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Materials Science and Engineering A452–453 (2007) 210–218The relation between microstructure and corrosion behaviorof GTA welded AZ31B magnesium sheetG.Ben-Hamu a,D.Eliezer a,∗,C.E.Cross b,Th.B¨o llinghaus ba Ben-Gurion University of the Negev,Beer-Sheva,Israelb Federal Institute for Materials Research&Testing(BAM),Berlin,GermanyReceived31July2006;received in revised form16October2006;accepted19December2006AbstractWelding of AZ31B magnesium alloy was carried out using gas-tungsten arc(GTA)welding.The microstructure and the corrosion behavior of welded magnesium AZ31B alloy were investigated.ac and dc polarization tests were carried out on the welded Mg sheet.The microstructure was examined using optical and electron microscopy(TEM and SEM),X-ray analysis and EDS.Scanning Kelvin probe force microscopy(SKPFM) was used in order to measure the V olta potential of different phases relative to the matrix.The results showed that the GTA process effected both the microstructure and the corrosion behavior.These results can be explained by the effects of the process on microstructure of AZ31B Mg alloy sheet such as grain size and precipitates caused by the change in precipitation and recrystallization behavior.© 2007 Elsevier B.V. All rights reserved.Keywords:AZ31B;GTA weld;Microstructure;Corrosion behavior;SKPFM;Intermetallics1.IntroductionThe challenges of significant weight reduction in the auto-mobile industry in the context of significant fuel savings, recyclability and emission reduction has promoted focus on lightweight metals such as aluminium and magnesium[1,2].As a general means of material manufacturing,welding can be used to optimize product design and minimize the costs of production. Up to now,information published on magnesium alloy welding is still limited[3–6].Many welding methods such as GMA, GTA,laser,electron beam,and friction welding have already been applied to weld magnesium alloys[7,8].GTA welding was selected for use in this study because it produces very high qual-ity welds as opposed to other methods(e.g.GMA or laser), thus minimizing complicating effects arising from weld defects such as undercuts,porosity,and weld spatter.GMA is partic-ularly prone to problems with weld spatter and arc instability. Laser welds highly susceptible to weld bead undercutting and pore entrapment associated with laser keyhole instability.When welding magnesium alloys in particular,all of these problems are exasperated by the high vapour pressure of magnesium.The ∗Corresponding author.Tel.:+97286461467;fax:+97286472931.E-mail address:deliezer@bgu.ac.il(D.Eliezer).corrosion of magnesium alloys is one of the major problems that limit the use of magnesium alloys for transportation applications.The main objective of this research is to investigate the microstructure and the corrosion behavior of welded Mg alloys. AZ31B Mg alloy sheet with thickness of2.3mm was selected as the base metal in the present study,an all-purpose wrought alloy with good strength and ductility.ac and dc polarization tests were carried out on welded Mg sheet.The microstructure was examined using optical and electron microscopy(TEM and SEM),X-ray analysis and EDS.SKPFM used in order to mea-sure the V olta potential of different phases in the BM,HAZ and WM.2.Experimental procedurePlates(215mm×63mm×2.3mm)of AZ31B-H24magne-sium alloy were used as the target for welding,whose chemical composition is listed in Table1.Autogenous single pass welds were made using a pulsed gas-tungsten arc(GTA)welding process with straight polarity(electrode negative),and argon shielding gas at aflow rate of20l/min.Back-side and trailing shielding gas was also employed(12l/min,Ar).The welding travel speed was4.0mm/s,and the arc voltage varied between 11and8V for high and low current,respectively.Current was pulsed between140and60A at a frequency of3.3Hz(100ms0921-5093/$–see front matter© 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2006.12.122G.Ben-Hamu et al./Materials Science and Engineering A 452–453 (2007) 210–218211Table1Chemical composition of the base metal(AZ31B-H24)(in wt.