**3. Results**

#### *3.1. Hardness Measurements*

The hardness measurement results and their standard deviations in Figure 2 show that the hardness of the as-delivered material was 58 ± 2 HV1. Changing the cooling rate affected the hardness of samples heat-treated at 300 ◦C. The hardness increased by more than 20 HV1 for the water-quenched sample compared with the material that was furnace-cooled from the same temperature (300 ◦C). This is due to the eutectoid transformation which occurred at 275 ◦C. Faster cooling promoted the formation of finer (α + η) eutectoid structures from the γ phase, while slower cooling allowed the alloy to form a coarser eutectoid structure, which translated into a lower hardness. The increased hardness due to the increased cooling rate realized from the beginning crystallization temperature and microstructure refinement has been observed by other Authors [27].

To provide a comparison, heat treatment was also carried out at a temperature lower than the eutectoid transformation, i.e., 250 ◦C, and various cooling rates were also used. The cooling rate had no effect on the material hardness at this temperature, which indicates that the formed microstructure was stable. The slight differences in the hardness values were within the standard deviation.

**Figure 2.** Hardness values obtained for heat-treated Zn-4Al alloy at 250 and 300 ◦C using various cooling rates.

#### *3.2. Microstructural Examination*

The microstructure of the material in delivered state was typical of hypoeutectic Zn-Al alloys (Figure 3). Dendrites of the Zn-base solid solution (η) and an (α + η) eutectic lamellar structure were visible. The microstructure contained the product of eutectoid decomposition because the γ phase was transformed into (α + η) phase at 275 ◦C, as shown in Figure 1. On the other hand, rod-like eutectic features that may have been formed due to rapid quenching were not observed [11].

**Figure 3.** Microstructure of examined Zn-4Al alloy (as-delivered). Visible dendrites of η phase and a eutectic lamellar morphology (α + η). Etched with 10% NaCl solution. (**a**) Light Microscopy, (**b**) SEM.

After heat treatment at 300 ◦C, the effect of the cooling rate on the phase distribution in the eutectoid structure was examined (Figures 4 and 5). During the applied heat treatment, only the morphology of the microconstituents inside the lamellar structure was affected by eutectoid decomposition. Some divorced eutectic structure was also observed along the grain boundaries. Despite the eutectoid decomposition, the morphology of the interdendritic lamellar eutectic structure was not affected because it was not subjected to any solid-state transformation (Figure 4).

**Figure 4.** Microstructure of Zn-4Al alloy after heat treatment at 300 ◦C: (**a**) furnace-cooled, (**b**) air-cooled, (**c**) water-quenched. The visible morphology of the eutectic structure was not affected by the eutectoid transformation. Etched with Nital. Light Microscopy.

**Figure 5.** Microstructure of the Zn-4Al alloy after heat treatment at 300 ◦C: (**a**) furnace-cooled, (**b**) air-cooled, (**c**) water-quenched. A finer (α + η) eutectoid phase was formed from the γ phase. Etched with Nital. SEM.

#### *3.3. Electrochemical Examinations*

Figure 6 shows a comparison between the polarization curves of investigated samples. The corrosion current density and corrosion potential were estimated from the polarization curves using the Tafel extrapolation method (Table 2). The corrosion test results of the as-delivered material are presented as the average of four measurements. For the heat-treated samples measurements are shown as the average of three measurements. As expected, the Zn-4Al alloy had a negative corrosion potential, and the four curves obtained for the as delivered material were similar. This value was consistent with the results of other Authors [6,16]. A strong increase in the current density during the initial stage of the anodic curve was found, which indicates highly intense electrochemical processes.

The microstructure of the investigated material was composed of two phases—an α aluminum-rich solid solution and a η zinc-rich solid solution. The resultant electrochemical potential was closely related to the phase heterogeneity of the zinc alloy, i.e., to the corrosion potential of each phase. The phases with various electrode potentials became anodic and cathodic during contact between the alloy and the electrolyte [28]. Al has a nobler electrochemical behavior than Zn [17,29], and similar behavior should be attributed to Al-base and Zn-base solid solutions. It was previously shown that the anodic nature of the η phase depends on the pH of the corrosive agen<sup>t</sup> [16,24,30]. In slightly acidic or neutral environments, the α phase is nobler than the η phase, so it may act as a cathode. Conversely, in alkaline environments, the α phase may play the role of the anode [24,30]. Shihirova at al. [31] indicated that the electrochemical behavior of phases may be associated with local pH changing and their thermodynamic stability in this corrosive environment. In this study, experiments were carried out at a slightly alkaline pH of 7.5. However, the anodic processes lead to a local reduction pH due to the H<sup>+</sup> produced from the hydrolysis of Al3+ [32].

