154.03 171.58 172.72 225.52 235.47
135.3 141.6 158.2 162.9 173.4
3.34 3.73 3.75 4.90 5.11
839.2 546.5 526.3 483.5 370
0.15
30 35 40 45 50
−1537 −1500 −1515 −1491 −1475
210.80 255.51 303.27 351.00 370.3
140.1 145.7 158.8 160.9 184.5
4.58 5.55 6.59 7.63 8.05
568.8 492.3 398.7 353.4 239.2
0.2
30 35 40 45 50
−1500 −1485 −1483 −1511 −1497
432.55 457.04 489.13 687.77 709.98
189.5 214.7 181.4 211.3 171.5
9.40 9.93 10.63 14.75 15.43
247.0 268.9 248.6 230.0 226.3
0.25
30 35 40 45 50
−1526 −1515 −1488 −1510 −1494
520.41 553.44 577.18 608.31 792.81
190.4 192.9 188.0 209.2 186.4
11.31 12.03 12.54 13.22 17.23
435.1 337.4 271.6 221.9 180.3
Please cite this article as: M.G. Acharya and A.N. Shetty, The corrosion behavior of AZ31 alloy in chloride and sulfate media – A comparative study through electrochemical investigations, Journal of Magnesium and Alloys, https:// doi.org/ 10.1016/ j.jma.2018.09.003
ARTICLE IN PRESS
JID: JMAA
[m5+;December 28, 2018;15:54]
M.G. Acharya and A.N. Shetty / Journal of Magnesium and Alloys xxx (xxxx) xxx
5
Table 3 Electrochemical polarization parameters for the corrosion of AZ31 alloy in Na2 SO4 medium with different temperatures. Concentration (mol dm−3 )
Temperature(°C)
Ecorr (mv/ SCE)
icorr (μA cm−2 )
β c (mv dec−1 )
υ corr (mm y−1 )
Rp (ohm cm2 )
0.05
30 35 40 45 50
−1483 −1470 −1502 −1465 −1481
63.33 110.24 164.74 179.03 190.03
93.0 110.8 125.9 154.8 144.8
1.37 2.39 3.58 3.89 4.13
1080.0 877.0 810.5 800.8 766.0
0.1
30 35 40 45 50
−1522 −1528 −1485 −1481 −1456
71.20 157.60 191.24 202.70 208.41
117.8 134.6 140.9 150.6 157.3
1.54 3.42 4.15 4.40 4.53
803.0 754.0 657.0 580.0 582.8
0.15
30 35 40 45 50
−1457 −1457 −1469 −1446 −1448
218.14 322.28 445.19 461.63 546.62
138.1 142.7 151.6 158.2 170.2
4.74 7.00 9.67 10.03 11.88
854.0 763.0 610.0 503.5 445.1
0.2
30 35 40 45 50
−1505 −1493 −1489 −1472 −1480
235.63 354.34 470.87 542.13 672.86
152.2 158.0 164.9 172.7 197.9
5.12 7.70 10.23 11.78 14.62
527.3 511.9 451.9 439.0 405.0
0.25
30 35 40 45 50
−1451 −1411 −1438 −1486 −1463
372.80 497.12 516.71 583.03 727.89
147.1 158.6 170.2 190.9 201.6
8.10 10.80 11.23 12.67 15.82
793.1 600.8 315.8 279.5 270.2
lution of the deposited corrosion product, thereby increasing the anodic dissolution of the alloy. The oxidation and the corresponding corrosion of magnesium in aqueous solution can be represented by the following reactions [33]: Mg → Mg+ + e−
(3)
Mg → Mg2+ + 2e−
(4)
The steady state working potential of magnesium is about –1.5 V, even though the standard electrode potential of magnesium is –2.38 V. The change in potential is due to the formation of Mg(OH)2 film on the surface of the alloy [34]. However, this hydroxide layer is discontinuous and only partially covers the alloy surface, without effectively effective protecting the alloy surface. The anodic dissolution of magnesium and its alloys involve two oxidation processes. At more active potentials of about –2.78 V (vs SCE) magnesium undergoes oxidized to monovalent magnesium ion (Mg+ ) and at slightly higher potentials of about –1.56 V (vs SCE), magnesium undergoes oxidation to divalent magnesium ion (Mg2+ ), in parallel with the former oxidation [35]. The monovalent magnesium ion is unstable and undergoes oxidation to divalent magnesium ion through a series of reactions involving unstable intermediates like magnesium hydride as shown in equations below: Mg+ + 2H+ + 3e− → MgH2
(5)
MgH2 + 2H2 O → Mg2+ + 2OH− + 2H2
(6)
Mg2+ + 2OH− → Mg(OH )2
(7)
2Mg+ + 2H2 O → Mg2+ + Mg(OH )2 + H2
(8)
The microstructure of AZ31 alloy consists of the α-Mg matrix, distributed with the secondary β-phase, comprising of intermetallic compound Mg17 Al12 . The secondary β-phase is cathodic to the Mg matrix and is with a good passive behavior over a broad range of pH [21]. However, the role of β-phase in the corrosion process depends upon its size and distribution [36,37]. When the grain size of magnesium alloy is small and the mass fraction of β-phase is high, the distribution of the β-phase on the α-Mg matrix is continuous; providing a corrosion protection by a barrier layer effect. On the other hand, when the grain size is larger and the β-phases cannot cover the α-phase completely, galvanic corrosion results with the α-phase acting as anode and undergoing corrosion. The corrosion product, Mg(OH)2 , precipitates over the α-Mg matrix and this surface film is only partially selective as the continuity of the film is interrupted by the presence of secondary phase. In addition, the secondary oxidation results in chemical liberation of hydrogen gas at the anode. This rapid liberation of hydrogen gas at the anodic sites causes the breakdown of the surface film at higher anodic overvoltage; and accounts for the inflection in anodic branch of Tafel plots [38]. The corrosion product film on the magnesium surface is reported to be multilayered, with an inner layer of MgO–
Please cite this article as: M.G. Acharya and A.N. Shetty, The corrosion behavior of AZ31 alloy in chloride and sulfate media – A comparative study through electrochemical investigations, Journal of Magnesium and Alloys, https:// doi.org/ 10.1016/ j.jma.2018.09.003
JID: JMAA
6
ARTICLE IN PRESS
[m5+;December 28, 2018;15:54]
M.G. Acharya and A.N. Shetty / Journal of Magnesium and Alloys xxx (xxxx) xxx
Fig. 3. Nyquist plots for the corrosion of AZ31 alloy in NaCl solutions of different concentrations at 50 °C.
Mg(OH)2 , an intermediate thin film of MgO and an outer thick porous layer of Mg(OH)2 [15]. Further studies have confirmed the partial protective nature of the surface film and occurrence of corrosion reactions at the breaks and imperfections of the film [16,39,40]. The results also indicate that the corrosion rate in NaCl medium is higher than that in Na2 SO4 medium. This fact can be related to the higher conductivity of NaCl, which in turn has less resistivity for the flow of ions in the solution. Though sulfate is a mild corrosive compared to chloride, sulfate has been reported to possess an appreciable influence on the electrochemical behavior of pure magnesium and some of its alloys [41,34]. 3.2. Electrochemical impedance spectroscopy The electrochemical impedance spectra in the form of Nyquist plots for the corrosion of AZ31 alloy in NaCl solutions of different concentrations, at 50 °C are presented in Fig. 3. The Nyquist plots for the corrosion of the alloy in Na2 SO4 solutions of different concentrations, at 40 °C are shown in Fig. 4. Similar plots were obtained at other temperatures also. All the Nyquist plots consist of two capacitive loops at the higher and the medium frequencies, and the beginning of an inductive loop at the lower frequency region. The higher region frequency (hf) semicircle corresponds to the charge transfer of corrosion process and oxide film effects, and the medium frequency (mf) semicircle corresponds to the mass transport (diffusion of magnesium ions) through the
corrosion product layer of Mg(OH)2 . The relaxation of surface adsorbed species like Mg(OH)+ and Mg(OH)2 is considered to be the genesis of the lower frequency (lf) inductive loop. Though there are different versions interpreting the impedance of magnesium alloy corrosion processes, the current explanation has been one of the most adopted [34,39,42]. It is observed from Figs. 3 and 4 that the diameter of the capacitive loops decreases with the increase in the concentrations of chloride ions and sulfate ions, respectively, implying that the corrosion rate increases with increase in the concentrations of these ions. The impedance results are best analyzed and interpreted in terms of equivalent electrical circuit models obtained by simulating the electrochemical behavior of alloy-medium interface. The circuit fitment was done by ZSimpWin software of version 3.21. The equivalent electrical circuit (EEC), shown in Fig. 5 can be used to analyse the impedance data points, neglecting the lf inductive loop. The simulation of impedance data points is presented in Fig. 6. The hf region of the impedance spectra can be simulated by a series of two parallel resistance – constant phase element (R-CPE) networks; consisting of the charge transfer resistance (Rct ) in parallel with the double layer CPE (Qdl ) and the surface film resistance (Rf ) in parallel with the CPE (Qf ) of the film. The mf response can be simulated with a parallel network of resistance (Rdif ) and CPE (Qdif ) associated with diffusion [43]. The constant phase element (Qdl ) is substituted for the ideal capacitive element in oder to account for the inhomogeneity and porosity of the electrode surface [44].
