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

Contents lists available at ScienceDirect

Materials Science and Engineering B

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orrosion and impedance studies on magnesium alloy in oxalate solution

.M. Fekry ∗, Riham H. Tammamhemistry Department, Faculty of Science, Cairo University, Gamaa Street, Giza 12613, Egypt

r t i c l e i n f o

rticle history:eceived 12 July 2010eceived in revised form 8 February 2011ccepted 27 March 2011

a b s t r a c t

Corrosion behavior of AZ91E alloy was investigated in oxalate solution using potentiodynamic polariza-tion 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 Na2C2O4 solution at 298 K,

eywords:Z91E alloyxalateIS

was investigated. It was found that the corrosion rate of 0.1 M oxalate solution containing silicate ionis lower than the blank (0.1 M Na2C2O4). 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.

olarizationEM

. Introduction

Magnesium is the 8th most abundant element on the earth [1].t has a high thermal conductivity, good electromagnetic shieldingharacteristics and good machinability. Magnesium is the light-st of all metals in practical use, and has a density of 1.74 g cm−3

2]. Magnesium alloys have many unique properties comparedith other metals. Magnesium can form intermetallic phases withost alloying elements, the stability of this phase increases with

he electronegativity of the other element [3]. Aluminum (Al) hadlready become the most important alloying element for signifi-antly increasing the tensile strength, specifically by forming thentermetallic phase Mg17Al12. Similar effects can be achieved withinc (Zn) and manganese (Mn). The AZ-based Mg system has beenhe basis of the most widely used magnesium alloys [4]. Amonghese alloys, AZ91 is the most successful alloy having excellent

echanical properties. It is used in car parts, notebook PCs, portableelephones and other products. Nevertheless, a serious limitationor the wide-spread use of several magnesium alloys is their suscep-ibility to general and localized (pitting) corrosion [4,5]. This makestudying the corrosion and corrosion control of Mg alloys an inter-sting point of research which can enable extending the potentialse of these important materials in a broad range of many tech-ical and innovative applications. The Pourbaix diagram for theg–water system is shown in Scheme 1 [6]. The whole domain

f stability of magnesium is well below that of water. Magnesiumherefore dissolves as Mg+ and Mg+2 with accompanying hydrogenvolution.

∗ Corresponding author.E-mail address: hham4@hotmail.com (A.M. Fekry).

921-5107/$ – see front matter. Published by Elsevier B.V.oi:10.1016/j.mseb.2011.03.014

An oxalate (ethanedioate) is a salt or ester of oxalic acid thatis used in low technical applications throughout industry [7]. Thecharge on oxalate allows it to act as a chelator of various positivelycharged metal ions. The present work aims to attain more infor-mation concerning the electrochemical reactivity and corrosionbehavior of AZ91E Mg-based alloy in oxalate solution containingBr−, Cl− or SiO3

2− anions under various environmental conditionsincluding electrolyte type and concentration. Techniques employedwere potentiodynamic polarization and impedance spectroscopy(EIS).

2. Experimental

An extruded magnesium aluminum alloy AZ91E donated fromDepartment of mining, Metallurgy and Materials Engineering, LavalUniversity, Canada with chemical composition (wt%): 9.0 Al, 0.7Zn, 0.13 Mn, 0.03 Cu, 0.01 Si, 0.006 Fe, 0.004 Ni, 0.0007 Be andbalance Mg. The sample was divided into small coupons. Eachcoupon was welded to an electrical wire and fixed with Aralditeepoxy resin in a glass tube leaving cross-sectional area of the spec-imen 0.196 cm2. The solutions were prepared using Analar gradereagents (sodium oxalate, sodium bromide, sodium chloride andsodium silicate) and triply distilled water. The solubility of sodiumoxalate is 3.99 g/100 g H2O. The surface of the test electrode wasmechanically polished by emery papers with 400 up to 1000 grit toensure the same surface roughness. Subsequently, the specimenswere 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 elec-trode (CE), saturated calomel (SCE) as a reference electrode (RE) andAZ91E alloy as the working electrode (WE). Cathodic and anodicpolarization curves were scanned from −2.5 V to −1.5 V with a

A.M. Fekry, R.H. Tammam / Materials Science and Engineering B 176 (2011) 792–798 793

stpsTZewmmm

3

3

Aacottarch

tdcTclrbfqrat

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

can rate of 1 mV s−1. The impedance diagrams were recorded athe free immersion potential (OCP) by applying a 10 mV sinusoidalotential through a frequency domain from 100 kHz to 100 mHzuch that there was five measured frequency points per decade.he instrument used is the electrochemical workstation IM6eahner-elektrik, GmbH (Kronach, Germany). The electrochemicalxperiments were always carried inside an air thermostat whichas kept at 298 K, unless otherwise stated. All potentials wereeasured and given with respect to SCE (E = 0.241 V). The SEMicrographs were obtained using a JEOL JXA-840A electron probeicroanalyzer.

