The effect of sodium silicate concentration on microstructure and corrosion properties of MAO-coated magnesium alloy AZ31 in simulated body f l uid

Department of Materials and Engineering,Sharif University of Technology,P.O.Box 11155-9466,Tehran,Iran

The effect of sodium silicate concentration on microstructure and corrosion properties of MAO-coated magnesium alloy AZ31 in simulated body f l uid

B.Salami*,A.Afshar,A.Mazaheri

Department of Materials and Engineering,Sharif University of Technology,P.O.Box 11155-9466,Tehran,Iran

In recent years,magnesium and its alloys are considered as biodegradable implants.However magnesium implants may rapidly corrode before the natural healing process of the tissue is completed.In this investigation,micro arc oxidation process has been studied for avoiding primary corrosion of the magnesium alloy in simulated body f l uid.Anodized coating was formed on AZ31 alloy in nontoxic silicate-alkaline solution at constant current.The effects of silicate concentration and conductivity of electrolyte solution on microstructure and corrosion properties of coating were evaluated.Scanning electron microscopy showed that a thick and condensed coating is formed after enough anodizing period.Energy dispersive spectroscopy showed that Si,O and Mg are the main components of the coating.Corrosion resistance of the coated and uncoated samples was assessed using potentiodynamic polarization and electrochemical impedance spectroscopy tests in SBF at 37°C and pH of 7.4.Maximum corrosion resistance was achieved at 30 g/L concentration of sodium silicate in anodizing solution.It was observed that further increase in silicate concentration decreased the corrosion resistance.

Magnesium alloy;Micro arc oxidation;Sodium silicate;Conductivity;Corrosion properties

1.Introduction

Researches on biocompatible,biodegradable and bioactive implants are highly popular in the f i eld of corrosion and biomaterial engineering.Currently,permanent metallic implants are titanium,stainless steel and poly-L Lactic acid as a polymer implant material[1,2].For the metallic implants, there is a high probability of toxic ion release and a secondary surgery is needed after the complete healing of the tissue,inorder to remove the implant[3].Polymer implants do not have the desired mechanical strength and their applications are limited[4].Magnesium and its alloys are widely used in industries where high strength to weight ratio is required. Fortunately,magnesium can be a proper choice as an implant due to its biodegradable and non-toxic characteristics and also good physical and mechanical properties similar to bones [5,6].Magnesium is the eleventh element in the body in case of mass abundance.Magnesium ions are vital in all live cells of the body and about half of these ions are inside bones[7].A considerable amount of magnesium enters the body daily which is benef i cial for strength and growth of bones.However, the most important problem of magnesium is its fast corrosion rate in body which reduces its physical and mechanical properties.In aqueous environments,magnesium decomposes into crystalline f i lm of magnesium hydroxide and hydrogen gas.It is also possible that magnesium particles separate from the material body due to stress corrosion cracking,which isalso called the chunk effect[2,8].Magnesium surface modif i cation is an effective method to improve corrosion resistance of the implants.Laser Surface melting,by ref i ning β-phase in Mg-alloys and increasing aluminum density can increase corrosion resistance of magnesium alloy[9].In addition by aging at high temperature,the heat treatment can modify magnesium surface and decrease corrosion rate[10].Conversion coating,electroplating and anodizing are the most common surface treatment methods.However,when the coating is needed to be used in a bio-environments,some considerations should be adopted.The coating should be thick, strong and non-toxic.Anodizing is an electrolyte oxidation process in which,the metal acts as the anode and its surface is transformed into a thick,stable oxide layer which can enhance corrosion properties.Anodizing can increase the f i lm thickness,strength,and corrosion resistance and wear resistance making a better substrate for cohesion of the primers comparing to naked metal[2].The behavior of magnesium anodizing is highly dependent on the applied voltage,substrate,current density and electrolyte[11].The aim of the present investigation is evaluating the effect of silicate ion concentration on the Micro Arc Oxidation(MAO)process, microstructure,electrolyte conductivity and corrosion properties of MAO coating in SBF.The constituents of the constant current density anodizing electrolyte were sodium hydroxide (NaOH),borax(Na2B4O7)and various concentration of sodium silicate(Na2SiO3).Polarization and impedance electrochemical methods indicated that anodizing can improve corrosion resistance of AZ31 alloy.SEM,EDS and XRD analysis showed that the formed layer on the surface was rough and porous,containing mostly magnesium,oxygen and silicon in form of MgO and Mg2SiO4.

2.Experimental

Rectangular specimens with the dimension of 20 × 15 × 5 mm3were cut from an ingot of AZ31 magnesium alloy. Chemical composition of the ingot is shown in Table 1.Prior to MAO process,the specimens were polished by SiC papers up to mesh 3000,cleaned and degreased ultrasonically in acetone for 5 min,rinsed with distilled water and dried in air at room temperature.

