Corrosion resistance of pulsed laser modifie AZ31 Mg alloy surfaces

S.Fajardo,L.Miguélez,M.A.Arenas,J.de Damborenea,I.Llorente,S.Feliu

Department of Surface Engineering,Corrosion and Durability,National Centre for Metallurgical Research (CENIM-CSIC),Madrid 28040,Spain

Abstract The effect of laser surface melting on the corrosion resistance of AZ31 Mg alloy in 0.1 M NaCl solution was investigated using different laser processing conditions (energy densities of 14 and 17 J cm-2).Laser treatment induced rough surfaces primarily composed of oxidized species of Mg.XPS analysis revealed that the surface concentration of Al increased significantl as a consequence of LSM.Electrochemical impedance spectroscopy showed that the laser treatment remarkably increased the polarization resistance of the AZ31 Mg alloy and induced a passive-like region of about 100 mV,as determined by potentiodynamic polarization.Analysis of the results obtained provide solid evidence that within the immersion times used in this study,LSM treatment increased the corrosion resistance of AZ31 Mg alloy under open circuit conditions and anodic polarization.

Keywords: Mg alloys;laser surface melting (LSM);Electrochemical impedance;Potentiodynamic polarization;corrosion resistance.

1.Introduction

Among all the metallic materials used for structural applications,magnesium (Mg) alloys have the lowest density(2/3 of aluminum and 1/4 of steel).This makes them very attractive materials for the aerospace and land transportation industries.The lighter weight of a Mg-based vehicle reduces fuel consumption and the emission of greenhouse gasses.For instance,it is estimated that the replacement of multiple components of a car with 72 kg of Mg alloy can provide a weight reduction of 48.5 kg and 19.5 kg and a fuel saving of 0.25 and 0.1 L per 100 km in the cases of steel and aluminum,respectively [1].More recently,it has been reported that substitution of steel,aluminum and mild steel by magnesium implies a weight reduction of about 62%,10-41% and 75-81%,respectively [2].In addition,Mg alloys present a high strength-to-weight ratio,excellent formability,castability,machinability and weldability [3].Furthermore,the interest in the use of Mg alloys as a material for temporary implants is increasing due to a good combination of mechanical strength,similar elastic modulus to bone,biocompatibility,biodegradability and the ability to promote osteogenesis [4-6].However,their low resistance to corrosion and wear remains one of the main limitations in the use of Mg alloys for transportation applications [7,8].

Corrosion resistance is commonly associated with the chemical activity of a metal surface and the protective properties of surface films In the case of Mg alloys,the combination of a high reactivity in aqueous environments and the poor protection provided by the native oxide layer are responsible for their low resistance against corrosion [3,9].However,it is possible to improve the corrosion resistance of these materials by appropriate surface modification inducing microstructural and compositional changes in the outermost region of the metal [10].There are several methods available for surface modificatio of Mg alloys with the aim of improving their corrosion resistance such as the use of conversion coatings,anodization,sol-gel coatings,ionic implantation,laser surface melting,friction stir processing,chemical passivation,application of organic coatings and plasma electrolytic oxidation [8,11-16].Among them,laser surface modificatio and specially laser surface melting (LSM) offer numerous advantages compared to more conventional protection techniques like fast and fl xible processing,easy automation,excellent reproducibility and the lack of surface pretreatment necessity[8,17].Furthermore,LSM is a fast solidificatio method that can promote the generation of very thin melted surface layers without altering the bulk metal significantl [8].From a microstructural perspective,the improved corrosion resistance using this method is primarily associated with the formation of fin dendritic grains in the modifie region.These microstructural changes may promote the dissolution and redistribution of second phases (i.e.intermetallic particles),hindering the formation of microgalvanic couples thus improving the corrosion resistance of Mg alloys [8,18].Regarding the oxide surface fil modification even though LSM does not normally induce significan compositional changes in the treated surface,evaporation of any of the alloying elements is possible [8].For instance,selective evaporation of Mg and Zn in the AZ Mg alloy series would result in an Al enriched surface,enhancing the corrosion resistance of the laser treated regions [6].

The use of different types of laser for LSM treatments,such as excimer lasers or continuous-wave (CW) or long pulsed lasers,result in different solidificatio microstructures,which leads to different mechanisms of improving corrosion resistance of Mg alloys.Liu et al.[12] and Wang et al.[16] used continuous-wave CO2laser (10 kW) to improve the corrosion properties of AM60B and AZ91 Mg alloys,respectively,by changing the microstructure,such as grain refinement Al element enrichment [12] and homogeneous redistribution of the intermetallicβ-Mg17Al12phase in the alloys after treatment.Also,LSM treatment of MEZ alloy(0.5 wt.% Zn,0.1 wt.% Mn,0.1 wt.% Zr,2 wt.% RE,Mg balance) using continuous-wave CO2laser (10 kW) enhanced the corrosion resistance attributed to the combined influenc of grain refinement dissolution of intermetallic phases,and the presence of rare earth elements in the extended solid solution [19].Abbas et al.[20] studied laser surface melting of AZ31,AZ61 and WE43 Mg alloys with a 2kW continuouswave CO2laser beam and observed an improvement of 30%,66% and 87% improvement in corrosion resistance for AZ31,AZ61 and WE43 Mg alloys,respectively.In agreement with Liu et al.[12] and Wang et al.[16],this was attributed to a refinemen in the microstructure of the alloy,and a more homogenous redistribution of theβ-phase.However,Banerjee et al.[21] indicated that the corrosion resistance of ZE41 Mg alloy was not significantl improved by an Nd:YAG laser operating in a continuous wave mode,although microstructural refinemen was achieved.This is due to absence of beneficia alloying elements such as Al.