The test results show that the corrosion current density and corrosion potential change as the microstructural morphology changes. The other morphologies were obtained due to di fferent cooling rates during the eutectoid reaction. A very important factor in galvanic corrosion is the ratio of the anodic to cathodic area. If the surface of the cathode is larger than the anode, then more oxygen reduction or another cathodic reaction can occur, which increases the galvanic current. However, in this case, it remained at the same level, but the distance between the anode and cathode changed.

In this case, we had a corrosion microcell, in which the anodes and cathodes were separated by just a few microns. Previous electrochemical research determined that finer structures show a lower Icorr compared with a coarse structure. Ecorr remained rather constant, although it showed a slight decrease. It can be observed that the furnace-cooled structure was related to a corrosion current density and a corrosion potential of 7.01 μA/cm<sup>2</sup> and −1.06 V (vs. Ag/AgCl), respectively, compared with 4.74 μA/cm<sup>2</sup> and −1.07 V (vs. Ag/AgCl), respectively, for the water-cooled structure. Increasing the dispersion of cathode inclusions usually increases the cathode activity. However, if anode passivation occurs or a surface film of corrosion products forms, its activity can be decreased, and the anode process will be inhibited. On the other hand, the short phase distances typical of eutectoid structures may have protected the anode phase. This e ffect may be clearer due to the finer eutectoid structure.

The Ecorr value was more electronegative than EOCP. The di fferences between the EOCP and Ecorr values were due to the di ffusive nature of the cathode potential curve, which has been previously observed during anodic polarization [29].


**Table 2.** The electrochemical parameters obtained for as delivered samples, as well as samples heat-treated at 300 ◦C and subjected to di fferent cooling rates.

**Figure 6.** Example potentiodynamic polarization curves of the as-delivered Zn-4Al alloy and heat-treated at 300 ◦C in 5% NaCl solution. In the curve of the sample polarized to the highest potential value, the potential values (relative to Ecorr) were marked where polarization was stopped.

#### *3.4. SEM Surface Evaluation after Corrosion Tests*

The surfaces of the as-delivered material after electrochemical study were examined using SEM. Samples whose anodic polarization was terminated at different potential values were examined in order to illustrate the corrosion progress in chloride-containing media. The results were discussed in relation to the structural features of the alloys.

Corrosion began locally with the formation of aluminum-rich corrosion products (Figure 7, Table 3). The microscopic observations of the sample tested after reaching a potential of +150 mV versus Ecorr, did not permit the determination of which structural features underwent corrosion at this stage of development. However, the high aluminum content in the corrosion products on the surface suggested that degradation mainly involved eutectic areas. The formation of aluminum-rich corrosion products first may be unfavorable from the point of view using the alloy as a biomaterial.

Previous works have reported the preferential oxidation of Al-rich areas [29,33]. Other authors have shown that the α phase was protected at the initial stages of corrosion due to the formation of a corrosion product surface film that contained various aluminum-rich phases [24,34–36]. The presence of chlorine indicates that chlorides play an active role in the formation of corrosion products (Table 3). The simultaneous presence of Zn, Al, and Cl in the EDX spectra may be attributed to the formation of Zn2Al(OH)6Cl·2H2O, which has been reported to form during the early stage of corrosion [18]. Other Authors have observed an Al2(OH)5Cl·2H2O phase [35,36]. In this case, zinc may be associated with the base material. It is believed that, regardless of the chemical composition, these phases provide excellent protection against further corrosion.

Based on the electrochemical tests and the above literature data, it can be hypothesized that the finer Al-base phase in the eutectoid structure may result in the formation of a more compact corrosion product film that increases the temporary corrosion protection. The formation of a corrosion product film on the α phase can help reduce the corrosion current density as the distance between eutectoid components decreases. Consequently, the finer distribution of the two phases that formed during eutectoid decomposition in the eutectic mixture tended to decrease their corrosion rate.

**Figure 7.** (**a**) SEM image of the surface of a sample after polarization up to a potential of +150 mV vs. Ecorr. Corrosion initiation areas are visible. The corrosion products are rich in aluminum and chlorine (marked with point 1 and summarized in Table 3; (**b**) characteristic X-ray emission spectrum obtained from point 1 in Figure 7a.