Please cite this article as: M.G. Acharya and A.N. Shetty, The corrosion behavior of AZ31 alloy in chloride and sulfate media – A comparative study through electrochemical investigations, Journal of Magnesium and Alloys, https:// doi.org/ 10.1016/ j.jma.2018.09.003
JID: JMAA
ARTICLE IN PRESS
[m5+;December 28, 2018;15:54]
M.G. Acharya and A.N. Shetty / Journal of Magnesium and Alloys xxx (xxxx) xxx
7
Fig. 4. Nyquist plots for the corrosion of AZ31 alloy in Na2 SO4 solutions of different concentrations at 40 °C.
where, ωn m is the frequency at which the imaginary part of the impedance (Z// ) has a maximum. The polarization potential (Rp ) is calculated from the following expression: Rp = Rct + Rf + Rdif
Fig. 5. Electrical equivalent circuit used for the simulation of experimental data for the corrosion of AZ31 alloy in sodium chloride and sodium sulphate medium.
The impedance of the constant phase is given by the following equation [45]: ZQ = Y0 −1 ( jω )−n
(9)
where Y0 is the CPE constant, ω represents angular frequency (in rad s−1 ), j2 = –1 is the imaginary number and n is a CPE exponent which is measures the heterogeneity or roughness of the surface. The value of n is given by (–1 ≤ n ≤ 1); and the CPE simulates an ideal capacitor when n = 1, an ideal inductor for n = –1, and an ideal resistor for n = 0. The capacitance is deduced from the CPE using following equation [45]: C = Y0 (ωn m )−1
(10)
(11)
The values of Rp are listed in Tables 2 and 3 for the corrosion of AZ31 alloy in NaCl medium and Na2 SO4 mediium, respectively. The Rp value is inversely related to the corrosion rate. The decrease in the values Rp with the increase in the concentration of chloride and sulfate ions indicate the increase in the corrosion rate. The trend is in line with the one observed in the case of potentiodynamic polarization studies. The trend can be explained by taking into account of the tendency of anions like sulfate and chloride to destabilize the Mg(OH)2 surface film by dissolution, when they are present in higher concentrations in the corrosion media, thereby negating the partial protection provided by the surface film of the corrosion product. 3.3. Effect of temperature The effect of temperature on the corrosion of AZ31 alloy in the chloride and the sulfate media of different concentrations, were evaluated by measuring the corrosion rates at different temperatures. Figs. 7 and 8 represent the potentiodynamic polarization curves for the corrosion of AZ31 alloy at different temperatures in 0.25 M NaCl solution and 0.1 m Na2 SO4 solution respectively. Figs. 9 and 10 represent the corresponding
Please cite this article as: M.G. Acharya and A.N. Shetty, The corrosion behavior of AZ31 alloy in chloride and sulfate media – A comparative study through electrochemical investigations, Journal of Magnesium and Alloys, https:// doi.org/ 10.1016/ j.jma.2018.09.003
JID: JMAA
8
ARTICLE IN PRESS
[m5+;December 28, 2018;15:54]
M.G. Acharya and A.N. Shetty / Journal of Magnesium and Alloys xxx (xxxx) xxx
Fig. 6. The simulation of experimental impedance data points with theoretical model for the corrosion of ZE41 alloy specimen in NaCl solution.
Fig. 7. Potentiodynamic polarization plots for the corrosion of AZ31 alloy in 0.25 M NaCl at different temperatures.
Nyquist plots. Similar plots were obtained in other concentrations of the NaCl and Na2 SO4 solutions also. It is evident from the figures that with the increase in the medium temperature, the polarization curves shift to the higher current density region and the diameter of the capacitive loops in Nyquist plots decreases, both indicating an enhancement in the rate of corrosion. The same is reflected by the data listed in Tables 2 and 3. Though the kinetic effect of temperature is quite evident, the fact that the basic shapes of the polarization curves and Nyquist plots remain unaltered, illustrating that the
temperature changes only the rate of corrosion reactions and not the mechanisms. The activation energy (Ea ) for the corrosion process of the alloy was evaluated from the Arrhenius equation: ln υcorr = B −
Ea RT
(12)
where B is a constant which depends on the metal type, and R is the universal gas constant. The activation energy values were obtained from the slope (–Ea /R) of the straight line
Please cite this article as: M.G. Acharya and A.N. Shetty, The corrosion behavior of AZ31 alloy in chloride and sulfate media – A comparative study through electrochemical investigations, Journal of Magnesium and Alloys, https:// doi.org/ 10.1016/ j.jma.2018.09.003
JID: JMAA
ARTICLE IN PRESS
[m5+;December 28, 2018;15:54]
M.G. Acharya and A.N. Shetty / Journal of Magnesium and Alloys xxx (xxxx) xxx
Fig. 8. Potentiodynamic polarization plots for the corrosion of AZ31 alloy in 0.1 M Na2 SO4 at different temperatures.
Fig. 9. Nyquist plots for the corrosion of AZ31 alloy in 0.25 M NaCl at different temperatures.
Please cite this article as: M.G. Acharya and A.N. Shetty, The corrosion behavior of AZ31 alloy in chloride and sulfate media – A comparative study through electrochemical investigations, Journal of Magnesium and Alloys, https:// doi.org/ 10.1016/ j.jma.2018.09.003
9
JID: JMAA
10
ARTICLE IN PRESS
[m5+;December 28, 2018;15:54]
M.G. Acharya and A.N. Shetty / Journal of Magnesium and Alloys xxx (xxxx) xxx
Fig. 10. Nyquist plots for the corrosion of AZ31 alloy in 0.1 M Na2 SO4 at different temperatures.
Fig. 11. Arrhenius plots for the corrosion of AZ31 alloy in NaCl solutions of different concentrations.
Please cite this article as: M.G. Acharya and A.N. Shetty, The corrosion behavior of AZ31 alloy in chloride and sulfate media – A comparative study through electrochemical investigations, Journal of Magnesium and Alloys, https:// doi.org/ 10.1016/ j.jma.2018.09.003
JID: JMAA
ARTICLE IN PRESS
[m5+;December 28, 2018;15:54]
M.G. Acharya and A.N. Shetty / Journal of Magnesium and Alloys xxx (xxxx) xxx
11
Fig. 12. Arrhenius plots for the corrosion of AZ31 alloy in Na2 SO4 solutions of different concentrations.
Fig. 13. ln(ʋcorr /T) vs. 1/T plots for the corrosion of AZ31 alloy in NaCl solutions of different concentrations.
Please cite this article as: M.G. Acharya and A.N. Shetty, The corrosion behavior of AZ31 alloy in chloride and sulfate media – A comparative study through electrochemical investigations, Journal of Magnesium and Alloys, https:// doi.org/ 10.1016/ j.jma.2018.09.003
JID: JMAA
12
ARTICLE IN PRESS
[m5+;December 28, 2018;15:54]
M.G. Acharya and A.N. Shetty / Journal of Magnesium and Alloys xxx (xxxx) xxx
Fig. 14. ln(ʋcorr /T) vs. 1/T plots for the corrosion of AZ31 alloy in Na2 SO4 solutions of different concentrations.
Fig. 15. SEM image of a freshly polished surface of AZ31 alloy.
