Effect of Al Content on the Microstructure and Properties of Zn-Al Solder Alloys

Abstract

Zn-Al alloy with the addition of Al (5–25 wt.%) was fabricated into as–cast and rod–shaped alloys. SEM/EDS and XRD technology were used to examine the impacts of the Al–element content on the alloys’ microstructure, mechanical characteristics, electrical conductivity, wetting ability, and corrosion resistance. The findings demonstrate how the Zn-Al alloy’s microstructure is dramatically altered by the different additions of the Al content. When the Al content reaches 15 wt.%, the eutectoid structures of the as–cast Zn-Al alloy are the finest, and the microhardness and tensile strength of the extruded–state alloy reach their maximum and exhibit the best corrosion resistance. The spreading area of the Zn-15Al solder alloy achieves its maximum on the 6061 Al plate, while it reaches its minimum on the T2 Cu plate. Furthermore, the electrical resistivity of the Zn-Al alloy continuously decreases as the Al content increases.

1. Introduction

Cu is widely utilized in electric power, refrigeration, the automotive industry, electronics, and other areas due to its high thermal conductivity, electrical conductivity, and corrosion resistance. However, the high density and high cost of Cu are the main factors limiting its application. The density and price of Al are much lower than those of Cu, and the electrical conductivity and thermal conductivity of Al are similar to those of Cu, so the Cu/Al hybrid structure can be used in some electrical conductivity and thermal conductivity occasions; this can reduce production costs and weight without compromising the performance of the structure [1,2,3,4].

In recent years, many researchers have investigated the microstructure and properties of Cu/Al joints by using refill friction stir spot welding, electric current–assisted ultrasonic welding, and ultrasonic dynamic stationary–shoulder friction stir welding and other techniques [5,6,7,8,9,10,11], and these new processing methods can improve the quality of Cu/Al joints. However, these techniques are complicated and difficult to apply quickly. Brazing has become a popular process for creating Cu/Al hybrid structures due to its little influence on the base material, its smooth and flat connections, and its ease of automation in product manufacturing [4,12,13].

However, brazing also brings a series of problems. The physical and chemical properties of Cu and Al are quite different, and the dense oxide film on the surface of the Al plate makes the brazing process very difficult, so in order to obtain good welded joints, the selection of suitable solders and brazing fluxes is very important [14]. To ensure joint performance and meet brazing process criteria, the solder must fulfill the following fundamental characteristics [15]:

(1) The solder has a suitable melting temperature range, with a melting point lower than that of the base material.

(2) The solder has a good wetting effect at brazing temperature and can fully fill the joint gap.

(3) The solder can meet the joint physical, chemical, and mechanical properties and other requirements.

(4) The ingot can be made into wire, rod, and other states of solder by extrusion and other processes.

Currently, Sn–Zn [16], Zn-Al [1], and Al–Si [17] solders are the solders most often utilized in Cu/Al brazing. Since Zn-Al alloys have a lower melting point than Al–Si alloys and the shear strength of the Cu/Al joint brazed with Zn-Al filler metals is much higher than that brazed with Sn–Zn filler metals, it has emerged as the most promising solder for Cu/Al welding [2,12,13].

The effect of Al content on the microstructure and properties of Zn-Al solder alloys has been investigated by previous researchers, but most of the related studies have focused on Zn-Al alloys in the drawn–state [18,19,20,21,22,23]. However, in the actual production process, we found that even though the Zn-Al solder has good plasticity, if it is not annealed from the treatment the solder is easy to fracture after multiple drawings, which obviously greatly extends the production cycle of the solder products. In order to shorten the production cycle and avoid the fracture of the solder, we consider the preparation of solder products only by extrusion.

