Electrochemical investigation on the effects of sulfate ion concentration, temperature and medium ph on the corrosion behavior of mg–al–zn–mn alloy in aqueous ethylene glycol

Electrochemical investigation on the effects of sulfate ion concentration, temperature and medium pH on the corrosion behavior of Mg–Al–Zn–Mn alloy in aqueous ethylene glycol Full Length Article Elect[.]

ARTICLE IN PRESS Available online at www.sciencedirect.com H O S T E D BY ScienceDirect Journal of Magnesium and Alloys ■■ (2017) ■■–■■ www.elsevier.com/journals/journal-of-magnesium-and-alloys/2213-9567 Full Length Article Electrochemical investigation on the effects of sulfate ion concentration, temperature and medium pH on the corrosion behavior of Mg–Al–Zn–Mn alloy in aqueous ethylene glycol H Medhashree, A Nityananda Shetty * Department of Chemistry, National Institute of Technology Karnataka, Surathkal, Srinivasnagar, Mangalore, Karnataka 575025, India Received 15 September 2016; revised 28 December 2016; accepted 28 December 2016 Available online Abstract The effects of sulfate ion concentration, temperature and medium pH on the corrosion of Mg–Al–Zn–Mn alloy in 30% aqueous ethylene glycol solution have been investigated by electrochemical techniques such as potentiodynamic polarization and electrochemical impedance spectroscopy methods Surface morphology of the alloy was examined before and after immersing in the corrosive media by scanning electron microscopy (SEM) and energy dispersion X-ray (EDX) analysis Activation energy, enthalpy of activation and entropy of activation were calculated from Arrhenius equation and transition state theory equation The obtained results indicate that, the rate of corrosion increases with the increase in sulfate ion concentration and temperature of the medium and decreases with the increase in the pH of the medium © 2017 Production and hosting 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/) Keywords: Mg–Al–Zn alloy; EIS; SEM; EDX Introduction The high strength to weight ratio, ease of machinability, castability and weldability etc., have made magnesium alloys to be of great utility in several industrial fields for various applications [1] Automotive industry is one such field, where the total weight of the vehicle matters for fuel consumption and environmental pollution Therefore nowadays, attempts are made to replace most of the aluminum alloys in structural applications with low density material like magnesium alloys But the use of magnesium alloy has been limited for outdoor applications mainly due to its poor corrosion resistance [2] Magnesium forms an oxide film on the surface which can give limited protection The existence of such a surface film was confirmed by filiform corrosion of high purity magnesium in 3.5% NaCl solution saturated with Mg(OH)2 The extent of formation of surface film is decided by the various medium conditions, including medium pH [3] Liu et al showed that the magnesium film formed on the surface, consisted of a porous * Corresponding author Department of Chemistry, National Institute of Technology Karnataka, Surathkal, Srinivasnagar, Mangalore, Karnataka 575025, India Fax: +92 8242474033 E-mail address: nityashreya@gmail.com (A.N Shetty) hydroxide layer of (Mg(OH)2) upon an inner layer of MgO [4] The MgO has the Pilling-Bedworth ratio of 0.81 which indicates the insufficient oxide to cover the metal and is unprotective, whereas Mg(OH)2 has the Pilling-Bedworth ratio of 1.77, indicating the poor oxidation resistance due to cracking and spalling [5,6] Compared to pure magnesium its alloy with aluminum is having better resistance to corrosion because of the inclusion of aluminum in the matrix of the surface film Magnesium as well as its alloys has poor pitting resistance in the media containing aggressive ions such as chlorides, sulfate and bromides This is due to the breakdown of the oxide and hydroxide layer by aggressive ions [5] It is well established in the literature that chloride ion increases the rate of magnesium corrosion in aqueous medium and the rate of corrosion increases with the increase in the concentration of the chloride ions [7–9] Many literatures explain the effect of sulfate ion concentration on the corrosion behavior of pure magnesium and its alloy in aqueous medium Song et al studied the anodic