Please cite this article as: M.G. Acharya and A.N. Shetty, The corrosion behavior of AZ31 alloy in chloride and sulfate media – A comparative study through electrochemical investigations, Journal of Magnesium and Alloys, https:// doi.org/ 10.1016/ j.jma.2018.09.003
ARTICLE IN PRESS
JID: JMAA
[m5+;December 28, 2018;15:54]
M.G. Acharya and A.N. Shetty / Journal of Magnesium and Alloys xxx (xxxx) xxx
13
Table 5 Activation parameters for the corrosion of AZ31 alloy in different concentrations of Na2 SO4 . [Na2 SO4 ] (mol dm−3 )
Ea [kJ mol−1 ]
H− [kJ mol−1 ]
S# [J mol−1 K−1 ]
0.05M 0.10 M 0.15 M 0.20 M 0.25 M
44.17 39.64 35.65 35.21 24.39
40.04 33.78 28.33 23.87 18.81
−108.74 −126.03 −137.09 −149.98 −164.69
Fig. 16. EDAX spectra of a freshly polished surface of AZ31 alloy. Table 4 Activation parameters for the corrosion of AZ31 alloy in different concentrations of NaCl. [NaCl] (mol dm−3 ) Ea [kJ mol−1 ] H# [kJ mol−1 ] S# [J mol−1 K−1 ] 0.05M 0.10 M 0.15 M 0.20 M 0.25 M
54.00 24.55 23.57 20.67 16.94
34.66 20.97 18.70 18.08 15.30
−128.69 −0.162.88 −174.42 −167.27 −0.175.67
of the plot of ln(ʋcorr ) vs reciprocal of absolute temperature (1/T). Figs. 11 and 12 presents the Arrhenius plots for the corrosion of AZ31 alloy specimen in different concentrations of NaCl and Na2 SO4 solutions, respectively. The transition state theory equation (Eq. (9)) was used to evaluate the values of enthalpy of activation (H# ) and entropy of activation (S# ) values for the corrosion process. RT S# −H # e R e RT (13) Nh where h is plank’s constant, and N is Avogadro’s number and R is the ideal gas constant. A plot of ln(ʋcorr /T) vs 1/T gives a straight line with slope = –H# /R and intercept = ln(R/Nh) + (S# /R). Figs. 13 and 14, respectively, show the plots of ln(υ corr /T) vs 1/T for the corrosion of AZ31 alloy in different concentrations of NaCl and Na2 SO4 solutions. The activation parameters calculated in NaCl and Na2 SO4 media are summarized in Tables 4 and 5, respectively. The Ea value, which is the measure of the energy barrier for the occurrence of corrosion reaction decreases with the increase υcorr =
Fig. 18. EDAX spectra of AZ31 alloy surface immersed in 0.25 M NaCl for 3 hours at 30 °C.
in the salt concentrations, implying that the corrosion is thermodynamically more favoured in the concentrated media. The negative values of S# imply that the activated complex in the rate-determining step represents association rather than dissociation, indicating a decrease in randomness that takes place on going from the reactants to the activated complex [46]. 3.4. Surface morphology The SEM images were used to compare the morphology of the alloy surfaces under non corroded and corroded conditions. EDAX was employed to evaluate the compositions of the respective alloy surfaces. The SEM image of the freshly polished surface of AZ31 alloy is shown in Fig. 15. The microstructure of the alloy, as evident from the ESM image, consists of randomly distributed sub-micron sized β-phase in the main body of the α-phase. The average grain size
Fig. 17. SEM image of the corroded AZ31 alloy surface, immersed in different concentrations of NaCl for 3 hours at 30 °C. Please cite this article as: M.G. Acharya and A.N. Shetty, The corrosion behavior of AZ31 alloy in chloride and sulfate media – A comparative study through electrochemical investigations, Journal of Magnesium and Alloys, https:// doi.org/ 10.1016/ j.jma.2018.09.003
JID: JMAA
14
ARTICLE IN PRESS
[m5+;December 28, 2018;15:54]
M.G. Acharya and A.N. Shetty / Journal of Magnesium and Alloys xxx (xxxx) xxx
Fig. 19. SEM image of AZ31 alloy surface immersed in different concentrations of Na2 SO4 for 3 hours at 30 °C.
• The results of electrochemical studies and surface analysis confirm the formation of Mg(OH)2 film on the corroding alloy surface. • The Mg(OH)2 surface film provides only a partial protection due to breaks and non- uniformity in it. Higher ionic concentrations and temperatures further destabilize the surface film possibly by dissolution and increases the corrosion rate. • The corrosion kinetics follows Arrhenius law. • The rate of corrosion of the alloy is higher in the chloride medium than in sulfate medium. Fig. 20. EDAX spectra of AZ31 alloy surface immersed in 0.20 M Na2 SO4 for 3 hours at 30 °C.
was obtained to be 2.8 μm. The EDAX spectra (Fig. 16) of the freshly polished surface shows the presence of the constituent elements of the alloy. The SEM image of the alloy surface immersed in 0.25 M NaCl medium for 3 h is shown in Fig. 17 and the corresponding EDAX spectra in Fig. 18. The SEM image and EDAX spectra of the alloy surface after immersion in 0.2 M Na2 SO4 medium for 3 h are presented in Figs. 19 and 20, respectively. The SEM images of the alloy surfaces clearly show the deterioration in the presence of the corrosive media; and the microstructure is hardly visible hinting at the deposition occurred on the surface as a film. The appearance of predominant oxygen peaks in the EDAX spectra, indicating the presence of oxygen on the alloy surface due to the presence of corrosion product, Mg(OH)2 on the surface. 4. Conclusions From the results available, the following conclusion are drawn: • The environmental factors like concentrations of sulfate and chloride ions in the corrosion media and temperature have a significant influence on the rate of corrosion of magnesium alloy AZ31. • The rate of corrosion of AZ31 alloy increases with increase in the concentration of chloride ions and sulfate ions; and also with the increase in temperature.
Conflict of interest The authors declare no conflicts of interest. References [1] Anon, Technical Communication, Dow Chemical Company, Dow North America, Freeport, TX, 1994, p. 77541. [2] E.F. Emley, Principle of Magnesium Technology, Pergamon Press, London, UK, 1966. [3] R.F. Decker, Adv. Mater. Process. 154 (1998) 31–33. [4] J.R. Thomas, L.A. Darryl, Adv. Mater. Process. 145 (1994) 28–32. [5] F.H. Froes, D. Eliezer, E. Aghion, J. Miner. Met. Mater. Soc. 5 (1998) 30–34. [6] G.L. Makar, J. Kruger, Int. Mater. Rev. 38 (1993) 138–153. [7] A.L. Rudd, C.B. Breslin, F. Mansfeld, Corros. Sci. 42 (2000) 275–288. [8] G. Song, B. Johannesson, S. Hapugoda, D. St John, Corros. Sci. 46 (2004) 955–977. [9] G. Song, D. St John, Corros. Sci. 46 (2004) 1381–1399. [10] G. Song, A.L. Bowles, D. St John, Mater. Sci. Eng. A366 (2004) 74–86. [11] H. Huo, Y. Li, F. Wang, Corros. Sci. 46 (2004) 1467–1477. [12] L.H. Chiu, C.C. Chen, C.F. Yang, Surf. Coat. Technol. 191 (2005) 181–187. [13] Y. Zhang, C. Yan, F. Wang, W. Li, Corros. Sci. 47 (2005) 2816–2831. [14] X.W. Guo, W.J. Ding, C. Lu, C.Q. Zhai, Surf. Coat. Technol. 183 (2004) 359–368. [15] G. Song, A. Atrens, Adv. Eng. Mater. 1 (1999) 11–33. [16] G. Song, A. Atrens, Adv. Eng. Mater. 5 (2003) 837–858. [17] G.E. Totten, L. Xie, K. Funatani (Eds.), Handbook of Mechanical Alloy Design, Marcel Dekker, Inc., New York, 2004, pp. 487–538. [18] N.S. McIntyre, C. Chen, Corros. Sci. 40 (1998) 1697–1709. [19] C. Chen, S.J. Splinter, T. Do, N.S. McIntyre, Surf. Sci. 382 (1997) L652–L657. [20] W.K. Miller, E.F. Ryntz, Society of Automotive Engineers, Warrendale, PA, 1984, pp. 2.524–2.542. [21] Y. Cheng, T. Qin, H. Wang, Z. Zhang, Trans. Nonferrous Met. Soc. China 19 (2009) 517–524.
Please cite this article as: M.G. Acharya and A.N. Shetty, The corrosion behavior of AZ31 alloy in chloride and sulfate media – A comparative study through electrochemical investigations, Journal of Magnesium and Alloys, https:// doi.org/ 10.1016/ j.jma.2018.09.003
JID: JMAA
ARTICLE IN PRESS M.G. Acharya and A.N. Shetty / Journal of Magnesium and Alloys xxx (xxxx) xxx
[22] O. Lunder, J.E. Lein, T.K. Aune, K. Nisancioglu, Corrosion 45 (1989) 741–748. [23] T.J. Warner, N.A. Thorne, G. Nussbaum, W.M. Stobbs, Surf. Interface Anal. 19 (1992) 386–392. [24] F. Hehmann, F.H. Froes, W. Young, J. Met. 39 (1987) 14–21. [25] J. Banhart, Prog. Mater. Sci. 46 (2001) 559–632. [26] C. Blawert, N. Hort, K.U. Kainer, Trans. Indian Inst. Met. 57 (2004) 397–408. [27] M. Gupta, N.M.L. Sharon, Magnesium Alloys and Magnesium Composites, John Wiley & Sons, NewJersy, 2011. [28] L. Wang, T. Shinohara, B.P. Zhang, Corros. Eng. Jpn. 58 (2009) 105–110. [29] L. Wang, T. Shinohara, B.P. Zhang, J. Alloys Compd. 496 (2010) 500–507. [30] N. Wang, R. Wang, C. Peng, Y. Feng, X. Zhang, Trans. Nonferrous. Met. Soc. China 20 (2010) 1936–1943. [31] M.C. Zhao, M. Liu, G. Song, A. Atrens, Corros. Sci. 50 (2008) 3168–3178. [32] R.C. Zeng, Y. Hu, S.K. Guan, H.Z. Cui, E.H. Han, Corrosion science 86 (2014) 171–182.