The MAO electrolyte was an aqueous solution of 500 mL of 1 M NaOH+30 g/L Na2B4O7+(X)g/L Na2SiO3(X=15 g/L,30 g/L and 60 g/L).The AZ31 specimen connected to an aluminum rod acted as the anode and a sheet of stainless steel which was shaped into a circular cylinder, formed the cathode.The MAO process was carried out by high voltage DC power supply(VOKAM 400 V,100 mA)at constant current density of 10 mA cm-2for 30 min.During the process,the electrolyte was agitated by magnetic stirrer.Thetemperature of the system was f i xed at 25 ± 2°C.The conductivity of electrolyte was measured using a digital conductivity meter(EDT,ModelRE387).In addition,during each MAO process,variations of supplied voltage vs.time was recorded using a digital multimeter.Then,the anodized specimens were washed with double distilled water and dried at room temperature.The thickness of anodized layer was measured using an eddy current based thickness gauge(Fisher, Dual scope Model MP40).

Table 1The amount of element present in AZ31 magnesium alloy.

Fig.1.The effect of silicate concentration on V-t curve during MAO treatment.

Electrochemical measurements(Potentiodynamic polarization&Electrochemical impedance spectroscopy(EIS))were carried out to study the corrosion behavior of anodized specimens and uncoated specimen in simulated body f l uid(SBF)at 37°C ± 1.The composition and method of preparation of SBF is reported in Ref.[12].A three-electrode cell was used for electrochemical measurements:a platinum sheet as a counter electrode,the anodized/uncoated specimens as the working electrode and a saturated calomel electrode(SCE)as a reference electrode.The exposed area of working electrode with the SBF solution was 1 cm2.

Prior to each measurements the samples were kept in solution for 30 min to establish the open circuit potential(OCP). The potentiodynamic polarization measurements were carried out at a scan rate of 1 mV s-1from-0.2 V to 0.8 V(vs.OCP). The EIS measurements were carried out in a scan range of 100 kHz up to 10 mHz with a sinusoidal amplitude voltage of 10 mV.

3.Results and discussion

3.1.The silicate concentration effect on MAO process

Fig.1 shows the diagrams of supplied voltage vs.time during the MAO process for different concentration ofNa2SiO3.The increase in silicate concentration shifts the breakdown voltage to higher values which may enhance the properties of MAO coating[13].On the other hand,the increase in silicate concentration decreases the conductivity of electrolyte which reduces the intensity of discharge channels (Table 2).This leads to decrease in the amount of localized melting and solidif i cation cycles and consequently diminishesthe quality of MAO coating.Therefore,there is an optimum concentration of silicate for MAO process.

Table 2Electrical conductivity of electrolyte in various silicate concentration.

Table 3Atomic and weight percentage of element in MAO coating.

Fig.2.SEM micrograph and EDS analysis of MAO coating in various silicate concentration a)15 b)30 and c)60 g/L.

Fig.3.Thickness of MAO coating in various silicate concentration.

3.2.The silicate concentration effect on morphology and thickness of coating

The result of EDS analysis of anodized coatings at different concentration of silicate ion is shown in Table 3.It was found that by increasing the silicate concentration from 15 g/L to 60 g/L,the ratio of oxygen to magnesium increases from 2.3 to 4.1 as well as increasing the weight percentage of silicon(Si) in chemical composition of anodized layer.These results indicate that the increase in silicate concentration of electrolyte bath decreases the ratio of wt.%of MgO to wt.%of Mg2SiO4in the resultant coating(Fig.2)leading to the formation of more uniform and denser MAO coating[14].This is due to the benef i cial effect of silicate on the uniformity of emitted sparks[15].As mentioned above,increasing the concentration of silicate reduces the conductivity of the electrolyte and therefore repression of ion exchange.Thus, discharge channels occur more severely promoting the formation of large pores that is detrimental to the properties of the coating as shown in Fig.2.It is also shown in Fig.3 that upon increasing the concentration of silicate,the thickness of the MAO coating increases and reaches to a maximum value of 32 μm in the present study.

Fig.4.XRD diffraction pattern of MAO coating.

Fig.5.Potentiodynamic polarization curves of MAO coating in at different silicate concentration in SBF(Temp.37°C,pH 7.4).

3.3.Phase composition of MAO coating

The XRD pattern of MAO coating is shown in Fig.4.It is obvious that the MAO coating is mainly composed of the biocompatible Mg2SiO4(Forsterite).In addition,oxide of magnesium(MgO)is the second phase detected in the MAO coating.The appearance of strong magnesium peaks in the pattern suggests the relatively thin and porous nature of the MAO coating that the X-ray could penetrate into the substrate.

