Effect of crystallographic texture and twinning on the corrosion behavior of Mg alloys:A review

Ehsn Gershi ,Rez Alizdeh,? ,Terence G.Lngdon

a Department of Materials Science and Engineering,Sharif University of Technology,Tehran,Iran

b Materials Research Group,Department of Mechanical Engineering,University of Southampton,Southampton SO17 1BJ,United Kingdom

Abstract Magnesium and its alloys have gained significant popularity due to their light weight and their potential for use as bioresorbable materials.However,their application is limited in practice due to their relatively poor corrosion resistance.Several methods are available for improving the corrosion resistance of Mg alloys for bio-applications such as using different coatings,alloying,and modifying the microstructural parameters such as the grain size and the crystallographic texture.This review provides a comprehensive summary of the effects of crystallographic texture and twinning,as one of the most important deformation mechanisms of Mg and Mg alloys,on the corrosion behavior.Regarding the crystallographic texture,it is shown that theoretically the basal planes should exhibit a lower corrosion rate but in some cases,such as when there is a galvanic effect or when corrosion film control the overall corrosion behavior,different results may take place.Also,there are contradictory results concerning the effect of twinning on the corrosion behavior.Thus,in some cases twinning may provide preferential sites for corrosion due to the higher energies of atoms located in the twin region by comparison with normal atomic positions in the crystalline lattice whereas there are also other examples where experiments show that twins produce more protective film than in the surrounding matrix.

Keywords: Bio-materials;Corrosion;Magnesium;Texture;Twinning.

1.Introduction

In the last two decades,magnesium alloys have been widely investigated for different applications (as in the automobile industry,metallic implants,electronic devices,etc.)due to their special properties,where the most important are the low density (1.738 g/cm3),high specific strength,good cast-ability and weld-ability,and reasonable cost.However,poor corrosion resistance of Mg alloys remains an unsolved issue that limits the application of these alloys in applications which need considerable corrosion resistance.

One of the main applications of Mg and Mg alloys which is almost compatible with their poor corrosion resistance,and is of special interest in recent years,is their application as implants in medicine [1].The regenerative medicine is one of the forefront research fields in healthcare that is now becoming increasingly important as a consequence of the aging and overall longevity of the world’s population.In this connection,metallic implants are required for the replacement of tissues having a structural function such as bone and for supporting high stresses where polymeric replacements are not sufficient With the aim of addressing the disadvantages of permanent metallic implants in the human body,where secondary surgery is often needed to remove the implants from the patient,Mg alloys have gained significant attention in recent years because of their use as biodegradable implants and their potential for providing several favorable properties [2,3].

In practice,there is a long history of using Mg in medical applications because of their biocompatibility and biodegradability [4].Mg is the lightest structural metal with a density of 1.74 g/cm3and an elastic modulus of～45 GPa,which is only slightly higher than cortical bone.However,pure Mg has a low yield strength of～30 MPa and a very fast corrosion rate of～2.89 mm/year [5] with a corrosion potential of about-2.3 V versus standard hydrogen electrode (SHE) [6] and a hydrogen evolution rate of～56.5 ml/cm2/day in simulated body fluid (SBF).By comparison with the desired characteristics for resorbable bone fixtures such as a compressive strength in the range of 130-180 MPa [7] (similar to compressive strength of the cortical bone,to minimize the stress shielding effect),an elongation to failure higher than～10%,a corrosion rate less than～0.5 mm/year in SBF at 37 °C and a hydrogen evolution of less than 0.48 ml/cm2/day [8],it appears that pure Mg is too soft and corrodes too quickly within the human body thereby leading to excessive H2production and a local increase in pH that may cause death of the tissue cells.

It follows therefore that,in order to use Mg in biodegradable applications including reabsorption,it is necessary to develop new Mg alloys with tailored mechanical properties and having appropriate corrosion resistance in physiological environments.Several approaches have been used to overcome these limitations including alloying [9-11] and modifying the microstructure by thermo-mechanical processing [12].In practice,deformation of the materials by thermo-mechanical processes results in beneficial microstructural changes which help to improve the mechanical and corrosion properties.These include grain refinement attaining a uniform distribution and refinement of the secondary phases,and also development of some special crystallographic textures [13-16].These special textures may lead to an increased corrosion resistance by,for example,placing planes having lower surface energy,such as basal (0001) planes,more abundant at the outer surface of the deformed samples [17].In addition,a material with smaller grain size can show better corrosion resistance due to the denser corrosive product layer,formed in the early stages of exposure to the corrosive medium.Such layer can protect the substrate better and thus the overall corrosion rate would be less,though it was higher in early stages in comparison with a material with coarser grain size [18].Thus,the role of microstructural features like grain size and crystallographic texture in controlling the corrosion rate of Mg alloys is complicated and needs more clarification While the effects of grain refinement on the corrosion behavior of Mg alloys have been reviewed [19],there are no comprehensive reports or reviews on the effects of the crystallographic texture.

