Improved Corrosion Resistance of 5 XXX Aluminum Alloy by Homogenization Heat Treatment

The corrosion resistance of homogenized Al-Mg (6.5%) alloy—adding Si, Zn, Mn, and Fe (0.2%) to improve various properties—was observed. Differential scanning calorimetry (DSC) and a JMatPro simulation revealed that the optimal homogenization temperature was 450 ◦C. The homogenization was carried out at 450 ◦C for 3, 6, 12, 18, 24, and 30 h in order to view the corrosion resistance change. Corrosion resistance was analyzed by a polarization test in 3.5 wt % NaCl solution. The corrosion resistance improved with increasing homogenization time up to 24 h, but there was no change with longer time periods. To observe the reason for the change in corrosion resistance, scanning electron microscopy coupled with energy dispersive X-ray spectroscopy (SEM-EDS), X-ray diffraction (XRD), and transmission electron microscopy coupled with energy dispersive X-ray spectroscopy (TEM-EDS) analyses were performed. Precipitates containing Mg, such as Al3Mg2 and Mg32(Al, Zn)49, decreased at the grain boundary. After homogenization, the amount of Mg measured by SEM-EDS at the grain boundary decreased from 36% to 8%, while Si increased. Generally, the potential difference between the grain boundary and the grains leads to intergranular corrosion. Reduction of Mg, whose standard electrode potential is lower than that of Al, and an increase of Si, which is present in higher concentration than Al at the grain boundaries, improved the corrosion resistance of 5XXX Al alloy by reducing the intergranular corrosion.


Introduction
As the automotive industry has gradually become more sophisticated and subdivided, fuel economy regulations are becoming more stringent to address environmental issues.Accordingly, fuel efficiency improvement technology has been attracting a great deal of attention [1,2].Among ways to improve fuel efficiency, adopting lighter weight vehicles is thought to be the most promising approach because it is expected to greatly improve the fuel efficiency through changes of structure, replacement of heavy materials with lightweight materials and reduction of the number of parts.Currently, non-ferrous metals such as Al and Mg are the mainly used light metals [3].Among them, Al is 33% lighter than Fe, has higher strength and is excellent in terms of machining, thermoelectric conductivity, and low temperature brittleness [4].Among the Al alloys, the Al 5000 series is an alloy containing Mg added to conventional Al.Mg has a high solubility limit in the Al matrix and a strong solid solution strengthening effect.At the same time, it has excellent ductility [5,6].Also, Al 5000 alloys are generally used as an extruded material, but they contain Fe and Si and can also be used as casting materials.Alloys that can be used for both casting and extrusion improve the recyclability.However, when 3% or more of magnesium is added, β(Al 3 Mg 2 ) phase, precipitated continuously at the grain boundaries, undergoes stress corrosion cracking and intergranular corrosion [7][8][9].To solve this problem, Zn is added to the Al 5000 series alloy.This leads to precipitation of τ(Mg 32 (Al, Zn) 49 ) phases and prevention of β phases at the grain boundaries, thereby improving the corrosion resistance and enhancing the mechanical properties [10].In addition, alloy elements can be added to fulfill various goals.For example, Si improves the fluidity for casting [11].Mn improves the tensile strength without decreasing the corrosion resistance.Fe prevents adhesion of liquid Al to the mold [12,13].With these many aims, a new complex-composition Al alloy was investigated.Homogenization of the Al alloy distributes the secondary phase and thereby improves the tensile properties [14] and corrosion resistance [15].However, research into the effect of homogenization on the corrosion behavior of Al-Mg-Zn alloys with Si, Fe, and Mn elements has not been reported.Therefore, in this study, the corrosion resistance of a new Al alloy was investigated depending on the homogenization time.