. Results and discussion

.1. EIS measurements

The impedance measurements recorded after 2 h of immersingZ91E electrode in oxalate solution with different concentrations,s shown in Fig. 1a and b as Bode and Nyquist plots, respectively. Asan be seen in Fig. 1a, impedance value decreases with increasingxalate concentration from 0.005 to 0.1 M. At high frequency rangehe solution resistance (RS) dominates and appears as a horizon-al plateau as log RS, however, at low frequency range log(RS + Rp)ppears as a horizontal plateau [8], where Rp is the polarizationesistance. The changing of the solution conductivity with the con-entration of oxalate results in different impedance responses atigh and low frequency ranges [9].

Nyquist plots (Fig. 1b) show two semicircles indicating twoime constants and these two semicircles increase in diameter withecreasing oxalate concentration. From the figure at high frequen-ies, the impedance entirely created by the solution resistance, RS.he frequency reaches its high limit at the leftmost end of the semi-ircle, where the semicircle touches the x-axis. At the low frequencyimit, the impedance value corresponds to (RS + Rp). The frequencyeaches its low limit at the rightmost end of the semicircle. RS cane determined from the frequency independent limit of |Z| at highrequencies and (RS + Rp) from the corresponding limit at low fre-

uencies [8]. These two time constants at high and low frequencyegion means that the film consists of two layers. The loop diametert high and low frequency decrease with increasing oxalate concen-ration indicating a decrease in impedance value and increasing

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

corrosion rate. The impedance data were thus simulated to theappropriate equivalent circuit for the case with two time constants(Fig. 2). Analyses of the experimental spectra were made by bestfitting to the corresponding equivalent circuit using Thales soft-ware provided with the workstation where the dispersion formulasuitable to each model was used [7]. This is the simulation thatgave a reasonable fit using the minimum amount of circuit with anaverage error of about 3%. In this model, RS is the solution resis-tance, Rp is charge transfer resistance of the porous layer, Rb is theresistance of the barrier oxide layer, Cp is the capacitance of theporous 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 cor-responds to the polarization resistance including charge-transferresistance at the electrode/solution interface, along with resistanceof the oxide layers and/or corrosion products. The total capacitanceC is equal to (Cb + Cp) and 1/C corresponds to the relative thicknessof the film. A constant-phase element (CPE) [11] representing a shiftfrom the ideal capacitor was used instead of the capacitance itself,for simplicity. The impedance (ZCPE) of a constant phase elementis defined as ZCPE = [C(jw)˛]−1, where −1 ≤ ˛ ≤ 1, j = (−1)1/2, w = 2�fis 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 com-plex formula an empirical exponent (˛) is introduced to accountfor the deviation from the ideal capacitive behavior due to surface

794 A.M. Fekry, R.H. Tammam / Materials Science

Fe

ivaffu

pytcaifcaart

bromide on the magnesium alloys. Magnesium-based alloys would

ig. 2. Equivalent circuit model representing two parallel time constants for anlectrode/electrolyte solution interface.

nhomogeneties, roughness factors and adsorption effects [2]. Thealue of ˛ is associated with the non-uniform distribution of currents a result of roughness and surface defects. In all cases, good con-ormity between theoretical and experimental results was obtainedor the entire frequency range. The resistance and capacitance val-es of the porous and barrier layers are given in Table 1.