%)Al 3.01Zn0.91Mn0.41Fe<0.01Cu0.008Ni<0.002high current,200ms low current),which corresponds to an aver-age linear heat input of approximately208kJ/m.The behavior of GTA welded AZ31B-H24magnesium alloys was investigated by using potentiodynamic polarization (PD)measurements,linear polarization(L.P)measurements (dc polarization)and electrochemical impedance spectroscopy (EIS).All the electrochemical measurements were performed in 3.5%NaCl saturated with Mg(OH)2with a pH(10.5)at which Mg can cover itself with more or less protective oxide or hydrox-ide which checks the dissolution reaction[9].The electrochemical testing was employed to study the main features of the processes taking place at the alloy/solution interface.The effect of welding process on the AZ31B-H24mag-nesium alloy corrosion resistance was studied.The corrosion resistance of Mg alloys was pointed out by EIS measurements performed under polarization and the effect of the different alloy-ing elements was studied.Corrosion rates were derived from polarization data by the common method[10].For all measure-ments a three-electrode electrochemical cell was used,with an SCE as reference electrode and a platinum counter electrode. The working electrode was prepared from the Mg alloy sam-ples.Mg alloy samples were embedded in an acrylic resin to provide electrical isolation of the sample surface.The samples were air dried at room temperature.The potentiodynamic and linear curves were obtained using a263EG&G Potentiostat/Galvanostat,with a voltage scan rate of1mV/min.The impedance measurements were carried out using a PARSTAT2263frequency response analyser coupled with the potentiostat.All the experiments were controlled by a PC,which was also used for the acquisition,storage and plotting of data.The scanned frequency ranged from6mHz to100kHz and the perturbation amplitude was of5mV(it was observed that a variation of the amplitude did not change the frequency response of the electrode/electrolyte interface).The impedance measurements were performed at open circuit potential(E oc).A partial datafitting made with the Boukamp circuit equiva-lent software[11]for the charge transfer process produced the R p(polarization resistance)and C dl(double-layer capacitance) values.The topographic and potential maps of GTA welded AZ31B-H24magnesium alloy were recorded using the scanning Kelvin probe force microscopy(SKPFM)technique with a Nanoscope III Multimode AFM equipped with an extender TM electronic module[12].The SKPFM characterisation was carried out in air at room temperature and a relative humidity between20and 40%.The scan height in the lift mode was100nm.The topog-raphy and the potential were sampled with a pixel density of 256×256and with a scan frequency of0.1Hz.For all mea-surements,commercially available n+-silicon tips coated with PtIr5were used.Prior to each experiment,the tip was checked by performing a potential measurement on a reference sample, consisting of aluminium partially covered with a thin gold layer. The samples were ultrasonically cleaned in ethanol and air dried to avoid any effect of adsorbed species on the potential value.Microstructure studies were carried out using optical microscopy,transmission electron microscopy,and scanning electron microscopy with an energy-dispersive spectroscope (SEM-EDS).Specimens for TEM analyses were prepared as follows.First,slices with diameter of6mm and thickness of 0.2mm were cut from the center of the fusion zone.Then, they were mechanically polished and cut intoØ3mm disks,and dimpled to about10␮m in the center.Final preparation of the specimens was ion milling on a Gatan precision ion polishing system691under conditions of5.0kV and incident angle of 6–4◦.Conventional TEM analyses were conducted on a JEOL 2010TEM equipped with a Gatan Multiscan CCD digital cam-era.The software for the CCD camera is Digital Micrograph2.5.8.3.Results and discussionThe wrought alloy ASTM AZ31B-H24was received in the form of a sheet,2.3mm thick.The chemical composition of the material is presented in Table1.It is seen that the sheet material contains0.41wt.