Materials Science and Engineering B 176 (2011) 792–798

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Corrosion and impedance studies on magnesium alloy in oxalate solution A.M. Fekry ∗ , Riham H. Tammam Chemistry Department, Faculty of Science, Cairo University, Gamaa Street, Giza 12613, Egypt

a r t i c l e

i n f o

Article history: Received 12 July 2010 Received in revised form 8 February 2011 Accepted 27 March 2011 Keywords: AZ91E alloy Oxalate EIS Polarization SEM

a b s t r a c t Corrosion behavior of AZ91E alloy was investigated in oxalate solution using potentiodynamic polarization and electrochemical impedance measurements (EIS). The effect of oxalate concentration was studied, where the corrosion rate increases with increasing oxalate concentration. The effect of added ions (Br− , Cl− or SiO3 2− ) on the electrochemical behavior of magnesium alloy in 0.1 M Na2 C2 O4 solution at 298 K, was investigated. It was found that the corrosion rate of 0.1 M oxalate solution containing silicate ion is lower than the blank (0.1 M Na2 C2 O4 ). This was confirmed by scanning electron microscope (SEM) observations. However, for the other added ions Br− or Cl− , the corrosion rate is higher than the blank. Published by Elsevier B.V.

1. Introduction Magnesium is the 8th most abundant element on the earth [1]. It has a high thermal conductivity, good electromagnetic shielding characteristics and good machinability. Magnesium is the lightest of all metals in practical use, and has a density of 1.74 g cm−3 [2]. Magnesium alloys have many unique properties compared with other metals. Magnesium can form intermetallic phases with most alloying elements, the stability of this phase increases with the electronegativity of the other element [3]. Aluminum (Al) had already become the most important alloying element for significantly increasing the tensile strength, specifically by forming the intermetallic phase Mg17 Al12 . Similar effects can be achieved with zinc (Zn) and manganese (Mn). The AZ-based Mg system has been the basis of the most widely used magnesium alloys [4]. Among these alloys, AZ91 is the most successful alloy having excellent mechanical properties. It is used in car parts, notebook PCs, portable telephones and other products. Nevertheless, a serious limitation for the wide-spread use of several magnesium alloys is their susceptibility to general and localized (pitting) corrosion [4,5]. This makes studying the corrosion and corrosion control of Mg alloys an interesting point of research which can enable extending the potential use of these important materials in a broad range of many technical and innovative applications. The Pourbaix diagram for the Mg–water system is shown in Scheme 1 [6]. The whole domain of stability of magnesium is well below that of water. Magnesium therefore dissolves as Mg+ and Mg+2 with accompanying hydrogen evolution.

∗ Corresponding author. E-mail address: [email protected] (A.M. Fekry). 0921-5107/$ – see front matter. Published by Elsevier B.V. doi:10.1016/j.mseb.2011.03.014

An oxalate (ethanedioate) is a salt or ester of oxalic acid that is used in low technical applications throughout industry [7]. The charge on oxalate allows it to act as a chelator of various positively charged metal ions. The present work aims to attain more information concerning the electrochemical reactivity and corrosion behavior of AZ91E Mg-based alloy in oxalate solution containing Br− , Cl− or SiO3 2− anions under various environmental conditions including electrolyte type and concentration. Techniques employed were potentiodynamic polarization and impedance spectroscopy (EIS). 2. Experimental An extruded magnesium aluminum alloy AZ91E donated from Department of mining, Metallurgy and Materials Engineering, Laval University, Canada with chemical composition (wt%): 9.0 Al, 0.7 Zn, 0.13 Mn, 0.03 Cu, 0.01 Si, 0.006 Fe, 0.004 Ni, 0.0007 Be and balance Mg. The sample was divided into small coupons. Each coupon was welded to an electrical wire and fixed with Araldite epoxy resin in a glass tube leaving cross-sectional area of the specimen 0.196 cm2 . The solutions were prepared using Analar grade reagents (sodium oxalate, sodium bromide, sodium chloride and sodium silicate) and triply distilled water. The solubility of sodium oxalate is 3.99 g/100 g H2 O. The surface of the test electrode was mechanically polished by emery papers with 400 up to 1000 grit to ensure the same surface roughness. Subsequently, the specimens were degreased in acetone, rinsed with ethanol and dried in air. The cell used was a typical three-electrode one fitted with a large platinum sheet of size 15 mm × 20 mm × 2 mm as a counter electrode (CE), saturated calomel (SCE) as a reference electrode (RE) and AZ91E alloy as the working electrode (WE). Cathodic and anodic polarization curves were scanned from −2.5 V to −1.5 V with a

