**1. Introduction**

Zinc-based alloys have good tribological properties, relatively high mechanical strength and hardness values, and show good castability due to their low melting points. These features make them good candidates for use in automotive and electronics applications, and they have also been used in the production of small components and plain bearings. Studies have shown that these alloys have superior wear resistance to common copper-bearing alloys. Adhesion and smearing are the main wear mechanisms of zinc-based alloys, while abrasive wear is the predominant wear mode in bronzes [1–5]. In the last decade, zinc has been extensively studied as a potential biocompatible and biodegradable metal for medical applications [4,6–10].

Zinc has one of the lowest electrode potentials, and machines made from it are highly susceptible to electrochemical corrosion due to the formation of a galvanic cell. The presence of extensive corrosion may also affect other co-working components made of different materials. The resulting corrosion products affect the pH of the surrounding environment, which, in turn, may accelerate the degradation of lubricants. On the other hand, a low electrochemical potential gives zinc and its alloys broad application prospects as cathodic protection coatings. Thus, Zn-Al alloys may be used to replace traditional zinc galvanic coatings [11–18] The commercial Galfan alloy has found broad applications in this area [18] but it exhibits several serious drawbacks, including a low creep resistance, low shape stability associated with aging, insufficient corrosion resistance in acidic and alkaline environments, and a low cavitation erosion resistance [5,19–21]

The main alloying elements in Zn-based alloys are aluminum, magnesium, and copper. Cast Zn-Al alloys are commercially available under the Zamak tradename, the most popular of which is the Zamak 3 alloy which has a nominal composition of 4% Al. This aluminum content classifies this alloy as hypoeutectic (Figure 1) whose microstructure is composed of a η-Zn(Al) dendrite solid solution and (α+η) eutectic phases, in which the α solid solution is Al(Zn).

**Figure 1.** Zn-Al phase diagram adapted from [5].

Chloride ions are one of the primary catalysts responsible for the corrosion of zinc and its alloys. Components made from these alloys are exposed to chloride ions in both seawater and also in seaside environments. Micron-sized salt aerosol particles can be deposited on elements located as far as 10 km from the shoreline. The threat in engineering practice may be intergranular corrosion by chloride ions [10,22,23], which was the main reason that a solution rich in these ions was used as a corrosive environment.

Analyzing the corrosion mechanisms of Zn-based alloys is challenging due to their complex microstructures. However, previous studies have only focused on the influence of factors affected by the crystallization conditions (from the temperature of the liquid phase). This translates into an effect on the size and branching of dendrites, as well as eutectic dispersion. Thus, practical considerations are important during casting. The grain size and microstructure morphology (affected by the crystallization conditions) [5,16,17,19,24] and cooling rate have been shown to affect the corrosion resistance of zinc alloys [12,24–27]. Finer dendrites were shown to improve the corrosion resistance of hypoeutectic alloys, whereas a coarse microstructure was more preferable for hypereutectic alloys [5,17]. The aim of this work was to determine the effect of the cooling rate during the eutectoid transformation on the corrosion resistance of a hypoeutectic Zn-4Al alloy. These changes apply to the crystallized alloy and are relevant to determining a heat treatment process. As a part of this research, samples were heat-treated at temperatures higher and lower than the eutectoid transformation, and electrochemical studies were combined with metallographic studies to confirm the effect of heat treatment on the alloy's microstructure. The surface condition of the alloy was assessed after electrochemical tests to determine the role of microstructure during corrosion.

#### **2. Materials and Methods**

The investigated material was a Zn-4Al alloy that was fabricated by melting and casting pure elements (99.995% Zn and 99.7% Al) in a PIT10 induction furnace. The obtained material was subjected to heat treatment by annealing for 1 h at 250 ◦C and 300 ◦C, followed by cooling. The samples were quenched in water and cooled in air or in a furnace. The scheme of the research design is shown in Table 1.

Hardness measurements were performed using the Vickers method. Microscopic examinations were carried out using a stereoscopic microscope (Leica M205 C, Leica Microsystems, Wetzlar, Germany), a light microscope (Nikon Eclipse MA 200, Nikon Instruments Inc., Tokyo, Japan), and a scanning electron microscope (SEM) (Phenom World ProX, Thermo Fisher Scientific, Waltham, Massachusetts, USA). Light microscopy was used to examine metallographic sections to identify microstructural features after Nital etching (3% nitric acid in ethanol) and 5% NaCl solution. Stereoscopic and SEM microscopes were used after electrochemical measurements to illustrate the corrosion progress.


**Table 1.** Scheme of the research design.

Polarization tests were performed using a three-electrode cell with a potentiostat (ATLAS 0531 ELEKTROCHEMICAL UNIT & IMPEDANCE ANALYSER, Atlas-Sollich, Gdansk, Poland). The auxiliary electrode was made of austenitic stainless steel, while a saturated Ag/AgCl electrode was used as the reference electrode. Just before the experiments, samples were subjected to mechanical grinding with 800 SiC emery papers. The surface area of the working electrode (the sample) was 0.785 cm2. Before experiments, each sample was immersed for 20 min in 250 mL of a 5% NaCl solution at room temperature. After that, the open circuit potential (EOCP) was measured. Polarization tests were conducted in the same solution by stepping the potential in the anodic direction using a scanning rate of 1 mV/s from −250 mV relative to the open-circuit potential. The pH of the applied solution was 7.5. Four anodic and cathodic polarization curves were recorded for the as-delivered material. The initial potential value was 200 mV lower than the EOCP value. The polarization of each tested sample was terminated at different potential values. Thus, the potentiodynamic curves were stopped at potentials of +150, +225, +300, and +450 mV vs. Ecorr. Polarization curves were also obtained for samples heat-treated at 300 ◦C. Three curves per series were determined for the heat-treated alloy. The polarization curves were plotted using an automatic data acquisition system, and the corrosion potential (Ecorr) and corrosion current density (Icorr) were estimated by Tafel plot extrapolation.
