3.2. Microstructures
Figure 4 depicts the representative microstructures and EDS line-scan results of the Al/Mg joint. In
Figure 4a, the SZ of the Al/Mg joint is constituted by a chaotic morphology on the top and a uniform morphology on the bottom. The magnified view of the R1 region in
Figure 4b reveals the existence of vortex-like Al–Mg IMCs in the SZ. The elemental compositions at points 1–3 in
Figure 4b are listed in
Table 2. The gray zone, dark zone, and white zone, marked as points 1, 2, and 3 are primarily Al
3Mg
2, an Al matrix with little Al
2O
3, and Al
12Mg
17 with a lot of Al
2O
3 and MgO, respectively. Similar results can also be found in other studies of Al/Mg FSW joints [
24,
25]. The detection of metallic oxides may have originated from two sources [
26]: (1) the thermal interaction between oxygen and the base materials and (2) the remaining oxide layers on the base materials, which were mixed into the SZ. The magnified view of the R2 region in
Figure 4c exhibits an intercalated microstructure with a wide transition zone at the Mg alloy/SZ interface. The magnified view of the R3 region in
Figure 4d indicates that there existed a lot of irregular Al–Mg IMCs in the transition zone. The corresponding EDS line-scan results in
Figure 4e show the formation of abrupt changes in elemental contents of Al and Mg between adjacent plateaus, suggesting the presence of stable Al–Mg IMCs. The Al–Mg IMCs were formed by the constitutional liquation phenomenon via the eutectic reactions of L = Al + Al
3Mg
2 at 450 °C and L = Mg + Al
12Mg
17 at 437 °C, respectively [
27,
28,
29].
Figure 5 demonstrates the representative microstructures and EDS line-scan results of the Al/Zr/Mg joint. As shown in
Figure 5a, the smashed Zr interlayer distributed in the SZ was intermixed with the base materials, while the undamaged part remained stable at the Al/Mg interface. The magnified view of the R4 region in
Figure 5b displayed three different morphologies of white plates, dark strips, and gray matrix in the SZ. The elemental compositions at points 4–6 in
Figure 5b are summarized in
Table 3. The white plates, dark strips, and gray matrix marked as points 4, 5, and 6 are Zr with little ZrO
2, Al matrix with a lot of Al
2O
3 and MgO, Al matrix with little Al
2O
3, respectively. The magnified view of the R5 region in
Figure 5c indicates the existence of a thin transition zone at the Mg alloy/SZ interface. The corresponding EDS line-scan results in
Figure 5d exhibit a smooth transition in Al and Mg content with no abrupt changes from the Mg alloy to the SZ.
In comparison with the Al/Mg joint (see
Figure 4 and
Table 2), the Al/Zr/Mg joint revealed more favorable elemental distributions with reduced Al–Mg IMCs (see
Figure 5 and
Table 3). This microstructure improvement may be explained by the synergetic effects of the Zr interlayer: (1) The smashed Zr intermixed in the SZ exhibited a chemical modification effect to hinder the strong reaction between the base materials, as evidenced by the much lower Mg content in the R4 region than that in the R1 region; and (2) the undamaged Zr at the Al/Mg interface segregated the contact of the base materials, which showed a thermal barrier effect in mitigating the mutual diffusion of Al and Mg.
3.3. Corrosion Behavior
Figure 6 presents the OCP curves for the representative regions of the Al/Mg and Al/Zr/Mg joints in the 3.5% NaCl solution. The OCP curves of the experimental samples exhibited a rising trend at the initial stage and then became steady at the final stage, indicating that the chemical status on the surface gradually became stable with the increasing immersion time. The highest and lowest OCP values can be found for the Al alloy and Mg alloy, respectively. The OCP values of the Al/Mg-HAZ, Al/Mg-SZ, Al/Zr/Mg-HAZ, and Al/Zr/Mg-SZ samples were within the range of the base materials. This variation in OCP values may be related to the passivation layers on the surface. The compact Al
2O
3 layer on the Al alloy surface had some anti-corrosion ability, while the porous MgO layer on the Mg alloy surface was easily dissolved by the corrosive media. The passivation layers of the representative regions (HAZ and SZ) were composed of metallic oxide mixtures, such as Al
2O
3, MgO, and ZrO
2, which presented a mediate passivation effect. Moreover, there appeared to be some fluctuations in the OCP curves, which were caused by the dissolution–regeneration process of the passivation layers [
30].