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Scheme 1. Potential-pH (Pourbaix) diagram for magnesium in water at 25 ◦ C.

scan rate of 1 mV s−1 . The impedance diagrams were recorded at the free immersion potential (OCP) by applying a 10 mV sinusoidal potential through a frequency domain from 100 kHz to 100 mHz such that there was five measured frequency points per decade. The instrument used is the electrochemical workstation IM6e Zahner-elektrik, GmbH (Kronach, Germany). The electrochemical experiments were always carried inside an air thermostat which was kept at 298 K, unless otherwise stated. All potentials were measured and given with respect to SCE (E = 0.241 V). The SEM micrographs were obtained using a JEOL JXA-840A electron probe microanalyzer. 3. Results and discussion

Fig. 1. (a and b) Bode and Nyquist plots of AZ91E alloy in sodium oxalate solution of different concentrations at 298 K.

3.1. EIS measurements The impedance measurements recorded after 2 h of immersing AZ91E electrode in oxalate solution with different concentrations, as shown in Fig. 1a and b as Bode and Nyquist plots, respectively. As can be seen in Fig. 1a, impedance value decreases with increasing oxalate concentration from 0.005 to 0.1 M. At high frequency range the solution resistance (RS ) dominates and appears as a horizontal plateau as log RS , however, at low frequency range log(RS + Rp ) appears as a horizontal plateau [8], where Rp is the polarization resistance. The changing of the solution conductivity with the concentration of oxalate results in different impedance responses at high and low frequency ranges [9]. Nyquist plots (Fig. 1b) show two semicircles indicating two time constants and these two semicircles increase in diameter with decreasing oxalate concentration. From the figure at high frequencies, the impedance entirely created by the solution resistance, RS . The frequency reaches its high limit at the leftmost end of the semicircle, where the semicircle touches the x-axis. At the low frequency limit, the impedance value corresponds to (RS + Rp ). The frequency reaches its low limit at the rightmost end of the semicircle. RS can be determined from the frequency independent limit of |Z| at high frequencies and (RS + Rp ) from the corresponding limit at low frequencies [8]. These two time constants at high and low frequency region means that the film consists of two layers. The loop diameter at high and low frequency decrease with increasing oxalate concentration indicating a decrease in impedance value and increasing

corrosion rate. The impedance data were thus simulated to the appropriate equivalent circuit for the case with two time constants (Fig. 2). Analyses of the experimental spectra were made by best fitting to the corresponding equivalent circuit using Thales software provided with the workstation where the dispersion formula suitable to each model was used [7]. This is the simulation that gave a reasonable fit using the minimum amount of circuit with an average error of about 3%. In this model, RS is the solution resistance, Rp is charge transfer resistance of the porous layer, Rb is the resistance of the barrier oxide layer, Cp is the capacitance of the porous layer and Cb is the capacitance of the barrier oxide layer [10]. The total resistance (RT ) is equal to 1/((1/Rb ) + (1/Rp )) which corresponds to the polarization resistance including charge-transfer resistance at the electrode/solution interface, along with resistance of the oxide layers and/or corrosion products. The total capacitance C is equal to (Cb + Cp) and 1/C corresponds to the relative thickness of the film. A constant-phase element (CPE) [11] representing a shift from the ideal capacitor was used instead of the capacitance itself, for simplicity. The impedance (ZCPE ) of a constant phase element is defined as ZCPE = [C(jw)˛ ]−1 , where −1 ≤ ˛ ≤ 1, j = (−1)1/2 , w = 2f is the angular frequency in rad/s, f is the frequency in Hz = s−1 , ˛ is a fit parameter which is an empirical exponent varies between 1 for a perfect capacitor and 0 for a perfect resistor. In this complex formula an empirical exponent (˛) is introduced to account for the deviation from the ideal capacitive behavior due to surface