Studies reported by Taltavull et al.[18] on the high power diode laser (HPDL) processing indicate that changes in corrosion resistance depended on processing operating parameters as well as on the surface roughness.In summary,enhanced corrosion resistance was observed at increasing laser beam power and decreasing laser scanning speed.Coy et al.[10] investigated the effect LSM on the microstructure and corrosion behavior of AZ91D Mg alloy using an excimer laser.Improvement of the corrosion resistance of the alloy was associated with the large dissolution of intermetallic phases and the enrichment of aluminum within the highly homogeneous and refine melted microstructure.Khalfaoui et al.[22] observed improved corrosion behavior of a LSM treated ZE41 Mg alloy using an excimer KrF laser.The melted Mgalloy presented a homogenous distribution of the alloying elements in the magnesium matrix.In contrast,Dubé et al.[23] reported that the corrosion of AZ91D and AM60B Mg alloys melted with the use of pulsed Nd:YAG laser was not significantl reduced even though the microstructure of both alloys was refined

In addition to the laser surface melting technique,electron beam processing (EBP) is one of the most effective way to improve the corrosion resistance of magnesium alloys[24,25].EBP exhibits essential advantages over laser and ion beams with its great efficien y,simplicity,and reliability[25].K.Zhang et al.[26] used a high-current pulsed electron beam(HCPEB) to process as-cast WE43 Mg alloy,and the disappearance of second phase,the formation of homogeneous dispersedβ’ nano-precipitates and the chemically homogenized melted layer improved its corrosion resistance in vitro.Recently,Liu et al.[27] found that Mg-4Sm alloy treated by HCPEB shows the lowest corrosion current density among all samples,which can be attributed to the homogenous microstructure and composition with fewer defects in the surface layer after repeated melting and rapid solidification Finally,Lee et al.[28] observed that selective evaporation of Mg induced by large pulsed electron beam(LPEB)irradiation led to the formation of an Al-enriched re-solidifie layer with nanograined structure consisting of Mg3.1Al0.9metastable phase,promoting a decrease in the corrosion rate of AZ31B Mg alloy in 3.5% NaCl solution.

In a previous work,it was found that an Al enrichment(of about 0.5 and 0.8 wt.%) within a surface layer of approximately 10 μm substantially improved the corrosion resistance on a AZ31 Mg alloy manufactured by twin-cast rolling method [29].The present paper aims to further explore the effect of LSM using a nanosecond low power Nd:YAG laser under different laser processing conditions on the Al enrichment of AZ31 Mg alloy and its corrosion behavior in 0.1 M NaCl solution.Even though laser treatment may also induce microstructural changes in the outermost region of the test specimens,this work focuses on the changes in the surface chemistry as a consequence of LSM.Furthermore,an AZ31 Mg alloy was used with the aim of avoiding the well-known galvanic effect of the second phases that form with high volume fraction for increasing concentrations of Al (primarily Mg17Al12),and act as local cathodic sites during corrosion.Surface analysis and electrochemical methods were used to evaluate the efficien y and suitability of LSM treatments for improving the corrosion resistance of Mg alloys.

2.Materials and methods

Commercially available AZ31 Mg alloy of composition 3.1 wt.% Al,0.73 wt.% Zn,0.25 wt.% Mn (balance Mg)supplied by Magnesium Elektron Ltd.was used as test specimen.Plates of 20 × 20 × 5 mm were cut and successively ground using SiC emery paper to a 2000 grit finis under ethanol and polished to a 1 μm mirror finis before laser treatment.The test solution was 0.1 M NaCl (pH～6) prepared from laboratory grade reagents and high purity water of 18.2MΩcm resistivity (MilliporeTMsystem).

A pulsed Nd:YAG laser(LOTIS LS 2147)with a frequency of 10 Hz and with Q-Switch mode was used.The pulse duration was 12-18 ns,operating in the fundamental mode of 1064 nm.The beam,with an average output energy up to 830 mJ (45 J pumping energy),was directed by a mirror to a convex bending lens with 150 mm focal length.The beam impinges the sample that was located inside a reaction chamber.The chamber was on an XY table controlled by a specifi in-house developed software.The treatment was carried out in a protective argon atmosphere.The scanning speed was 1 mm/s and the overlapping of 250 μm with respect to the width of the bead.The laser fluenc was of 14 and 17 J cm-2.Lower and higher energy densities (10 and 20 J cm-2)were also tested.However,these conditions were finall discarded.For 10 J cm-2,the surface was not homogeneously melted,leaving parts of the specimen untreated;whereas 20 J cm-2was determined to go beyond the damage threshold,often producing deep holes on the surface that protruded into the metallic matrix.For comparative purposes,a mirror finis polished non-LSM treated AZ31 specimen was also tested.