Figure 7 displays the potentiodynamic polarization curves for the representative regions of the Al/Mg and Al/Zr/Mg joints in the 3.5% NaCl solution. The corresponding electrochemical parameters of E
corr and
icorr are listed in
Table 4. Usually, the E
corr is a thermodynamic indicator to estimate the corrosion probability, and a larger E
corr represents a higher surface stability. On the other hand, the
icorr is a kinetic indicator to evaluate the corrosion degree once corrosion occurs, and a smaller
icorr indicates a lower corrosion rate [
31,
32]. The Al alloy had a higher E
corr and a lower
icorr compared with the Mg alloy, suggesting that superior corrosion resistance was found for the Al alloy. The E
corr of the Al/Mg-HAZ, Al/Mg-SZ, Al/Zr/Mg-HAZ, and Al/Zr/Mg-SZ samples were between those of the Al alloy and the Mg alloy, which were the mixed potentials of the base materials. The
icorr increased in the order of Al alloy < Mg alloy < Al/Zr/Mg-HAZ < Al/Mg-HAZ < Al/Zr/Mg-SZ < Al/Mg-SZ samples, implying the same change trend in the corrosion rate. For both Al/Mg and Al/Zr/Mg joints, the corrosion rate of the representative regions decreased in the order of SZ > HAZ > base materials. This phenomenon may have been owing to the following two reasons: (1) The SZ and HAZ, incorporating Al and Mg dissimilar alloys, experienced galvanic corrosion at the Al/Mg interface because of the potential differences, which induced faster corrosion than the base materials; and (2) different from the HAZ, the SZ possessed a more complex microstructure with heterogeneous phases of Al–Mg IMCs, metals, and metallic oxides, which accelerated the corrosion rate via building more local corrosion cells. In addition, the corrosion behavior of the Al/Mg joint was improved by the Zr interlayer, as could be seen from the smaller
icorr at the HAZ and SZ for the Al/Zr/Mg joint. The Zr interlayer reduced the formation of Al–Mg IMCs in the SZ and segregates the contact of the base materials at the interface (see
Figure 5 and
Table 3), which mitigated the galvanic corrosion effect between the Al and Mg alloys.
Figure 8 shows the corrosion morphologies of the Al/Mg joint after immersion in the 3.5% NaCl solution for 4 h. The corresponding elemental compositions of points 7–11 are summarized in
Table 5. It must be pointed out that SEM-EDS is a semi-quantitative measuring technique, and it may cause deviations in quantifying the minor elements. In order to reduce this inaccuracy, the contents of minor elements (Cl, Si, and Zn) are added up and listed as the “others”. The corrosion behavior of the joints were analyzed by comparing the contents of the major elements (Al, Mg, O, and Zr). This similar analysis method has also been reported in [
29], where the phase constitutions of Al/Mg dissimilar FSW joints were investigated via the major elements (Al and Mg) with the minor elements (Fe, Mn, Si, Zn, etc.) added as a whole. As shown
Figure 8a, the Al/Mg joint was corroded to form diverse corrosion morphologies at different regions. The magnified view of the R6 region in
Figure 8b indicates that the Al alloy with a flat surface had a higher corrosion resistance than the Mg alloy with a rough surface, which was due to the galvanic corrosion between active Mg and noble Al [
15]. The corrosion products at the Al/Mg interface (point 7) were mainly composed of O and Mg with a small amount of Al, implying the formation of Mg oxides. The corroded surface of the Mg alloy (point 8) was filled with blocky corrosion products, whose composition had slightly lower Al and O with higher Mg compared with the Al/Mg interface (point 7). The magnified view of the R7 region in
Figure 8c is featured with two distinct corrosion forms: the uniform corrosion at the Al alloy (point 9) and the discrete corrosion on the top of the SZ (point 10). The granular corrosion morphology at point 9 contained 60.01 Al, 4.85 Mg, 29.46 O, and 5.68 minor elements (in wt.%) due to the corrosion of the Al alloy, while the dendritic corrosion morphology at point 10 included 49.85 Al, 31.27 Mg, 17.28 O, and 1.60 minor elements (in wt.%), owing to the combined corrosion of the base materials and Al–Mg IMCs. The magnified view of the R8 region in
Figure 8d demonstrates that the SZ on the bottom had a compact corrosion morphology, whereas the adjacent Mg alloy was badly corroded to form many cracks and some holes. High O and Mg with trace Al were detected on the bottom of the SZ (point 11), suggesting the existence of Mg oxides formed by the corrosion of the Mg alloy.