**Table 3.** Chemical composition obtained from EDX analysis of point 1 in Figure 7a.

As corrosion progressed and the potential increased to +225 mV vs. Ecorr, the alloy selectively dissolved. At this stage, due to the formation of an electrochemical cell between the α and η phases, the eutectoid (α + η) became susceptible to corrosion (Figure 8). Thus, the anode phase was present only in eutectoid areas, which suggested the α phase. When immersed in the corrosive solution, the hypoeutectic Zn-4Al alloy displayed Al-rich regions (the phase of the eutectic structure) which acted as anodic barriers that protected the η phase. Corrosion gradually occurred throughout the entire eutectic area (Figure 9), which was also reflected by a macroscopically visible color change over the sample surface where eutectics formed. The selective dissolution of eutectic areas has also been documented in other works [18,24,37]. Despite this, the local dissolution of η phase dendrites was also observed at higher magnifications (Figure 9a).

**Figure 8.** SEM image of the surface of a sample after polarization up to a potential of +225 mV vs. Ecorr. Selective dissolution of the eutectoid (α + η) in eutectic areas of the Zn-4Al alloy is visible.

**Figure 9.** (**a**) SEM image of the surface of a sample after polarization up to a potential of +225 mV vs. Ecorr. Selective dissolution of the Zn-4Al alloy is visible. The dark areas represent areas in which the eutectic structure has dissolved. (**b**) Magnified image.

The sample polarized up to a potential of +300 mV vs. Ecorr experienced more extensive corrosion of the eutectic areas over its entire surface (Figure 10). At this stage, the dissolution of the η phase and the revealed α crystals (or products of its corrosion), was observed at the macroscopic scale (Figure 11). This is consistent with the observations that the lamellar structure enables the storage of corrosion products in areas of the corroded α phase, thereby delaying the corrosion process in the eutectic η phase [36]. A higher oxygen content was observed in the dendritic regions (Figure 12). The dendritic η phase dissolved and underwent anodic dissolution reactions [29] which resulted in a constant increase of the current density with the increased polarization potential (Figure 6).

**Figure 10.** SEM image of the surface of a sample after polarization up to a potential of +300 mV vs. Ecorr. Corrosion initiation locations of the Zn-4Al alloy are visible. The dark areas represent areas of the eutectic alloy affected by corrosion.

**Figure 11.** Stereoscopic image of a sample after polarization up to a potential of +300 mV vs. Ecorr. Selective dissolution of the dendritic η phase at the macroscopic scale.

**Figure 12.** SEM and EDX images showing the distribution of elements on the sample surface after polarization up to the potential +300 mV vs. Ecorr: (**a**) SEM image merged with zinc and aluminum, (**b**) zinc, (**c**) aluminum, (**d**) oxygen. Element-rich areas are darker in the image.

After reaching a potential of −675 mV vs. Ag/AgCl (+450 mV vs. Ecorr) the current density decreased on the potentiodynamic curve (Figure 6). The SEM observations of samples tested at a higher potential revealed that at this stage, corrosion extended to all structural constituents (Figure 13). Due to the selective dissolution of the η phase and the macroscopic exposure of (α + η) eutectoid areas, surface topography was observed (Figure 14). These microscopic observations sugges<sup>t</sup> the anodic character of the η phase relative to the α phase in the corrosive solution at this stage of corrosion. Zn<sup>+</sup> ions are formed in the Zn-rich phase (η) due to anodic reactions: Zn → Zn2<sup>+</sup> + 2e<sup>−</sup>, while the Al-rich phase (α) is expected to be responsible for the cathodic reactions: O2 + 2H20 +4e− → 4OH<sup>−</sup>. This indicates that despite the initiation of corrosion in areas of the α phase, there is a change in the η phase polarity and its corrosion. This is most likely due to the formation of a corrosion product film on the surface of the α phase that protects it from further corrosion, in accordance with other works [18,24,34]. Thus, changing the anodic zone polarity due to the formation of a protective film can be used in corrosion protection [38].

**Figure 13.** The surface of a sample after polarization up to a potential of +450 mV vs. Ecorr. Changes on the sample surface due to corrosion are visible over the entire alloy surface. SEM.

**Figure 14.** 3D SEM topography of the sample surface after polarization up to a potential of +450 mV vs. Ecorr. The image based on "shape from shading" technology shows the selective dissolution of the η-phase and a macroscopic exposure of the eutectic structure. Blue indicates the lowest areas, while red represents the highest features.