In addition to development of a specific crystallographic texture and also grain refinement thermomechanical processing,especially at low temperatures,generally results in activation of twinning mode of deformation in Mg and Mg alloys,due to the lack of enough active slip systems in the hcp structure.Twinning,as one of the lattice faults,not only has a great influence on the mechanical properties of metals but also can affect the corrosion behavior [12].After deformation processing as in rolling and extrusion,twinning is introduced to regulate the texture and microstructure [20,21].Twinning may also assist slip by the development of new crystallographic orientations as a result of lattice rotation during twinning.The {102} extension twins and {101} contraction twins with rotation angles of～86 °and～56 ° in respect to the matrix,respectively,are the most common twin modes in magnesium alloys [22].The energy of a twin depends strongly on its orientation relationships,but as a general rule,the energy of atoms placed in a twinned region are more than those placed in normal lattice sites.This means corrosion can occur more readily in the twinned regions and thus the presence of twins in the microstructure can affect the corrosion behavior of Mg alloys.However,as discussed in the following,the effect of twining on the corrosion resistance of Mg alloys is more complicated and needs consideration of several factors.

For this reason,the role of crystallographic texture and twinning on the corrosion behavior of Mg alloys needs a much greater clarification and accordingly this limitation is specifically addressed in the present review.

2.Background

2.1.Texture development in Mg alloys

While the crystallographic texture is not usually considered or discussed for as-cast Mg alloys,it is of particular interest and importance for the wrought products.Depending on the mode of plastic deformation,and also the dominant slip system,special textures develop in Mg and Mg alloys after plastic deformation using processes such as rolling [21,23],extrusion [24,25] and also severe plastic deformation (SPD)techniques [26,27].Regarding extrusion as one of the most conventional forming techniques for Mg alloys,after extrusion there will be a bar with two directions,the transverse direction (TD) and longitudinal (LD) or extrusion direction(ED).It has been reported that a conventional texture in extruded samples consists of basal planes aligned parallel with the ED.The extrusion parameters such as the speed (or rate)of extrusion,the temperature and also the extrusion ratio can affect the final texture of the extruded bar.For example,it has been reported that extruding Mg-Gd-Y-Zr alloys using high extrusion ratios (19:1) will lead to the development of a special and unusual texture in the extruded bars,where the basal planes are aligned perpendicular to the extrusion axis[15].This is contrary to the usual texture reported for most Mg alloys after low-ratio extrusion,where the basal planes align parallel with respect to the extrusion axis.In addition to deformation parameters,changing composition by the addition of alloying elements such as Ca [28],Al [29],Zn [30],Mn [31],Sr [32] and/or rare earths [33,34] has also been reported to modify the extrusion texture.The alloying elements can weaken or strengthen some specific orientations in the Mg alloys.For example,it was reported that through the addition of Al to an Mg-Mn alloy the basal texture intensity was decreased [35].

2.2.Twinning in Mg alloys

Due to the lack of sufficient active slip systems,twinning is one of the most important deformation mechanisms of Mg and Mg alloys,especially at lower deformation temperatures where there is a very large difference between the critical resolved shear stress (CRSS) for basal slip and for other slip modes such as prismatic and pyramidal.Twinning,as a secondary mode of deformation,contributes less in the total strain by comparison with slip,but instead it enables slip to occur more homogenously by providing new crystallographic orientations as the result of crystal rotation during twinning.Both mechanical [22] and annealing [36] twins have been observed in Mg alloys.

Atoms are more active on the twin region in comparison with atoms placed on normal lattice sites.Also,different orientation of the lattice in the twinned region in comparison with the surrounding matrix results in some differences in properties which are orientation dependent (like corrosion as discussed in this paper).Noting that usually a considerable volume fraction of twins exists in the microstructure of Mg alloys after plastic deformation,especially at low temperatures,the effect of twinning on the corrosion behavior is clearly important.However,it should be noted that the twin boundaries are not always considered as high energy boundaries.In FCC metals,it is possible for the twin boundary to align parallel with the twin plane.In this case,a “coherent twin boundary” is formed,which is an extremely low energy special boundary.However,this can happen mainly in the case of the annealing twins and not the mechanical twins,which are the main observed twining mode in Mg alloys.