%Mn,usually added for improved corrosion resistance.The AZ31B-H24sheet had a partially recrystallized microstructure andfine grain size of about8␮m(Fig.1).Dis-tinctly different second phase particles were observed in this material.Energy-dispersive spectroscopic(EDS)analysis in the scanning electron microscope(SEM)indicated that the coarse, angular particles contained mainly Mn,Al and Mg.The coarse particles are Mn–Al intermetallic compounds.The Mg–Al phase diagram shows that the major phase present in Mg-rich alloys is␣-Mg,a solid solution containing Al [13].Under equilibrium conditions a eutectic reaction occurs at 437◦C where liquid decomposes into a two-phase structure con-sisting of␣-Mg and an intermetallic compound,␤-Mg17Al12. In systems containing eutectic reactions,because of the fast, non-equilibrium cooling rates in welds and low solid state dif-fusivities,solidifiying dendrites increases continually in solute content with respect to the material previously solidified,result-ing in microsegregation[14].Furthermore,the solute,which is aluminium in this case,is“pushed ahead”at the solidification front enriching the remainder of the liquid to be solidified,and thus thefinal solidification happens around the eutectic compo-sition.The consequence of this microsegregation is that the last liq-uid to solidify has a eutectic composition and solidifies as two phases.The particles at the dendrite boundaries are present as a divorced eutectic containing Mg–Al as well as some Zn.Pre-liminary semi-quantitative analysis indicates these particles are similar in composition to the Al–Zn–Mg particles located within the grains.212G.Ben-Hamu et al./Materials Science and EngineeringA 452–453 (2007) 210–218Fig.1.(a)Macrograph of the GTA weld joint;(b and c)optical micrographs of the fusion zone and HAZ.The dashed line in(c)indicates the fusion line. 4.Microstructural characterisationFig.1(a)presents a cross-sectional view of the GTA welded test plate.The acetic-acid etchant clearly reveals the solidi-fication structure.Fig.1(b)and(c)are higher magnification views of the areas outlined by frames in the weld metal in Fig.1(a),showing weld metal and the fusion line.Thefine, columnar–dendritic,two-phase structure of the fusion zone is clearly evident.As noted above,the two phases are␣-Mg and Mg–Al–Zn.Fig.1(c)shows the microstructure of the HAZ.Over a distance of∼300␮m from the fusion line,the original grain structure present in the base material is still discernible,but within the grains,dark-etching particles,similar in appearance to the second phase particles in the fusion zone,but muchfiner, are apparent.Some of these particles appear to have formed on twin bound-aries.Microchemical analysis in the SEM confirmed that these particles contain Al,Zn and Mg,as do the particles in the fusion zone.It is concluded that these particles formed upon solidification of locally melted material adjacent to the fusion line.Localized variation in solute content caused by segregation invariably present in a commercial alloy causes a point to point variation in effective melting point[14].When the instantaneous temperature adjacent to the fusion line exceeds the effective melting point,localized melting occurs[14].The microstructures of the rolled and welded magnesium alloy AZ31B are compared in the optical micrographs in Fig.1.The microstructure of the welded alloy is significantly coarser than the rolled structure.Samples from the2.3mm thick area of the rolled component show an average grain size of 8.12±3.22␮m.For the welded alloy,an average grain size of 111.27±38.01␮m was determined.Between the dendrites,an intermetallic phase is present at the dendrite boundaries.The intermetallic phase is surrounded by a region of highly super-saturated magnesium solid solution,which appears in the optical micrographs as a darker gray compared to the␣-Mg dendrite. The supersaturation of Mg with Zn and Al was verified by energy-dispersive X-ray(EDX)in an SEM.According to the ternary phase diagram,two ternary inter-metallic equilibrium phases are known in the Mg–Zn–Al system: the magnesium-rich phase(Mg5Zn2Al2)and the␶phase (Mg32(Al,Zn)49)[15].The␶phase possesses a wide homogene-ity range near the center of the ternary phase diagram.X-ray diffraction of the samples gave no clear evidence which inter-metallic phase is present in the alloy.Therefore,a TEM analysis was performed to clarify the structure and the chemical composi-tion of the grain-boundary phase.EDX analysis atfive different intermetallic particles at the grain-boundaries showed an average composition of Mg–42at.