dissolution of pure magnesium in chloride, sulfate and hydroxide solution and concluded that the rate of corrosion was more in chloride solution than that in sulfate solution [10] Chen et al have studied the corrosion behavior of an AZ91 magnesium alloy in 0.1 M sodium sulfate solution at various immersion times [5] However, the effect of sulfate ion concentration on http://dx.doi.org/10.1016/j.jma.2016.12.003 2213-9567/© 2017 Production and hosting 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/) Please cite this article in press as: H Medhashree, A Nityananda Shetty, Electrochemical investigation on the effects of sulfate ion concentration, temperature and medium pH on the corrosion behavior of Mg–Al–Zn–Mn alloy in aqueous ethylene glycol, Journal of Magnesium and Alloys (2017), doi: 10.1016/j.jma.2016.12.003 ARTICLE IN PRESS H Medhashree, A.N Shetty / Journal of Magnesium and Alloys ■■ (2017) ■■–■■ Table Composition of Mg–Al–Zn–Mn alloy Element Aluminum Zinc Manganese Silicon Copper Iron Magnesium Weight (%) 8.3 0.6 0.35 0.2 0.12 0.2 Balance the corrosion rate of Mg–Al–Zn–Mn alloy in aqueous organic medium like ethylene glycol is not found in literature The aqueous ethylene glycol medium is present in engine coolant of an automobile, the components of which are preferably made up of magnesium alloys Presently, the main composition of the coolant is 30–70% by volume of aqueous ethylene glycol It was found by Song and St John that pure ethylene glycol was almost inert toward the corrosivity of magnesium and the rate of corrosion depended on the water content of the solution This is because pure ethylene glycol has very low electrical conductivity, and acts as an insulator for the movement of electrons and corrosion products As the volume of the water increases, the hydrolysis of the hydroxyl groups of the ethylene glycol leads to the increased conductivity of the solution and thus increases the rate of corrosion as well And also, the presence of contaminants like chloride, sulfate, etc., increases the rate of corrosion in aqueous ethylene glycol Therefore, water quality should have remarkable influence on the corrosion rate of magnesium [11] Fekry and Fatayerji studied the effect of concentration of chloride and fluoride ions on the corrosion rate of AZ91D alloy and also have explained the effect of concentration of ethylene glycol on the corrosion rate of AZ91D alloy [12] Huang et al have reported the corrosion performance of the magnesium based rare earth containing alloy GW103 in 50 vol% ethylene glycol [13] Similarly, Slavcheva et al have studied the influence of chloride ion on the corrosion of AZ91 magnesium alloy in 50 wt% water and ethylene glycol; they have discussed the effect of chloride ion on the dissolution of magnesium alloy [14] Seifzadeh and Basharnavaz have studied the corrosion behavior of AZ91 alloy in 30% aqueous ethylene glycol at 25 °C, 50 °C and 75 °C [15] The present investigation is aimed to study the effect of sulfate ion concentration, temperature and pH of the medium on the corrosion properties of Mg–Al–Zn alloy in 30% aqueous ethylene glycol water Sulfate ion concentration was varied from mM to 10 mM with an increment of mM in each trail The effect of temperature was studied using calibrated thermostat at a range of 30 °C to 50 °C with an increment of °C in each trail 2.3 Electrochemical measurements All the experiments were carried out in a conventional three electrode compartments Pyrex glass cell The electrochemical cell setup consists of platinum as counter electrode, saturated calomel as reference electrode and Mg–Al–Zn–Mn alloy as working electrode All the electrochemical measurements were carried out using electrochemical work station, Gill AC having ACM instrument Version software 2.3.1 Potentiodynamic polarization studies Initially the working electrode was allowed to attain open circuit potential (OCP) After the attainment of OCP, it was polarized by applying a potential of −250 mV cathodically and +250 mV anodically with respect to the OCP A scan rate of mV s−1 was applied for all potentiodynamic polarization studies Polarization studies were carried out immediately after the impedance measurement without any further surface treatment 2.3.2 Electrochemical impedance spectroscopy (EIS) studies Periodic small amplitude