When the electrode was immersed in oxalate solution, two com-etitive processes occur. The first one is oxide formation whichields a compact magnesium oxide film with good corrosion resis-ance (Rb). The second one is the formation of magnesium oxalateomplexes, which yields a thick porous film as in case of aluminumlloys [12] with expected low corrosion resistance (Rp), where its well known that oxalate ions are bidentate ligands capable oforming strong surface complexes. With increasing of oxalate con-entration and decreasing the pH value from 6.9 at 0.005 M to 6.3t 0.1 M oxalate solution (blank). This leads to an increase in the

lternation of the compact oxide film by porous one. Thus corrosionate increases where Rb or Rp value decreases and also the relativehickness decreases. However, the relative thickness of porous film

Fig. 3. Bode plots of AZ91E alloy in 0.1 M sodium oxalate solution (

and Engineering B 176 (2011) 792–798

(1/Cp) is higher than that for barrier film (1/Cb) and this means thatthe relative thickness of porous film is higher than barrier film butits resistance is low due to pores formed on film make it like spongeand thus lowering its resistance that is become easy to be broken.For the empirical exponent (˛), it was found to be higher for com-pact magnesium oxide film than that of porous magnesium oxalatefilm. For both layers, ˛ value decreases with increasing oxalateconcentration.

Since the passive oxide film can be considered as a dielectricplate capacitor, the passive film thickness (d) in cm is related to thecapacitance (C) by the equation [7]:

d = εrεoA

C(1)

where d is the film thickness, εr is the relative permittivity ofthe 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 thefilm is difficult to estimate, a change in C can be used as an indica-tor for change in the film thickness (d). Hence, the reciprocal totalcapacitance (1/C) of the passive film is directly proportional to itsthickness [7].

AZ91E electrode is used in order to study the effect of 0.1 M forBr−, Cl− or SiO3

2− anions as additives in 0.1 M Na2C2O4 solution onthe EIS characteristics. The Bode plots measured are presented inFig. 3. The general features of the impedance diagrams remain thesame as in Fig. 1a; and the estimated impedance and polarizationparameters were evaluated and given in Table 2, after fitting theexperimental diagrams using the model in Fig. 2. It was found thatthe addition of only silicate anion caused a decrease in the corrosionrate than the blank; however, the other anions increase the corro-sion rate of the tested alloy relative to the blank. These results werenot surprising in light of the deleterious effect for chloride [9] or

be degraded to magnesium chloride or bromide [10]. However, theobserved corrosion rate of the chloride ion is higher than that of thebromide ion. This effect can be attributed to the more aggressive

blank) containing 0.1 M of Br− , Cl− or SiO32− anions at 298 K.

A.M. Fekry, R.H. Tammam / Materials Science and Engineering B 176 (2011) 792–798 795

Table 1Impedance 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 26.5 1.72 12.5 0.90 0.77 10.1 0.83 0.53 0.044 5.1 −1.730.010 13.9 1.21 19.8 0.89 0.68 11.3 0.83 0.43 0.032 6.6 −1.740.050 5.1 1.03 20.7 0.88 0.52 13.9 0.81 0.34 0.029 10.6 −1.800.100 3.7 0.17 38.7 0.85 0.10 15.2 0.79 0.06 0.019 131.1 −1.84

Table 2Impedance 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 3.7 0.17 38.7 0.85 0.10 15.2 0.79 0.06 0.019 131.1 −1.84− 30

243

nbitllstbtcttattTtr

Ft

Cl 1.67 0.06 55.8 0.81 0.05Br− 2.03 0.07 41.3 0.83 0.06SiO3

2− 1.69 27.7 2.7 0.96 8.10

ature of the chloride anion and its higher electronegativity thanromide anion [13] as shown in Fig. 4 (Nyquist plot for chloride ion

n comparison to the blank). The negative part of Nyquist plot is dueo chloride ion adsorption–desorption mechanism on alloy surfaceeading to higher corrosion rate and lower resistance. However, initerature, the addition of oxysalt in electrolyte improved the corro-ion resistance of magnesium alloys [1,14–16]. The results indicatehat the corrosion resistance of surface film was greatly improvedy the addition of sodium silicate (pH in the range of 11.5–12.8), dueo changing the medium from neutral to alkaline. Sodium silicateould be chosen as the optimum addition of the secondary oxysalto sodium oxalate. Sodium silicate could contribute to the forma-ion of a surface oxide film with the best anti-corrosion property,nd the different concentrations of sodium silicate would causehe change of film structure, and the optimum additive concentra-ion in electrolyte solution was 0.1 M as observed by Chai et al. [1].

hat is, the corrosion rate decreases with increasing silicate concen-ration till 0.1 M. However, at higher concentrations, the corrosionate increases or the total resistance decreases as shown in Fig. 5.

ig. 4. Nyquist plots of AZ91E alloy in 0.1 M sodium oxalate solution (blank) con-aining 0.1 M of Cl− anions at 298 K.