EIS measurements were performed potentiostatically in 0.1 M NaCl solution at the open circuit potential (OCP).A sinusoidal AC wave of ±10 mV (rms) amplitude was applied from 100 kHz to 0.1 Hz collecting 7 points per decade.Fitting of the EIS spectra was carried in out using the ZView?software (Scribner Associates).Potentiodynamic polarization(PDP)measurements were performed polarizing the test specimens in the anodic direction starting at the OCP to+700 mV vs.OCP at a scan rate of 1 mV/s.

All electrochemical measurements used a three-electrode configuratio electrochemical cell,where the test specimen was the working electrode,a Ag/AgCl (3 M KCl) electrode(SSC) was the reference electrode and a platinum wire acted as the counter electrode.With the aim of maximizing the utility of the LSM treated surfaces for electrochemical testing,the syringe cell method developed by Panindre et al.[30] was used.This method allows for conventional electrochemical testing on small surface areas.Furthermore,the method has proved to be extremely suitable for assessment of localized corrosion susceptibility and is easy to set up.Details are available elsewhere [30].In the present investigation,a standard 20 mL syringe with an inner diameter of the syringe mouth(tip of the barrel) of 1 mm was employed.This resulted in an exposed area of about 3.14 mm2.All experiments were replicated several times as will be commented along with the corresponding reported results for clarity.

Table 1.

Table 1 Chemical surface composition expressed in atomic percentage (at.%) observed by XPS on the AZ31 Mg alloy without LSM treatment [24] and after the LSM treatment at energy density values of 14 and 17 J cm-2.

All electrochemical tests were carried out using a Gamry Instruments Interface 1000E potentiostat/galvanostat controlled by the Gamry Framework.Prior to polarization,all specimens were left at the OCP and the potential monitored until a stable value was observed.

Fig.1.Surface appearance of the AZ31 Mg alloy after LSM treatment at 14 and 17 J cm-2.

Microstructural characterization of LSM treated AZ31 Mg alloys was performed using optical microscopy (OM).A Hitachi S 4800 scanning electron microscope (SEM) equipped with an Oxford Instruments energy dispersive X-ray (EDX)microanalyzer was used for surface and elemental analysis of the specimens after treatment.XPS measurements were carried out using a VG Microtech model MT 500 spectrometer equipped with a non-monochromatic MgKα1.2anode X-ray source operating at 300 W.A pressure below 10-9torr was maintained during data collection.Adventitious C was used as binding energy (BE) reference,with 285.0 eV for C 1 s.

Surface profil roughness (Ra) was measured using a TR-200 stylus instrument by Innovatetst Europe with a cut-off length of 0.8 mm and total measurement length of 4 mm.

3.Results and discussion

3.1.Surface and chemical characterization after LSM treatment

Fig.1 shows the surface appearance of the AZ31 Mg alloy after the LSM treatment at energy density values of 14 and 17 J cm-2.Even though the non-treated specimen presented some submicrometer pits from the polishing (not shown),a smooth surface without corrosion products was observed.On the contrary,the LSM treated samples at 14 and 17 J cm-2showed roughened surfaces.Fig.2 shows the optical micrographs of the AZ31 Mg alloy after the LSM treatment at energy density values of 14 and 17 J cm-2.It is evident from the micrographs in Fig.2 that the laser impacts can be seen on the surface.This is due to the non-homogeneous energy distribution of the laser beam which is not a TEM00.Laser treated zones with maximum energy produces melting and rapid cooling and the impact spots of the laser were easily located,particularly at the lowest energy applied.The average value of Ra was determined from an average of 5 measurements with a deviation between the different measurements lower than 5%.While Ra for the untreated sample was 0.25 μm,Ra for the LSM treated samples was about 0.62 μm.

Fig.2.Optical micrographs at different magnification of the AZ31 Mg alloy after LSM treatment at 14 and 17 J cm-2.