3.4.Electrochemical measurements

3.4.1.Potentiodynamic polarization

Fig.5 shows the polarization plots of MAO coatings produced in electrolytes with different silicate concentrations.As can be seen in Table 4,raising of silicate concentration in MAO process to 30 g/L has led to improvement of corrosion resistance of the coating due to accelerated coating growth as a result of incorporation of Si into the coating structure and formation of complex Mg-Si-O phases.A similar phenomenon has been reported by A.L.Yerokhin et al.[11].But as mentioned before increasing the silicate concentration morethan 30 g/L can have improper effect on MAO coating. Consequently,the silicate concentration of MAO electrolyte plays a major role in characterization of the f i nal coating and can lead to the formation of a two-phase structure in which the weight percentage of each phase is strongly dependent on silicate concentration.

Table 4Potentidynamic polarization data for specimen without and with coating in various silicate concentration.

Based on prior observations,the corrosion process of MAO coatings can be divided to three stages[13]:

1 Penetration of electrolyte via porosities of the oxide layer, reaching the interface between barrier layer and porous layer

2 Transformation of MgO phase to a less dense Mg(OH)2phase

3 Formation of micro cracks in barrier layer and initiation of corrosion of magnesium alloy substrate

Fig.6.Nyquist and bode plot of MAO coating at different silicate concentration in SBF(temp.37,pH 7.4).

Assuming the corrosion process of MAO coatings as above three stages,it can be anticipated that the corrosion resistance of MAO coatings strongly depend on porosity and the thickness of the coating,in a manner that the increase in the thickness of the coating and decrease in porosity leads to enhancement of corrosion resistance of MAO coating[13].

3.4.2.Electrochemical impedance spectroscopy(EIS)

The Nyquist plots and Bode plots of as-coated and bare specimens are presented in Fig.6.As can be seen in Fig.6,the Nyquist plots of as-coated and bare specimens are two-loop plots.The two-loop plots of as-coated specimens are indicative of presence of two-layer coating in which,the outer one is a porous layer and the inner one is a compact and uniform adjacent to the substrate surface[15].The two-loop plot of bare specimen may be illustrative of formation of calcium--phosphate precipitations on the surface of specimen during the test[16].EIS data analysis has been carried out by f i tting it to an equivalent electric circuit model shown in Fig.7 which has good compliance with the resulting data.Due to nonuniformity of surface and diffusion factors,the capacitor in the equivalent electric circuit can be replaced by a constant phase element(CPE)to improve simulation of the impedance. The admittance of CPE is presented in equation(1):

Fig.7.Cross section SEM micrograph of coating and Electrochemical equivalent circuit used for curved f i tting.

Table 5Electrochemical f i tting values for MAO coating and bare specimen from EIS technic.

In equation(1),Ais a constant,jis imaginary unit (j2=-1),fis frequency of input AC signal andnis the power of the constant phase element(CPE).Ifn=0,the CPE is equivalent of resistance;ifn=1,the CPE is equivalent of capacitor and ifn=0.5,the CPE is equivalent of Warburg impedance.As can be seen in Fig.7,the equivalent electric circuit of MAO system consists of the following elements: solution resistance(Rs),resistance of porous outer layer(Rp), constant phase element of porous layer(CPEp),resistance of compact inner layer(Rf)and constant phase element of compact layer(CPEf)[15,17].The quantities of these parameters of equivalent circuit which are extracted from impedance data are presented in Table 5.As can be seen in Fig.6 and Table 5,with raising the silicate concentration up to 30 g/L,the resistance of the coating increases.Further increasing of the silicate in electrolyte,deteriorates corrosion resistance of the f i lm.

4.Conclusion

1.In MAO process,by increasing the silicate concentration of electrolyte,the breaking voltage of the coating increases from 100 V to 110 V.Further increasing of silicate concentration,raises the breaking voltage to 120 V.The increase in breaking voltage is advantageous for MAO process due to accumulation of the energy in the system and the increase in kinetic of coating formation.On the other hand,the increase of breaking voltage which exceeds a limiting point may impair the quality of MAO coating by resultant increase of discharging channels and increase in girth of pores.

2.The XRD analysis showed that the MAO coating consists of two bio-ceramic phases;which are MgO and Mg2SiO4.

3.The most corrosion resistant coating was achieved in the electrolyte containing 30 g/Lof Na2SiO3.The corrosion current density of the Mg alloy was decreased from 214 μA cm-2(thedensity ofbare specimen)to 2.73 μA cm-2which demonstrates the ability of MAO coating to improve the corrosion resistance of the surface of biodegradable magnesium alloy.

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Received 10 January 2014;revised 7 February 2014;accepted 8 February 2014 Available online 24 April 2014

*Corresponding author.Tel.:+98 915 571 1726;fax:+98 21 6600 5717. E-mail address:bsalami@gmail.com(B.Salami).

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

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http://dx.doi.org/10.1016/j.jma.2014.02.002.

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