Experimental Details
The chemical composition of the Al 5XXX alloy is shown in Table 1.Specimens were prepared by wire cutting as 10 × 10 × 20 mm 3 plates.In order to determine the optimum homogenization temperature, DSC analysis and JMatPro simulation were performed.The DSC analysis, performed by increasing the temperature by 1 • C/min up to 500 • C, recorded a change of enthalpy.Homogenization was carried out using a furnace by heating up to 450 • C at a rate of 100 • C/h and maintaining the temperature at 450 • C for 3, 6, 12, 18, 24, and 30 h; samples were then air cooled.For the polarization test, the surface of the specimen was polished with #2000 emery paper and washed with distilled water; this was followed by drying using nitrogen gas.The polarization test conditions are shown in Table 2. Saturated calomel and graphite were used for the standard and counter electrodes, respectively.The electrolytic solution was water containing 3.5% NaCl.The experiment was carried out from −1.3 V to −0.2 V at a 0.5 mV/s scanning speed.Various methods were used to identify the cause of the corrosion resistance change after homogenization.An optical microscope was employed to observe the microstructure.The etching solution was Keller reagent (2 mL HF, 3 mL HCl, 5 mL HNO 3 and 190 mL water), which is mainly used for Al-Mg-Zn [16].The volume fraction of the grain boundaries was calculated using software from MEDIA cybernetics, Image pro 5.0.XRD was utilized to observe the secondary phase change.SEM-EDS and TEM-EDS were used to analyze the secondary phase present at the grain boundaries.The specimen for TEM was a 10 um thick foil prepared using FIB.The change in Mg content at the grain boundaries was investigated using TEM-EDS line scanning.

Results and Discussion
Figure 1 shows the results of the DSC analysis before homogenization.Power compensated DSC measures the energy required to maintain the temperature difference between the sample and the reference.Therefore, the peak at around 472.8 • C indicates heat flow compensating for the endothermic reaction.Since the melting point of Al is higher than 600 • C, this peak does not mean Al melting; rather, it indicates the dissolution of a secondary phase.The homogenization temperature was hence decided to set at below 470 • C [17].

Results and Discussion
Figure 1 shows the results of the DSC analysis before homogenization.Power compensated DSC measures the energy required to maintain the temperature difference between the sample and the reference.Therefore, the peak at around 472.8 °C indicates heat flow compensating for the endothermic reaction.Since the melting point of Al is higher than 600 °C, this peak does not mean Al melting; rather, it indicates the dissolution of a secondary phase.The homogenization temperature was hence decided to set at below 470 °C [17].Figure 2 shows the JMatPro simulation results illustrating the secondary phase formation with temperature change.The possible precipitates in Al 5XXX alloys are β(Al3Mg2), τ(Mg32(Al, Zn)49), Al6(Mn, Fe), AlMgCuZn, and Mg2Si.The dissolution at 478.2 °C, shown in Figure 1, corresponds to β and/or τ phase.Since the dissolution temperature of the β phase in the ordinary Al-Mg alloy is near 470 °C, the effective homogenization temperature for the β, τ phase reduction without Mg2Si dissolution was considered to be 450 °C.