[m5+;December 28, 2018;15:54]
15
[33] M. Baghni, Y. Wu, J. Li, W. Zhang, Trans. Nonferrous. Met. Soc. China 14 (2004) 1–10. [34] S. Mathieu, C. Rapin, J. Hazan, P. Steinmetz, Corros. Sci. 44 (2002) 2737–2756. [35] M.G. Lopez, B. Natta, Corrosion 712 (2001) 5006–5015. [36] G. Song, A. Andrej, D. Mathew, Corros. Sci. 41 (1999) 249–273. [37] N. Dinodi, A.N. Shetty, J. Magn. Alloys 1 (2013) 201–209. [38] J.H. Nordlien, S. Ono, N. Masuko, K. Nisancioglu, Corros. Sci. 39 (1997) 1397–1414. [39] G. Song, A. Atrens, Adv. Eng. Mater. 9 (2007) 177–183. [40] G. Song, A. Atrens, D. St John, X. Wu, J. Nairn, Corros. Sci. 39 (1997) 10–11. [41] G. Baril, N. Pebere, Corros. Sci. 43 (2001) 471–484. [42] H. Ardelean, I. Frateur, P. Marcus, Corros. Sci. 50 (2008) 1907–1918. [43] S. Fletcher, J. Electrochem. Soc. 141 (1994) 1823–1826. [44] F. Mansfeld, C.H. Tsai, H. Shih, Computer Modeling in Corrosion, ASTM, Philadelphia, 1992, pp. 186–196. [45] N. Pebere, C. Riera, F. Dabosi, Electrochim. Acta 35 (1990) 555–561. [46] F. Bentiss, M. Lebrini, M. Lagrenee, Corros. Sci. 47 (2005) 2915–2931.
Please cite this article as: M.G. Acharya and A.N. Shetty, The corrosion behavior of AZ31 alloy in chloride and sulfate media – A comparative study through electrochemical investigations, Journal of Magnesium and Alloys, https:// doi.org/ 10.1016/ j.jma.2018.09.003
JID: JMAA
[m5+;December 28, 2018;15:54]
Available online at www.sciencedirect.com
Journal of Magnesium and Alloys xxx (xxxx) xxx www.elsevier.com/locate/jma
Full Length Article
The corrosion behavior of AZ31 alloy in chloride and sulfate media – A comparative study through electrochemical investigations M. Gururaj Acharya, A. Nityanansda Shetty∗ Department of Chemistry, National Institute of Technology Karnataka, Surathkal, Srinivasnagar, Mangalore, Karnataka 575 025, India Received 8 June 2018; received in revised form 6 September 2018; accepted 19 September 2018 Available online xxx
Abstract The magnesium alloys are considered to be the best structural materials, because of their advantageous weight to strength ratio. But, the limitation in their real field applications lies on the fact that magnesium alloys are highly susceptible for corrosion. The corrosion behaviour of AZ31 alloy was investigated by electrochemical methods in sodium chloride and sodium sulphate of different concentrations at different temperatures. The corrosion rate was monitored by potentiodynamic polarization technique and electrochemical impedance spectroscopy method. The surface morphology and surface composition of the freshly polished surface of the alloy was compared with that of the corroded surface by recording their SEM images and EDS, respectively. The results showed that the corrosion rate of AZ31 alloy increased with the increase in the temperature of the medium and also with the increase in the salt concentration of the medium. The activation parameters for the corrosion process were calculated and interpreted. © 2019 Published by Elsevier B.V. on behalf of Chongqing University. This is an open access article under the CC BY-NC-ND license. (http://creativecommons.org/licenses/by-nc-nd/4.0/) Peer review under responsibility of Chongqing University Keywords: AZ31 alloy; Polarization techniques; EIS; SEM.
1. Introduction In the last few years, magnesium alloys have been the materials of great interest in automobile industry. Magnesium, with a specific gravity of 1.74 is one of the lightest structural metal, extracted from its principal ores dolomite, magnesite and carnallite [1]. Magnesium alloys possess many excellent properties, such as light mass, high specific strength, high thermal conductivity, electromagnetic interference resistance and ability to be recycled [2–5]. But the poor corrosion resistance has limited their wider applications. Magnesium is highly susceptible for corrosion in most of the inorganic acidic and neutral solutions high [6–14]. The localized corrosion in Mg alloys is considered to be irregular, which spread laterally and cover the whole surface, without going deep inside. There are two main reasons for poor corrosion resistance; the internal galvanic corrosion by second phases
∗
Corresponding author. E-mail address: [email protected] (A.N. Shetty).
or impurities, and the poor stability of the hydroxide film formed above the surface of magnesium alloy [15]. One of the important alloying elements for Magnesium is aluminum [16]. Usually commercial AZ alloys contain less than 10 wt% aluminum, with zinc and manganese as secondary alloying elements. Mg-Al-Zn series alloys are majorly used for structural applications, particularly in automobile industries [17]. The alloying elements significantly influence the mechanical, physical and chemical properties of magnesium alloys. Aluminum significantly improves the tensile strength by the formation of an intermetallic β- phase (Mg17 Al12 ) [15]. The presence aluminum also increases the ambient compressive and fatigue strength and improves the castability of the alloy [18,19]. In AZ series alloys, the secondary alloying element, Zn solidifies as a sufficiently fine grain to meet most of the physical and chemical property requirements [20]. Manganese improves the corrosion resistance of magnesium alloys by removing iron and other heavy metal elements to avoid the formation of harmful intermetallic compounds. Also, manganese refines the grain size and improves the welding properties of magnesium alloys [21].
https://doi.org/10.1016/j.jma.2018.09.003 2213-9567/© 2019 Published by Elsevier B.V. on behalf of Chongqing University. This is an open access article under the CC BY-NC-ND license. (http://creativecommons.org/licenses/by-nc-nd/4.0/) Peer review under responsibility of Chongqing University Please cite this article as: M.G. Acharya and A.N. Shetty, The corrosion behavior of AZ31 alloy in chloride and sulfate media – A comparative study through electrochemical investigations, Journal of Magnesium and Alloys, https:// doi.org/ 10.1016/ j.jma.2018.09.003
ARTICLE IN PRESS
JID: JMAA
2
[m5+;December 28, 2018;15:54]
M.G. Acharya and A.N. Shetty / Journal of Magnesium and Alloys xxx (xxxx) xxx Table 1 Composition of AZ31 alloy (% by weight). Chemical composition
Al
Zn
Mn
Si
Cu
Ni
Fe
Mg
(Mass fraction)
2.96
0.83
0.43
0.004
0.004
<0.001
0.002
Balance
However, there are many differing reports about the influence of aluminum on the corrosion resistance of magnesium alloy. Lunder and his coworkers reported that the corrosion resistance of magnesium alloy could be greatly improved when the aluminum content reaches 8% (mass fraction) [22]. The reports are also available, indicating that 5% Al in magnesium alloy improved the corrosion resistance significantly [23]. The studies carried out on the corrosion behavior of magnesium alloys with aluminum content of 9.0%−62.3% revealed that aluminum content from 9.6% to 23.4% was beneficial to improving the corrosion resistance of magnesium alloy [24]. AZ31 alloy is one of the best structural alloy used in aerospace and automobile industries, for its straight sheet like shape, which has been used as brackets and structural bones of aircrafts where they get exposed to different temperatures, aqueous medium such as acid rain, salt water, etc. When compared with other commercially available magnesium alloys, AZ31 alloy has excellent properties, such as ultra-low density, good energy absorption and excellent damping performance [5,25–27]. But, attention must be paid to its corrosion properties as it has low corrosion resistance. There are only a few reports in the literature, on the corrosion behavior of AZ31 magnesium alloy in dilute sodium chloride solutions and sodium sulfate solutions [28,29]. However, a detailed study on the effect of concentration of the media and temperature are not reported. This paper reports the detailed study on the corrosion behavior of AZ31 alloy in NaCl and Na2 SO4 media, by varying the concentrations of salts and also by varying the temperature of the corrosive media. A comparison on the corrosion behavior of the alloy in the two media are discussed herein.
2. Experimental 2.1. Material The specimen studied was magnesium alloy AZ31. The composition of the alloy sample is given in Table 1. The working electrode received as a sheet was cut and embedded in epoxy resin, exposing a definite open surface area of 0.69 cm2 . This coupon was polished as per standard metallographic practice, belt grinding, followed by polishing on emery paper of grade 600, 800, 1000, 1200, 1500, 2000; and finally on polishing wheel using legated alumina abrasive to obtain a mirror finish. The polished specimen was washed with double distilled water, degreased with acetone and dried before immersing in the sodium chloride medium.