of 10 mV over a wide range of frequency from 100 kHz to 0.01 Hz were applied for the impedance measurements Impedance data were analyzed by circuit fitment to the obtained Nyquist plots, using ZSimpWin software of version 3.21 For all the above measurement, minimum of three similar results were considered and average values of the results have been reported 2.4 Surface characterization JEOL JSM-6380LA analytical scanning electron microscope was used for recording SEM and EDX spectra of the freshly polished surface and corroded surfaces of the Mg–Al– Zn–Mn alloy Experimental Results and discussion 2.1 Electrodes 3.1 Potentiodynamic polarization measurements In all the experiments Mg–Al–Zn–Mn alloy was used as the test electrode The composition of the alloy is given in Table A cylindrical test coupon, mounted in epoxy resin was used as the working electrode, with an area of exposure of 0.834 cm2 Before immersing in the electrolyte media, the working electrode was abraded on SiC papers of grades 600–2000 and polished on polishing wheel using legated alumina to get mirror finish It was followed by washing in double distilled water and acetone The working electrode was properly dried before immersing in the corrosive medium The potentiodynamic polarization studies of Mg–Al– Zn–Mn alloy in 30% aqueous ethylene glycol were carried out in a range of conditions such as different concentrations of sulfate ions, temperature and with varying medium pH using Tafel extrapolation method For Mg and Mg alloy, Tafel extrapolation method cannot be applied for the determination of corrosion rate in corrosive medium containing aggressive ions [16,17] This is because the formed surface film breaks down at more negative potential than their OCP, thus Tafel plots not follow the Tafel equation around their OCPs However, in ethylene glycol, the polarization curve shows the breakdown potential more positive than their corrosion potential and therefore, Tafel extrapolation method is applicable in ethylene glycol medium [13] 2.2 Electrolyte media Electrolyte medium was prepared by using analytical grade sodium sulfate, AR grade ethylene glycol and double distilled Please cite this article in press as: H Medhashree, A Nityananda Shetty, Electrochemical investigation on the effects of sulfate ion concentration, temperature and medium pH on the corrosion behavior of Mg–Al–Zn–Mn alloy in aqueous ethylene glycol, Journal of Magnesium and Alloys (2017), doi: 10.1016/j.jma.2016.12.003 ARTICLE IN PRESS H Medhashree, A.N Shetty / Journal of Magnesium and Alloys ■■ (2017) ■■–■■ anodic polarization curve shows the inflection point with two different slopes at potential more positive than corrosion potential This is due to some sort of kinetic barrier effect, due to the deposition of corrosion product surface film followed by its dissolution at higher anodic potential [18–20] As seen in Fig 1, the nature of the polarization curves remain the same at higher concentration of sulfate ion, which indicates the insignificant role of sulfate ion on the mechanism of corrosion But the polarization curves shifted to higher current density region with the increase in the concentration of the sulfate ion, indicating an increase in the corrosion rate with the increase in the concentration of sulfate ion Electrochemical parameters were calculated by extrapolating the linear Tafel region of the cathodic polarization curve to OCP Potentiodynamic polarization parameters such as corrosion current density (icorr), corrosion potential (Ecorr) and cathodic slope (bc) are tabulated in Table The corrosion rate was calculated from corrosion current density using the following equation [6]: Fig Potentiodynamic polarization plots for the corrosion of Mg–Al–Zn–Mn alloy in 30% aqueous ethylene glycol containing different concentrations of sulfate ions at 50 °C υcorr = Fig shows the potentiodynamic polarization plots for the corrosion of Mg–Al–Zn–Mn alloy in 30% aqueous ethylene glycol containing different concentrations of sulfate ion at 50 °C In polarization plots, the anodic polarization curve corresponds to the anodic dissolution of magnesium alloy, and the cathodic polarization curve represents the hydrogen evolution through reduction of water The cathodic polarization curves are characterized by distinctly linear Tafel