Results and Discussion
Figure 1 shows the results of the DSC analysis before homogenization.Power compensated DSC measures the energy required to maintain the temperature difference between the sample and the reference.Therefore, the peak at around 472.8 °C indicates heat flow compensating for the endothermic reaction.Since the melting point of Al is higher than 600 °C, this peak does not mean Al melting; rather, it indicates the dissolution of a secondary phase.The homogenization temperature was hence decided to set at below 470 °C [17].In Figure 3a, the potential can be seen not to change with increasing homogenization time.This is because there was no significant change in the alloy composition with homogenization.The corrosion rate expressed in Equations ( 1) and ( 2) in ASTM G102 can be calculated from the measured current density in the Tafel test [18].
Corrosion rate (mmpy) = 3.27 (1) where i corr , current density [µA/cm 2 ]; mmpy, mm/year (in corrosion depth); f i , the mass fraction of the i th element in the alloy; W i , the atomic weight of the element; n i , the number of electrons required to oxidize an atom of the element in the corrosion process, i.e., the valence of the element.
When the homogenization was carried out at 450 • C for up to 24 h, the corrosion rate decreased steadily and it was saturated after 24 h.The optimum homogenization time for improving the corrosion resistance was thus determined to be 24 h.
Figure 3 illustrates the results of the polarization test.Tafel test was carried out to determine the change of potential and current density with homogenization time.In Figure 3a, the potential can be seen not to change with increasing homogenization time.This is because there was no significant change in the alloy composition with homogenization.The corrosion rate expressed in Equations ( 1) and ( 2) in ASTM G102 can be calculated from the measured current density in the Tafel test [18].
Corrosion rate mmpy 3.27 10 density g cm where , current density [μA/cm 2 ]; mmpy, mm/year (in corrosion depth); , the mass fraction of the th element in the alloy; , the atomic weight of the element; , the number of electrons required to oxidize an atom of the element in the corrosion process, i.e., the valence of the element.
When the homogenization was carried out at 450 °C for up to 24 h, the corrosion rate decreased steadily and it was saturated after 24 h.The optimum homogenization time for improving the corrosion resistance was thus determined to be 24 h. Figure 4 demonstrates the change in microstructure with homogenization time.In order to numerically express the grain boundary volume fraction, program calculation was carried out and the results are shown in Figure 4h.Due to the grain growth, the grain boundary volume fraction decreases as the homogenization time increases to 24 h.After 24 h, the decrease slows, as anticipated by Fick's law; a similar result was reported by Li et al. [19].The shapes in Figures 3b and 4h were very similar.Therefore, it can be considered that the corrosion rate and grain boundary are closely correlated.
Figure 5 displays X-ray diffraction analysis results confirming the existence of a secondary phase during the homogenization.Before the homogenization, peaks (8 8 0) and (10 6 2), which can be indexed to the β phase, and peak (6 3 1), which is ascribed to the τ phase [20], are observed.After 24 h of homogenization, the β and τ phases disappeared due to the dissolution of the second phases.
Figure 6 and Table 3 show the SEM-EDS analysis results of the secondary phase before homogenization.In Figures 2 and 6 and Table 3, we observed β(Al3Mg2), τ(Mg32(Al, Zn)49), Mg2Si and Al6(Mn, Fe).Phase A is considered to be a mixture of β and τ phases with a small amount of Cu.Phases B and D, frequently observed around phase A, are similar to Al6(Mn, Fe).Phase C is Mg2Si.
Figure 7 and Table 4 present SEM-EDS images and the composition of the secondary phases at the grain boundary after homogenization.The main secondary phase in the grain boundaries was   4h.Due to the grain growth, the grain boundary volume fraction decreases as the homogenization time increases to 24 h.After 24 h, the decrease slows, as anticipated by Fick's law; a similar result was reported by Li et al. [19].The shapes in Figures 3b and 4h were very similar.Therefore, it can be considered that the corrosion rate and grain boundary are closely correlated.
Figure 5 displays X-ray diffraction analysis results confirming the existence of a secondary phase during the homogenization.Before the homogenization, peaks (8 8 0) and (10 6 2), which can be indexed to the β phase, and peak (6 3 1), which is ascribed to the τ phase [20], are observed.After 24 h of homogenization, the β and τ phases disappeared due to the dissolution of the second phases.
Figure 6 and Table 3 show the SEM-EDS analysis results of the secondary phase before homogenization.In Figures 2 and 6 and Table 3, we observed β(Al 3 Mg 2 ), τ(Mg 32 (Al, Zn) 49 ), Mg 2 Si and Al 6 (Mn, Fe).Phase A is considered to be a mixture of β and τ phases with a small amount of Cu.Phases B and D, frequently observed around phase A, are similar to Al 6 (Mn, Fe).Phase C is Mg 2 Si.