The corrosion morphologies of the Al/Zr/Mg joint after immersion in the 3.5% NaCl solution for 4 h are exhibited in
Figure 9.
Table 6 lists the corresponding elemental compositions of points 12–17. In
Figure 9a, an inhomogeneous corrosion morphology can be observed for the Al/Zr/Mg joint with the appearance of some Zr pieces. The magnified view of the R9 region in
Figure 9b shows a sandwiched microstructure with the Al and Mg alloys being separated by the Zr interlayer, which blocks the contact of the base materials and retards the galvanic corrosion at the Al/Mg interface to some degree. The corroded surface of the Zr interlayer (point 12) was characterized by high O and Zr with some Al and Mg, indicating that the main corrosion products were Zr oxides. The blocky corrosion products of the Mg alloy (point 13) have higher Mg and comparable Al and O compared with point 8 (see
Figure 8b), implying that the corrosion of the Mg alloy was slightly mitigated. The magnified view of the R10 region in
Figure 9c presents a corrosion morphology similar to that of the R7 region of the Al/Mg joint. The corrosion products of the Al alloy at point 14 were composed of 81.37 Al, 1.89 Mg, 14.53 O, and 2.21 minor elements (in wt.%), indicating that a lower corrosion degree than point 9 (see
Figure 8c) was obtained. The corroded surface on the top of the SZ (point 15) showed a relatively uniform and continuous corrosion morphology with comparable compositions of 47.39 Al, 29.43 Mg, 22.29 O, and 0.89 minor elements (in wt.%) to those of point 10 (see
Figure 8c). The magnified view of the R11 region in
Figure 9d shows that many irregular Zr pieces were embedded in the middle of the SZ, whose composition primarily consisted of Zr and O with little Al and Mg (point 16). The magnified view of the R12 region in
Figure 9e exhibits a corrosion morphology similar to that of the R8 region of the Al/Mg joint. The corrosion degree of the SZ on the bottom (Point 17) was comparable to that of point 11 (see
Figure 8d), as can be seen by their similar compositions.
Figure 10 displays the corrosion morphologies of the Al/Mg joint after immersion in the 3.5% NaCl solution for 60 h. The corresponding elemental compositions of points 18–20 are listed in
Table 7. As shown in
Figure 10a, the corrosion degree of the Al/Mg joint was aggravated as the immersion time increased to 60 h. The magnified view of the R13 region in
Figure 10b shows that more severe corrosion was found for the Al and Mg alloys compared with the Al/Mg interface. The corroded surface at the Al/Mg interface (point 18) had a similar composition with that of point 7 (see
Figure 8b), indicating that no further corrosion was obviously observed at the Al/Mg interface as immersion time increased. The magnified view of the R14 region in
Figure 10c exhibits a discrete corrosion morphology on the top of the SZ, which produced a discontinuous surface with many irregular grooves. The dendritic corrosion morphology at point 19 had a heavier corrosion degree than point 10 (see
Figure 8c), as evidenced by its lower Al, Mg, and higher O. The magnified view of the R15 region in
Figure 10d demonstrates that the Mg alloy corroded faster than the SZ on the bottom to form an uneven surface with a large height difference. Relatively higher Al and lower Mg were detected on the bottom of the SZ (point 20) than at point 11 (see
Figure 8d), which resulted from the further corrosion of the Mg alloy.