Chemical surface composition of the LSM treated samples at 14 and 17 J cm-2was analyzed by X-ray photoelectron spectroscopy (XPS).As an example of the fitting Fig.3 shows the high resolution XPS spectra for O 1 s(Fig.3a),Mg 2p (Fig.3b) and Al 2p (Fig.3c) of the LSM treated specimen at 14 J cm-2.Spectrum for O 1 s shows only one peak associated with the presence of oxide species at binding energies of 531.4 eV.Spectra corresponding to Mg 2p (50.4 eV) and Al 2p (73.5 eV),also show one peak assigned to oxidized species of Mg2+and Al3+,respectively.It is interesting to note that no metallic components were observed for any of the LSM treated AZ31 surfaces.This indicates that the laser treatment generated a surface layer composed mainly of oxides of Mg and Al.Neither Zn nor Mn were detected in the surface oxide film

Table 1 summarizes the concentration of the elements determined by XPS on the LSM treated samples at 14 and 17 J cm-2,expressed in atomic percentages (at.%).For comparative purposes,Table 1 also includes the elemental composition of the AZ31 Mg alloy in polished condition as determined by our group in a previous study [31].It is observed that the non LSM treated specimen presented a high content of C,which may be attributed to environmental contamination due to air and the ethanol used during polishing for surface preparation.On the contrary,the concentration of C on both the LSM treated specimens decreased significantl,likely due to the laser treatment.In the case of O,even though surface laser treatment was performed in the presence of Ar to minimize oxidation,greater concentrations of O were measured.

Given that the surface fil was mainly composed of oxidized species of Mg and Al,the relative contribution of each element on the LSM specimens was analyzed by their corresponding cationic fraction (shown in Table 1).As an example,the cationic fraction for Al was calculated as follows:Alcat=Al3+/ (Mg2++Al3+).It can be observed that the surface cationic fraction of Al increased remarkably after the AZ31 Mg alloy was subjected to LSM treatment with respect to the polished specimen.This,as will be shown later,resulted in significan increased corrosion resistance in NaCl solution.Even though the cationic fraction of Al determined for both LSM treated specimens were similar,those corresponding to the surface treated at 17 J cm-2exhibited slightly greater values.This is consistent with the notion that during laser treatment partial evaporation of Mg occurred,reducing its surface concentration and resulting in an enrichment of Al oxidized species.

Finally,it is reasonable that the laser treatment resulted in a non-uniform chemical composition in the surface oxide layer.Please note that energy density transmitted to the specimen during laser treatment is not homogeneous within the impact spot(unpublished work).In brief,maximum energy is reached at the beam center whereas decreased energies are found moving radially from the center towards the perimeter of the beam.Consequently,temperature gradients exist in the irradiated region,probably leading to non-homogeneous evaporation of Mg.Temperature gradients due to non-homogeneous energy densities within the irradiated spots also explain the roughened surfaces shown in Figs.1 and 2.

3.2.Electrochemical impedance spectroscopy (EIS)measurements

Fig.3.High resolution XPS spectra for O 1 s,Mg 2p and Al 2p corresponding to the AZ31 Mg alloy after LSM treatment at 14 J cm-2.

Fig.4.Nyquist and Bode plots for the AZ31 Mg alloy without surface laser treatment in 0.1 M NaCl solution at room temperature.All curves correspond to independent replicates using the same experimental conditions measured at different regions of the surface.

Fig.4 shows the Nyquist and Bode plots of the AZ31 Mg alloy without LSM treatment in 0.1 M NaCl solution.Results from a total of fi e replicates under the same experimental conditions are presented.A very good reproducibility was obtained,confirmin the suitability of the syringe cell method for the electrochemical testing of Mg alloys.The presence of a capacitive and an inductive arc is observed in the Nyquist plot.The capacitive arc is represented in Fig.4 by a negative component of the imaginary part (Z’’) and the increase of the real component (Z’) with the decrease in frequency to intermediate values.On the contrary,the inductive arc is represented by a positive component of Z’’ and the decrease of Z’with the decrease of the frequency as it approaches to zero.This inductive behavior has been described previously in numerous independent studies in which the impedance response of Mg and its alloys was investigated [32-34].

Fig.5.Nyquist and Bode plots for the AZ31 Mg alloy after surface laser treatment at 14 J cm-2 in 0.1 M NaCl solution at room temperature.All curves correspond to independent replicates using the same experimental conditions measured at different regions of the surface.

Fig.6.Nyquist and Bode plots for the AZ31 Mg alloy after surface laser treatment at 17 J cm-2 in 0.1 M NaCl solution at room temperature.All curves correspond to independent replicates using the same experimental conditions measured at different regions of the surface.

Figs.5 and 6 show the Nyquist and Bode plots of the AZ31 Mg alloy after the LSM treatment at energy density values of 14 and 17 J cm-2in 0.1 M NaCl solution.As in the case of the AZ31 Mg alloy in the absence of LSM treatment,results from 5 replicated tests at each energy density carried out under identical experimental conditions are presented.It is evident that a wide dispersion of the results was obtained for both LSM treatment energies.Given that an excellent reproducibility was observed for the polished AZ31 Mg alloy (see Fig.4),the most reasonable explanation is that dispersion of the data was due to the high heterogeneity shown by the LSM treated surfaces (Figs.1 and 2).These may be associated with local compositional changes due to non-homogeneous energy distribution during laser application.However,it is also likely that the differences in the EIS plots at each LSM energy density applied originated due to topographical variations.For instance,small variations of the surface area exposed to the electrolyte would result in capacitive arcs of varying diameter.Accurate determination of the exact area exposed during testing is not trivial.Due to this experimental limitation,all curves shown in Figs.4-6 were normalized by the nominal area exposed to the electrolyte,define by inner diameter of the syringe mouth.It should be mentioned that the curves shown in Figs.5 and 6 correspond to 5 representative measurements of a higher number of tests performed on these surfaces where,in any case,curves with capacitive semicircle diameters above or below the limiting curves shown in Figs.5 and 6 were obtained.Consequently,it is reasonable to assume that the global response of the LSM treated surfaces will be within the limits of these curves,and that the mean values of the electric elements that describe the impedance spectra (with their respective standard deviations) obtained from the fittin provide a reliable measure of the electrochemical behavior of the studied AZ31 Mg alloys LSM treated surfaces.