Figure 7 and Table 4 present SEM-EDS images and the composition of the secondary phases at the grain boundary after homogenization.The main secondary phase in the grain boundaries was likely Si and/or Mg 2 Si, as can be identified in Table 4.This means that β(Al 3 Mg 2 ), τ(Mg 32 (Al, Zn) 49 ) phases are reduced during homogenization at the grain boundaries and Si and/or Mg 2 Si are formed.The difference in the composition of Si and Mg 2 Si between 24 h and 30 h was not clear.
Coatings 2018, 8, 39 5 of 9 likely Si and/or Mg2Si, as can be identified in Table 4.This means that β(Al3Mg2), τ(Mg32(Al, Zn)49) phases are reduced during homogenization at the grain boundaries and Si and/or Mg2Si are formed.The difference in the composition of Si and Mg2Si between 24 h and 30 h was not clear.likely Si and/or Mg2Si, as can be identified in Table 4.This means that β(Al3Mg2), τ(Mg32(Al, Zn)49) phases are reduced during homogenization at the grain boundaries and Si and/or Mg2Si are formed.The difference in the composition of Si and Mg2Si between 24 h and 30 h was not clear.In Equation (3), the diffusion activation energy Q of Si is larger than those of Mg, Mn, and Fe [21].Accordingly, the distribution of Si at the grain boundaries increases with temperature.The presence of Si at the grain boundary at high temperature prevents the precipitation of other phases and increases the potential of the grain boundary via the high potential of Si after homogenization [22].This potential increase can decelerate the intergranular corrosion.In addition, the β phase is known to form easily at grain boundaries as well as at the periphery of Al6(Mn, Fe) [23].The reduction of the Al6(Mn, Fe) phase at the grain boundary by homogenization can decrease the precipitation of β phases at the grain boundary because β phases prefer to precipitate around Al6(Mn, Fe).
As shown before in Figures 6 and 8, amounts of Mg, Mn, Fe, and Zn decrease with homogenization.This means β, τ, Al6(Mn, Fe) phases decrease.On the contrary, Si increases by homogenization at the grain boundary.The corrosion of β and τ, which mainly contain Mg, activate galvanic corrosion because the standard reduction potential of Mg is lower than that of Al [24,25].As reported by Zeng et al., in the Al6000 alloy, Mg accelerated inter-granular corrosion with a standard reduction potential, which is lower than that of Al; Si has with a higher standard reduction potential than discontinuous Mg2Si, which acts as a sacrificial anode to improve corrosion resistance, especially at the grain boundary [26].Similarly, for the Al 5XXX alloy in this study, Mg containing β and τ phases in the grain boundary significantly lowers the grain boundary potential, but Si, with higher potential and sacrificial Mg2Si, can increase the grain boundary potential.A decrease in Mg and increases in Si and Mg2Si at the grain boundary by homogenization, therefore, would be the main reason for the improvement of the corrosion resistance.In Equation (3), the diffusion activation energy Q of Si is larger than those of Mg, Mn, and Fe [21].Accordingly, the distribution of Si at the grain boundaries increases with temperature.The presence of Si at the grain boundary at high temperature prevents the precipitation of other phases and increases the potential of the grain boundary via the high potential of Si after homogenization [22].This potential increase can decelerate the intergranular corrosion.In addition, the β phase is known to form easily at grain boundaries as well as at the periphery of Al 6 (Mn, Fe) [23].The reduction of the Al 6 (Mn, Fe) phase at the grain boundary by homogenization can decrease the precipitation of β phases at the grain boundary because β phases prefer to precipitate around Al 6 (Mn, Fe).
As shown before in Figures 6 and 8, amounts of Mg, Mn, Fe, and Zn decrease with homogenization.This means β, τ, Al 6 (Mn, Fe) phases decrease.On the contrary, Si increases by homogenization at the grain boundary.The corrosion of β and τ, which mainly contain Mg, activate galvanic corrosion because the standard reduction potential of Mg is lower than that of Al [24,25].As reported by Zeng et al., in the Al6000 alloy, Mg accelerated inter-granular corrosion with a standard reduction potential, which is lower than that of Al; Si has with a higher standard reduction potential than discontinuous Mg 2 Si, which acts as a sacrificial anode to improve corrosion resistance, especially at the grain boundary [26].Similarly, for the Al 5XXX alloy in this study, Mg containing β and τ phases in the grain boundary significantly lowers the grain boundary potential, but Si, with higher potential and sacrificial Mg 2 Si, can increase the grain boundary potential.A decrease in Mg and increases in Si and Mg 2 Si at the grain boundary by homogenization, therefore, would be the main reason for the improvement of the corrosion resistance.

