*

Faculty of Engineering,Bu-Ali Sina University,Hamedan 65178-38695,Iran

Semiconducting behavior of the anodically passive f i lms formed on AZ31B alloy

A.Fattah-alhosseini*,M.Sabaghi Joni

Faculty of Engineering,Bu-Ali Sina University,Hamedan 65178-38695,Iran

This work includes determination of the semiconductor character and estimation of the dopant levels in the passive f i lm formed on AZ31B alloy in 0.01 M NaOH,as well as the estimation of the passive f i lm thickness as a function of the f i lm formation potential.Mott-Schottky analysis revealed that the passive f i lms displayed n-type semiconductive characteristics,where the oxygen vacancies and interstitials preponderated.Based on the Mott-Schottky analysis,it was shown that the calculated donor density increases linearly with increasing the formation potential.Also,the electrochemical impedance spectroscopy(EIS)results indicated that the thickness of the passive f i lm was decreased linearly with increasing the formation potential.The results showed that decreasing the formation potential offer better conditions for forming the passive f i lms with higher protection behavior,due to the growth of a much thicker and less defective f i lms. Copyright 2014,National Engineering Research Center for Magnesium Alloys of China,Chongqing University.Production and hosting by Elsevier B.V.All rights reserved.

AZ31B alloy;Mott-Schottky;Formation potential;Donor density

1.Introduction

Magnesium alloys have wide applications in many industries due to their high strength/weight ratio,low density (=1.7 g/cm3:about one-fourth the density of iron and onethird that of aluminum),high stiffness/weight ratio,and ease of machinability.Generally,the low density of magnesium alloys makes them useful wherever low weight is very important[1-5].

According to the potential-pH diagram for magnesium and water,this metal corrodes over a wide range of potential and pH.Indeed,the principal drawback of magnesium alloys is the low corrosion resistance,which is generally;much lower when compared to many other competing materials,like steels or Al alloys[6,7].Nevertheless,at high pH(values over 11),in the region identif i ed as passive,the surface f i lm is protective [8].

In the last decade,many studies investigated the passive behavior of magnesium alloys in alkaline solutions.Mott--Schottky analysis indicated that the passive f i lm formed on magnesium alloys, exhibits semiconducting properties, because of its non-stoichiometric nature[6,9-11].

Despite the extensive works published on the passivity of the magnesium alloys,little information about the effect of the formation potential on the semiconductive behavior of the passive f i lm formed on these alloys was available.In this work,EIS and Mott-Schottky analysis of AZ31B alloy in alkaline solution(0.01 M NaOH)have been performed to determine the semiconductor character and estimate the dopant levels in the passive f i lm,as well as to estimate the f i lm thickness as a function of the formation potential.

2.Experimental procedures

The specimens were fabricated from a thick plate of AZ31B alloy with the chemical composition(%,wt.):Al 2.3, Zn 0.89,Mn 0.43,Sn 0.002,and balance Mg.All samples were ground to 2500 grit and cleaned by deionized water prior to tests.

All the electrochemical measurements were performed in a conventional three-electrode f l at cell under aerated conditions by using an Autolab potentiostat/galvanostat system.The counter electrode was a Pt plate,while the reference electrode was Ag/AgCl saturated in KCl.Also,the alkaline solution (0.01 M NaOH)was used as the test solution at 25 ± 1°C.

The potentiodynamic polarization curves were measured potentiodynamically at a scan rate of 1 mV/s starting from -0.25 VAg/AgCl(vs.Ecorr)to 1.0 VAg/AgCl.Five formation potentials(-0.2,0.0,0.2,0.4 and 0.6 VAg/AgCl)within the passive region were chosen for EIS and Mott-Schottky measurements.Films were grown at each potential for 1 h to ensure that the system was in steady-state.After each passive f i lm growth period,EIS and Mott-Schottky measurements were performed.The impedance spectra were measured in a frequency range of 100 kHz-100 mHz at an AC amplitude of 10 mV(rms)[5,6].For the EIS data modeling and curve-f i tting method,the NOVA impedance software was used.Mott--Schottky analyses were done by measuring the frequency response at 1 kHz during a 25 mV/s negative potential scan from each selected potential to-1.4 VAg/AgCl.