2.2. Medium The electrolyte media of five concentrations 0.05 M, 0.1 M, 0.15 M, 0.2 M, 0.25 M were prepared by dissolving appropriate amounts of, analytical grade samples of sodium chloride salt and sodium sulfate salt, separately, in double distilled water. The corrosion studies were carried out at temperatures 30 °C, 35 °C, 40 °C, 45 °C and 50 °C in a calibrated thermostat. 2.3. Electrochemical measurements Electrochemical measurements were carried out using electrochemical work station, Gill AC having ACM instrument Version 5 software. The arrangement employed was a conventional three-electrode Pyrex glass cell with a platinum counter electrode, a saturated calomel electrode (SCE) as reference and the AZ31 alloy specimen as the working electrode. All the values of potential reported are referred to the SCE. The polarization studies were carried out immediately after the EIS studies on the same exposed electrode surface without any additional surface treatment. 2.3.1. Potentiodynamic polarization studies Well-polished AZ31 alloy specimen coupon was exposed to the corrosion medium and allowed to establish a steadystate open circuit potentials (OCP). The potentiodynamic current potential curves (Tafel curves) were recorded by polarizing the specimen to −250 mV cathodically and +250 mV anodically, relative to the OCP at a scan rate of 1 mV s−1 . 2.3.2. Electrochemical impedance spectroscopy (EIS) studies Impedance measurements were performed at open circuit potential (OCP) by the application of a periodic small amplitude (10 mV) ac voltage signal with a wide spectrum of frequency ranging from 100 kHz to 0.01 Hz. The impedance data were analysed using Nyquist plots. In all the above measurements, at least three similar results were considered and their average values have been reported. 2.4. Scanning electron microscopy (SEM) analysis The surface morphology of the fresh polished surface and the corroded surface of the AZ31 alloy, in the corrosive media were analyzed through the SEM images recorded using JEOL JSM-6380LA analytical scanning electron microscope. The surface composition of the fresh surface and corroded surface were determined from their corresponding EDX spectra.
Please cite this article as: M.G. Acharya and A.N. Shetty, The corrosion behavior of AZ31 alloy in chloride and sulfate media – A comparative study through electrochemical investigations, Journal of Magnesium and Alloys, https:// doi.org/ 10.1016/ j.jma.2018.09.003
ARTICLE IN PRESS
JID: JMAA
[m5+;December 28, 2018;15:54]
M.G. Acharya and A.N. Shetty / Journal of Magnesium and Alloys xxx (xxxx) xxx
3
Fig. 1. Potentiodynamic polarization curves for the corrosion of AZ31 alloy in NaCl solutions of different concentrations at 50 °C.
3. Results and discussions 3.1. Potentiodynamic polarization measurements The corrosion behavior of AZ31 alloy was investigated sodium chloride and sodium sulfate media of different concentrations, at different temperatures by potentiodynamic polarization method. The potentiodynamic polarization curves for the corrosion of AZ31 alloy in sodium chloride media of different concentrations are presented in Fig. 1. The corresponding curves in sodium sulfate media of different concentrations at 40 °C presented in Fig. 2. Similar curves were obtained at other temperatures also. It can be observed from Figs. 1 and 2 that the polarization curves are shifted to the higher current density region, indicating an increase in the corrosion rate with the increase in chloride and sulfate concentration in the corrosion media. The anodic polarization curves represent the anodic oxidation of the magnesium alloy, while the cathodic curves represent the hydrogen evolution through reduction of water at the cathode. The anodic curves show the inflection points, characterized by two different slopes, indicating a kinetic barrier effect, possibly due to the deposition of a surface film of magnesium hydroxide, followed by its dissolution at higher anodic potential [30,31]. It is also observed from Figs. 1 and 2 that there is no significant change in overall shapes of the Tafel branches with varying salt concentration in the corrosion media, indicating that medium concentration only alters the rate, without altering the mechanism of corrosion reaction [30,31].
The corrosion current density (icorr ) was deduced by extrapolating the cathodic branch of the polarization curves to the OCP, as the anodic curves do not possess distinct Tafel regions. The corrosion rate (υ corr ) was calculated using the following equation [32]: υcorr (mmy−1 ) =
K × icorr × E W ρ
(1)
where, the constant, K = 0.00327, defines the unit of corrosion rate (mm y−1 ), icorr is the corrosion current density in μA cm−2 , ρ is the density of the corroding material, 1.84 g m−3 , EW is the equivalent weight of the alloy calculated using equation shown below: 1 EW = ni × f i
(2)
wi
where fi is the weight fraction of the ith element in the alloy, wi is the atomic weight of the ith element in the alloy and ni is the valence of the ith element of the alloy. The potentiodynamic polarization parameters, including corrosion potential (Ecorr ), corrosion current (icorr ), cathodic slopes (bc ) and corrosion rate (υ) are summarized in Tables 2 and 3 for the corrosion of AZ31 alloy in NaCl and Na2 SO4 media, respectively. It is evident from the data in Tables 2 and 3, that the corrosion rate increases with the increase in the concentration of chloride / sulfate in the corrosion medium. These ions are corrosive towards magnesium and its alloys because of their tendency to cause surface film breakdown by the disso-
Please cite this article as: M.G. Acharya and A.N. Shetty, The corrosion behavior of AZ31 alloy in chloride and sulfate media – A comparative study through electrochemical investigations, Journal of Magnesium and Alloys, https:// doi.org/ 10.1016/ j.jma.2018.09.003
ARTICLE IN PRESS
JID: JMAA
4
[m5+;December 28, 2018;15:54]
M.G. Acharya and A.N. Shetty / Journal of Magnesium and Alloys xxx (xxxx) xxx
Fig. 2. Potentiodynamic polarization curves for the corrosion of AZ31 alloy in Na2 SO4 solutions of different concentrations at 40 °C.
Table 2 Electrochemical polarization parameters for the corrosion of AZ31 alloy in NaCl medium with different temperatures. Concentration (mol dm−3 )
Temperature (°C)
Ecorr (mV/ SCE)
icorr (μA cm−2 )
β c (mV dec−1 )
ʋcorr (mm y−1 )
Rp (ohm cm2 )
0.05
30 35 40 45 50
−1514 −1490 −1500 −1503 −1493
41.11 87.10 96.09 125.20 145.39
99.7 117.1 119.1 114.2 130.3
0.89 1.89 2.08 2.72 3.16
870.9 860.5 615.4 612.6 584.8
0.1
30 35 40 45 50
−1483 −1489 −1502 −1516 −1490
154.03 171.58 172.72 225.52 235.47
135.3 141.6 158.2 162.9 173.4
3.34 3.73 3.75 4.90 5.11
839.2 546.5 526.3 483.5 370
0.15
30 35 40 45 50
−1537 −1500 −1515 −1491 −1475
210.80 255.51 303.27 351.00 370.3
140.1 145.7 158.8 160.9 184.5
4.58 5.55 6.59 7.63 8.05
568.8 492.3 398.7 353.4 239.2
0.2
30 35 40 45 50
−1500 −1485 −1483 −1511 −1497
432.55 457.04 489.13 687.77 709.98
189.5 214.7 181.4 211.3 171.5
9.40 9.93 10.63 14.75 15.43
247.0 268.9 248.6 230.0 226.3
0.25
30 35 40 45 50
−1526 −1515 −1488 −1510 −1494
520.41 553.44 577.18 608.31 792.81
190.4 192.9 188.0 209.2 186.4
11.31 12.03 12.54 13.22 17.23
435.1 337.4 271.6 221.9 180.3
Please cite this article as: M.G. Acharya and A.N. Shetty, The corrosion behavior of AZ31 alloy in chloride and sulfate media – A comparative study through electrochemical investigations, Journal of Magnesium and Alloys, https:// doi.org/ 10.1016/ j.jma.2018.09.003
ARTICLE IN PRESS
JID: JMAA
[m5+;December 28, 2018;15:54]
M.G. Acharya and A.N. Shetty / Journal of Magnesium and Alloys xxx (xxxx) xxx
5
Table 3 Electrochemical polarization parameters for the corrosion of AZ31 alloy in Na2 SO4 medium with different temperatures. Concentration (mol dm−3 )
Temperature(°C)
Ecorr (mv/ SCE)
icorr (μA cm−2 )
β c (mv dec−1 )
υ corr (mm y−1 )
Rp (ohm cm2 )
0.05
30 35 40 45 50
−1483 −1470 −1502 −1465 −1481
63.33 110.24 164.74 179.03 190.03
93.0 110.8 125.9 154.8 144.8
1.37 2.39 3.58 3.89 4.13