Conclusions
In this study, a new Al-Mg-Zn-Mn-Fe-Si Al 5XXX alloy was homogenized and the corrosion resistance was investigated.Homogenization was conducted at 450 °C for 3, 6, 12, 18, 24, and 30 h.
The corrosion resistance improved steadily with homogenization time and saturated after 24 h.With homogenization, the grain boundary volume fraction decreased and the corrosion rate decreased.The second phases recognized at the grain boundary are mainly β(Al3Mg2), τ(Mg32(Al, Zn)49), and Al6(Mn, Fe) before homogenization and Mg2Si and Si after homogenization.
The secondary phase present in the grain boundary had a large amount of Mg before homogenization and it is thought that the Mg, with a lower standard reduction potential, would intensify the intergranular corrosion.
With homogenization, the β and τ phase with high Mg content decreased at the grain boundary by diffusion into grains, while Si preferentially diffused to the grain boundary at high temperature homogenization can remain in Si and/or Mg2Si at the grain boundary.Furthermore, the high reduction potential of Si and sacrificial Mg2Si would reduce the intergranular corrosion, resulting in the better corrosion resistance of the newly developed Al 5XXX alloy.

Conclusions
In this study, a new Al-Mg-Zn-Mn-Fe-Si Al 5XXX alloy was homogenized and the corrosion resistance was investigated.Homogenization was conducted at 450 • C for 3, 6, 12, 18, 24, and 30 h.
The corrosion resistance improved steadily with homogenization time and saturated after 24 h.With homogenization, the grain boundary volume fraction decreased and the corrosion rate decreased.The second phases recognized at the grain boundary are mainly β(Al 3 Mg 2 ), τ(Mg 32 (Al, Zn) 49 ), and Al 6 (Mn, Fe) before homogenization and Mg 2 Si and Si after homogenization.
The secondary phase present in the grain boundary had a large amount of Mg before homogenization and it is thought that the Mg, with a lower standard reduction potential, would intensify the intergranular corrosion.
With homogenization, the β and τ phase with high Mg content decreased at the grain boundary by diffusion into grains, while Si preferentially diffused to the grain boundary at high temperature homogenization can remain in Si and/or Mg 2 Si at the grain boundary.Furthermore, the high reduction potential of Si and sacrificial Mg 2 Si would reduce the intergranular corrosion, resulting in the better corrosion resistance of the newly developed Al 5XXX alloy.

Figure 2
Figure 2 shows the JMatPro simulation results illustrating the secondary phase formation with temperature change.The possible precipitates in Al 5XXX alloys are β(Al 3 Mg 2 ), τ(Mg 32 (Al, Zn) 49 ), Al 6 (Mn, Fe), AlMgCuZn, and Mg 2 Si.The dissolution at 478.2 • C, shown in Figure 1, corresponds to β and/or τ phase.Since the dissolution temperature of the β phase in the ordinary Al-Mg alloy is near 470 • C, the effective homogenization temperature for the β, τ phase reduction without Mg 2 Si dissolution was considered to be 450 • C.

Figure 2
Figure2shows the JMatPro simulation results illustrating the secondary phase formation with temperature change.The possible precipitates in Al 5XXX alloys are β(Al3Mg2), τ(Mg32(Al, Zn)49), Al6(Mn, Fe), AlMgCuZn, and Mg2Si.The dissolution at 478.2 °C, shown in Figure1, corresponds to β and/or τ phase.Since the dissolution temperature of the β phase in the ordinary Al-Mg alloy is near 470 °C, the effective homogenization temperature for the β, τ phase reduction without Mg2Si dissolution was considered to be 450 °C.

Figure 2 .
Figure 2. JMatPro simulation analyzing the second phase formation.Figure 2. JMatPro simulation analyzing the second phase formation.

Figure 2 .
Figure 2. JMatPro simulation analyzing the second phase formation.Figure 2. JMatPro simulation analyzing the second phase formation.

Figure 3
Figure3illustrates the results of the polarization test.Tafel test was carried out to determine the change of potential and current density with homogenization time.In Figure3a, the potential can be seen not to change with increasing homogenization time.This is because there was no significant change in the alloy composition with homogenization.The corrosion rate expressed in Equations (1) and (2) in ASTM G102 can be calculated from the measured current density in the Tafel test[18].