Prior to the analysis of the main features of the Nyquist plots presented in Figs.4-6,the consistency of the experimental data was evaluated using the Kramers-Kronig (K-K)transforms [35,36]:

where Z’(x) and Z’’(x) are the real and imaginary parts of the impedance,respectively,as a function of angular frequency (x),with 0＜x ＜∞;Z’(ω) and Z’’(ω) are the real and imaginary parts of the impedance,respectively,for the angular frequencyx=ω.The KK transforms provide a measure of the robustness of the experimental results.In summary,using the K-K transforms it is possible to calculate the imaginary part of the impedance from the values of the real part obtained experimentally and compare them with the values of the imaginary part obtained experimentally.Alternatively,it is possible to calculate the values of the real part from the experimental values of the imaginary part.Fig.7 shows the comparison between the experimental data and those calculated using the K-K transforms for one of each LSM treatment presented in Figs.4-6 (i.e.no LSM treatment and LSM treatment at 14 and 17 J cm-2).An excellent correlation between the experimental and the KK-transformed data was obtained.Even though only one plot is shown per condition for simplicity,all the curves presented in Figs.4-6 exhibited a very good consistency between the experimental and the transformed data.Additionally,a statistical error function was used to compare the data obtained using the K-K transforms and the experimental data [37].The errors calculated for all replicated tests were lower than 2.2% for the AZ31 Mg alloy without LSM treatment and 0.7 and 1.3%for the AZ31 Mg alloy subjected to LSM treatment at energy densities of 14 and 17 J cm-2,respectively.The low calculated errors (less than 3%),and the good correlation shown in Fig.7,confir the validity of the experimental data.

Fig.8 shows the equivalent electrical circuits (EEC) used to analyze the experimental EIS data of the AZ31 Mg alloy in the absence of LSM treatment (Fig.8a) and the LSM treated AZ31 Mg alloys at energy density values of 14 and 17 J cm-2(Fig.8b) in 0.1 M NaCl solution.The EEC in Fig.8a consists of a resistor (Rs) that simulates the resistance of the electrolyte between the working electrode (the Mg alloy) and the reference electrode,associated in series with the parallel combination of a constant phase element (CPE) and a resistor (R1),and the series association of an inductance(L) and a resistance (R2) [29,38].A CPE was used instead of a capacitor to adequately simulate the capacitive response of the system.The CPE is an empirical admittance function that is usually used when the system cannot be described by an ideal capacitor,primarily due to the heterogeneous nature of the surface.The impedance of a CPE is define by the following expression:

Fig.7.Comparison of typical experimental impedance data for the AZ31 Mg alloy (a) without laser treatment,(b) after laser treatment at 14 J cm-2,and (c) after laser treatment at 17 J cm-2,from Figs.4-6 and the impedance calculated using the Kramers-Kronig (KK) transforms.

Fig.8.Equivalent electrical circuits (EEC) used to fi the EIS spectra of the AZ31 Mg alloy (a) without LSM treatment,(b) after LSM treatment at 14 J cm-2 and 17 J cm-2 in 0.1 M NaCl.

where Y is the value of the admittance (S cm-2sn),ωis the angular frequency (rads-1),j is the imaginary number(j2=(-1)) and n is a dimensionless fractional exponent that takes values between -1 and 1.Whenn=1,the CPE represents an ideal capacitor;whenn=0,the CPE represents a resistance;and whenn=-1,the CPE represents an inductor.In the EEC of Fig.8a,the CPE is associated with the capacitance of the electrochemical double layer and R1 is the charge transfer resistance (Rct).The series combination of L and R2 was used to simulate the inductive response of the system,and is normally associated with the presence of reaction intermediates or adsorbed species on the electrode surface during the corrosion reaction.A detailed explanation of the physical meaning of the electrical elements that simulate the inductive response in the EEC of Fig.8a is not trivial and is currently the subject of debate.Recently,Yuwono et al.[39] studied the dissolution mechanism of Mg and the concomitant anomalous hydrogen evolution using first-principle density functional theory calculations and proposed the existence of intermediate species adsorbed on the metal surface that would be involved in these phenomena.It is possible that the presence of the inductive behavior observed in this investigation and in a large number of independent works on the corrosion of Mg is related to the presence of those intermediates.