%Zn–19at.%Al.This concentration varied by±1at.%between the measurements at different posi-tions.The chemical composition is located in the homogeneity range of the crystalline␶phase which is not expected to occur in AZ31B alloy.One selected area diffraction pattern obtained from the inter-metallic grain-boundary phase of a welded sample is shown in Fig.2(point1).The phase at point2is Mg2Si.The diffraction pattern(from Fig.2point1)reveals periodic distances between the diffraction spots indicating a structure without translation periodicity.These observations show that the grain-boundary phase is the crystalline equilibrium␶phase with cubic crystal structure[16].This ternary phase having162atom per unit cell in T5h space group with a wide range of solubility and Zn/Al ratio[17,18].Another important feature of the AZ31B weld metal microstructure is the supersaturated areas near the dendrite boundaries.According to the ternary Mg–Zn–Al phase diagram, the solidification of the AZ31B alloy starts with the crystal-lization of␣-Mg with a dendritic structure.In the next step Mg17Al12solidifies in a binary eutectic reaction.The Mg17Al12 phase is not stable during further cooling and reacts with theG.Ben-Hamu et al./Materials Science and Engineering A 452–453 (2007) 210–218213Fig.2.TEM image of Mg 32(Al,Zn)49—eutectic constituent from AZ31-WM (point 1)and electron diffraction pattern obtained from point 1(point 2—Mg 2Si particle).liquid in a transformation reaction to form the ternary inter-metallic.The microstructure results presented in this paper may have important consequences for the mechanical behavior of welded AZ31B alloys.It can be argued that the superior mechanical strength of these alloys at elevated temperatures is related to the better thermal and mechanical stability of the grain-boundary phase compared to Mg 17A 12[19–21].4.1.Electrochemical testsThe polarization curves of the investigated alloy in 3.5%NaCl saturated with Mg(OH)2solution are shown in Fig.3.Magnesium alloys normally start pitting corrosion at a potential slightly more negative than their corrosion potentials in a NaCl solutions.On a polarization curve obtained by the potentiody-namic method,and starting potential scanning from a cathodic potential,the pitting potential E pitt is usually indicated by asud-Fig.3.Polarization curves of GTA welded AZ31B Magnesium alloy in 3.5%NaCl saturated with Mg(OH)2.den drop in cathodic current which is immediately followed by anodic current around the corrosion potential.At the pitting potential,the initiation of pitting corrosion is connected with visible hydrogen evolution from the corroding electrode surface during the measurement.The E pitt is an important electrochem-ical parameter.It indicates the tendency to localized corrosion.A more positive value of E pitt means less likely localized cor-rosion [22].Some small pits were observed to initiate at the critical potential and at potentials more positive than the critical pitting potential,while no pits were observed at more negative potentials.It was also noticed that hydrogen evolution occurred mainly of nucleated pits.All polarization curves did not exhibit Tafel behavior in the anodic branch,while cathodic branch represented linear Tafel behavior for potentials more negative than E pt.Corrosion poten-tial values recorded for investigated alloys were quite similar and are situated in the range from –1611mV for the weld metal to –1595mV for heat affected zone (HAZ)(Table 2).In all cases,E corr was slightly more positive than E pt and both values showed similar