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(1/Cp ) is higher than that for barrier film (1/Cb ) and this means that the relative thickness of porous film is higher than barrier film but its resistance is low due to pores formed on film make it like sponge and thus lowering its resistance that is become easy to be broken. For the empirical exponent (˛), it was found to be higher for compact magnesium oxide film than that of porous magnesium oxalate film. For both layers, ˛ value decreases with increasing oxalate concentration. Since the passive oxide film can be considered as a dielectric plate capacitor, the passive film thickness (d) in cm is related to the capacitance (C) by the equation [7]: d= Fig. 2. Equivalent circuit model representing two parallel time constants for an electrode/electrolyte solution interface.

inhomogeneties, roughness factors and adsorption effects [2]. The value of ˛ is associated with the non-uniform distribution of current as a result of roughness and surface defects. In all cases, good conformity between theoretical and experimental results was obtained for the entire frequency range. The resistance and capacitance values of the porous and barrier layers are given in Table 1. When the electrode was immersed in oxalate solution, two competitive processes occur. The first one is oxide formation which yields a compact magnesium oxide film with good corrosion resistance (Rb ). The second one is the formation of magnesium oxalate complexes, which yields a thick porous film as in case of aluminum alloys [12] with expected low corrosion resistance (Rp ), where it is well known that oxalate ions are bidentate ligands capable of forming strong surface complexes. With increasing of oxalate concentration and decreasing the pH value from 6.9 at 0.005 M to 6.3 at 0.1 M oxalate solution (blank). This leads to an increase in the alternation of the compact oxide film by porous one. Thus corrosion rate increases where Rb or Rp value decreases and also the relative thickness decreases. However, the relative thickness of porous film

εr εo A C

(1)

where d is the film thickness, εr is the relative permittivity of the passive film and εo is the permittivity of the free space (8.85 × 10−12 F cm−1 ). Although the actual value of εr within the film is difficult to estimate, a change in C can be used as an indicator for change in the film thickness (d). Hence, the reciprocal total capacitance (1/C) of the passive film is directly proportional to its thickness [7]. AZ91E electrode is used in order to study the effect of 0.1 M for Br− , Cl− or SiO3 2− anions as additives in 0.1 M Na2 C2 O4 solution on the EIS characteristics. The Bode plots measured are presented in Fig. 3. The general features of the impedance diagrams remain the same as in Fig. 1a; and the estimated impedance and polarization parameters were evaluated and given in Table 2, after fitting the experimental diagrams using the model in Fig. 2. It was found that the addition of only silicate anion caused a decrease in the corrosion rate than the blank; however, the other anions increase the corrosion rate of the tested alloy relative to the blank. These results were not surprising in light of the deleterious effect for chloride [9] or bromide on the magnesium alloys. Magnesium-based alloys would be degraded to magnesium chloride or bromide [10]. However, the observed corrosion rate of the chloride ion is higher than that of the bromide ion. This effect can be attributed to the more aggressive

Fig. 3. Bode plots of AZ91E alloy in 0.1 M sodium oxalate solution (blank) containing 0.1 M of Br− , Cl− or SiO3 2− anions at 298 K.

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Table 1 Impedance and polarization parameters for sodium oxalate solution with different concentrations at 298 K. Conc. (M)

RS ( cm2 )

Rb (k cm2 )

Cb (␮F cm−2 )

˛1

Rp (k cm2 )

Cp (␮F cm−2 )

˛2

RT (k cm2 )

1/CT (␮F−1 cm2 )

icorr (␮A cm−2 )

Ecorr (V)

0.005 0.010 0.050 0.100

26.5 13.9 5.1 3.7

1.72 1.21 1.03 0.17

12.5 19.8 20.7 38.7

0.90 0.89 0.88 0.85

0.77 0.68 0.52 0.10

10.1 11.3 13.9 15.2

0.83 0.83 0.81 0.79

0.53 0.43 0.34 0.06

0.044 0.032 0.029 0.019

5.1 6.6 10.6 131.1

−1.73 −1.74 −1.80 −1.84

Table 2 Impedance and polarization parameters for 0.1 M sodium oxalate solution (blank) containing 0.1 M concentration for Br− , Cl− or SiO3 2− at 298 K. Anion added