In recent years, many researchers have carried out a number of studies on Zn-Al alloys in the extruded state. Zhang et al. [24] heated the extruded–state Zn-Al alloy above the eutectoid point and held it for 10 h before cooling it with the furnace. The phenomena of annealing hardening and an increase in conductivity were observed in extruded specimens with an aluminum content of 5 wt.% to 25 wt.%. Yan et al. [25] found that after homogenization, the coarse structure in extruded Zn–15Al was eliminated and the structure became more homogeneous. Sun et al. [26] presented the finding that Zn–15Al in the extruded state has the highest strength and plasticity at casting temperatures of 530 °C and 560 °C.

However, few researchers have combined the application of extruded Zn-Al alloys and solders. So, in this work, the effects of Al content (5–25 wt.%) on the microstructure, mechanical properties, electrical conductivity, wetting ability, and corrosion resistance of Zn-Al alloys in the extruded state were investigated comparatively for the first time, laying the foundation for further research and application of Zn-Al alloys as copper–aluminum solders.

2. Experimental Details

2.1. Preparation of Samples

Zn ingots (purity of 99.99 wt.%) and Al ingots (purity of 99.8 wt.%) were added into the graphite crucible, proportionally, and melted at 650 °C by a box–type resistance furnace. Then the melted metal was cooled to 560 °C and cast into an ingot of Φ110 mm × 200 mm. Eventually, the ingot was heated to 250 °C for hot extrusion after removing the oxide scale, and a rod sample of Φ13.5 mm was obtained with an extrusion ratio of 58:1.

2.2. Experimental Procedure

The microstructure of samples was observed by a scanning electron microscope (SEM, Zeiss Gemini SEM 300, Zeiss, Oberkochen, Germany). An X-ray diffractometer (XRD, Smart Lab, Rigaku, Tokyo, Japan) was used to analyze the physical phase of alloys, and the anode type of XRD was the copper target. Samples of 8 mm × 8 mm were taken from as–cast and rod–shaped alloys.

Measurement of the hardness of extruded–state Zn-Al alloys was carried out by means of the Vickers hardness test method. Samples were first ground and polished using a series of different SiC papers. Then the microhardness of extruded–state alloys was measured by a microhardness tester (Zwick/Rell ZHμ, Zwick Roell, Ulm, Germany). Each sample was subjected to a regular load of 0.1 kg and a dwell time of 10 s, and at least 10 readings of indentations were taken for each sample at room temperature to obtain the mean value.

The conductivity of specimens was measured by an electrical conductivity meter (Portable Conductivity Meter FD-101, FIRST, Beijing, China). Samples of Φ13.5 mm × 10 mm were cut and ground to 800 mesh.

Before the spreading experiment, the 6061 Al and T2 Cu plates were sanded with 400-mesh sandpaper to remove surface oil and oxide. The solder (0.15 g) was placed on the specimens with a non-corrosion CsF-AlF₃ flux (the melting point is 480 °C). Afterward, the specimens were heated at 530 °C for 50 s in an electrical resistance furnace. When the furnace temperature dropped to 350 °C, the sample was transferred to the atmosphere for cooling. Then the spreading area was photographed by a digital camera and calculated using Image-J (v1.8.0.345). In order to ensure the accuracy of the data, the spreading test was repeated five times for each composition of the alloy, and the average values were reported. A schematic of the spreading test and substrate dimensions is shown in Figure 1.

Uniform corrosion testing was carried out at a temperature of 25 ± 2 °C and the soaking time was 168 h. The size of the samples was Φ13.5 mm × 18 mm, and the corrosive solution was 3.5 wt.% NaCl. The corrosion rate of Zn-Al alloy solder was calculated according to the following equation:

$$\mathrm{R}=\frac{8.76\times {10}^7\times (\mathrm{M}-{\mathrm{M}}_{\mathrm{t}})}{\mathrm{S}\times \mathrm{T}\times \mathrm{D}}$$

(1)

where $\mathrm{R}$ is the corrosion rate of the samples (mm/a), $\mathrm{M}$ is the weight of the samples before testing (g), ${\mathrm{M}}_{\mathrm{t}}$ is the weight of the sample after testing (g), $\mathrm{S}$ is the total area of the sample (cm²), $\mathrm{T}$ is the testing time (h), and $\mathrm{D}$ is the density of the material (kg/m³).