2.3.General corrosion mechanism of Mg alloys

The firs reason why Mg has a relatively low corrosion resistance is that it has a high electrode potential (about-2.37 V versus standard hydrogen electrode [6]) which makes this element very active and allows it to be susceptible to corrosion even when there is no oxygen.The second reason is that the film (e.g.oxides and hydroxides) which form on the corroded surface of Mg in corrosive medias are usually weak and cannot protect the underlying surface because they are highly defective and soluble in most aqueous solutions [37].In physiological aqueous environments such as the human body,the corrosion product layers contain MgO(inner thick layer) and Mg(OH)2(external heavy layer).The former cannot protect the surface as it is not sufficiently dense and the Pilling-Bedworth ratio (molar volume of the elementary unit cell of MgO/molar volume of the Mg metal) is less than 0.8.Also,the latter (Mg(OH)2) is usually dissolved in these environments [7].

Oxygen plays an important role in the atmospheric corrosion,however,in aqueous medias like physiological environments Mg is degraded by an electrochemical reaction with water and in this case Mg(OH)2and hydrogen gas (H2) is produced.Also,it should be noted that micro-galvanic coupling between the cathodic and anodic areas in aqueous environments is the reason for the corrosion attack.The whole reaction in the flowing form is obtained:

However this reaction can be divided into three separate reactions [38] as in the anodic reaction

the cathodic reaction

and the film formation reaction

In the human body,due to the presence of large amounts of different ions (oxygen,electrolyte and also protein),Mg is expected to be corroded severely,considering its high electrochemical potential.As a result,ions migrate from the Mg surface to the surrounding body fluid Normally thick Mg(OH)2layers are deposited on the Mg samples after the occurrence of corrosion (Eq.(3)) [7].

2.4.Surface energy

The term “surface energy” is used to describe the increase in energy at the surface of the materials as a result of the formation of broken bonds when new surfaces are created.Accordingly,the surface energy may be described as the difference in energy of atoms on the surface by comparison with the atoms in the bulk.

According to the definition of the surface energy given above,in addition to the bonding strength,it will depend also on the numbers of atomic bonds on the proposed surface.In metals with an hcp crystal structure,like Mg,the planar density of the basal planes is the highest (1.13×1019atoms/m2)and after that there are the(110)plane (6.94×1018atoms/m2) and then the(100)plane (5.99×1018atoms/m2)[17].According to the model based on empirical electron theory,the surface energy of Mg on the (0001),(100)and(110)planes is 1.808,1.868 and 2.156 eV/nm2,respectively[39].

Such differences in the surface energy values can obviously affect the corrosion behavior of Mg and Mg alloys in corrosive medias.In theory at least,an atom placed on a plane with high planar density has a high coordination number and,thus,less available broken bonds when placed on the exterior surfaces of the metal.Hence,metallic ions are expected to be dissolved with more difficult from a plane with high planar density.In this respect,the basal (0001) planes in Mg should show less tendency to be corroded in different solutions by comparison with the prismatic and pyramidal planes.

2.5.Dissolution rate

The dissolution rate of Mg or its corrosion rate can be correlated to its surface energy.According to detailed reports[40,41],the electrochemical dissolution rate (Ia) may be expressed as:

In this equationnis the number of electrons involved in the electrochemical reaction,kis a reaction constant,F is the Faraday constant,Ris the universal gas constant,Tis the absolute temperature,Eis the electrode potential,αis a transit coefficient which is based on using surface energy in the calculations instead of the activation energy,and finallyQis the activation energy required for the escape and dissolution of the metallic ions in the solution.Thus,Qcan simply be related to the metal surface energy (Es) as Q=Q0-Es(where Q0is a constant).It is believed that the planar density of the planes affects the ease with which ions escape from the surface of the metal and thus theQvalue.The activation energy for dissolution of metallic ions is higher for the more compact planes.In other words,atoms on a plane with a low surface energy are expected to dissolve with more difficult and thus more slowly.

Referring again to Eq.(5),and with the aim of comparing the electrochemical dissolution rate for different planes with the help of this equation,thenandkparameters can satisfactorily be considered the same for different planes.Thus,assuming the sameα,E,nandkvalues for different planes,the electrochemical dissolution rates of the(1010)andplanes,respectively,can be written in the following form by comparison with the dissolution rate of the basal plane

It is now clearly inferred that the dissolution rate of the prismatic (1010) and (1120) planes are much larger than the dissolution rate of the (0001) basal plane,due to the higher surface energy of these planes.Also,it can be seen that the difference in surface energy exponentially affects the relative dissolution rate.