region, and the In the above equation K = 0.00327 a constant, which defines the unit of corrosion rate (mm y−1), icorr is the corrosion current density (μA cm−2), ρ is the density of the specimen and EW is the equivalent weight of the alloy The low corrosion rate value of the Mg–Al–Zn–Mn alloy in aqueous ethylene glycol medium indicates the adsorption of ethylene glycol molecule on the surface of the alloy Ethylene glycol molecule, being larger than water with two electron rich oxygen atoms, gets adsorbed on the surface of the alloy and thus reduces the corrosion rate due K × icorr × EW ρ (1) Table Electrochemical parameters for the corrosion of Mg–Al–Zn–Mn alloy in 30% aqueous ethylene glycol containing different concentration of sulfate ions at different temperatures Concentration of sulfate ions (mM) Temperature (°C) Ecorr vs SCE (mV) ‒bc (mV dec−1) icorr (μA cm−2) ʋcorr (mm y−1) Rhf (ohm cm2) 30 35 40 45 50 30 35 40 45 50 30 35 40 45 50 30 35 40 45 50 30 35 40 45 50 −1546 −1556 −1468 −1464 −1470 −1545 −1538 −1457 −1459 −1455 −1566 −1519 −1512 −1500 −1445 −1481 −1534 −1457 −1463 −1464 −1530 −1520 −1458 −1458 −1462 183 206 199 197 208 220 248 212 213 218 173 254 261 284 229 351 262 206 207 206 284 326 227 211 225 3.937 4.382 4.813 5.657 7.090 4.997 6.435 7.142 8.660 9.411 5.720 6.757 7.824 9.363 10.623 6.729 8.445 9.671 10.898 11.403 7.966 9.809 11.045 12.063 13.176 0.085 0.094 0.103 0.122 0.152 0.107 0.138 0.153 0.186 0.202 0.124 0.145 0.168 0.201 0.228 0.145 0.182 0.208 0.234 0.245 0.171 0.211 0.237 0.259 0.283 3700 3612 3520 2991 2799 3621 3546 3229 2641 2472 3601 3033 2721 2505 2273 3429 3004 2637 2383 2155 3067 2873 2498 2275 1969 10 Please cite this article in press as: H Medhashree, A Nityananda Shetty, Electrochemical investigation on the effects of sulfate ion concentration, temperature and medium pH on the corrosion behavior of Mg–Al–Zn–Mn alloy in aqueous ethylene glycol, Journal of Magnesium and Alloys (2017), doi: 10.1016/j.jma.2016.12.003 ARTICLE IN PRESS H Medhashree, A.N Shetty / Journal of Magnesium and Alloys ■■ (2017) ■■–■■ to water [12] The corrosion of magnesium alloys in aqueous medium proceeds through an electrochemical reaction between magnesium and water to produce magnesium hydroxide and hydrogen as per the following equation: Mg + 2H2O → Mg ( OH )2 + H2 (2) The anodic dissolution of magnesium proceeds through the oxidation of magnesium into monovalent Mg+ ions and divalent Mg2+ ions as summarized by the following reactions, represented in Eqs (3) and (4) [21] Mg → Mg + + e − (3) Mg → Mg 2+ + 2e − (4) At more active potentials around −2.78 V (vs SCE) magnesium is oxidized to monovalent magnesium ion and at slightly higher potentials of −1.56 V (vs SCE) oxidation to divalent magnesium ion takes place in parallel with the former oxidation [22] Being unstable, monovalent magnesium ion undergoes oxidation to divalent magnesium ion through a series of reactions involving unstable intermediates like magnesium hydride as shown in equations (5)–(8) below: Mg + + 2H + + 3e − → MgH2 (5) MgH2 + 2H2O → Mg 2+ + 2OH − + 2H2 (6) Mg 2+ + 2OH − → Mg ( OH )2 (7) 2Mg + 2H2O → Mg + Mg ( OH )2 + H2 (8) + 2+ Mg–Al–Zn–Mn alloys are dual phase alloys consisting of microstructure with a primary α-phase and a divorced eutectic β-phase, distributed along the grain boundaries [23] The α-Mg matrix is α-Mg–Al–Zn solid solution with the same crystal structure as pure magnesium and the β-phase is made up of Mg17Al12 They are also found to have intermetallic inclusions of MnAl2 [24] The α-Mg matrix, associated with a very negative free corrosion potential, acts as anodic with respect to the β-phase of Mg17Al12, and undergoes corrosion by the micro galvanic coupling between the anodic α-Mg phase and cathodic β-Mg17Al12 phase [25–31] However, the β-Mg17Al12 phase may also have a different role in reducing the corrosion as a barrier film if it is in the form of a continuous network [27,28] However, the corrosion of the alloy in the aqueous ethylene glycol media in the presence of sulfate ions indicates the discontinuities in the β-phase According to the literature, it is a well-known fact that magnesium alloys exhibit higher corrosion resistance than pure magnesium [32] The higher corrosion resistance of Mg alloys in the presence of alloying elements has been attributed to