The polarization curves of the samples were obtained by using a Gamry electrochemical apparatus (INTERFACE 1000 Em Gamry Instruments, Philadelphia, PA, USA) to evaluate the corrosion characteristics. A saturated calomel electrode (SCE) was used as a reference electrode, and a platinum electrode was used as an auxiliary electrode. The corrosive medium was consistent with the uniform corrosion test, and the experimental temperature was about 25 ± 2 °C. The potential scanning range was between −1.35_VSCE and 0.4_VSCE, and the scanning rate was 1 mV/s. At an open-circuit potential in the frequency range of 0.01 Hz to 100,000 Hz, an AC signal’s electrochemical impedance spectrum (EIS) with an amplitude of 10 mV was recorded.

3. Results and Discussion

3.1. Microstructure Analysis

According to the binary phase diagram of Zn-Al (Figure 2), the whole transformation process of each component alloy phase is analyzed as follows:

(1) Zn-5Al

L→ Eutectic (η + β) → Eutectic (η + Eutectoid (α + η)).

(2) Zn-15Al

L→L1 + Proeutectic α → L2 + Peritectic β → Peritectic β + Eutectic (η + β) → Eutectoid (α + η) + Eutectic (η + Eutectoid (α + η)).

(3) Zn-22Al

L → L1 + Proeutectic α → L2 + Peritectic β → Peritectic β → Eutectoid (α + η).

(4) Zn-25Al

L → L1 + Proeutectic α → L2 + Peritectic β → Peritectic β → α +Peritectic β → α + η_II + Eutectoid (α + η).

The solidification process of the alloys is usually non-equilibrium solidification, and scanning electron microscopy (SEM) was essential to fully understand the microstructure of the samples. The following subsections provide a detailed analysis of the changes in microstructure.

3.1.1. Microstructure of as-Cast Alloys

Figure 3 shows the scanned images of the as-cast samples, and

Table 1. Elemental composition of different locations in Figure 3f (at.%).

Location	Zn	Al
1	97.26	2.74
2	96.02	3.98
3	74.69	25.31
4	75.14	24.86

lists the elemental compositions of locations 1–4. Combining the XRD analysis results of the as-cast alloys (Figure 4), it can be found that four Zn-Al alloys are composed of Al-based solid solution (α-Al) and Zn-based solid solution (η-Zn), with no compounds present. Zn-5Al is composed entirely of a coarse eutectic structure [24]. When the total content of Al is 15 wt.%, the alloy consists of both eutectoid (α+η) and eutectic (η+β) structures [25,26,27,28,29]. Furthermore, Zn-15Al can be found to have the finest eutectoid structures compared to the other three alloys [30]. As the Al content continues to increase, it is found that the alloy consists of the α-Al phase, eutectoid (α+η) and eutectic (η+β) phases, and a small amount of η-Zn phase. The eutectoid structures in the alloys become progressively coarser compared with Zn-15Al, which occurs due to the fact that the growth of the Al-based solid-solution incipient phase is accelerated by the elevated concentration of Al atoms. In the supercooled liquid alloy, Zn atoms and Al atoms are easily adsorbed on the primary phase of a larger size, and it is not easy to produce new nuclei, so the number of nuclei is less, and the size of the primary phase of the Al-based solid formed is coarser. When the temperature is lowered to the eutectoid point, the eutectoid reaction occurs in the Al-based solid of the eutectoid composition, resulting in the eutectoid structure. Due to the large size of the primary phase of the Al-based solid solution, the size of the eutectoid structure formed by the solid-state phase transformation is also large, as shown in Figure 3c,d. Eutectic reaction does not occur in alloys when Al content exceeds 17.2 wt.%, whereas eutectic structure can still be seen in Zn-22Al and Zn-25Al, due to the occurrence of dendritic segregation during solidification of the alloys, and the eutectic line in non-equilibrium solidification is prolonged from 17.2 wt.% to more than 50 wt.%. In addition, due to the increasing deviation from the eutectic point, there is less and less eutectic structure in the alloy and a gradual transition to the η-Zn phase.