The corrosion morphologies of the Al/Zr/Mg joint after immersion in the 3.5% NaCl solution for 60 h are presented in
Figure 11.
Table 8 summarizes the corresponding elemental compositions of points 21–24. As shown in
Figure 11a, relatively mild corrosion with a flatter corroded morphology was found in the Al/Zr/Mg joint compared with that of the Al/Mg joint after 60 h of immersion time (see
Figure 10a). The magnified view of the R16 region in
Figure 11b shows that the Zr interlayer remained stable at the Al/Mg interface to balance the corrosion of the base materials, as can be seen from the inconspicuous height difference between the Al and Mg alloys. The corroded surface of the Zr interlayer (point 21) had a higher Al and Mg and a much lower Zr than that at point 12 (see
Figure 9b), implying the further corrosion of the Zr interlayer. The magnified view of the R17 region in
Figure 11c exhibits that the SZ on the top was corroded to form some small pits with the existence of dendritic corrosion products and Zr pieces. The dendritic corrosion products at point 22 were characterized by a much lower Al, higher O, and similar Mg compared with that of point 19 (see
Figure 10c), suggesting that the Al alloy was further corroded to form metallic oxides on the top of the SZ. The Zr pieces at point 23 had similar compositions with those at point 16 (see
Figure 9d), indicating that no further corrosion was observed for the Zr pieces. The magnified view of the R18 region in
Figure 11d demonstrates that there was a transverse crevice on the bottom of the SZ, which may have provided passages for corrosive media to further corrosion. The corrosion products on the bottom of the SZ (point 24) had comparable compositions with those at point 20 (see
Figure 10d), suggesting a similar corrosion degree for both sites.
Figure 12 presents the XRD patterns for the corrosion products of the Al/Mg and Al/Zr/Mg joints. No obvious differences in XRD patterns were observed between the Al/Mg and Al/Zr/Mg joints, suggesting that the phase constitutions of the corrosion products for both joints were similar. The diffraction peaks of the corrosion products were identified as Mg(OH)
2, Al(OH)
3, and Mg
2Al(OH)
7 phases by MDI Jade 6.0 software, which correspond to the standard PDF #83-0114, #20-0011, and #35-1275, respectively. The corrosion of Al and Mg alloys usually produces metallic oxides, which are easily transformed to metallic hydroxides under humid environment [
33,
34]. Furthermore, no Zr-containing phase was detected because of its low content. The detection of metallic hydroxides by the XRD patterns was inconsistent with the EDS results, which was attributed to the inability of EDS to detect H.
Figure 13 illustrates the weight losses and corresponding change rates of the Al/Mg and Al/Zr/Mg joints during the immersion tests in the 3.5% NaCl solution for 60 h. The weight loss of the Al/Mg joint linearly increased to 176.42 ± 12.22 mg/cm
2 in the initial 28 h and slowly increased to 268.72 ± 13.44 mg/cm
2 in the last 32 h. The weight loss of the Al/Zr/Mg joint reached 231.08 ± 4.81 mg/cm
2 during the whole 60 h and exhibited a 14% reduction compared with that of the Al/Mg joint, which evidenced that the corrosion resistance was enhanced by the Zr interlayer. Moreover, the weight loss rate of the Al/Mg joint slowly increased to 6.30 ± 0.44 mg/cm
2/h with increasing immersion time to 28 h and then decreased to 4.48 ± 0.22 mg/cm
2/h at the end of 60 h. A similar change trend in weight loss rate was found for the Al/Zr/Mg joint except for the slightly lower value at each immersion time. The slowdown in weight loss rate may be attributed to the gradual deposition of corrosion products on the surface, which can act as a protective layer to suppress further corrosion [
35].