1080.0 877.0 810.5 800.8 766.0
0.1
30 35 40 45 50
−1522 −1528 −1485 −1481 −1456
71.20 157.60 191.24 202.70 208.41
117.8 134.6 140.9 150.6 157.3
1.54 3.42 4.15 4.40 4.53
803.0 754.0 657.0 580.0 582.8
0.15
30 35 40 45 50
−1457 −1457 −1469 −1446 −1448
218.14 322.28 445.19 461.63 546.62
138.1 142.7 151.6 158.2 170.2
4.74 7.00 9.67 10.03 11.88
854.0 763.0 610.0 503.5 445.1
0.2
30 35 40 45 50
−1505 −1493 −1489 −1472 −1480
235.63 354.34 470.87 542.13 672.86
152.2 158.0 164.9 172.7 197.9
5.12 7.70 10.23 11.78 14.62
527.3 511.9 451.9 439.0 405.0
0.25
30 35 40 45 50
−1451 −1411 −1438 −1486 −1463
372.80 497.12 516.71 583.03 727.89
147.1 158.6 170.2 190.9 201.6
8.10 10.80 11.23 12.67 15.82
793.1 600.8 315.8 279.5 270.2
lution of the deposited corrosion product, thereby increasing the anodic dissolution of the alloy. The oxidation and the corresponding corrosion of magnesium in aqueous solution can be represented by the following reactions [33]: Mg → Mg+ + e−
(3)
Mg → Mg2+ + 2e−
(4)
The steady state working potential of magnesium is about –1.5 V, even though the standard electrode potential of magnesium is –2.38 V. The change in potential is due to the formation of Mg(OH)2 film on the surface of the alloy [34]. However, this hydroxide layer is discontinuous and only partially covers the alloy surface, without effectively effective protecting the alloy surface. The anodic dissolution of magnesium and its alloys involve two oxidation processes. At more active potentials of about –2.78 V (vs SCE) magnesium undergoes oxidized to monovalent magnesium ion (Mg+ ) and at slightly higher potentials of about –1.56 V (vs SCE), magnesium undergoes oxidation to divalent magnesium ion (Mg2+ ), in parallel with the former oxidation [35]. The monovalent magnesium ion is unstable and undergoes oxidation to divalent magnesium ion through a series of reactions involving unstable intermediates like magnesium hydride as shown in equations below: Mg+ + 2H+ + 3e− → MgH2
(5)
MgH2 + 2H2 O → Mg2+ + 2OH− + 2H2
(6)
Mg2+ + 2OH− → Mg(OH )2
(7)
2Mg+ + 2H2 O → Mg2+ + Mg(OH )2 + H2
(8)
The microstructure of AZ31 alloy consists of the α-Mg matrix, distributed with the secondary β-phase, comprising of intermetallic compound Mg17 Al12 . The secondary β-phase is cathodic to the Mg matrix and is with a good passive behavior over a broad range of pH [21]. However, the role of β-phase in the corrosion process depends upon its size and distribution [36,37]. When the grain size of magnesium alloy is small and the mass fraction of β-phase is high, the distribution of the β-phase on the α-Mg matrix is continuous; providing a corrosion protection by a barrier layer effect. On the other hand, when the grain size is larger and the β-phases cannot cover the α-phase completely, galvanic corrosion results with the α-phase acting as anode and undergoing corrosion. The corrosion product, Mg(OH)2 , precipitates over the α-Mg matrix and this surface film is only partially selective as the continuity of the film is interrupted by the presence of secondary phase. In addition, the secondary oxidation results in chemical liberation of hydrogen gas at the anode. This rapid liberation of hydrogen gas at the anodic sites causes the breakdown of the surface film at higher anodic overvoltage; and accounts for the inflection in anodic branch of Tafel plots [38]. The corrosion product film on the magnesium surface is reported to be multilayered, with an inner layer of MgO–
Please cite this article as: M.G. Acharya and A.N. Shetty, The corrosion behavior of AZ31 alloy in chloride and sulfate media – A comparative study through electrochemical investigations, Journal of Magnesium and Alloys, https:// doi.org/ 10.1016/ j.jma.2018.09.003
JID: JMAA
6
ARTICLE IN PRESS
[m5+;December 28, 2018;15:54]
M.G. Acharya and A.N. Shetty / Journal of Magnesium and Alloys xxx (xxxx) xxx
Fig. 3. Nyquist plots for the corrosion of AZ31 alloy in NaCl solutions of different concentrations at 50 °C.
Mg(OH)2 , an intermediate thin film of MgO and an outer thick porous layer of Mg(OH)2 [15]. Further studies have confirmed the partial protective nature of the surface film and occurrence of corrosion reactions at the breaks and imperfections of the film [16,39,40]. The results also indicate that the corrosion rate in NaCl medium is higher than that in Na2 SO4 medium. This fact can be related to the higher conductivity of NaCl, which in turn has less resistivity for the flow of ions in the solution. Though sulfate is a mild corrosive compared to chloride, sulfate has been reported to possess an appreciable influence on the electrochemical behavior of pure magnesium and some of its alloys [41,34]. 3.2. Electrochemical impedance spectroscopy The electrochemical impedance spectra in the form of Nyquist plots for the corrosion of AZ31 alloy in NaCl solutions of different concentrations, at 50 °C are presented in Fig. 3. The Nyquist plots for the corrosion of the alloy in Na2 SO4 solutions of different concentrations, at 40 °C are shown in Fig. 4. Similar plots were obtained at other temperatures also. All the Nyquist plots consist of two capacitive loops at the higher and the medium frequencies, and the beginning of an inductive loop at the lower frequency region. The higher region frequency (hf) semicircle corresponds to the charge transfer of corrosion process and oxide film effects, and the medium frequency (mf) semicircle corresponds to the mass transport (diffusion of magnesium ions) through the
corrosion product layer of Mg(OH)2 . The relaxation of surface adsorbed species like Mg(OH)+ and Mg(OH)2 is considered to be the genesis of the lower frequency (lf) inductive loop. Though there are different versions interpreting the impedance of magnesium alloy corrosion processes, the current explanation has been one of the most adopted [34,39,42]. It is observed from Figs. 3 and 4 that the diameter of the capacitive loops decreases with the increase in the concentrations of chloride ions and sulfate ions, respectively, implying that the corrosion rate increases with increase in the concentrations of these ions. The impedance results are best analyzed and interpreted in terms of equivalent electrical circuit models obtained by simulating the electrochemical behavior of alloy-medium interface. The circuit fitment was done by ZSimpWin software of version 3.21. The equivalent electrical circuit (EEC), shown in Fig. 5 can be used to analyse the impedance data points, neglecting the lf inductive loop. The simulation of impedance data points is presented in Fig. 6. The hf region of the impedance spectra can be simulated by a series of two parallel resistance – constant phase element (R-CPE) networks; consisting of the charge transfer resistance (Rct ) in parallel with the double layer CPE (Qdl ) and the surface film resistance (Rf ) in parallel with the CPE (Qf ) of the film. The mf response can be simulated with a parallel network of resistance (Rdif ) and CPE (Qdif ) associated with diffusion [43]. The constant phase element (Qdl ) is substituted for the ideal capacitive element in oder to account for the inhomogeneity and porosity of the electrode surface [44].
Please cite this article as: M.G. Acharya and A.N. Shetty, The corrosion behavior of AZ31 alloy in chloride and sulfate media – A comparative study through electrochemical investigations, Journal of Magnesium and Alloys, https:// doi.org/ 10.1016/ j.jma.2018.09.003
JID: JMAA
ARTICLE IN PRESS
[m5+;December 28, 2018;15:54]
M.G. Acharya and A.N. Shetty / Journal of Magnesium and Alloys xxx (xxxx) xxx
7
Fig. 4. Nyquist plots for the corrosion of AZ31 alloy in Na2 SO4 solutions of different concentrations at 40 °C.
where, ωn m is the frequency at which the imaginary part of the impedance (Z// ) has a maximum. The polarization potential (Rp ) is calculated from the following expression: Rp = Rct + Rf + Rdif
Fig. 5. Electrical equivalent circuit used for the simulation of experimental data for the corrosion of AZ31 alloy in sodium chloride and sodium sulphate medium.