3.Effect of crystallographic texture on the corrosion behavior of Mg alloys

The theoretical dissolution rates of the prismatic {1010}and {110} planes are about 18-20 times higher than for the (0001) basal plane [17,40,42-46].Furthermore,this behavior (having higher corrosion rates for the less compact planes) is not limited to hcp metals and has been reported also for other structures such as fcc metals [47-49].However,there are also other parameters which may affect the overall behavior.For example,experimental results show that for a surface composed of both prismatic and basal planes,the corrosion resistance can be decreased in comparison with a surface composed only of prismatic planes and this effect has been attributed to galvanic corrosion [50-52].In galvanic corrosion one metal corrodes preferentially when it is in electrical contact with another metal.This process can occur also in a singular metal (or alloy) in which there is a difference in electrical potential from different constituents within the crystalline matrix such as grains having different orientations.For example,a comparison of the orientation of the grains in the transverse and longitudinal surfaces (TS and LS,respectively) of an as-extruded Mg-3Al-1Zn (AZ31) alloy is shown in Fig.1,where the data are collected by the electron back scattered diffraction (EBSD) method [50,51].Depending on whether the exposure surface is TS or LS,it can be anticipated that different corrosion behavior can be observed,due to the different textures of the grains in these two samples.Corrosion may occur more easily in the {1010} and{110} oriented grains when galvanized with the (0002) oriented grains,in comparison with the samples which contain only{1010}and{110}oriented grains.This means that having differently oriented grains in one sample simultaneously may lead to a deterioration in the corrosion resistance due to the galvanic effect between the basal and prismatic planes.

In another study on the same AZ31 alloy[51]it was shown that not only the corrosion behavior of the base material was dependent upon the crystallographic texture but also the protectiveness of the corrosive film depended on the orientation of the planes.For the surface with high density of the {1010}and {110} prismatic planes,it was observed that the formed corrosive film in a phosphate buffer saline (PBS) solution degraded very slightly (after 48 h immersion) only at some specific regions.This means that the film maintained a good protection for the underlying substrate.Nevertheless,in the case which a combination of {0002} basal planes together with{1010} and {110} prismatic planes were present on the surface,the degradation of the product film happened more extensively and thus a decrease in corrosion resistance was observed.The effects of crystallographic texture on the development of passive film are discussed in more detail in Section 3.1.

As was discussed earlier,the most important impact of the crystallographic orientation is on the anodic dissolution of metals.However,it has been shown that different crystallographic planes with different energies can also influence the cathodic hydrogen evolution process (Eq.(3) in Section 2.3).This means that a surface with a higher energy leads to more water adsorption and thus more hydrogen evolution.However,the surface energy affects the hydrogen reaction and metal dissolution reaction differently and in fact,the effect is minor in the case of hydrogen reaction [40,53].The orientation dependence of the current density is almost negligible in the cathodic region (in the polarization curve) [44].

The polarization curve measurements also indicate that the open circuit potential (OCP) of the (0001) plane shows additional positivity in comparison with that of the(100)and(110)planes.The observed differences in OCPs has been related to the formation of micro-galvanic cells between grains with different orientations which can result in preferential dissolution in(1010)and(110)orientated grains [40].

Fig.1.EBSD data of the as-extruded AZ31 alloy bar:The EBSD orientation maps are presented in (a,b) and the {0002} pole figure are presented in (c,d).The data have been collected for both the longitudinal (a,c) and transverse (b,d) surfaces [50].

As already noted,the crystallographic texture can significantly affect the corrosion behavior of magnesium alloys.However,in many cases it is difficult to completely isolate the effect of texture from other metallurgical parameters such as the effect of intermetallic phases,grain size,film formation on the surface and the dislocation density.For example,it was reported that the galvanic effect due to the presence of the particles may dominate the influence of the crystallographic texture,an effect which results in almost similar corrosion behaviors on the rolling surface and cross-sectional surfaces[41].In the following,cases are discussed where the main focus is on the effect of crystallographic texture or twining on the corrosion resistance.

3.1.Film formation

In general,the overall corrosion behavior of a metal depends on the corrosion behavior of both the substrate and the film It has been shown that different oxide film can markedly alter the corrosion behavior of Mg alloys [37,54-59].The process of film formation on a metal surface depends not only on the environment facing the metal but also on the metallurgical properties of the substrate including the grain size and the crystallographic texture,where the latter example is of particular interest in the present paper.

Crystallographic texture can affect both the corrosion of the substrate and also characteristics of the corrosion film and understanding the sum effect on the overall corrosion rate is complicated and needs a consideration of several facts:(1)after formation of a stable and protective corrosion film the orientation differences between different grains would not affect the corrosion,however,if the corrosion film does not possess enough protectiveness so the crystallographic texture of the substrate would still affect the corrosion,(2) the composition of the corrosion film does not depend on the grain orientation,and (3) the compactness,thickness and resistance of the corrosion film varies from grain to grain,because of the different electrochemical activities of grains with different orientations.By decreasing the corrosivity of the solution (or increasing the passivity of the metal),the surface film would be thinner,more compact,more resistant and more uniform (and so less different),and thus their capability to cover the influence of grain orientation would be improved.