3.1.2. Microstructure of Alloys in the Extruded State

Figure 5 shows the microstructure of the extruded-state alloy, and the XRD analysis results in Figure 4 and Figure 6 show that the types of phases in the alloys did not change after extrusion. From Figure 5a, it can be noticed that the eutectoid structure of a rather large size appears in Zn-5Al. As the Al content continues to rise, the size of the eutectoid structure in the alloy gradually becomes smaller. The continuous precipitation transition mostly occurs in the eutectic structure [25,26]. The metastable Al atoms tend to be present on the supersaturated η-Zn phase, and the α-Al phase precipitates more readily in plastic deformation at eutectic structures with higher Al content [19,22,23,24]. The elemental compositions of locations 5–9 are shown in

Table 2. Elemental composition of different locations in Figure 7a (at.%).

Location	Zn	Al
5	92.57	7.43
6	93.12	6.88
7	35.67	64.33
8	34.82	65.18
9	26.79	73.21

. Comparing locations 1–4 in Figure 3f, locations 5–6 in Figure 7a show less Zn content in the white η-Zn phase and higher Al content in the point-like precipitates in the η-Zn phase (locations 7 and 8 in Figure 7a). The following reactions are inferred to be possible [31]:

$${\eta }_s\to {\beta }_E+{\eta }_E$$

(2)

where the ${\beta }_E$ as stable punctate precipitates has the same fcc structure as both β-ZnAl and Al-based solid solution. η-Zn transformed from the supersaturated state (${\eta }_s$) into the equilibrium ${\eta }_E$.

The shearing action of extrusion destroys the α-Al phases of large sizes in the as-cast alloy, leading to their redistribution in the matrix. And as the alloy composition deviates from the eutectic point, the amount of α-Al precipitated from the eutectic decreases to a negligible amount. Therefore, the black α-Al phases uniformly distributed in Zn-22Al and Zn-25Al are mainly formed by the crushing of large-size α-Al phases in the as-cast alloys. In addition, some large black zones (Al clusters) are found in Zn-22Al and Zn-25Al, whose compositions are shown in location 9 of Table 2. The data indicate that there is a significant difference in the Al content in this region compared to the other regions. This may be due to the phenomenon of grain boundary segregation that occurs in the alloys during the extrusion process, resulting in the accumulation of Al elements at the grain boundaries. This phenomenon may have a negative impact on the mechanical properties of the alloy [29,32].

3.2. Mechanical Properties

The mechanical properties of the solder will directly affect the quality and performance of the brazed joint, and in this section the mechanical properties of the solder are evaluated by means of a microhardness and a tensile test.

3.2.1. Microhardness Test

Figure 8 shows the microhardness of the extruded-state Zn-Al alloy. The results show that the microhardness of the alloys exhibits a tendency to increase and then decrease with an increase in Al content. A related study found that the hardness of α-phase is 143.5 HV and the hardness of η-Zn-phase is 63.2 HV, and the microhardness of the eutectoid structure should be in between the α-phase and η-phase, indicating that α is a harder phase than η-phase and the eutectoid structure [24]. Figure 5 shows that the Al content in Zn-5Al is less, and its microhardness mainly depends on the η phase, so the microhardness value is lower. When the Al content reaches 15 wt.%, the microstructure of the alloy is basically composed of eutectic and eutectoid structures, so Zn-15Al has a higher value of microhardness. As the Al content continues to increase, the α-Al phase gradually increases in microhardness number. From the distribution of the phases, the microhardness of the alloy should increase with the rise in Al content. However, from the test results, the microhardness tends to decrease when the Al exceeds 15 wt.%. This is because there is also a close relationship between the grain size and the microhardness of the alloy. The as-cast Zn-15Al has the finest eutectoid structure among the four alloys, indicating that its grain size may be the smallest. The grain size may be further refined, and the number of grain boundaries increases after extrusion, while the more grain boundaries there are, the greater the resistance to plastic deformation and the higher the hardness and strength of the alloy at room temperature, which is in accordance with the Hall–Petch relationship. And the coarser grains of as-cast Zn-22Al and Zn-25Al may make it difficult to refine the grains sufficiently during the extrusion process, resulting in a reduction in the microhardness of the alloys. Thus, Zn-15Al has the highest microhardness.