trends at different zones.Always E pt was located within the linear region around E corr .The minimum difference between E corr and E pt 41mV was observed for the weld metal.For base metal the difference was equal to 101mV .The cathodic Tafel slope b c was measured from the Tafel region of cathodic branch of polarization curves.There were no distinct differences between b c values obtained for the base metal and HAZ,whichTable 2Corrosion rate of GTA welded AZ31B Magnesium alloyE corr (mV)E pt (mV)b c (mV)I corr(␮A/cm 2)E pt −E corr (mV)Corrosion rate (mpy)BM −1601−150019335.54101124.3HAZ −1595−152018936.5275127.7WM−1611−157034274.6441261.9214G.Ben-Hamu et al./Materials Science and EngineeringA 452–453 (2007) 210–218Fig.4.Corrosion rate(∼1/R p)from EIS measurements and Corrosion rate from linear polarization for AZ31Weldment in3.5%NaCl saturated with Mg(OH)2. indicates on the same character of hydrogen evolution mecha-nism on the surface.Cathodic Tafel slopes varied from189to 193mV.For the weld metal,the cathodic Tafel slope was higher then another zones342mV.This indicates that the character of hydrogen evolution mechanism on the surface of welding zone was different than the base metal and HAZ.The lowest corrosion current density was noticed for the base metal,whereas the worst corrosion resistance in3.5%NaCl saturated with Mg(OH)2solution was measured for the weld metal.The rolled AZ31B alloy showed higher corrosion resistance than welded and HAZ alloys due to presence of large quantity of Al–Mn phase.Corrosion behavior of Mg–Al alloys depends significantly on the size and distribution of second phases in the microstructure[23].In addition,linear polarization(L.P)and electrochemical impedance spectroscopy(EIS)tests were used in this work in order to understand the effect of the welding process in the corrosion resistance of rolled AZ31B magnesium alloy.Fig.4shows the effect of welding on the corrosion rate of rolled AZ31B alloy.As shown in thefigure,the weld metal (WM)exhibits the lowest corrosion resistance compared to the rolled metal and the heat affected zone(HAZ).Due to the electrochemical tests results,in order to under-stand the effect of welding process on the corrosion behavior of welded AZ31B alloy;this work employed the SKPFM tech-nique to measure the V olta potential of second phase particles present in an rolled and welded AZ31B magnesium alloy.4.2.Potential difference4.2.1.Rolled-base metal(BM)Fig.5shows an SEM micrograph of the surface of rolled magnesium alloy AZ31B-H24,which exhibits one phase present in this alloy:Al8Mn5.This phase is distributed homogeneously at the surface.4.2.1.1.Al8Mn5intermetallics.Fig.5shows a few Al8Mn5 intermetallics as identified by EDXS analyses.The spectrum indicates(not provided)that the intermetallic contains Al,Mn and Fe.The at.%concentration calculated from the spectrumare Fig.5.SEM micrographs of the microstructure of rolled AZ31B-H24alloy. 56.2%for Al,40.9%for Mn and2.1%for Fe.They correspond to an Al/Mn ratio of1.37,which is close to the stoichiometric ratio(1.6)of the Al8Mn5intermetallics observed in magnesium alloys[24].The presence of Fe is explained by the possibility of partial substitution of Al or Mn with Fe due to the presence of a small quantity of Fe(0.005wt.%)in the alloy composition.Fig.6(a)is the V olta potential map of the same area and Fig.6(b)is the V olta potential line scan across one of the Al8Mn5 intermetallics in Fig.5.The V olta potential map exhibits a strong contrast between the phase and the matrix.The measurements of V olta potential difference might be affected by the height difference between the intermetallics and the matrix[12].However,the potential differences measured for rolled magnesium alloy AZ31B-H24 are rather large(ranging from about100mV to a few hundreds millivolts).Therefore,it can be considered that the height dif-ference between the second phase and the matrix gives only a minor contribution to the V olta potential difference between this phase and the matrix.The main contribution to the potential dif-ferences results from the different composition