In corrosive solutions,different corrosion behavior of the substrate grains(due to different crystallographic orientations)is considered as the main factor and the difference in properties of the corrosion film are believed to further enlarge such differences.On the other hand,in less aggressive solutions or strong passivating solutions,the corrosion film are thin,well protecting the substrate,with small differences between the film formed on different grains.In such conditions,the corrosion of the substrate grains is considerably reduced and thus the effects of grain orientation on the corrosion would be limited.In a solution containing a strong passivator such as chromate together with some aggressive species like chloride,although a very thin and compact film is expected to be formed (which in normal conditions should eliminate the effect of crystallographic texture),a strong dependency on grain orientation has been observed due to the fact that the Cl-ions can amplify localized small differences in the film and the substrate grain.

The chloride ions locally tend to attack more the special parts of the film which are formed on crystallographic defects of the substrate Mg alloy,such as defects like dislocations,twins and grain boundaries.In such solutions,the competition between the corrosive and passivating species can result in special corrosion morphologies like the filiform damage[60] which will be discussed in another section.

It was reported that the film developed on basal planes of pure Mg is slightly thinner but perhaps slightly more compact in comparison with the film formed on the prismatic plane in both an Mg(OH)2saturated solution and a 1 M NaOH solution [60].The formation of thinner oxide film on basal planes was also reported for other hcp metals such as Ti and Zr.In the case of Zr,the oxidation rate of the basal plane was slower than that of the prismatic planes,and this difference was attributed to the different oxygen diffusion kinetics through the high packed and low packed planes [61,62].In another study on pure Mg [63],a notable dependency of the thickness of the oxide film on the crystallographic texture was reported in different solutions.In all studied environments,the thinnest and thickest oxide film were formed on the basal planes and the low index planes,respectively,while the thickness of this oxide was inversely proportional to the corrosion rate.

In addition to film thickness,the nature of the film may be different also on grains with different crystallographic orientations.Thus,it was reported that the Pilling-Bedworth (PB)ratio,which describes the effectiveness of an oxide film depends on the crystallographic orientation and increases with atomic density in pure Mg.The value of PB for the sample with the higher amount of basal plane or (0002) oriented grains is higher than for other samples.This suggests that the stability of the corrosion film and the corrosion resistance in the basal planes would be improved [45].Also,an incorporation of an MgO and Mg(OH)2passive film (in a 3.5 wt.%NaCl solution) on pure Mg surfaces with low-index planes is facilitated and could enhance the corrosion resistance by forming a more effective barrier against the penetration of chloride ions [64].

Finally,it can be concluded that in the surfaces containing basal planes the formed oxide film is always thinner by comparison with that formed on the pyramidal or prismatic planes but there is a contrary result about their stability and effectiveness.In Table 1,the effect of texture on the film formation is summarized for the corrosion of Mg and its alloys in corrosive media.In this summation,the effect of texture on the corrosion film (given in the fourth column from the left) is summarized for different materials (firs column) in different solutions(second column)and then evaluated by different methods (third column) from the available references(last column).

Table 1Summary of texture effect on the corrosion film of Mg and its alloys in different corrosive media.

Table 2Summary of the effect of twinning on the corrosion resistance of the Mg alloys.

3.2.Filiform corrosion

Filiform corrosion,which is made as the result of active galvanic cells across the surface,is a type of corrosion usually observed in alloys like steel,aluminum alloys and Mg alloys under certain conditions [65].This kind of corrosion is quite common in magnesium alloys exposed to air and NaCl solution[66-70].For example,Fig 2.shows the orientation dependent morphologies of corrosion in pure Mg exposed to 0.6 M NaCl (after the samples were cleaned with CrO3).The filiform corrosion was observed on the low index plane {010}and not significantly on the planes with high index [63].Localized corrosion in the form of rapid propagating filament is especially probable in solutions which contain simultaneously chloride and dichromate ions [71].In Mg alloys,the pitting and filiform type corrosion usually occur at the same time.The filiform corrosion is believed to initiate from the pits on the surface and then extend along the active planes.In corrosion of Mg alloys,the hydrogen plays a more vital role in comparison with the oxygen (unlike many other alloys such as steels),as was described in Section 2.3.The filiform corrosion of magnesium is due to the different concentrations of oxygen at the filament head compared with the filament tail,and this suggests a filiform corrosion model for the Mg alloys [70].It has been reported that the filiform corrosion in Mg alloys is strongly orientation dependent [71,72].