The impedance of the constant phase is given by the following equation [45]: ZQ = Y0 −1 ( jω )−n
(9)
where Y0 is the CPE constant, ω represents angular frequency (in rad s−1 ), j2 = –1 is the imaginary number and n is a CPE exponent which is measures the heterogeneity or roughness of the surface. The value of n is given by (–1 ≤ n ≤ 1); and the CPE simulates an ideal capacitor when n = 1, an ideal inductor for n = –1, and an ideal resistor for n = 0. The capacitance is deduced from the CPE using following equation [45]: C = Y0 (ωn m )−1
(10)
(11)
The values of Rp are listed in Tables 2 and 3 for the corrosion of AZ31 alloy in NaCl medium and Na2 SO4 mediium, respectively. The Rp value is inversely related to the corrosion rate. The decrease in the values Rp with the increase in the concentration of chloride and sulfate ions indicate the increase in the corrosion rate. The trend is in line with the one observed in the case of potentiodynamic polarization studies. The trend can be explained by taking into account of the tendency of anions like sulfate and chloride to destabilize the Mg(OH)2 surface film by dissolution, when they are present in higher concentrations in the corrosion media, thereby negating the partial protection provided by the surface film of the corrosion product. 3.3. Effect of temperature The effect of temperature on the corrosion of AZ31 alloy in the chloride and the sulfate media of different concentrations, were evaluated by measuring the corrosion rates at different temperatures. Figs. 7 and 8 represent the potentiodynamic polarization curves for the corrosion of AZ31 alloy at different temperatures in 0.25 M NaCl solution and 0.1 m Na2 SO4 solution respectively. Figs. 9 and 10 represent the corresponding
Please cite this article as: M.G. Acharya and A.N. Shetty, The corrosion behavior of AZ31 alloy in chloride and sulfate media – A comparative study through electrochemical investigations, Journal of Magnesium and Alloys, https:// doi.org/ 10.1016/ j.jma.2018.09.003
JID: JMAA
8
ARTICLE IN PRESS
[m5+;December 28, 2018;15:54]
M.G. Acharya and A.N. Shetty / Journal of Magnesium and Alloys xxx (xxxx) xxx
Fig. 6. The simulation of experimental impedance data points with theoretical model for the corrosion of ZE41 alloy specimen in NaCl solution.
Fig. 7. Potentiodynamic polarization plots for the corrosion of AZ31 alloy in 0.25 M NaCl at different temperatures.
Nyquist plots. Similar plots were obtained in other concentrations of the NaCl and Na2 SO4 solutions also. It is evident from the figures that with the increase in the medium temperature, the polarization curves shift to the higher current density region and the diameter of the capacitive loops in Nyquist plots decreases, both indicating an enhancement in the rate of corrosion. The same is reflected by the data listed in Tables 2 and 3. Though the kinetic effect of temperature is quite evident, the fact that the basic shapes of the polarization curves and Nyquist plots remain unaltered, illustrating that the
temperature changes only the rate of corrosion reactions and not the mechanisms. The activation energy (Ea ) for the corrosion process of the alloy was evaluated from the Arrhenius equation: ln υcorr = B −
Ea RT
(12)
where B is a constant which depends on the metal type, and R is the universal gas constant. The activation energy values were obtained from the slope (–Ea /R) of the straight line
Please cite this article as: M.G. Acharya and A.N. Shetty, The corrosion behavior of AZ31 alloy in chloride and sulfate media – A comparative study through electrochemical investigations, Journal of Magnesium and Alloys, https:// doi.org/ 10.1016/ j.jma.2018.09.003
JID: JMAA
ARTICLE IN PRESS
[m5+;December 28, 2018;15:54]
M.G. Acharya and A.N. Shetty / Journal of Magnesium and Alloys xxx (xxxx) xxx
Fig. 8. Potentiodynamic polarization plots for the corrosion of AZ31 alloy in 0.1 M Na2 SO4 at different temperatures.
Fig. 9. Nyquist plots for the corrosion of AZ31 alloy in 0.25 M NaCl at different temperatures.
Please cite this article as: M.G. Acharya and A.N. Shetty, The corrosion behavior of AZ31 alloy in chloride and sulfate media – A comparative study through electrochemical investigations, Journal of Magnesium and Alloys, https:// doi.org/ 10.1016/ j.jma.2018.09.003
9
JID: JMAA
10
ARTICLE IN PRESS
[m5+;December 28, 2018;15:54]
M.G. Acharya and A.N. Shetty / Journal of Magnesium and Alloys xxx (xxxx) xxx
Fig. 10. Nyquist plots for the corrosion of AZ31 alloy in 0.1 M Na2 SO4 at different temperatures.
Fig. 11. Arrhenius plots for the corrosion of AZ31 alloy in NaCl solutions of different concentrations.
Please cite this article as: M.G. Acharya and A.N. Shetty, The corrosion behavior of AZ31 alloy in chloride and sulfate media – A comparative study through electrochemical investigations, Journal of Magnesium and Alloys, https:// doi.org/ 10.1016/ j.jma.2018.09.003
JID: JMAA
ARTICLE IN PRESS
[m5+;December 28, 2018;15:54]
M.G. Acharya and A.N. Shetty / Journal of Magnesium and Alloys xxx (xxxx) xxx
11
Fig. 12. Arrhenius plots for the corrosion of AZ31 alloy in Na2 SO4 solutions of different concentrations.
Fig. 13. ln(ʋcorr /T) vs. 1/T plots for the corrosion of AZ31 alloy in NaCl solutions of different concentrations.
Please cite this article as: M.G. Acharya and A.N. Shetty, The corrosion behavior of AZ31 alloy in chloride and sulfate media – A comparative study through electrochemical investigations, Journal of Magnesium and Alloys, https:// doi.org/ 10.1016/ j.jma.2018.09.003
JID: JMAA
12
ARTICLE IN PRESS
[m5+;December 28, 2018;15:54]
M.G. Acharya and A.N. Shetty / Journal of Magnesium and Alloys xxx (xxxx) xxx
Fig. 14. ln(ʋcorr /T) vs. 1/T plots for the corrosion of AZ31 alloy in Na2 SO4 solutions of different concentrations.
Fig. 15. SEM image of a freshly polished surface of AZ31 alloy.
Please cite this article as: M.G. Acharya and A.N. Shetty, The corrosion behavior of AZ31 alloy in chloride and sulfate media – A comparative study through electrochemical investigations, Journal of Magnesium and Alloys, https:// doi.org/ 10.1016/ j.jma.2018.09.003
ARTICLE IN PRESS
JID: JMAA
[m5+;December 28, 2018;15:54]
M.G. Acharya and A.N. Shetty / Journal of Magnesium and Alloys xxx (xxxx) xxx
13
Table 5 Activation parameters for the corrosion of AZ31 alloy in different concentrations of Na2 SO4 . [Na2 SO4 ] (mol dm−3 )
Ea [kJ mol−1 ]
H− [kJ mol−1 ]
S# [J mol−1 K−1 ]
0.05M 0.10 M 0.15 M 0.20 M 0.25 M
44.17 39.64 35.65 35.21 24.39
40.04 33.78 28.33 23.87 18.81
−108.74 −126.03 −137.09 −149.98 −164.69
Fig. 16. EDAX spectra of a freshly polished surface of AZ31 alloy. Table 4 Activation parameters for the corrosion of AZ31 alloy in different concentrations of NaCl. [NaCl] (mol dm−3 ) Ea [kJ mol−1 ] H# [kJ mol−1 ] S# [J mol−1 K−1 ] 0.05M 0.10 M 0.15 M 0.20 M 0.25 M
54.00 24.55 23.57 20.67 16.94
34.66 20.97 18.70 18.08 15.30
−128.69 −0.162.88 −174.42 −167.27 −0.175.67
of the plot of ln(ʋcorr ) vs reciprocal of absolute temperature (1/T). Figs. 11 and 12 presents the Arrhenius plots for the corrosion of AZ31 alloy specimen in different concentrations of NaCl and Na2 SO4 solutions, respectively. The transition state theory equation (Eq. (9)) was used to evaluate the values of enthalpy of activation (H# ) and entropy of activation (S# ) values for the corrosion process. RT S# −H # e R e RT (13) Nh where h is plank’s constant, and N is Avogadro’s number and R is the ideal gas constant. A plot of ln(ʋcorr /T) vs 1/T gives a straight line with slope = –H# /R and intercept = ln(R/Nh) + (S# /R). Figs. 13 and 14, respectively, show the plots of ln(υ corr /T) vs 1/T for the corrosion of AZ31 alloy in different concentrations of NaCl and Na2 SO4 solutions. The activation parameters calculated in NaCl and Na2 SO4 media are summarized in Tables 4 and 5, respectively. The Ea value, which is the measure of the energy barrier for the occurrence of corrosion reaction decreases with the increase υcorr =
Fig. 18. EDAX spectra of AZ31 alloy surface immersed in 0.25 M NaCl for 3 hours at 30 °C.
in the salt concentrations, implying that the corrosion is thermodynamically more favoured in the concentrated media. The negative values of S# imply that the activated complex in the rate-determining step represents association rather than dissociation, indicating a decrease in randomness that takes place on going from the reactants to the activated complex [46]. 3.4. Surface morphology The SEM images were used to compare the morphology of the alloy surfaces under non corroded and corroded conditions. EDAX was employed to evaluate the compositions of the respective alloy surfaces. The SEM image of the freshly polished surface of AZ31 alloy is shown in Fig. 15. The microstructure of the alloy, as evident from the ESM image, consists of randomly distributed sub-micron sized β-phase in the main body of the α-phase. The average grain size
Fig. 17. SEM image of the corroded AZ31 alloy surface, immersed in different concentrations of NaCl for 3 hours at 30 °C. Please cite this article as: M.G. Acharya and A.N. Shetty, The corrosion behavior of AZ31 alloy in chloride and sulfate media – A comparative study through electrochemical investigations, Journal of Magnesium and Alloys, https:// doi.org/ 10.1016/ j.jma.2018.09.003
JID: JMAA
14
ARTICLE IN PRESS
[m5+;December 28, 2018;15:54]
M.G. Acharya and A.N. Shetty / Journal of Magnesium and Alloys xxx (xxxx) xxx
Fig. 19. SEM image of AZ31 alloy surface immersed in different concentrations of Na2 SO4 for 3 hours at 30 °C.