Figure 3 .
Figure 3. (a) Polarization potential obtained from Tafel analysis; (b) Corrosion rate calculated from the polarization current density of Tafel analysis.

Figure 3 .
Figure 3. (a) Polarization potential obtained from Tafel analysis; (b) Corrosion rate calculated from the polarization current density of Tafel analysis.

Figure 4
Figure4demonstrates the change in microstructure with homogenization time.In order to numerically express the grain boundary volume fraction, program calculation was carried out and the results are shown in Figure4h.Due to the grain growth, the grain boundary volume fraction decreases as the homogenization time increases to 24 h.After 24 h, the decrease slows, as anticipated by Fick's law; a similar result was reported by Li et al.[19].The shapes in Figures3b and 4hwere very similar.Therefore, it can be considered that the corrosion rate and grain boundary are closely correlated.Figure5displays X-ray diffraction analysis results confirming the existence of a secondary phase during the homogenization.Before the homogenization, peaks (8 8 0) and (10 6 2), which can be indexed to the β phase, and peak (6 3 1), which is ascribed to the τ phase[20], are observed.After 24 h of homogenization, the β and τ phases disappeared due to the dissolution of the second phases.Figure6and Table3show the SEM-EDS analysis results of the secondary phase before homogenization.In Figures2 and 6and Table3, we observed β(Al 3 Mg 2 ), τ(Mg 32 (Al, Zn) 49 ), Mg 2 Si and Al 6 (Mn, Fe).Phase A is considered to be a mixture of β and τ phases with a small amount of Cu.Phases B and D, frequently observed around phase A, are similar to Al 6 (Mn, Fe).Phase C is Mg 2 Si.Figure7and Table4present SEM-EDS images and the composition of the secondary phases at the grain boundary after homogenization.The main secondary phase in the grain boundaries was

Figure 5 .
Figure 5. XRD analyses with no and 24 h homogenization.

Figure 5 .
Figure 5. XRD analyses with no and 24 h homogenization.Figure 5. XRD analyses with no and 24 h homogenization.

Figure 5 .
Figure 5. XRD analyses with no and 24 h homogenization.Figure 5. XRD analyses with no and 24 h homogenization.

Figure 8 9 Figure 8
Figure 8 displays the SEM-EDS composition map after 24 h homogenization.The map confirms again that secondary phases such as β and τ almost completely disappear at the grain boundary and, instead, levels of Mg 2 Si and Si increased after homogenization.This can be explained by diffusion, following the Arrhenius equation below D = D 0 exp − Q RT (3) where D 0 , diffusion coefficient [m 2 /s]; R, mole gas constant; Q, diffusion activation energy [kJ/mole]; T, thermodynamic temperature.

Figure 9 9 Figure 9 Figure 9 .
Figure9shows results of TEM-EDS line scanning of the interface between the Al matrix and the secondary phase after homogenization for 24 h at 450 • C.After the homogenization, a remainder of Mg 2 Si phase was confirmed.Coatings 2018, 8, 39 8 of 9Figure9shows results of TEM-EDS line scanning of the interface between the Al matrix and the secondary phase after homogenization for 24 h at 450 °C.After the homogenization, a remainder of Mg2Si phase was confirmed.

Figure 9 .
Figure 9. (a) TEM image; (b) TEM-EDS line scanning profile of the interface between Al matrix and second phase after 24 h homogenization at 450 • C.

Table 1 .
Chemical composition of the Al 5XXX alloy (all in wt %).

Table 3 .
Chemical composition of secondary phases in Figure6before homogenization.

Table 4 .
Chemical composition of secondary phases in Figure7after homogenization.

Table 3 .
Chemical composition of secondary phases in Figure6before homogenization.

Table 3 .
Chemical composition of secondary phases in Figure6before homogenization.

Table 4 .
Chemical composition of secondary phases in Figure7after homogenization.

Table 4 .
Chemical composition of secondary phases in Figure7after homogenization.