3.Results and discussion

3.1.Polarization measurement

Fig.1 shows the potentiodynamic polarization curve of AZ31B alloy in 0.01 M NaOH.According to this f i gure,the current density was found to increase slowly with potential during the early stage of passivation and no obvious current peak was observed.Also,it is seen that the passive potential range extending from the corrosion potential.This alloy in alkaline solutions exhibits the same curve shapes[6],where the current changes smoothly and linearly around the rest potential manifesting cathodic and anodic Tafel behavior.

Fig.1.Polarization curve of AZ31B in 0.01 M NaOH in the anodic direction at 1 mV/s.

Fig.2.Mott-Schottky plots of passive f i lm formed at different formation potential on AZ31B in 0.01 M NaOH.

3.2.Mott-Schottky analysis

Based on the Mott-Schottky analysis,the charge distribution at the semiconductor/solution is usually determined by measuring electrode capacitanceCSC,as a function of electrode potential(E)[6,12]:

where ε is the electron charge,NDis the donor density(cm-3), ε is the dielectric constant of the passive f i lm(ε=9.6[6,12]),KBis the Boltzmann constant,Tis the absolute temperature andEFBis the f l at band potential.Fig.2 shows the Mott--Schottky plots of AZ31B in 0.01 M NaOH at selected formation potentials.As can be seen,C-2clearly decrease with increasing the formation potential.In this Figure,the positive slopes are attributed to n-type behavior.

Fig.3.Donor densities of the passive f i lms formed on AZ31B in 0.01 M NaOH as a function of f i lm formation potential.

Fig.3 shows the calculated donor density of the passive f i lm formed on AZ31B in 0.01 M NaOH at selected formation potentials.The orders of magnitude are around 1020cm-3and increases linearly with the formation potential.The values are comparable to the calculated donor density of the passive f i lm formed on AZ31B in NaOH solutions at open circuit potential [6].According to the point defect model(PDM)[13-16],the f l ux of oxygen vacancy and cation interstitials through the passive f i lm is essential to the f i lm growth process.In this concept(n-type behavior),the dominant point defects in the passive f i lm are considered to be oxygen vacancies and/or cation interstitials acting as electron donors.

3.3.EIS measurements

The EIS response of AZ31B in 0.01 M NaOH at selected formation potentials are presented as Bode plots in Fig.4.In this f i gure,the Bode plots show a resistive behavior at high frequencies,but in the middle to low frequency range there was a marked capacitive response.The Bode-phase curves show one time constant.The phase angles values(remained very close to 80°)revealed the formation and growth of a passive f i lm.Similar plots for the EIS response of AZ31B in NaOH solutions at open circuit potential are observed[6].

Fig.4.(a)Bode and(b)Bode-phase plots of AZ31B in 0.01 M NaOH measured at different formation potential.

Fig.5.Effect of the formation potential on the passive f i lm thickness of AZ31B in 0.01 M NaOH.

Fig.5 shows a linear relationship between the passive f i lm thickness(d)and the formation potential.This relationship between the passive f i lm thickness and the formation potential has been reported by Macdonald[14].The f i lm thickness was calculated from the capacitance measured at 1 kHz after each 1 h constant potential growth.At this frequency,the impedance is largely capacitive in nature,with the measured capacitance being almost independent of frequency.The parallel plate expression was used for calculating the f i lm thickness from the measured capacitance[6]:

Also,Cis calculated from Eq.(3)as follows[17]:

whereZimgis the imaginary component of the impedance.The calculated thickness ranges from about 37.2 nm at-0.2 VAg/AgClto 31.1 nm at 0.6 VAg/AgCl.Therefore,it is clear that decreasing the formation potential give better conditions for forming the passive f i lms with higher protection behavior,due to the growth of a much thicker and less defective f i lms.