With respect to the AZ31 Mg alloys after LSM treatment at energy density values of 14 and 17 J cm-2,the Nyquist plots in Figs.5 and 6 show three different capacitive arcs(i.e.at high,medium and low frequencies).Consequently,a three-time constant EEC (Fig.8b) was used to model the electrochemical response of the specimens during testing.The EEC in Fig.8b shows a multi-component nested Randle’s circuits where the minimum number of capacitive time-constants to reliably represent the data was used [33,34].The physical meaning of each of the electrical elements used will be briefl discussed later.Interestingly,no inductive behavior was observed for any of the LSM treated Mg alloys in the same frequency range where the specimens in the absence of LSM treatment exhibited a clear inductive response.For this reason,no series association of an inductance and a resistance were included in the EEC.This is likely due to a greater corrosion resistance associated with the formation of a protective surface fil that hindered dissolution.All curves in Figs.4-6 show an excellent correlation between the experimental and the fitte data.

Tables 2-4 show the results obtained from the fittin of the Nyquist plots in Figs.4-6 using the EECs shown inFig.8.Each Table shows the fittin results of the 5 measurements presented in Figs.4-6.While the CPE parameters (Y and n) provide information on the capacitive behavior of the system,the combination of resistances is associated with the polarization resistance (Rp).In brief,the Stern-Geary equation predicts that for a metal under activation control Rpis inversely related to the corrosion current density (icorr) by the proportionality constant B,which depends exclusively on the Tafel slopes [40].King et al.[33] demonstrated the importance of accounting for the inductive behavior of dissolving Mg for the accurate determination of Rpand,in consequence,of icorr.Assuming that the B constant does not change dramatically with the LSM treatment,it is possible to evaluate the corrosion resistance of the AZ31 Mg alloys without LSM treatment and LSM treated at energy densities of 14 and 17 J cm-2by simply comparing their respective Rpvalues obtained from the EIS fitting

Table 2 Fitting results of EIS experimental data for the polished AZ31 Mg alloy without LSM treatment in 0.1 M NaCl solution.Data from 5 replicated measurements are presented.

Table 3 Fitting results of EIS experimental data for the LSM treated AZ31 Mg alloy at an energy density of 14 J cm-2 in 0.1 M NaCl solution.Data from 5 replicated measurements are presented.

Table 4 Fitting results of EIS experimental data for the LSM treated AZ31 Mg alloy at an energy density of 17 J cm-2 in 0.1 M NaCl solution.Data from 5 replicated measurements are presented.

Tables 5 and 6 summarize the fittin results presented in Tables 2-4 showing the mean values and their corresponding standard deviation for each of the elements in the EEC shown in Fig.8.Please note that in Tables 5 and 6 no CPE admittance values (Y) are presented.Instead,effective capacitances(Ceff) are provided.The admittance of a CPE is not expressed in units of capacitance and,as shown in Eq.(4),is dependent of an exponent.Small deviations of that exponent with respect to 1 (ideal capacitor) can result in large errors if Y is used as a capacitance.However,it is possible to convert Yinto Ceffusing the expression developed by Brug et al.[41]:

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Table 5 Fitting results of EIS experimental data for the AZ31 Mg alloy without LSM treatment in 0.1 M NaCl solution.Mean values and their corresponding standard deviation for each of the elements in the EEC shown in Fig.8a are presented.CPE elements expressed as Ceff calculated using Eq.(5).

Table 6 Fitting results of EIS experimental data for the AZ31 Mg alloy after LSM treatment at energy densities of 14 and 17 J cm-2 in 0.1 M NaCl solution.Mean values and their corresponding standard deviation for each of the elements in the EEC shown in Fig.8b are presented.CPE elements expressed as Ceff calculated using Eq.(5).

where RFis the faradaic resistance in parallel with the capacitance to be determined and the rest of the terms have already been defined

It can be seen that for the AZ31 Mg alloy without LSM treatment,Ceff,1is of the order of 20 μF cm-2,which corresponds to typical values for the capacitance of an electrochemical double layer.The fact that the CPE1 in Fig.8 is associated with the capacity of the double layer confirm that R1 is associated with the charge transfer resistance (Rct).In the case of the LSM treated AZ31 Mg alloys,Ceff,1and Ceff,2presented similar values of the order of 10 μF cm-2,complicating their assignation to a certain relaxation process.Nevertheless,the slightly lower values of Ceff,1are consistent with the capacitance of an oxide film which is in agreement with the XPS surface analysis.On the other hand,even though Ceff,2values are moderately small for an electric double layer,they may be associated with the double layer capacitance.Finally,the time constant at the lowest frequencies(Ceff,3) presents values of approximately 2200 and 5300 μF cm-2for energies densities of 14 and 17 J cm-2,respectively.The physical meaning of these pseudo-capacitances is not evident.They may be associated with finit diffusional processes [42,43].However,it is possible that these pseudocapacitances were due to adsorption accompanied by charge transfer [42].This notion is in agreement with the mechanism proposed by Yuwono et al.[39].