of the phases as compared to the matrix.This was already proven by previous work on aluminium and magnesium alloys[12,25–27].As can be seen in Fig.6(a),one Al8Mn5intermetallic exhibits a higher potential relative to the matrix.The V olta potential difference relative to the magnesium matrix is162.05mV for the Al8Mn5intermetallics.The existence of a positive potential difference suggests that this intermetallic may have a cathodic behavior relative to the matrix.This is explained by the cathodic behavior of Al and Mn relative to Mg[28].Moreover,this type of intermetallic contains Fe that is strongly cathodic relative to Mg[28].Table3reports the average V olta potential difference,and its standard deviation for20Al8Mn5intermetallics.All Al8Mn5 Table3V olta differences for second phase particles in GTA welded AZ31B alloy Particle BM HAZ WMAl8Mn5214.38±16.57110.63±20.34404.68±30.83 Mg32(Al,Zn)49––471.89±15.78G.Ben-Hamu et al./Materials Science and Engineering A 452–453 (2007) 210–218215Fig.6.V olta potential map(a)and V olta potential line scan(b)for an Al8Mn5intermetallic in rolled AZ31B alloy(BM).Fig.7.SEM micrographs of the microstructure of AZ31B heat affect zone (HAZ)alloy.intermetallics present a higher potential relative to the matrix with an average value of214.38mV.Therefore,the Al8Mn5 intermetallics may behave as local cathodes when rolled AZ31B-H24magnesium alloy is immersed in an aggressive solution.The standard deviation is rather low,i.e.16.57mV.This is possibly caused by small changes in the Mn and Fe content measured for the Al8Mn5intermetallics,which might affect the potential dif-ferences,as previously observed for aluminium alloys[12].The maximum value of the potential difference(295.64mV)indi-cates that a number of Al8Mn5intermetallics present very high potential differences with the matrix.This is probably associ-ated to the intermetallics containing relatively high amounts of Fe,in accordance with the results of Lunder et al.[29].4.2.2.Heat affected zone(HAZ)Fig.7shows an SEM micrograph of the surface of HAZ in GTA welded magnesium alloy AZ31B,which exhibits one phases present in this zone:Al8Mn5.This phase distribute homo-geneous in the surface.4.2.2.1.Al8Mn5intermetallics.Fig.7shows a few Al8Mn5 intermetallics as identified by EDXS analyses.The spectrum indicates(not provided)that the intermetallic contains Al,Mn and Fe.The presence of Fe is explained by the possibility of partial substitution of Al or Mn with Fe due to the presence of a small quantity of Fe in the alloy composition.The phase present in this zone is not influenced by the welding process.Fig.8(a)is the V olta potential map of the same area and Fig.8(b)is the V olta potential line scan across one of the Al8Mn5 intermetallics in Fig.7.As can be seen in Fig.8(a),the Al8Mn5intermetallics exhibit a higher potential relative to the matrix.The V olta potential dif-ference relative to the magnesium matrix is93.99mV for one of the Al8Mn5intermetallics.The existence of positivepotential Fig.8.V olta potential map(a)and V olta potential line scan(b)for an Al8Mn5intermetallic in HAZ AZ31B alloy.216G.Ben-Hamu et al./Materials Science and EngineeringA 452–453 (2007) 210–218Fig.9.SEM micrographs of the microstructure of AZ31B welding zone (WZ)alloy.difference suggests that this intermetallic may have a cathodic behavior relative to the matrix.This is explained by the cathodic behavior of Al and Mn relative to Mg [28].Moreover,this type of intermetallic contains Fe that is strongly cathodic relative to Mg [28].Table 3reports the average V olta potential difference,and its standard deviation for 20Al 8Mn 5intermetallics.All Al 8Mn 5intermetallics present a higher potential relative to the matrix with an average value of 110.63mV .Therefore,the Al 8Mn 5intermetallics may behave as local cathodes when fusion zone in welded magnesium alloy AZ31B is immersed in an aggressivesolution.The standard deviation is rather high,i.e.20.34mV .This is possibly caused by small changes in the Mn and Fe content measured for the Al 8Mn 5intermetallics,which might affect the potential