4.Conclusions

In this work,the passivity of AZ31B in 0.01 M NaOH has been explored using EIS,and Mott-Schottky analysis.Conclusions drawn from the study are as follows

1.Mott-Schottky analysis revealed that the passive f i lms displayed n-type semiconductive characteristics,where the oxygen vacancies and interstitials preponderated.

2.Based on the Mott-Schottky analysis,it was shown that the calculated donor density increases linearly with increasing the formation potential.

3.The Bode plots show a resistive behavior at high frequencies,but in the middle to low frequency range there was a marked capacitive response.

4.Also,the EIS results indicated that the thickness of the passive f i lm was decreased linearly with increasing the formation potential.

5.The results showed that decreasing the formation potential offer better conditions for forming the passive f i lms with higher protection behavior,due to the growth of a much thicker and less defective f i lms.

[1]R.Zhu,J.Zhang,C.Chang,S.Gao,N.Ni,J.Magnes.Alloys 1(2013) 235-241.

[2]Q.Yang,B.Jiang,X.Li,H.Dong,W.Liu,F.Pan,J.Magnes.Alloys 2 (2014)8-12.

[3]X.Song,J.Lu,X.Yin,J.Jiang,J.Wang,J.Magnes.Alloys 1(2013) 318-322.

[4]B.Salami,A.Afshar,A.Mazaheri,J.Magnes.Alloys 2(2014)72-77.

[5]F.E.T.Heakal,A.M.Fekry,M.A.E.B.Jibril,Corros.Sci.53(2011) 1174-1185.

[6]A.Fattah-alhosseini,M.Sabaghi Joni,J.Magnes.Alloys 2(2014) 175-180.

[7]L.J.Liu,M.Schlesinger,Corros.Sci.51(2009)1733-1737.

[8]M.Pourbaix,Atlas of Electrochemical Equilibria in Aqueous Solutions, second ed.,NACE,Houston,1974.

[9]M.C.L.de Oliveira,V.S.M.Pereira,O.V.Correa,N.B.de Lima, R.A.Antunes,Corros.Sci.69(2013)311-321.

[10]T.Ishizaki,Y.Masuda,K.Teshima,Surf.Coat.Technol.217(2013)76-83.

[11]L.J.Zhang,J.J.Fan,Z.Zhang,F.H.Cao,J.Q.Zhang,C.N.Cao,Electrochim.Acta 52(2007)5325-5333.

[12]S.J.Xia,R.Yue,R.G.Rateick Jr.,V.I.Briss,J.Electrochem.Soc.151 (2004)B179-B187.

[13]D.D.Macdonald,M.Urquidi-Macdonald,J.Electrochem.Soc.137 (1990)2395-2402.

[14]D.D.Macdonald,J.Electrochem.Soc.153(2006)B213-B224.

[15]D.D.Macdonald,J.Nucl.Mater.379(2008)24-32.

[16]A.Fattah-alhosseini,M.A.Golozar,A.Saatchi,K.Raeissi,Corros.Sci. 52(2010)205-209.

[17]A.Fattah-alhosseini,A.Saatchi,M.A.Golozar,K.Raeissi,J.Appl. Electrochem.40(2010)457-461.

Received 30 May 2014;accepted 13 October 2014 Available online 5 December 2014

*Corresponding author.Fax:+98 811 8257400.

E-mail address:a.fattah@basu.ac.ir(A.Fattah-alhosseini).

Peer review under responsibility of National Engineering Research Center for Magnesium Alloys of China,Chongqing University.

http://dx.doi.org/10.1016/j.jma.2014.10.005.

2213-9567/Copyright 2014,National Engineering Research Center for Magnesium Alloys of China,Chongqing University.Production and hosting by Elsevier B.V.All rights reserved.