RS ( cm2 )

Rb (k cm2 )

Cb (␮F cm−2 )

˛1

Rp (k cm2 )

Cp (␮F cm−2 )

˛2

RT (K cm2 )

1/CT (␮F−1 cm2 )

icorr (␮A cm−2 )

Ecorr (V)

Blank Cl− Br− SiO3 2−

3.7 1.67 2.03 1.69

0.17 0.06 0.07 27.7

38.7 55.8 41.3 2.7

0.85 0.81 0.83 0.96

0.10 0.05 0.06 8.10

15.2 30.2 24.1 3.2

0.79 0.76 0.77 0.97

0.06 0.03 0.03 6.27

0.019 0.012 0.015 0.169

131.1 1626.5 1245.9 0.18

−1.84 −1.72 −1.71 −1.71

nature of the chloride anion and its higher electronegativity than bromide anion [13] as shown in Fig. 4 (Nyquist plot for chloride ion in comparison to the blank). The negative part of Nyquist plot is due to chloride ion adsorption–desorption mechanism on alloy surface leading to higher corrosion rate and lower resistance. However, in literature, the addition of oxysalt in electrolyte improved the corrosion resistance of magnesium alloys [1,14–16]. The results indicate that the corrosion resistance of surface film was greatly improved by the addition of sodium silicate (pH in the range of 11.5–12.8), due to changing the medium from neutral to alkaline. Sodium silicate could be chosen as the optimum addition of the secondary oxysalt to sodium oxalate. Sodium silicate could contribute to the formation of a surface oxide film with the best anti-corrosion property, and the different concentrations of sodium silicate would cause the change of film structure, and the optimum additive concentration in electrolyte solution was 0.1 M as observed by Chai et al. [1]. That is, the corrosion rate decreases with increasing silicate concentration till 0.1 M. However, at higher concentrations, the corrosion rate increases or the total resistance decreases as shown in Fig. 5.

However, at all silicate concentrations as additive for 0.1 M oxalate, it decreases the corrosion rate than the blank, due to alkalinity of the medium. This is confirmed by scanning electron micrographs where Fig. 6a is for the blank (0.1 M oxalate solution) which shows a regular smooth thin film. Fig. 6b shows SEM for 1.0 M silicate in 0.1 M oxalate solution, it seems to be a denser film with higher surface roughness than the blank. However Fig. 6c shows SEM for 0.1 M silicate in the blank, which contains much denser film than the blank and 1.0 M silicate containing solution. This confirms that as an additive 0.1 M silicate is the concentration that gives the lowest corrosion rate. Also, by studying this critical concentration (0.1 M silicate added to 0.1 M oxalate solution) with time for 140 h as shown in Fig. 7 as Bode plots of 0.0, 40, 140 h of immersion as example. It was found that the total corrosion resistance or the relative thickness increases with increasing immersion time till ∼40 h then become constant after that as shown in Fig. 8. AZ91E microstructure [13] is known to contain a uniform distribution of Mg17 Al12 precipitates which present favorable sites for corrosion attack. The Mg–Al–Zn matrix is active in relation to the nobler ␤-phase precipitates, which subsequently generates a large number of micro-galvanic couples within the microstructure. The relatively fine ␤-phase (Mg17 Al12 ) network and the Al enrichment produced on the corroded surface are the key factors limiting progression of the corrosion attack [13].

7

RT / k

cm

6

5

4

3

2 0.0

0.2

0.4

0.6

0.8

1.0

1.2

Csilicate / M Fig. 4. Nyquist plots of AZ91E alloy in 0.1 M sodium oxalate solution (blank) containing 0.1 M of Cl− anions at 298 K.

Fig. 5. Variation of the total resistance (RT ) of AZ91E alloy in 0.1 M sodium oxalate solution containing different concentrations of the silicate anion, at 298 K.

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Fig. 6. SEM micrographs of (a) 0.1 M sodium oxalate solution (blank); (b) 1.0 M sodium silicate in the blank and (c) 0.1 M sodium silicate in the blank.