3.2.2. Tensile Test

The mechanical properties of the four specimens under room-temperature tensile conditions are shown in Figure 9 and

Table 3. Mechanical property parameters of the samples.

Sample	Zn-5Al	Zn-15Al	Zn-22Al	Zn-25Al
Rm (MPa)	210	233	221	231
Rp 0.2(MPa)	152	167	161	165
Elongation (%)	115.2	116.88	139.8	107.48

. Among them, Zn-15Al has the highest tensile strength of 233 MPa, while Zn-22Al has the best plasticity. This is because the α-Al phase is the strengthening phase in the alloy, and appropriate amounts of α-Al can improve the tensile strength and the plasticity of the alloy. Zn-25Al has the lowest ductility, due to the fact that when the Al content is too high the Al element undergoes a lot of segregation in the alloy (Figure 5d). The segregation area has a different structure from the rest of the surrounding structures, which makes it easier for stress concentration to occur. Once a crack forms at the grain-boundary segregation, it will expand rapidly, causing premature fracture of the alloy, thus reducing the tensile strength of the alloy.

Figure 10 shows the SEM images of the fracture surfaces of the four Zn-Al alloys, and it can be seen that the fracture surface of each of them has dimples on it. This proves that the fracture modes of the alloys are all ductile fractures. When the Al content is 22 wt.%, the dimples of the fracture are numerous and deep, indicating the best plasticity, which is consistent with the results of the tensile tests above.

3.3. Electrical Property

High conductivity and low resistance are necessary to form good solder joints [22]. The conductivity of the sample was first measured using a conductivity meter and then converted to resistivity. The conversion formula is as follows:

$${\sigma }_r=\frac{{\sigma }_{20}}{{\sigma }_{Cu}}=\frac{{\rho }_{Cu}}{{\rho }_{20}}\times 100\%\mathrm{I}\mathrm{A}\mathrm{C}\mathrm{S}$$

(3)

${\sigma }_r$ is the international standard conductivity percentage for annealed Cu, %IACS; ${\sigma }_{20}$ is the conductivity of the specimen at 20 °C, Ω⁻¹. m⁻¹; ${\sigma }_{Cu}$—20 °C is the international standard conductivity for annealed Cu, 0.58 × 10⁸ Ω⁻¹. m⁻¹; ${\rho }_{Cu}$ is the international standard resistivity of annealed Cu at 20 °C, 1.724 × 10⁻⁸ Ω·m and ${ \rho }_{20}$ is the resistivity of the specimen at 20 °C, Ω·m.

The results obtained are shown in Figure 11. The resistivity of the alloy decreases with the rise in Al content, which indicates that the addition of Al elements has a positive effect on the electrical properties of the extruded Zn-Al alloy, which is consistent with previous research findings [24,33]. The electrical properties of alloys are usually affected by a combination of grain boundaries, dislocations, solid-solution atoms, and second-order relative electron scattering [34]. In general, the more alloying elements that are added to a certain metal, the more serious the lattice distortion caused, and the more serious the scattering of electrons that occurs, thus increasing the resistivity of the alloy. However, in this experiment, the alloy was extruded and dynamically recrystallized, which reduced the effect of lattice distortion, to a certain extent. The existence of the element in the alloy has an important relationship with the influence of the resistivity of the alloy. When the element exists in the alloy in the precipitation state, as opposed to the solid solution, its existence is more conducive to improving the electrical conductivity of the metal. But, as mentioned above, as the alloy deviates further and further from the eutectic point, less and less α-Al precipitates from the eutectic, so the redistribution of the α-Al phase after extrusion crushing is the main reason for the increase in alloy conductivity. Since the resistivity of Al is lower than that of Zn, the conductivity of the α-Al phase in the alloy may be higher than that of the η-Zn phase, and the higher the Al content, the more the α-Al phase, and the better the conductivity.