.2 0.76 0.03 0.012 1626.5 −1.72

.1 0.77 0.03 0.015 1245.9 −1.71

.2 0.97 6.27 0.169 0.18 −1.71

However, at all silicate concentrations as additive for 0.1 M oxalate,it decreases the corrosion rate than the blank, due to alkalinity ofthe medium.

This is confirmed by scanning electron micrographs whereFig. 6a is for the blank (0.1 M oxalate solution) which shows a reg-ular smooth thin film. Fig. 6b shows SEM for 1.0 M silicate in 0.1 Moxalate solution, it seems to be a denser film with higher surfaceroughness than the blank. However Fig. 6c shows SEM for 0.1 Msilicate in the blank, which contains much denser film than theblank and 1.0 M silicate containing solution. This confirms that asan additive 0.1 M silicate is the concentration that gives the lowestcorrosion rate.

Also, by studying this critical concentration (0.1 M silicate addedto 0.1 M oxalate solution) with time for 140 h as shown in Fig. 7as Bode plots of 0.0, 40, 140 h of immersion as example. It wasfound that the total corrosion resistance or the relative thicknessincreases with increasing immersion time till ∼40 h then becomeconstant after that as shown in Fig. 8. AZ91E microstructure [13] isknown to contain a uniform distribution of Mg17Al12 precipitateswhich present favorable sites for corrosion attack. The Mg–Al–Znmatrix is active in relation to the nobler �-phase precipitates, whichsubsequently generates a large number of micro-galvanic couples

within the microstructure. The relatively fine �-phase (Mg17Al12)network and the Al enrichment produced on the corroded surfaceare the key factors limiting progression of the corrosion attack [13].

Csilicate / M

0.0 0.2 0.4 0.6 0.8 1.0 1.2

RT

/ k c

m

2

3

4

5

6

7

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

796 A.M. Fekry, R.H. Tammam / Materials Science and Engineering B 176 (2011) 792–798

) 1.0 M

3

tFto

Fs

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 opencircuit in the respective solution for 2 h until a steady state freecorrosion potential was recorded. For all tested concentrations on

Fig. 6. SEM micrographs of (a) 0.1 M sodium oxalate solution (blank); (b

.2. Potentiodynamic polarization measurements

The potentiodynamic polarization behavior of the AZ91E elec-rode was studied in relation to concentration of oxalate electrolyte.

ig. 9 shows a linear sweep potentiodynamic traces for the elec-rode in oxalate solution with the highest and lowest concentrationf oxalate, as an example. The scanning was carried out at a rate

ig. 7. Bode plots of AZ91E alloy in a 0.1 M sodium oxalate solution containing 0.1 Milicate anion with time, at 298 K.

sodium silicate in the blank and (c) 0.1 M sodium silicate in the blank.

Time / hour

0 20 40 60 80 100 120 140 160

5.5

6.0

6.5

7.0

7.5

1/C

/µ F

-1 c

m2

0.164

0.166

0.168

0.170

0.172

0.174

0.176

0.178

0.180RT

1/C

R /

kΩ c

m2

T

Fig. 8. Variation of the total resistance (RT) and relative thickness (1/C) of AZ91Ealloy in a 0.1 M sodium oxalate solution containing 0.1 M silicate anion with time,at 298 K.

A.M. Fekry, R.H. Tammam / Materials Science and Engineering B 176 (2011) 792–798 797

Fo

AgtTsTotp[ttogmmt

iS

i

AfEs

Nsa(offharnci

Fig. 10. Potentiodynamic polarization curves of AZ91E alloy in a 0.1 M sodium

Fig. 11.