• The results of electrochemical studies and surface analysis confirm the formation of Mg(OH)2 film on the corroding alloy surface. • The Mg(OH)2 surface film provides only a partial protection due to breaks and non- uniformity in it. Higher ionic concentrations and temperatures further destabilize the surface film possibly by dissolution and increases the corrosion rate. • The corrosion kinetics follows Arrhenius law. • The rate of corrosion of the alloy is higher in the chloride medium than in sulfate medium. Fig. 20. EDAX spectra of AZ31 alloy surface immersed in 0.20 M Na2 SO4 for 3 hours at 30 °C.
was obtained to be 2.8 μm. The EDAX spectra (Fig. 16) of the freshly polished surface shows the presence of the constituent elements of the alloy. The SEM image of the alloy surface immersed in 0.25 M NaCl medium for 3 h is shown in Fig. 17 and the corresponding EDAX spectra in Fig. 18. The SEM image and EDAX spectra of the alloy surface after immersion in 0.2 M Na2 SO4 medium for 3 h are presented in Figs. 19 and 20, respectively. The SEM images of the alloy surfaces clearly show the deterioration in the presence of the corrosive media; and the microstructure is hardly visible hinting at the deposition occurred on the surface as a film. The appearance of predominant oxygen peaks in the EDAX spectra, indicating the presence of oxygen on the alloy surface due to the presence of corrosion product, Mg(OH)2 on the surface. 4. Conclusions From the results available, the following conclusion are drawn: • The environmental factors like concentrations of sulfate and chloride ions in the corrosion media and temperature have a significant influence on the rate of corrosion of magnesium alloy AZ31. • The rate of corrosion of AZ31 alloy increases with increase in the concentration of chloride ions and sulfate ions; and also with the increase in temperature.
Conflict of interest The authors declare no conflicts of interest. References [1] Anon, Technical Communication, Dow Chemical Company, Dow North America, Freeport, TX, 1994, p. 77541. [2] E.F. Emley, Principle of Magnesium Technology, Pergamon Press, London, UK, 1966. [3] R.F. Decker, Adv. Mater. Process. 154 (1998) 31–33. [4] J.R. Thomas, L.A. Darryl, Adv. Mater. Process. 145 (1994) 28–32. [5] F.H. Froes, D. Eliezer, E. Aghion, J. Miner. Met. Mater. Soc. 5 (1998) 30–34. [6] G.L. Makar, J. Kruger, Int. Mater. Rev. 38 (1993) 138–153. [7] A.L. Rudd, C.B. Breslin, F. Mansfeld, Corros. Sci. 42 (2000) 275–288. [8] G. Song, B. Johannesson, S. Hapugoda, D. St John, Corros. Sci. 46 (2004) 955–977. [9] G. Song, D. St John, Corros. Sci. 46 (2004) 1381–1399. [10] G. Song, A.L. Bowles, D. St John, Mater. Sci. Eng. A366 (2004) 74–86. [11] H. Huo, Y. Li, F. Wang, Corros. Sci. 46 (2004) 1467–1477. [12] L.H. Chiu, C.C. Chen, C.F. Yang, Surf. Coat. Technol. 191 (2005) 181–187. [13] Y. Zhang, C. Yan, F. Wang, W. Li, Corros. Sci. 47 (2005) 2816–2831. [14] X.W. Guo, W.J. Ding, C. Lu, C.Q. Zhai, Surf. Coat. Technol. 183 (2004) 359–368. [15] G. Song, A. Atrens, Adv. Eng. Mater. 1 (1999) 11–33. [16] G. Song, A. Atrens, Adv. Eng. Mater. 5 (2003) 837–858. [17] G.E. Totten, L. Xie, K. Funatani (Eds.), Handbook of Mechanical Alloy Design, Marcel Dekker, Inc., New York, 2004, pp. 487–538. [18] N.S. McIntyre, C. Chen, Corros. Sci. 40 (1998) 1697–1709. [19] C. Chen, S.J. Splinter, T. Do, N.S. McIntyre, Surf. Sci. 382 (1997) L652–L657. [20] W.K. Miller, E.F. Ryntz, Society of Automotive Engineers, Warrendale, PA, 1984, pp. 2.524–2.542. [21] Y. Cheng, T. Qin, H. Wang, Z. Zhang, Trans. Nonferrous Met. Soc. China 19 (2009) 517–524.
Please cite this article as: M.G. Acharya and A.N. Shetty, The corrosion behavior of AZ31 alloy in chloride and sulfate media – A comparative study through electrochemical investigations, Journal of Magnesium and Alloys, https:// doi.org/ 10.1016/ j.jma.2018.09.003
JID: JMAA
ARTICLE IN PRESS M.G. Acharya and A.N. Shetty / Journal of Magnesium and Alloys xxx (xxxx) xxx
[22] O. Lunder, J.E. Lein, T.K. Aune, K. Nisancioglu, Corrosion 45 (1989) 741–748. [23] T.J. Warner, N.A. Thorne, G. Nussbaum, W.M. Stobbs, Surf. Interface Anal. 19 (1992) 386–392. [24] F. Hehmann, F.H. Froes, W. Young, J. Met. 39 (1987) 14–21. [25] J. Banhart, Prog. Mater. Sci. 46 (2001) 559–632. [26] C. Blawert, N. Hort, K.U. Kainer, Trans. Indian Inst. Met. 57 (2004) 397–408. [27] M. Gupta, N.M.L. Sharon, Magnesium Alloys and Magnesium Composites, John Wiley & Sons, NewJersy, 2011. [28] L. Wang, T. Shinohara, B.P. Zhang, Corros. Eng. Jpn. 58 (2009) 105–110. [29] L. Wang, T. Shinohara, B.P. Zhang, J. Alloys Compd. 496 (2010) 500–507. [30] N. Wang, R. Wang, C. Peng, Y. Feng, X. Zhang, Trans. Nonferrous. Met. Soc. China 20 (2010) 1936–1943. [31] M.C. Zhao, M. Liu, G. Song, A. Atrens, Corros. Sci. 50 (2008) 3168–3178. [32] R.C. Zeng, Y. Hu, S.K. Guan, H.Z. Cui, E.H. Han, Corrosion science 86 (2014) 171–182.
[m5+;December 28, 2018;15:54]
15
[33] M. Baghni, Y. Wu, J. Li, W. Zhang, Trans. Nonferrous. Met. Soc. China 14 (2004) 1–10. [34] S. Mathieu, C. Rapin, J. Hazan, P. Steinmetz, Corros. Sci. 44 (2002) 2737–2756. [35] M.G. Lopez, B. Natta, Corrosion 712 (2001) 5006–5015. [36] G. Song, A. Andrej, D. Mathew, Corros. Sci. 41 (1999) 249–273. [37] N. Dinodi, A.N. Shetty, J. Magn. Alloys 1 (2013) 201–209. [38] J.H. Nordlien, S. Ono, N. Masuko, K. Nisancioglu, Corros. Sci. 39 (1997) 1397–1414. [39] G. Song, A. Atrens, Adv. Eng. Mater. 9 (2007) 177–183. [40] G. Song, A. Atrens, D. St John, X. Wu, J. Nairn, Corros. Sci. 39 (1997) 10–11. [41] G. Baril, N. Pebere, Corros. Sci. 43 (2001) 471–484. [42] H. Ardelean, I. Frateur, P. Marcus, Corros. Sci. 50 (2008) 1907–1918. [43] S. Fletcher, J. Electrochem. Soc. 141 (1994) 1823–1826. [44] F. Mansfeld, C.H. Tsai, H. Shih, Computer Modeling in Corrosion, ASTM, Philadelphia, 1992, pp. 186–196. [45] N. Pebere, C. Riera, F. Dabosi, Electrochim. Acta 35 (1990) 555–561. [46] F. Bentiss, M. Lebrini, M. Lagrenee, Corros. Sci. 47 (2005) 2915–2931.
Please cite this article as: M.G. Acharya and A.N. Shetty, The corrosion behavior of AZ31 alloy in chloride and sulfate media – A comparative study through electrochemical investigations, Journal of Magnesium and Alloys, https:// doi.org/ 10.1016/ j.jma.2018.09.003