a variety of factors such as refining of the β-phase with the formation of more continuous network, suppression of β-phase formation with the formation of a less harmful intermetallic and incorporation of the added elements into the protective film and thus increasing its stability [33–35] Small additions of Mn have been reported to increase the corrosion resistance of magnesium alloys and also reduce the effects of metallic impurities [36,37] Fig Nyquist plots for the corrosion of Mg–Al–Zn–Mn alloy in 30% aqueous ethylene glycol containing different concentrations of sulfate ions at 30 °C The corrosion rate in the presence of sulfate ions is less, compared to the rate in the presence of chloride ions [38] This is because, in the presence of sulfate ions Mg-Al alloys mainly forms MgAl2(SO4)4.22H2O by the following equation: Mg + + 2Al3+ + 4SO4 − + 22 H2 O → MgAl2 (SO4 )4 22H2 O (9) The so formed MgAl2(SO4)4 22H2O, as a corrosion product, owing to its low solubility, forms a semi-permeable film on the alloy surface and thus decreases the penetration of sulfate ions Also, sulfate ion is bigger than chloride ions in size, and hence, less prone to drill through the film Thus, both these factors decrease the penetration of sulfate ions to the fresh alloy surface [39] 3.2 Electrochemical impedance spectroscopy Fig shows the impedance spectra in the form of Nyquist plots for the corrosion of Mg–Al–Zn–Mn alloy in 30% aqueous ethylene glycol medium containing different concentrations of sulfate ions at 30 °C Similar plots were obtained at other temperatures also The Nyquist plots for the corrosion of Mg–Al– Zn–Mn alloy consist of two capacitive loops and beginning of an inductive loop at higher, medium and lower frequency regions, respectively The depressed semi-circle at the higher frequency region is due to the charge transfer reaction during the corrosion process The second capacitive loop pertains to the diffusion of ions through the corrosion product layer and relaxation of intermediate species The lower frequency inductive loop corresponds to the relaxation of adsorbed species like Mg(OH)+ and Mg(OH)2 adsorbed at the metal surface [40–42] The decrease in the diameter of the capacitive loop denotes the increased corrosion rate with the increase in the concentration of the sulfate ion [43,44] The quantitative analysis of the EIS experimental data was performed by fitting to equivalent circuit model by ZSimpWin software of version 3.21 The best fitting equivalent electrical Please cite this article in press as: H Medhashree, A Nityananda Shetty, Electrochemical investigation on the effects of sulfate ion concentration, temperature and medium pH on the corrosion behavior of Mg–Al–Zn–Mn alloy in aqueous ethylene glycol, Journal of Magnesium and Alloys (2017), doi: 10.1016/j.jma.2016.12.003 ARTICLE IN PRESS H Medhashree, A.N Shetty / Journal of Magnesium and Alloys ■■ (2017) ■■–■■ Fig Equivalent electrical circuit used for the simulation of experimental impedance data points circuit with less than 5% error, for the impedance data points, neglecting the low frequency inductive loop is as shown in the Fig The high frequency response can be simulated by a series of two parallel resistances – constant phase element (R-CPE) networks: the charge transfer resistance (Rct) in parallel with the double layer CPE (Qdl) and the resistance of the surface film (Rf) in parallel with the film CPE (Qf) The diffusion resistance (Zw), designated as Rdif in the equivalent circuit and CPE (Qdif) associated with diffusion, is assigned to the middle frequency response [45] The constant phase element (Qdl) is substituted for the ideal capacitive element to give a more accurate fit, as the lack of homogeneity and even porosity of the electrode surface can be accounted only by the introduction of constant phase element [8] The impedance of constant phase is given by: Z Q = Y0 −1 ( jw ) −n (10) where Y0 is the CPE constant, w is the angular frequency j = √-1 is the imaginary number, n is a CPE exponent which measures the heterogeneity or roughness of the surface Depending on the value of n = 0, 1, or −1 CPE can represent resistance, capacitance or inductance, respectively [46–49] The collective resistance associated with the high frequency loop (Rhf) is inversely related to the corrosion rate [50,51] The