0.0 0.2 0.4 0.6 0.8 1.0 1.2

i corr

rr /

µA c

m-2

0.16

0.18

0.20

0.22

0.24

0.26

0.28

0.30

0.32

0.34

0.36

0.38

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

Z91E alloy, the active dissolution parameters were estimated andiven in Table 1. The results indicate clearly that these parame-ers are dependent on the molar concentration of oxalate solution.he corrosion current (Icorr), which is proportional to the corro-ion rate, is given by the intersection of the cathodic and anodicafel lines extrapolation. Because of the presence of some degreef nonlinearity in the Tafel slope region of the obtained polariza-ion curves, the Tafel constants were calculated as the slope of theoints after Ecorr by ±50 mV using a computer least-square analysis7]. The corrosion currents were then determined by the intersec-ion of the cathodic Tafel line with the open circuit potential (i.e.he free steady Ecorr value). Obviously, increasing the concentrationf oxalate affected the corrosion current by increasing its value asiven in Table 1. Also, from the figure, it is clear that Ecorr tends toore negative values with increasing oxalate concentration. Thisay be due to formation of more porous film with an increase in

he oxalate concentration.Generally, it is well known that the polarization resistance Rpol

s related to the corrosion rate through Tafel slopes ˇa and ˇc bytern–Geary equation [17]:

corr = 1

2.303Rp((1/ˇa) + (1/∣∣ˇc

∣∣)

(2)

s given in Table 3, it can be seen that evaluated Rpol values obtainedrom Tafel measurements have the same trend as RT obtained fromIS measurements. Thus there is a good agreement between corro-ion rates determined by both techniques.

A study on the effect of added Br−, Cl− or SiO32− anions in 0.1 M

a2C2O4 solution on the electrochemical behavior of the magne-ium alloy at 298 K was performed. The scanning was carried outt a rate of 1 mV/s over the potential range from −2.5 to 1.5 VSCE). Prior to the potential sweep, the electrode was left underpen-circuit in the respective solution for nearly 2 h until a steadyree corrosion potential was recorded. In general, it can be noticedrom the results that the polarization curves of AZ91E electrodeave almost similar characteristics in all solutions. A change in thedditive anion concentrations affected on the cathodic and anodic

eactions observed on the electrode surface. In detail, Ecorr wasearly independent of the increasing Br−, Cl− or SiO3

2− ions con-entrations, that is the rate of cathodic and anodic polarizations nearly the same for all anions but for all are of more positive

oxalate solution containing 0.1 M of Br− , Cl− or SiO32− anions at 298 K.

potential than the blank as shown in Fig. 10. But shifting of thepotential to more noble or more active values cannot serve asa dependable criterion of decreasing or increasing the corrosionrate [18]. The nature of the formed film being protective or non-protective is more clarified based on icorr values and this can beconfirmed from EIS results. From Table 2, icorr of Cl− or Br− is higherthan the blank due to their aggressiveness, however, for silicate ion,it shows lower corrosion current than the blank. Also, Rpol valuesgiven in Table 3 have the same trend as icorr values, which havethe same trend of total resistance (RT) obtained from impedanceresults. So, polarization results confirm the impedance datawell.

A study of the effect of silicate concentration in 0.1 M oxalatesolution showed that 0.1 M silicate addition is the optimumconcentration as observed from impedance results shown in

Csilicate / M

Fig. 11. Variation of the corrosion current density (icorr) of AZ91E alloy in a 0.1 Msodium oxalate solution containing different concentrations of the silicate anion, at298 K.

798 A.M. Fekry, R.H. Tammam / Materials Science and Engineering B 176 (2011) 792–798

Table 3Polarization 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 2.06 5.1 Blank 0.28 131.1−

4

1

2

3

R

[[[

[

[14] S. Ono, K. Asami, T. Osaka, N. Masuko, J. Electrochem. Soc. 143 (1996) L62.

0.010 1.83 6.60.050 1.33 10.60.100 0.28 131.1

. Conclusions

. An increase in the oxalate concentration (up to 0.1 M) leads toan increase in the corrosion rate, as observed from impedanceor polarization measurements.

. It was found that the corrosion rate of 0.1 M oxalate solutioncontaining silicate ion is lower than the blank (0.1 M Na2C2O4).However, for the other added ions Br− or Cl−, the corrosion rateis higher than the blank. Thus, the resistance of the surface filmdecreases, with adding Br− or Cl− as an additive to the blank.

. Good agreement was observed between the results obtainedfrom electrochemical techniques using potentiodynamic polar-ization and ac impedance techniques.

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[[[[

Cl 0.22 1626.5Br− 0.21 1245.9SiO3

2− 118 0.18

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