3.4. Wetting Ability

A decent and effective soldered joint largely depends on the wetting ability of the solder alloy [15]. The reaction of Zn-Al solder on Cu plate is more complicated: take the solder wetting and spreading on the Cu surface as an example. The whole spreading process of the solder and the role played by the CsF-AlF₃ brazing flux in the wetting process can be summarized as shown in Figure 12: (1) when the temperature reaches the melting point of the brazing flux, the CsF-AlF₃ flux reacts with the CuO on the Cu surface, causing gaps in the oxide film on the surface of the base material, as shown in Figure 12a,b. (2) The Al and Zn elements in the solder penetrate into the membrane along the broken gap of the oxide film on the Cu surface and diffuse with the Cu atoms to form Cu-Zn and Cu-Al compounds to break the oxide film and push away the broken oxide film through the flow of liquid brazing material, which promotes the dissolution of the oxide film in the liquid solder, as shown in Figure 12c,d. (3) The brazing flux covering the surface of the liquid brazing material reduces the interfacial tension of the liquid brazing material, improves the wetting effect of the brazing material on the Cu base material, and can effectively prevent the oxidation of the base material and the solder.

Figure 13, Figure 14 and Figure 15 demonstrate the influence of Al element content on the spreading area and actual spreading situation of the solder on 6061 Al plate and T2 Cu under experimental settings at 530 °C. The test results show the following: (1) the spreading area of the solder on the Al substrate shows a tendency to increase and then decrease, and reaches its maximum when the Al content is 15 wt.%. This is because Zn and Al have a large mutual solubility, and when the solder spreads on the Al substrate it will penetrate into the base material at a fast rate, thus negatively affecting the spreading of the solder on the surface of the Al plate [14]. However, as the Al content increases, the Zn content decreases accordingly, and the diffusion rate of Zn in the base material slows down, which is conducive to the diffusion of the solder. But when the Al content exceeds 15 wt.%, the melting point of the solder increases, which leads to an increase in its viscosity and the wetting properties deteriorate [35,36]. (2) With increasing Al content, the spreading properties of the solder on Cu surfaces are significantly lower than those on Al surfaces, and the spreading regulation on Cu surfaces is opposite to that on Al surfaces. This is because Zn-5Al is a kind of eutectic alloy with a small temperature difference between the solid and liquid phases, and the liquid solder has good fluidity. When the Al content is higher, the Al element easily reacts with Cu to form compounds, and the free energy formed by the diffusion of Cu and Al atoms is one of the driving forces for the wetting and capillary phenomena [14,15]. Liquid solder can wet the substrate better if the solder and the substrate can dissolve each other or form compounds with the substrate, but when the intermetallic compounds formed are more prominent and are there in large quantities, the diffusion of the solder will be hindered, which reduces the wetting performance of the solder [14,15,37].

The cross-sectional structure of the specimens spread on Cu is shown in Figure 16. In Figure 16a, no obvious interfacial layer is found, but many black dots are found in the region of A on Cu. Combining the enlarged view at A in Figure 16a and the results of the energy spectrum analysis at its point 1 (Figure 16c,f and

Table 4. Results of EDS (at.%).