The potentiodynamic polarization behavior of the AZ91E electrode was studied in relation to concentration of oxalate electrolyte. Fig. 9 shows a linear sweep potentiodynamic traces for the electrode in oxalate solution with the highest and lowest concentration of oxalate, as an example. The scanning was carried out at a rate

of 1 mV/s over the potential range from −2.5 to −1.5 V vs. SCE. Prior to the potential sweep, the electrode was left under open circuit in the respective solution for 2 h until a steady state free corrosion potential was recorded. For all tested concentrations on

7.5 0.180

RT 1/C

0.178

7.0

0.176

RT /kΩ cm2

0.174

6.5

0.172 0.170

6.0

1/C /µ F-1 cm2

3.2. Potentiodynamic polarization measurements

0.168 0.166

5.5

0.164 0

20

40

60

80

100

120

140

160

Time / hour Fig. 7. Bode plots of AZ91E alloy in a 0.1 M sodium oxalate solution containing 0.1 M silicate anion with time, at 298 K.

Fig. 8. Variation of the total resistance (RT ) and relative thickness (1/C) of AZ91E alloy in a 0.1 M sodium oxalate solution containing 0.1 M silicate anion with time, at 298 K.

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Fig. 9. Potentiodynamic polarization curves of AZ91E alloy in 0.01 M and 0.005 M oxalate solutions at 298 K.

Fig. 10. Potentiodynamic polarization curves of AZ91E alloy in a 0.1 M sodium oxalate solution containing 0.1 M of Br− , Cl− or SiO3 2− anions at 298 K.

AZ91E alloy, the active dissolution parameters were estimated and given in Table 1. The results indicate clearly that these parameters are dependent on the molar concentration of oxalate solution. The corrosion current (Icorr ), which is proportional to the corrosion rate, is given by the intersection of the cathodic and anodic Tafel lines extrapolation. Because of the presence of some degree of nonlinearity in the Tafel slope region of the obtained polarization curves, the Tafel constants were calculated as the slope of the points after Ecorr by ±50 mV using a computer least-square analysis [7]. The corrosion currents were then determined by the intersection of the cathodic Tafel line with the open circuit potential (i.e. the free steady Ecorr value). Obviously, increasing the concentration of oxalate affected the corrosion current by increasing its value as given in Table 1. Also, from the figure, it is clear that Ecorr tends to more negative values with increasing oxalate concentration. This may be due to formation of more porous film with an increase in the oxalate concentration. Generally, it is well known that the polarization resistance Rpol is related to the corrosion rate through Tafel slopes ˇa and ˇc by Stern–Geary equation [17]:

potential than the blank as shown in Fig. 10. But shifting of the potential to more noble or more active values cannot serve as a dependable criterion of decreasing or increasing the corrosion rate [18]. The nature of the formed film being protective or nonprotective is more clarified based on icorr values and this can be confirmed from EIS results. From Table 2, icorr of Cl− or Br− is higher than the blank due to their aggressiveness, however, for silicate ion, it shows lower corrosion current than the blank. Also, Rpol values given in Table 3 have the same trend as icorr values, which have the same trend of total resistance (RT ) obtained from impedance results. So, polarization results confirm the impedance data well. A study of the effect of silicate concentration in 0.1 M oxalate solution showed that 0.1 M silicate addition is the optimum concentration as observed from impedance results shown in Fig. 11.

1

 

2.303Rp ((1/ˇa ) + (1/ ˇc )

0.38

(2)

As given in Table 3, it can be seen that evaluated Rpol values obtained from Tafel measurements have the same trend as RT obtained from EIS measurements. Thus there is a good agreement between corrosion rates determined by both techniques. A study on the effect of added Br− , Cl− or SiO3 2− anions in 0.1 M Na2 C2 O4 solution on the electrochemical behavior of the magnesium alloy at 298 K was performed. The scanning was carried out at a rate of 1 mV/s over the potential range from −2.5 to 1.5 V (SCE). Prior to the potential sweep, the electrode was left under open-circuit in the respective solution for nearly 2 h until a steady free corrosion potential was recorded. In general, it can be noticed from the results that the polarization curves of AZ91E electrode have almost similar characteristics in all solutions. A change in the additive anion concentrations affected on the cathodic and anodic reactions observed on the electrode surface. In detail, Ecorr was nearly independent of the increasing Br− , Cl− or SiO3 2− ions concentrations, that is the rate of cathodic and anodic polarization is nearly the same for all anions but for all are of more positive