As indicated in the previous section,the simultaneous presence of dichromate and chloride ions in a solution makes magnesium alloys susceptible to localized corrosion which appears in the form of filament that can propagate at very high rates.These filament are called filiform corrosion and are shown in Fig.3 [71].The corrosion behavior of pure magnesium in 0.01 M Cl-solutions with small additions of dichromate (10-4M) results in filiform corrosion.Based on the experimental results,this localized pitting has been shown to be dependent on the crystallographic texture and the minimum resistance to localized corrosion was recorded for the (0001)plane [72].In practice,the Mg basal plane exhibits corrosion susceptibility at its open circuit corrosion potential.However,the(100)and(120)planes are passive under open circuit conditions and localized attack is anticipated to initiate only upon polarization to potentials somewhat anodic to their open circuit potentials.The critical pitting potential for Mg single crystals increases in the order of (0001)＜(100)＜(120).The localized attack on all three surfaces was shown to be directional dependent [72].It should be noted here that these results are showing the crystallographic dependency for the pitting susceptibility of magnesium single crystals in a specialenvironment (0.01 M NaCl+10-4M Na2Cr2O7) considered specifically for increasing pitting.This is different from the general corrosion behavior described in section 3,in normal Cl-containing solutions (without the dichromate ions).The observed differences between the pitting susceptibility of the basal plane and prismatic planes in Mg single crystals [72] is similarly reported for corrosion of polycrystalline magnesium in the same solution.Looking at the surface of the polycrystalline magnesium samples after immersion tests revealed that the pitting indications were found only on grains oriented in such a way that the basal planes were exposed to solution[71].The morpholgy of the corrosion in these grains was as filament which propagated similar to those observed on the Mg single crystal (0001) surfaces.

Fig.2.Different morphologies of corrosion in various crystallographic orientations.(a,c) higher index grains with terraces,(b) low index plane with filiform corrosion [63].

3.3.Biocompatibility

While there are some reports on the dependency of biocompatibility on crystallographic texture in hcp metals[73-75],this effect is not well examined and the outcome is not clear in the case of Mg and Mg alloys.For example,it was reported that,regardless of the sample grain size,more cells were attached on the surface of titanium samples which contained more (0002) planes,and it was related to the different nature of the oxide film on these planes [73,74].Also,in an investigation of the effect of crystallographic texture of the other similar HCP alloys like Ti-6Al-4V on cell attachment,better cell attachment has been observed for the sample with dominant (100) texture in comparison with(120)[75].This behavior was attributed to the lower surface contact angle of the(100)orientation and accordingly to the higher wettability,as it was shown that increased surface wettability or hydrophilicity may enhance protein adsorption and cell spreading on biomaterials [76-78].By contrast,there are some reports indicating that the biocompatibility of pure Mg is less sensitive to texture [46].In this respect,there are some investigations which have studied the effect of different thermo-mechanical processing on the biocompatibility.Although these reports are not considered directly,the obtained results show indirectly that biocompatibility of the Mg alloys is not significantly affected by the texture since it is reasonable to assume that different textures are obtained after different thermo-mechanical routes [79-81].Accordingly,it can be concluded that controlling the texture may be used as a beneficial tool to control and improve the corrosion and mechanical properties of Mg and Mg alloys without too much loss in the overall biocompatibility.However,it is readily apparent that further research is required in this area.

Fig.3.(a) Observation of the propagation of the filiform corrosion in pure Mg after immersion in 0.01 M NaCl+10-4 M Na2Cr2O7 solution,obtained by optical microscopy (a),and SEM (b).Small red arrows in (a) indicate the propagation direction [71].

4.Effect of twinning on the corrosion behavior of Mg alloys

It is generally believed that atoms placed in a twinned region are more active (and thus possess more energy) than the atoms placed in normal lattice positions [41] and therefore it is expected that the existence of twins on exterior surfaces will have negative effects on the corrosion behavior.In practice,there are several studies which support this hypothesis and show an increased corrosion rate of the base Mg alloy in the presence of twins.For example,applying the equalchannel angular pressing (ECAP) process on an AZ31 alloy showed that,while the grain size hardly changed,a higher density of dislocations and twins were produced which led to a considerable decrease of the corrosion resistance [82].In another study investigating the corrosion behavior of a rolled AZ31B alloy,heat treatment at different temperatures was used to obtain different microstructures with and without twins[83].According to the results of this study,the corrosion was more serious for the highly twinned microstructures of the as-received samples and samples annealed at 200°C compared with coarse-grained samples annealed at 300 °C which contained almost no twins.Fig.4 shows the optical micrographs of these two samples [83].Also,studying the surface of the as-received samples after corrosion clarifies the role of twins in accelerating the intragranular corrosion.According to these experimental results,a high density of dislocations and twins is mainly responsible for the observed increased hardness and dissolution rate in these samples.In such conditions,the anodic dissolution is accelerated due to the local reduction of the equilibrium potential in the vicinity of dislocations.However,the main drawback of this work is that the effect of grain size was not considered.