values of Rhf for the corrosion of the alloy are listed in Table The Rhf value decreases with the increase in the concentration of sulfate ions, indicating an increase in the corrosion rate Fig Potentiodynamic polarization plots for the corrosion of Mg–Al–Zn–Mn alloy in 30% aqueous ethylene glycol containing mM sulfate ions at different temperatures ln (υcorr ) = B − ( Ea RT ) (11) where B = a constant, which depends on the type of the metal, R = Universal gas constant A straight line graph is obtained by plotting ln( υcorr) vs reciprocal of absolute temperature (1/T) The activation energy values were calculated from the slope = − Ea/R Fig shows the Arrhenius plots for the corrosion of Mg–Al–Zn–Mn alloy in 30% aqueous ethylene glycol containing different concentrations of sulfate ion at different temperatures The transition state theory equation was used to calculate the enthalpy of activation and entropy of activation υcorr = ( RT Nh) exp ( ΔS # R ) exp ( − ΔH # RT ) (12) 3.3 Effect of temperature on corrosion rate The rate of chemical reaction increases with the raise in temperature Since corrosion is an electrochemical reaction, the raise in temperature definitely increases the rate of corrosion [52] Fig shows the Tafel plots for the corrosion of Mg–Al– Zn–Mn alloy in 30% aqueous ethylene glycol solution containing mM sulfate ion at different temperatures From Fig 4, it is observed that the polarization curves shifted to the higher current density region with the rise in temperature This phenomenon is further confirmed by the electrochemical impedance spectroscopy results The nature of the Nyquist plots (Fig 5) remains unaltered at higher temperature, which indicates the influential role of temperature is only on the rate of corrosion and not on the mechanism of corrosion Further, the potentiodynamic polarization and EIS parameters listed in Table reveal the same trend Arrhenius equation was used to calculate the activation energy of the corrosion process Arrhenius equation is given by: Fig Nyquist plots for the corrosion of Mg–Al–Zn–Mn alloy in 30% aqueous ethylene glycol containing mM sulfate ions at different temperatures Please cite this article in press as: H Medhashree, A Nityananda Shetty, Electrochemical investigation on the effects of sulfate ion concentration, temperature and medium pH on the corrosion behavior of Mg–Al–Zn–Mn alloy in aqueous ethylene glycol, Journal of Magnesium and Alloys (2017), doi: 10.1016/j.jma.2016.12.003 ARTICLE IN PRESS H Medhashree, A.N Shetty / Journal of Magnesium and Alloys ■■ (2017) ■■–■■ Table Activation parameters for the corrosion of Mg–Al–Zn–Mn alloy in 30% aqueous ethylene glycol containing different concentrations of sulfate ions Concentration of sulfate ions (mM) Ea (kJ mol−1) ΔH# (kJ mol−1) ΔS# (J mol−1 K−1) 10 23.07 25.60 25.14 21.26 19.80 20.47 23.00 22.54 18.66 17.20 −198.30 −187.37 −188.03 −199.08 −202.59 Mg alloy consists of mainly Mg(OH)2 by the following equations: Mg → Mg 2+ + 2e − 2H2O + 2e − → H2 + (OH ) Fig Arrhenius plots for the corrosion of Mg–Al–Zn–Mn alloy in 30% aqueous ethylene glycol containing different concentrations of sulfate ions at different temperatures where N = Avogadro’s number and h = Plank’s constant A straight line graph is obtained by plotting ln( υcorr/T) vs 1/T (Fig 7) Enthalpy of activation and entropy of activation were calculated from the slope = − ΔH#/R and intercept = ln(R/ Nh) + ΔS#/R, respectively [53] The activation parameters for the corrosion process are as shown in Table 3.4 Effect of pH on corrosion rate (13) − Mg 2+ + ( OH ) → Mg ( OH )2 − (14) (15) The influence of pH on corrosion of Mg alloy needs to take into the account of Mg, Al, Zn Pourbaix diagram Fig shows the Pourbaix diagram of Mg, Al, Zn [32,54] From the Pourbaix diagram, it is clear that the corrosion product Mg(OH)2 is stable only in alkaline conditions, with pH above 10.5 In the pH range 4–9, Al is passive and above pH 9, formation of magnesium aluminates and stabilization of Mg(OH)2 occurs In the range between 8.5 and 10.5 Zn is also passive Thermodynamics and the Pourbaix diagram predict that there should be no film on Mg surface in acidic and neutral pH solutions However, even though the surface film is not thermodynamically stable at low pH values, the surface film may be formed provided the