Location	Zn	Cu	Al
A (Point 1)	56.20	38.06	5.74
B	66.61	24.92	8.47
C	17.10	44.12	38.78
D	8.88	39.03	52.09

), it can be found that the content of Zn on Cu is very high, which is attributed to the fact that Zn diffuses from the solder to Cu, reducing the interfacial energy between the liquid solder and Cu, which in turn makes the Zn-5Al have better wetting ability on the Cu surface. From the Al-Cu-Zn ternary phase diagram, when the Zn-Al solder spreads on Cu substrates it may generate phases such as CuZn, CuZn₂, CuAl₂, etc. [19]. From Table 4, it is found that the interface layer in Figure 16b contains a higher amount of Zn at point B, and the atomic ratio of Cu and Zn elements is close to 1:2, which indicates that the CuZn₂ phase may be formed. Cu-Zn compounds have higher melting points than Cu-Al compounds, and a solid, continuous inhibition layer forms between the substrate and the solder first, which restricts the mutual diffusion and chemical reaction between the solder and the base material, thereby worsening spreading performance, and thus Zn-15Al has poor spreading performance on Cu substrates. The as-mentioned inhibition layer is also naturally visible after solidification. The content of Zn in C and D is less, and Cu mainly forms compounds with Al. Figure 16d,e show the intermetallic compounds in Cu grow in the form of shoots and dendrites. However, the shoot and dendritic characteristics of the compounds in Zn-25Al are not as significant as those of Zn-22Al, and the number of interfacial compounds is less. Therefore, Zn-25Al has better spreading performance on Cu substrates.

3.5. Corrosion Resistance

Aging deterioration of the Zn-Al alloy resulting from intergranular corrosion in a hot and humid environment, which leads to a sharp rise in brittleness and the deterioration of fluidity, should not be neglected in many practices [31]. Therefore, in this section, the corrosion resistance of extruded-state Zn-Al solders is investigated.

3.5.1. Effect of Al Content on the Rate of Self-Corrosion of Zn-Al Alloys

The effect of Al content on the corrosion rate of the solder alloys in the immersion test is shown in Figure 17. It can be found that the corrosion rate of the solder shows a trend of decreasing and then increasing with the increase in Al content. After 168 h of immersion in a 3.5 wt.% NaCl solution, it was seen that the surface of the sample was covered with a layer of white powdery corrosion products, and that some of the corrosion products were detached from the surface of the sample and precipitated into the test solution. After drying the specimen, the corrosion products were analyzed by SEM and EDS. It was discovered that the corrosion products were mainly composed of Zn, Cl, Na, O, and Al. According to previous studies, the corrosion products were mainly Zn (OH)₂ and Al (OH)₃ [22,36,37].

From Figure 18, it can be seen that the effect of Al content on the morphology of the corrosion products of the solder is significant. The morphology of corrosion products on the Zn-5Al surface is rosette-shaped and needle-like, and when the Al content reaches 15 wt.% the corrosion morphology is gradually transformed into dense lamellar flakes, which indicates that the corrosion rate gradually slows down. When the Al content continues to rise, the morphology of the corrosion products gradually changes to an irregular shape and shows an irregular dispersion, which may result in the corrosion products not protecting the substrate and thus accelerating the corrosion rate. In addition, the presence of cracks was also found in the micrographs. The presence of white powdery corrosion products and cracks proves that the electrochemical corrosion of alloys is dominated by both grain boundary corrosion and selective phase corrosion [22,38,39].

3.5.2. Effect of Al Content on the Self-Corrosion Potential of Alloys

The Tafel curves of the samples are exhibited in Figure 19. As the Al content rises, the corrosion potential gradually moves to the positive pole, but the difference is very small and difficult to distinguish. Therefore, the self-corrosion tendency of an alloy can be determined by the magnitude of the corrosion current density (I_corr). Through

Table 5. Specimen corrosion results.

Sample	Icorr/μA·cm−2	Ecorr/VSCE
Zn-5Al	10.21	−1.31
Zn-15Al	5.64	−1.28
Zn-22Al	9.09	−1.28
Zn-25Al	11.47	−1.29

, we can find that Zn-15Al has the lowest current density, indicating that it has the best corrosion resistance.