0.36 0.34 0.32

icorrrr / µA cm-2

icorr =

0.30 0.28 0.26 0.24 0.22 0.20 0.18 0.16 0.0

0.2

0.4

0.6

0.8

1.0

1.2

Csilicate / M Fig. 11. Variation of the corrosion current density (icorr ) of AZ91E alloy in a 0.1 M sodium oxalate solution containing different concentrations of the silicate anion, at 298 K.

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Table 3 Polarization resistance calculated from Stern–Geary equation for (a) sodium oxalate solution with different concentrations and (b) for 0.1 M sodium oxalate solution (blank) containing 0.1 M concentration for Br− , Cl− or SiO3 2− , at 298 K. (a) Conc. (M)

Rpol (k cm2 )

icorr (␮A cm−2 )

(b) Anion added

Rpol (k cm2 )

icorr (␮A cm−2 )

0.005 0.010 0.050 0.100

2.06 1.83 1.33 0.28

5.1 6.6 10.6 131.1

Blank Cl− Br− SiO3 2−

0.28 0.22 0.21 118

131.1 1626.5 1245.9 0.18

4. Conclusions 1. An increase in the oxalate concentration (up to 0.1 M) leads to an increase in the corrosion rate, as observed from impedance or polarization measurements. 2. It was found that the corrosion rate of 0.1 M oxalate solution containing silicate ion is lower than the blank (0.1 M Na2 C2 O4 ). However, for the other added ions Br− or Cl− , the corrosion rate is higher than the blank. Thus, the resistance of the surface film decreases, with adding Br− or Cl− as an additive to the blank. 3. Good agreement was observed between the results obtained from electrochemical techniques using potentiodynamic polarization and ac impedance techniques. References [1] L. Chai, X. Yu, Z. Yang, Y. Wanga, M. Okido, Corros. Sci. 50 (2008) 3274.

[2] A.M. Fekry, M.Z. Fatayerji, Electrochim. Acta 54 (2009) 6522. [3] K.U. Kainer, Magnesium-Alloys and Technology, Wiley-VCH Verlag GmbH & Co. KG aA, Weinheim, 2003. [4] G. Song, A. Atrens, M. Dargusch, Corros. Sci. 41 (1999) 249. [5] R.K.S. Raman, Metall. Mater. Trans. A 35A (2004) 2525. [6] M. Pourbaix, Atlas of Electrochemical Equilibria in Aqueous Solutions, NACE International, Houston, TX, 1974, p. 139, 168. [7] A.M. Fekry, Electrochim. Acta 54 (2009) 3480. [8] G.W. Walter, Corros. Sci. 26 (1986) 681. [9] L. Kobotiatis, N. Pebere, P.G. Koutsoukos, Corros. Sci. 41 (1999) 941. [10] A.M. Fekry, M. Rabab, El-Sherif, Electrochim. Acta 54 (2009) 7280. [11] D.D. Macdonald, Electrochim. Acta 51 (2006) 1376. [12] S. Wernick, R. Pinner, P.G. Sheeasby, The Surface Treatment and Finishing of Aluminium and its Alloys, ASM International, 1987. [13] F.E. Heakal, A.M. Fekry, M.Z. Fatayerji, Electrochim. Acta 54 (2009) 1545. [14] S. Ono, K. Asami, T. Osaka, N. Masuko, J. Electrochem. Soc. 143 (1996) L62. [15] A.K. Sharma, Met. Finish. 91 (1993) 57. [16] O. Khaselev, J. Yahalom, J. Electrochem. Soc. 145 (1998) 190. [17] A.M. Fekry, Int. J. Hydrogen Energy 35 (2010) 12945. [18] F. El-Taib Heakal, A.M. Fekry, A.A. Ghoneim, Corros. Sci. 50 (2008) 1618.