Interestingly,there are also some reports indicating that twinning can increase the corrosion resistance.These contrary reports may occur because in different alloying systems the corrosion film have different abilities to protect the underlying matrix.For example,for an Mg-1Y alloy it was reported that extension twins result in improving the overall corrosion resistance by affecting the charge transfer resistance,film formation process and increasing the pitting potential [84].The surface energy of atoms on the {102} extension twinning plane was found to be higher than in the {0001} closely packed basal plane,due to a lower coordination number.This might indicate higher oxidation activity of the atoms on the extension twinning planes which provides a preferential formation of the oxidation film This oxide film can effectively protect the underlying Mg-1Y alloy which is equivalent to a larger charge transfer resistance,as was further confirmed later [85].

A study of the effects of twinning on the bio-corrosion of an extruded Mg-4 Zn alloy showed that the stress concentrations at the twin interface could have negative effects on the corrosion behavior [12].Nevertheless,and more importantly,there was a description of the possible formation of a compact oxide layer on the twinning plane which could improve the overall corrosion resistance.It is more evident in Fig.5,where twin containing grains (marked by arrows) show less corrosion pits compared with similarly oriented grains without twins.Thus,it was suggested that there may be an optimum volume fraction for twins to be beneficial for the improvement of corrosion resistance of Mg-4 Zn alloy [12].

Another contribution of twinning in the corrosion process can be its weakening effect on the “corrosion anisotropy”.Applying thermo-mechanical processing,as in the extrusion and rolling processes,normally causes corrosion anisotropy in Mg and its alloys,meaning that different directions (like TD,ND,RD after rolling) will exhibit different corrosion behaviors.After the rolling process,pre-straining the surfaces of the rolled plate by compression can introduce a large amount of twinning in the structure and thus influence the corrosion anisotropy between differently oriented surfaces [86].Twin boundaries are normally lower energy boundaries in comparison with the high-angle grain boundaries.For example,for pure Mg,the energy of the {102} extension twin boundary is 0.11-0.20 J/m2which is much lower than the energy of random high angle grain boundary (0.41-0.60 J/m2).It is generally believed that twin boundaries and grain boundaries can be considered as physical barriers to any corrosion attack.Consequently,the development of localized corrosion was effectively delayed in the pre-strained sample with high densities of {102} twins.Regarding decreasing the corrosion anisotropy,it should be mentioned that,after imposing the compressive stain along the rolling direction of the plate,high densities of{102}twins may be formed simultaneously in differently oriented grains.Accordingly,the favorable effect of twins on weakening the corrosion attacks would be similar in differently oriented grains,meaning that the corrosion anisotropy is decreased.In this regard,the corrosion anisotropy was significantly reduced in the as-rolled AZ31 Mg alloy with a dominant basal texture,due to the high twin density.According to the above discussion,it can be concluded that high densities of {102} twins not only can improve the corrosion resistance of Mg alloys,they can weaken the corrosion anisotropy.

Fig.4.The microstructure of the rolled AZ31B alloy after annealing at 200 °C (a) and 300 °C (b).Almost no twins can be detected in the sample after high temperature annealing at 300 °C [83].

Fig.5.The EBSD map of the Mg-4 Zn alloy after corrosion.Grains containing twins showed less corrosion pits.Some of the twins are indicated by black arrows [12].

In addition to experimental studies,the effect of twin boundaries (TB) on the anodic dissolution of Mg was also investigated theoretically [87] and it was found that twin boundaries (TB1 {101} [210],TB2 {102} [210],or TB3{103}[210]) accelerate the corrosion rate such that the accelerations of the current density is related to the TB interfacial length per area.This behavior was attributed to the fact that TBs in the microstructure increase the surface energy density.

Contrary to some reports on the great influence of twinning on the corrosion behavior,there are also some studies suggesting that twinning generally has no great influence on the corrosion behavior of Mg alloys.For example,in an investigation of a rolled and heat treated AZ31 alloy it was reported that twinning played no decisive role in the corrosion behavior [41].The effect of twining is therefore discussed in more detail in the following sections.

4.1.Direct role (galvanic effect)

The corrosion behavior of two samples having different orientations (transverse cross-section,TS,and longitudinal cross-section,LS) was compared in an as-extruded Mg-3Al-1Zn bar [50].It was shown that the corrosion attack in an orientation with a high fraction of twins (TS) was weaker than in an orientation with a low fraction of twins (LS).In some twinned regions,the activation of {102} twins caused localized galvanic corrosion but this could not significantly influence the corrosion resistance of the TS sample.This research firmly demonstrates that whenever the exterior surface is mainly composed of prismatic planes,then,the galvanic corrosion occurring between the twinned and untwinned areas can be weakened.

Fig.6.Optical microscopy micrographs of the EW75 alloy after(a)T4 treatment(solid solution treated),and(b)T3 treatment(compressed at room temperature)[88].