Moreover, the relationship between the rate of self-corrosion and the current density of alloys can be expressed by Faraday’s equation [40]:

$$\mathsf{\nu }=\frac{M\times i_{corr}}{nF}=\frac{3.73\times {10}^{-4}\times M\times i_{corr}}{n}$$

(4)

$\mathsf{\nu }$ is the corrosion rate (g/m²h), $M$ is the relative atomic mass of a metal, $i_{corr}$ is the corrosion current density (A/cm²), $n$ is the valence, and $F$ is the Faraday constant (1F = 96,500C = 26.8 A.h).

This equation shows that the rate of galvanic corrosion of a metal is directly proportional to the current density. The higher the current density, the higher the rate of self-corrosion. Therefore, in conjunction with the data in Table 5, it can be concluded that the corrosion resistance of the alloy increases and then decreases with increasing Al content, which is consistent with the results of the immersion experiments described above. Thus, it can be concluded that the corrosion resistance of the alloy increases and then decreases with the rise in Al content, which is consistent with the results obtained from the immersion experiments above.

However, the corrosion current density depends strongly on the range selected on the polarization curve, and has a random nature [41]. It is necessary to evaluate the corrosion resistance of solders with the electrochemical impedance spectra (EIS) [42]. Figure 20a shows that the electrochemical impedance spectra of the alloy are a typical impedance spectrum, with Warburg impedance. The Nyquist semicircular arcs in the high-frequency region represent the process of charge transfer between the solder and the NaCl solution, and the straight lines in the low-frequency region (Warburg impedance) represent the diffusion-controlled process of the electrode reactants or products. The radius of the semicircle (impedance spectrum) represents the resistance of the interface between the NaCl solution and the electrode, with a larger radius indicating a higher impedance and thus a lower corrosion rate [40]. Using Zview Version 3.4 impedance spectrum-analysis software to fit the raw data in the Nyquist curve based on the basic R-C circuit, Figure 20b is obtained, and it can be found that the corrosion pattern obtained is basically consistent with that obtained from the Tafel curve.

Galvanic corrosion of Zn-Al alloys occurs because the α-Al phase and Zn-Al eutectic structure in the alloy have a lower electrical potential compared to the other phases [35]. As the Al content increases, the amount of α-Al phase and Zn-Al eutectic structure in the alloy may increase and, therefore, corrosion is more likely to occur. In a corrosive medium, the α-Al phase of the alloy acts as the anode and the η-Zn phase acts as the cathode, and the following reaction occurs [22,43,44]:

Zn → Zn²⁺ + 2e⁻

(5)

Al → Al³⁺ + 3e⁻

(6)

2H₂O + O₂ + 4e⁻ → 4OH⁻

(7)

Combining the microstructure of the extruded-state alloy in Figure 5, it was found that the number of α-Al phases in the alloy increased with the rise of the Al content; this would significantly increase the potential difference between the α-phase and the η-phase and thus increase the driving force of the galvanic coupling corrosion process and make the corrosion more likely to occur [38]. In addition, because Zn-5Al has fewer added Al elements, it is difficult to generate a continuous Al₂O₃ protective film to inhibit the electrochemical corrosion process, and thus corrosion is also more likely to occur [22].

Through the above immersion experiments and electrochemical tests, it can be found that the corrosion resistance of Zn-Al alloys shows a tendency to increase and then decrease.

4. Conclusions

By studying the microstructure and properties of Zn-Al alloy solders in the extruded state, it can be found that Zn-15Al has the highest microhardness and tensile strength, the best corrosion resistance, the best wetting ability on Al plates, and moderate resistivity when the Al content is 5–25 wt.%. Therefore, Zn-15Al has the best application prospects for subsequent brazing. However, due to the poor wetting properties of Zn-15Al on Cu plates, which will have a significant adverse effect on the quality of brazed joints, further alloying of Zn-15Al is required to improve its wetting properties on Cu plates.