Regarding the Rpvalues,they can be estimated from the resistances in Tables 5 and 6 by simplifying the system for the case in which the frequency tends to zero (f →0):

where Eqns.(6) and (7) correspond to the case of the AZ31 Mg alloy without LSM treatment and the LSM treated AZ31 Mg alloys,respectively.

Fig.9 shows the calculated Rpvalues for the AZ Mg alloy without LSM treatment and LSM treated at 14 and 17 J cm-2in 0.1 M NaCl solution determined from the EIS spectra shown in Figs.4-6.The LSM treatment increased remarkably the values of Rpfor the AZ31 Mg alloy with respect to the non-treated surface,indicating lower corrosion rates.Furthermore,the LSM treatment at 14 J cm-2provided a slightly greater Rpvalues than that at 17 J cm-2.

3.3.Potentiodynamic anodic polarization measurements

Fig.9.Polarization resistance values obtained by EIS for the AZ31 Mg alloy without LSM treatment,and after LSM treatment at 14 J cm-2 and 17 J cm-2 in 0.1 M NaCl solution.Mean values from replicated experiments are presented.Error bars are standard deviation.

Fig.10.Potentiodynamic polarization curves of the AZ31 Mg alloy without LSM treatment,and after LSM treatment at 14 J cm-2 and 17 J cm-2 in 0.1 M NaCl solution.The scan rate was 1 mV/s.Typical results from replicated experiments are presented.

The anodic kinetics was studied by means of potentiodynamic polarization.Fig.10 shows the potentiodynamic anodic polarization curves of the AZ31 Mg alloy without LSM treatment and after the LSM treatment at energy density values of 14 and 17 J cm-2in 0.1 M NaCl solution.For simplicity,one typical curve from replicated tests is presented for each condition.Significan differences were observed between the three specimens.The AZ31 Mg alloy without LSM treatment showed an Ecorrof about -1.5 VSSCand no sign of passivity with increasing current densities at potentials above the Ecorr.This behavior,typical of an ideally non-polarizable electrode,has been previously observed for the AZ31 Mg alloy in many independent works [14,44-46].On the contrary,the AZ31 Mg alloy specimens previously subjected to LSM treatment exhibited more negative Ecorrvalues and clear passive-like regions of around 100 mV followed by a sharp increase of the current density,which is indicative of localized corrosion.These results are in agreement with the increased Rpvalues estimated using EIS (see Fig.9) and support the notion that LSM treatment enhances the corrosion resistance of AZ31 Mg alloy in a chloride containing solution.Furthermore,differences were observed between the LSM treated AZ31 Mg alloys at energy density values of 14 and 17 J cm-2.While the specimen treated at 14 J cm-2presented an Ecorrof about -1.60 VSSC,the AZ31 treated at 17 J cm-2exhibited a lower Ecorrvalue of approximately -1.65 VSSC.Another interesting difference between the two LSM energy densities applied was observed in the passive regions,where the specimen treated at the lower energy showed the lowest current densities thus the more protective surface against corrosion.Even though the cathodic kinetics was not investigated,it is clear that the LSM treatment decreased rate of HE.It is reasonable that this decreased cathodic kinetics was responsible for the lower values of Ecorrand the ability to observe the passive behavior in a potential region that was dominated by the cathodic reaction for the AZ31 Mg alloy without LSM treatment.

In summary,the potentiodynamic anodic polarization results confir the EIS observations where the LSM treatment on the AZ31 Mg alloy resulted in enhanced corrosion resistance.Furthermore,the lower current densities measured in the passive regions for the AZ31 Mg alloy treated at 14 J cm-2are in agreement with the EIS determined Rpvalues,where the AZ31 Mg treated at this energy density were the greatest.

Fig.11 shows the optical micrographs of the AZ31 Mg alloy specimens without LSM treatment,and after LSM treatment at 14 J cm-2and 17 J cm-2after anodic polarization in 0.1 M NaCl solution.All specimens exhibited the typical black corrosion products that are normally observed after Mg alloy corrosion.However,while the sample without prior LSM treatment showed circular-like attacked regions with the black corrosion product layer extending radially on the surface,a different morphology was observed for the AZ31 Mg alloy specimens that had been previously subjected to LSM treatment at 14 J cm-2and 17 J cm-2.In these cases,significantl less corroded surfaces were exhibited,with a number of localized corroded regions distributed on the sample.This is consistent with the XPS results that revealed an enrichment of Al in the surface after LSM treatment at both energy densities.Furthermore,the localized attack observed in the surface appearance of the previously LSM treated samples (Fig.11) is also in accordance with the passive regions found during potentiodynamic anodic polarization (Fig.10).