In another study [88],an Mg-5Y-7Gd-1Nd-0.5Zr (EW75 alloy) was compressed to obtain twins under a T3 treatment(compressed at room temperature) and this condition was compared with the solid solution treated samples with no twins (T4).Optical microscopy images of T3 and T4 samples are shown in Fig.6.It was found that in a corrosive solution the twins play a dual role.Accordingly,the microgalvanic corrosion between twins accelerates the corrosion rate and simultaneously promotes the formation of a surface film However,the formation of a compact surface film was more effective in an EW75 alloy.Thus,the twins in the EW75 alloy could improve the corrosion resistance.This dual effect was also further confirmed in another study [89] where the effect of twinning on the corrosion resistance was investigated for ZK60.The results showed that a small volume fraction of twins could decrease the corrosion resistance due to a galvanic effect while a high density of twins could improve the corrosion resistance of the material due to the preferential formation of an oxidation film (MgO).

In Table 2,the effect of twinning on the corrosion behavior is summarized.Thus,the effect of twinning on the corrosion resistance (fift column from the left) is summarized for different materials (firs column) prepared by different processes (second column),in different solutions (third column),and evaluated by different methods (fourth column) from the available references (last column).

4.2.The role of twinning shape in corrosion propagation in solutions containing dichromate

A study was conducted to examine the corrosion behavior of pure Mg in a dichromate containing solution [71].On a large-grained sample,the attack was observed to propagate at the twin boundaries.After polishing the corroded samples,defects such as twins,subtwins,and grain boundaries were visible.The results of orientation imaging microscopy indicated that the corrosion mainly propagated along the {0001}planes and in the prismatic directions.An example of the photos showing the interaction of twins and the corrosion path is presented in Fig.7 [71].It can be observed that a filiform-like corrosion,with approximate width of 1 mm,is formed at the twin in Fig.7a and has continued growth up to the intersection with the other twin.In Fig.7b it can be observed that this filiform-like corrosion has crossed the other twin and then followed it.It is interesting to note that,while the presence of twins significantly affected the corrosion path and propagation,sub-twins did not appear to have a similar effect.

Another important property of the twins which can affect the filiform corrosion is the size of the twin.An example of the interaction of the twins with filiform corrosion is presented in Fig.8.In this micrograph,which is selected from a special part of the sample to include twins with different sizes,an interaction of the corrosion filament with twins can be observed [71].It is observed that while thin twins (compared with the width of the filament appear not to affect significantly the propagation direction of the filament thicker twins change the growth direction.

Fig.7.In situ optical micrographs showing the influence of the twins on the filiform corrosion in pure Mg,after immersion in 0.01 M NaCl+10-4 M Na2Cr2O7 solution:(a) initiation and propagation of the filiform corrosion along twins,and (b) propagation of the filiform corrosion at the intersection of two twins [71].

Fig.8.Optical image showing the influence of the twin size on the severity of the interaction with the corrosion filament in pure Mg [71].

5.Summary and overall outlook

Magnesium is an active metal and the corrosion rate of Mg and Mg alloys is normally too high for most applications.There are some ways to control and decrease the corrosion rate,such as applying coatings,changing the chemical composition through the addition of alloying elements and/or by modifying the microstructure as in decreasing the grain size,introducing twins or changing the crystallographic texture.Among the possible controllable microstructural parameters,the effect of crystallographic texture and twinning on the corrosion behavior of pure Mg and its alloys has been reviewed in this paper.The most important conclusions may be effectively summarized as follows:

1.Theoretically,the basal planes have the most corrosion resistance among all the crystallographic planes.However,there are also contrary results mainly originating from the galvanic effect or the formation of more stable film on the non-basal planes which reduce the overall corrosion rate.

2.In addition to the corrosion behavior of the base material,the film formation process is also orientation dependent.The thickness and also the nature of the film depend on the crystallographic texture.In most cases,thinner but more compact film are formed on the basal planes by comparison with the less compact prismatic and pyramidal planes.

3.The effect of twinning on the corrosion behavior is complex and several results are contradictory.There are results confirming the deleterious effect of twins on the corrosion behavior but there are also other reports indicating an improved corrosion resistance in the presence of twins due to the formation of a better protective layer.In addition to this duality,there are data reporting galvanic corrosion between the twinned and untwined areas.The overall effect seems to be strongly composition dependent.In alloys with a stable protective film like Mg-Y alloys,the presence of twinning can increase the corrosion resistance.Twining was also reported to reduce the corrosion anisotropy in wrought samples.

4.The filiform-type corrosion is orientation dependent and is influence also by the presence of twins in the microstructure.The available results suggest that the corrosion filament interact differently with thin twins,thick twins and with subtwins.

Declaration of Competing Interest

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

Acknowledgment

The work of one of us was supported by the European Research Council under Grant Agreement No.267464-SPDMETALS (TGL).