differences,as previously observed for alu-minium alloys [30].The difference between the V olta potential results for the BM and the HAZ depend on the welding pro-cess.The same phase is present in both these areas,Al 8Mn 5,presenting a high potential difference to the magnesium matrix.However,the Al 8Mn 5intermetallics in the BM present higher potential difference due to the high residual stresses between the phases and the matrix (different crystal structure)caused by the rolling process.Probably,during the welding process the re-heating of the metal relives this residual stress and the potential decreases.In the other hand the Al 8Mn 5particles dissolution due to the welding process and more level concentration gradi-ent in the HAZ around particles.The different of concentration influenced on the potential.Furthermore,the difference in elec-trochemical results is caused by the change in the microstructure of these zones.The different average grain size (BM −8.12±3.22␮m and HAZ 32.42±16.87␮m)also effected the corrosion behavior of the welded AZ31B magnesium alloy.4.2.3.Weld metal (WM)Fig.9shows an SEM micrograph of the surface of weld metal in the GTA welded magnesium alloy AZ31B,which exhibits three phases present in this zone:Al 8Mn 5,Mg 32(Al,Zn)49and Mg 2Si.The direction of the three phases is similar to the weldingdirection.Fig.10.SEM micrograph (a);V olta potential map (b);V olta potential line scan (c)for an Al 8Mn 5intermetallic in AZ31B welding zone.G.Ben-Hamu et al./Materials Science and Engineering A 452–453 (2007) 210–2182174.2.3.1.Al8Mn5intermetallics.Fig.10(a)is an SEM micro-graph of the surface of weld metal in the GTA welded magnesium alloy AZ31B showing two Al8Mn5intermetallics as identified by EDXS analyses(points1and2).Fig.10(b)is the V olta potential map of the same area and Fig.10(c)is the V olta potential line scan across one of the Al8Mn5intermetallics in Fig.10(a)—point1.As can be seen in Fig.10(b),the two Al8Mn5intermetallics exhibit a higher potential relative to the matrix.The V olta poten-tial difference relative to the magnesium matrix is436.39mV for one of the Al8Mn5intermetallics,as shown in Fig.10(c). The existence of positive potential difference suggests that this intermetallic may have a cathodic behavior relative to the matrix. This is explained by the cathodic behavior of Al and Mn relative to Mg[28].Moreover,this type of intermetallic contains Fe that is strongly cathodic relative to Mg[28].Table3reports the average V olta potential difference,and its standard deviation for20Al8Mn5intermetallics.All Al8Mn5 intermetallics present a higher potential relative to the matrix with an average value of404.68mV.Therefore,the Al8Mn5 intermetallics may behave as local cathodes when the fusion zone in welded magnesium alloy AZ31B is immersed in an aggressive solution.The standard deviation is rather high,i.e.30.83mV.This is possibly caused by small changes in the Mn and Fe content measured for the Al8Mn5intermetallics,which might affect the potential differences,as previously observed for aluminium alloys[30].4.2.3.2.Mg32(Al,Zn)49intermetallics.Fig.11(a)is an SEM micrograph of the surface of weld metal in the GTA welded mag-nesium alloy AZ31B showing two Mg32(Al,Zn)49intermetallics (points1and2)as identified by EDXS analyses.The spectrum indicates(not provided)that the intermetallic contains Mg,Zn and Al.The at.%concentration calculated from the spectrum are39%for Mg,42%for Zn and19%for Al.This corresponds to a Mg/Zn and Mg/Al ratio of0.95and2.1,respectively,which is close to the stoichiometric ratio(1and2,respectively)of the Mg32(Al,Zn)49intermetallics observed in magnesium alloys [31].As can be seen in Fig.11(b),the Mg32(Al,Zn)49intermetallics exhibit a higher potential relative to the matrix.The V olta poten-tial difference relative to the magnesium matrix is408.32mV for Mg32(Al,Zn)49intermetallics,as shown in Fig.11(c).The existence of positive potential difference suggests that thisinter-Fig.11.SEM micrograph(a);V olta potential map(b);V olta potential line scan(c)for a Mg32(Al,Zn)49intermetallic in AZ31B welding zone.。