Fig.12 provides a closer look to the corroded regions on the surface of the AZ31 Mg alloy without LSM treatment.SEM micrographs show a dense and rather compact corrosion product layer with the presence of numerous cracks(Fig.12a).Higher magnificatio SEM micrographs reveal that the corrosion product exhibited a crystalline microstructure.Elemental analysis obtained by EDS on a number of sites on the corroded areas showed that the corrosion product consisted mainly of Mg and O at a O/Mg ratio equal to 1.8±0.2.This is very close to the expected value of 2 for Mg(OH)2,suggesting that the corrosion product fil was primarily composed of this species.

Fig.11.Optical micrographs of the AZ31 Mg alloy without LSM treatment,and after LSM treatment at 14 J cm-2 and 17 J cm-2 after potentiodynamic anodic polarization in 0.1 M NaCl solution..

Fig.12.BSE SEM micrographs of the corroded regions on the AZ31 Mg alloy without prior LSM after potentiodynamic anodic polarization in 0.1 M NaCl solution,shown in Fig.11a.

Fig.13.BSE SEM micrographs of the corroded regions on the AZ31 Mg alloy subjected to LSM treatment at 14 J cm-2 after potentiodynamic anodic polarization in 0.1 M NaCl solution,shown in Fig.11b.

Fig.14.EDX maps for elemental composition of the corroded regions on the AZ31 Mg alloy subjected to LSM treatment at 14 J cm-2 after potentiodynamic anodic polarization in 0.1 M NaCl solution.

Regarding the corroded regions on the AZ31 Mg alloy specimens previously subjected to LSM treatment,a nearly identical behavior was observed.For this reason,only the SEM micrographs corresponding to the corroded regions on the LSM sample treated at 14 J cm-2are shown in Fig.13.In contrast with the Mg alloy without LSM treatment,the fraction of the surface covered with corrosion products was significantl lower.This is consistent with a more corrosion resistant surface fil and is in agreement with the results from the electrochemical tests.Despite the differences in the amount of corrosion products,morphology of the corroded regions was similar to that observed for the Mg alloy without LSM treatment.A dense and cracked corrosion product layer was clearly exhibited.Furthermore,Fig.13 also shows the presence of a small amount of tiny and mostly elongated particles.EDS analysis (not shown here for brevity) confirme that they consisted of Al,Mn and Fe,which is consistent with Al-Mn-Fe intermetallic second phases.

Element distribution of the corroded regions on the LSM sample treated at 14 J cm-2was carried out by EDS elemental maps,shown in Fig.14.As previously commented,this is representative of both energy densities used for LSM treatment.Compositional analysis showed that the corrosion product fil exhibited a high concentration of oxygen.Interestingly,Mg concentration was lower in the corrosion products than in the non-corroded regions.Area analysis in randomly selected zones of the corroded and non-corroded regions revealed that Mg concentration was 41.0 ± 0.2 and 78 ± 1 at.%,respectively.This explains that,even though the corrosion product fil mainly consisted in oxidized species of Mg,no significan contrast in the Mg signal was observed in the EDS maps.Furthermore,O/Mg ratio in the corroded regions was about 1.38 ± 0.01,indicating a lower contribution of Mg(OH)2to the corrosion product than for the non-LSM treated specimen and suggesting that it was composed of a mixture of MgO and Mg(OH)2.Finally,the EDS elemental maps depicted in Fig.14 show that the other detected elements were homogenously distributed over the scanned area with no appreciable enrichment in the corrosion product film

4.Conclusions

The effect of LSM treatment on the corrosion resistance of AZ31 Mg alloy was investigated using surface characterization,electrochemical impedance spectroscopy and potentiodynamic polarization.Different laser processing conditions(energy densities of 14 and 17 J cm-2) were used.The main conclusions are:

?LSM treatment induced topographical and compositional changes with respect to a polished AZ31 Mg alloy specimen.Rougher surfaces composed of a surface layer enriched in Al were observed.

?Electrochemical impedance showed that LSM treatment significantl increased Rpin the AZ31 Mg alloy,leading to decreased values of corrosion rate.

?LSM treatment at both energy densities hindered anodic dissolution.Potentiodynamic polarization revealed a small passive-like region of about 100 mV.

?No significan differences in the corrosion resistance of the LSM treated AZ31 Mg alloys were observed between the different energy densities used for LSM treatment.

?These finding are in agreement with the notion that localized heating of the surface due to laser irradiation yields to preferential evaporation of Mg,resulting in greater surface concentrations of more protective Al.

?For the immersion times used in this study,LSM treatment increased the corrosion resistance of AZ31 Mg alloy under open circuit conditions and anodic polarization.

Declaration of Competing Interest

The authors declare that they have no known competing financia interests or personal relationships that could have appeared to influenc the work reported in this paper.

Acknowledgements

S.Fajardo expresses his gratitude to the State Research Agency (Ministry of Science and Innovation of Spain),the Spanish National Research Council (CSIC) and the European Regional Development Fund (ERDF) for the support under the project RYC2019-027006-I (AEI/FEDER/UE).The authors thank the CENIM Surface Characterization (XPS) and Electron Microscopy Laboratories.