The corrosion behavior of Mg–Al–Zn–Mn alloy is significantly influenced by pH and sulfate ion concentration The corrosion rate of Mg alloy is associated with the characteristics of its surface film In aqueous solution, the surface film of the Fig Plots of (lncorr /T) vs 1/T for the corrosion of Mg–Al–Zn–Mn alloy in 30% aqueous ethylene glycol containing different concentration of sulfate ions Fig Pourbaix diagram of Mg, Al, Zn Please cite this article in press as: H Medhashree, A Nityananda Shetty, Electrochemical investigation on the effects of sulfate ion concentration, temperature and medium pH on the corrosion behavior of Mg–Al–Zn–Mn alloy in aqueous ethylene glycol, Journal of Magnesium and Alloys (2017), doi: 10.1016/j.jma.2016.12.003 ARTICLE IN PRESS H Medhashree, A.N Shetty / Journal of Magnesium and Alloys ■■ (2017) ■■–■■ Fig Potentiodynamic polarization curves for the corrosion of Mg–Al– Zn–Mn alloy in 30% aqueous ethylene glycol containing 10 mM sulfate ions at different pH and at 30 °C dissolution kinetics is slower than the formation kinetics Furthermore, an alkaline pH zone develops in the electrolyte layer near a corroding magnesium electrode, as a result of the cathodic reaction of hydroxyl ion generation, even though the bulk solution has an acidic pH value of This alkalization is associated with the formation of surface film The corrosion rate of Mg–Al–Zn alloy increases with the decrease in the pH of the media and increase in sulfate ion concentration This trend is consistent with the corrosion behavior of Mg alloy, governed by a partially protective surface film Corrosion reaction occurs predominantly at the breaks or imperfections of the partially protective Mg(OH)2 film The fraction of film free surface increases with the decreasing pH of the media and with the increasing sulfate ion concentration However, the value of corrosion rate in alkaline media is small, but significant because the Mg(OH)2 surface film is with a pilling Bed-worth ratio ~ 0.81 and hence incapable of imparting complete passivity to the underlying metal [18,34,55] Fig 10 Nyquist plots for the corrosion of Mg–Al–Zn–Mn alloy in 30% aqueous ethylene glycol containing mM sulfate ions at different pH and at 30 °C Fig shows the potentiodynamic polarization curves for the corrosion of Mg–Al–Zn–Mn alloy in 30% aqueous ethylene glycol containing 10 mM sulfate ions at different pH, at 30 °C The polarization curves shift to the higher current density region with decreasing the medium pH from alkaline to acidic condition The effect of pH on the corrosion rate is further supported by the results of electrochemical impedance spectroscopy studies Fig 10 shows the Nyquist plots for the corrosion of Mg–Al– Zn–Mn alloy in 30% aqueous ethylene glycol containing mM sulfate ions at different pH and at 30 °C The results of potentiodynamic polarization and EIS studies at different pH of the media are tabulated in Table In impedance results, the value of Rhf increases with the increase in the pH of the media, indicating a decreasing trend in corrosion rate 3.5 Surface morphology and EDX analysis Mg–Al–Zn–Mn alloy mainly consists of α-Mg matrix and precipitates of Mg17Al12 as a second phase with dendritic and Table Electrochemical parameters for the corrosion of Mg–Al–Zn–Mn alloy in 30% aqueous ethylene glycol solution of different pH, containing different concentrations of sulfate ions at 30 °C Concentration of sulfate ions (mM) pH Ecorr vs SCE (mV) ‒bc (mV dec−1) icorr (μA cm−2) ʋcorr (mm y−1) Rhf (ohm cm2) mM 10 12 10 12 10 12 −1550 −1545 −1533 −1150 −1520 −1529 −1533 −1336 −1488 −1518 −1521 −1380 294 308 278 209 204 300 294 170 314 319 325 177 7.049 6.054 4.292 0.248 8.292 7.374 6.706 0.376 9.891 8.767 7.857 0.444 0.152 0.130 0.092 0.005 0.178 0.158 0.144 0.008 0.213 0.189 0.169 0.009 3597 4159 5723 64540 2891 3655 3777 62520 2743 3460 3658 57510 mM 10 mM Please cite this article in press as: H Medhashree, A Nityananda Shetty, Electrochemical investigation on the effects of sulfate ion concentration, temperature and medium pH on the corrosion behavior of Mg–Al–Zn–Mn alloy in aqueous ethylene glycol, Journal of Magnesium and Alloys (2017), doi: 10.1016/j.jma.2016.12.003 ARTICLE IN PRESS H Medhashree, A.N Shetty / Journal of Magnesium and Alloys ■■ (2017) ■■–■■ Fig 11 SEM image of freshly polished surface of Mg–Al–Zn–Mn alloy Fig 12 EDX spectrum of freshly polished surface of Mg–Al–Zn–Mn alloy Fig 13 XRD pattern of the Mg–Al–Zn–Mn alloy Fig 14 SEM image of Mg–Al–Zn–Mn alloy specimen, immersed in the medium of 30% aqueous ethylene glycol containing 10 mM sulfate ions for 24 h Please cite this article in press as: H Medhashree, A Nityananda Shetty, Electrochemical investigation on the effects of sulfate ion concentration, temperature and medium pH on the corrosion behavior of Mg–Al–Zn–Mn alloy in aqueous ethylene glycol, Journal of Magnesium and Alloys (2017), doi: 10.1016/j.jma.2016.12.003 ARTICLE IN PRESS H Medhashree, A.N Shetty / Journal of Magnesium and Alloys ■■ (2017) ■■–■■ Fig 15 EDX spectrum of Mg–Al–Zn–Mn alloy specimen immersed in the medium of 30% aqueous ethylene glycol containing 10 mM sulfate ions for 24 h island-like structures [56] In Mg alloys, most of the additional elements affect the corrosion resistance of magnesium alloys after they form second phases High aluminum content Mg alloys generally associated with β phase of Mg17Al12 which is cathodic with respect to α-Mg matrix This β-phase shows a passive behavior than either of its components aluminum and magnesium It was found that the distribution of the Mg17Al12 phase determines the corrosion resistance property of the alloy; if the volume fraction of the β-phase is high, it acts as a barrier and decreases the corrosion rate At the same time, if the volume fraction of the β-phase is less, then it acts as a cathode and accelerates the corrosion process [1] Fig 11 and Fig 12 show the SEM image and EDX spectrum of the fresh Mg–Al–Zn–Mn alloy, respectively The enlarged image of the SEM image clearly shows the presence of discontinuous precipitation in lamellar form of β-phase (Mg17Al12) The presence of β-phase was further confirmed by XRD pattern and is given in Fig 13 Fig 14 and Fig 15 show SEM image and EDX spectrum of the corroded alloy surface after immersing in the corrosive Fig 16 SEM images of Mg–Al–Zn–Mn specimen surface after 24 h immersion in 30% aqueous ethylene glycol containing 10 mM sulfate ions with (a) acidic pH of 4, (b) neutral pH of and (c) alkaline pH of 12 Please cite this article in press as: H Medhashree, A Nityananda Shetty, Electrochemical investigation on the effects of sulfate ion concentration, temperature and medium pH on the corrosion behavior of Mg–Al–Zn–Mn alloy in aqueous ethylene glycol, Journal of Magnesium and Alloys (2017), doi: 10.1016/j.jma.2016.12.003 ARTICLE IN PRESS H Medhashree, A.N Shetty / Journal of Magnesium and Alloys ■■ (2017) ■■–■■ 10 medium A strong peak of oxygen in the EDX spectrum confirms the presence of Mg(OH)2 surface film on the corroded surface of the alloy Fig 16 shows the SEM images of the corroded surface of Mg–Al–Zn–Mn alloy after immersing in the media containing 10 mM sulfate ions with pH 4, and 10, respectively The specimen immersed in the acidic medium is more corroded than that in the neutral medium, whereas specimen in basic medium is not having any visible roughened surface Conclusions From the results of the experiments, following conclusions have been drawn: 1) Environmental factors like temperature, sulfate ion concentration and medium pH have remarkable influence on rate of corrosion of Mg–Al–Zn–Mn alloy 2) The rate of corrosion increases with increase in the concentration of the sulfate ion and increase in the temperature of the medium 3) Experimental results indicate that the order of corrosion rate in sulfate ion containing solution with various pH values is pH > pH > pH 10 > pH 12 4) From the kinetic study, it is proved that the corrosion kinetics follows 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Electrochemical investigation on the effects of sulfate ion concentration, temperature and medium pH on the corrosion behavior of Mg–Al–Zn–Mn alloy in aqueous ethylene glycol, Journal of Magnesium and Alloys... Nityananda Shetty, Electrochemical investigation on the effects of sulfate ion concentration, temperature and medium pH on the corrosion behavior of Mg–Al–Zn–Mn alloy in aqueous ethylene glycol, ... temperature, sulfate ion concentration and medium pH have remarkable influence on rate of corrosion of Mg–Al–Zn–Mn alloy 2) The rate of corrosion increases with increase in the concentration of the sulfate