Effects of Ti and Zr elements addition on the microstructure and corrosion resistance of Zn-2.5Al-2Mg alloy

In this article, the effects of Ti and Zr elements addition on the microstructure and corrosion resistance of Zn-2.5Al-2Mg alloy were studied. The microstructure, micro hardness, corrosion resistance of Zn-2.5Al-2Mg alloy were investigated by x-ray diffraction (XRD), Scanning electron microscopy (SEM) equipped with energy dispersive spectroscopy (EDS), Rockwell hardness tester and Electrochemical workstation, respectively. The result shows that the solidified structure of Zn-2.5Al-2Mg alloys was refined by Al3Ti, Al3Zr and Al3(Ti, Zr) which act as a heterogeneous nucleation site. Moreover, the Rockwell hardness of Zn-2.5Al-2Mg alloys with the addition of Zr was significantly increased. The hardness of this alloy with the same amount of Zr and Ti addition is lower than that of the alloy with Zr addition alone, but obviously higher than that of the alloy with Ti alone. The corrosion resistance of the Zn-2.5Al-2Mg alloys with the same amount of Zr and Ti addition was improved significantly.


Introduction
Steel with excellent properties, such as high strength and hardness, wear resistance and easy processing, is one of the most widely used metallic materials in the world. Nevertheless, steel which was affected by various environments was easily corroded and invalid in practical situation. Hot-dip galvanizing is the most common way for steel anticorrosion, by forming a zinc layer on the surface of steel. The addition of Al element can improve of the corrosion resistance of Zn-based coating, and several Zn-Al alloy coating systems have been developed, such as Galfan (Zn-5Al-Re) and Galvalume (Zn-55Al-1.6Si) [1,2]. Recent years, the microstructure and corrosion resistance of Zn-Al-Mg alloy coatings have been widely reported [3][4][5][6][7]. And the hot-dip galvanized coatings, such as ZAM (Zn-6Al-3Mg), Super Dyma (Zn-11Al-3Mg-0.2Si) and PosMAC (Zn-2.5Al-3Mg) are extensively adopted for building industry, home appliance and automotive applications [8][9][10][11][12].
Solidification structure of alloy is the most significant for the performance and the surface appearance of the final products. A large number of scientific literature focused on the effect of alloying elements (Mg, Ti, Zr, Sb, Re, etc) on the microstructure and corrosion resistance of zinc-based coating [13][14][15][16][17][18][19]. It is believed that the corrosion and mechanical properties was strongly affected by the microstructure of zinc alloy coatings. It has been reported that the grain size in cast Al alloys with Ti and Zr addition was refined by the nucleation of Al 3 Ti and Al 3 Zr [20,21]. Moreover, the Ti addition evidently refined the dendrite structure of the primary Al phase [22]. The addition of Ti can form a compact oxide film on the alloy surface, which has a good self-repairing ability and can prevent corrosion well [23][24][25][26][27]. Liu et al [28] studied the effect of Ti addition on microstructure and corrosion property of Zn-5Al alloy. The results showed that the addition of Ti refined the microstructure and improved the corrosion resistance of the alloy. Zhou et al [29] investigated the effect of Zr cotent on microstructure and corrosion resistance of hot-dip galvanized Zn-0.1%Ni-Zr alloy coating. The results showed that the addition of Zr significantly improved the corrosion resistance of Zn-0.1%Ni alloy coating. Li et al [30] studied the effect of Ti on solidification microstructure of Super Dyma alloy. The results showed that the Al-rich Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. dendrites were obviously refined, the size of the MgZn2 phase was greatly reduced and the Zn/Al/MgZn2 ternary eutectic structure was significantly refined.
Additionally, Ti and Zr elements are the important alloying elements in Al-based alloys and the addition of Ti or Zr elements alone in Al based alloys has been widely studied [31][32][33]. It is generally believed that the Ti and Zr addition can effectively improve the corrosion resistance and the mechanical properties of Al-based alloys. However, there are few studies on the effect of same content of Ti and Zr additions on the electrochemical properties of Zn-Al-Mg alloy.
With the increasing service requirements, traditional hot-dip alloys cannot meet people's needs. In recent years, a variety of mult-elemental zinc-based coatings have been developed, and the Zn-Al-Mg alloy coating is the most concerned one. In this work, the effects of Ti and Zr elements on the microstructures and corrosion resistance of Zn-2.5Al-2Mg alloy were studied. The addition of Ti and Zr elements individually or in equal quantities together has been studied in detail.

Experiment
Model alloys Zn-2.5Al-2Mg-xTi-yZr were prepared with zinc (99.99%), aluminum (99.5%), magnesium (99.99%), and Al-Ti, Al-Zr master alloys. All the solutions were prepared with the analytical pure regents and the deionized water. Zn-2.5Al-2Mg-xTi-yZr alloys were melted at 720°C and kept at the temperature for 3 h to dissolve and homogenized in an electric resistance furnace under the argon atmosphere. Afterward, the melt was cooled to 550°C and kept at the temperature for 2 h. Subsequently, the melt was cast into an ingot under atmospheric conditions. The chemical compositions of the cast alloys were detected by x-ray fluorescence spectrometer (XRF, Bruker Tiger S8) and corresponding results were presented in Table 1.
The structure and composition of the prepared alloys were investigated by x-ray diffraction (XRD, D8-advanY), scanning electron microscope (SEM, FEG250) and energy dispersive spectroscopy (EDS), respectively. The Rockwell hardness of the tested samples were measured by Rockwell hardness tester (HBRV-187.5).
All the electrochemical tests were using an IM6d Zahner-Elektrik workstations in 5 wt% NaCl solution at room temperature, taking platinum foil as a counter electrode, saturated calomel electrode (SCE) as a reference electrode and Zn-2.5Al-2Mg-xTi-yZr alloy with an exposed area of 1 cm 2 as a working electrode. The electrochemical measurements performed after the open circuit potential (OCP) was steady. Impedance spectra was obtained in the frequency range of 10 −1 Hz −10 5 Hz and the signal amplitude was set to 10 mV. Moreover, the samples were polarized from −3.5 to 1.5 V versus SCE and the free corrosion potential at a rate of 10 mV s −1 .

Results and discussion
The XRD spectrum of Zn-2.5Al-2Mg alloy in figure 1(a) qualitatively indicated that the dominant phases were Zn, Al and MgZn 2 . The rest peaks were so weak that the existence of Mg 2 Zn 11 cannot be verified. In addition, it cannot satisfy the condition of phase quantitative analysis by XRD because the sample was as-casted and there was strong preferred orientation. The typical structures of Zn-2.5Al-2Mg alloy was shown in figure 1(b). The spherical and gray dendrites was Zn-rich grain, and the granular/lamellar region surrounding the dendrite was Zn/Al/MgZn 2 ternary eutectic. In addition, there are a small amount of Zn/MgZn 2 binary eutectic surrounding the Zn phase. The SEM images of Zn-2.5Al-2Mg-xTi (x=0.05, 0.1, 0.15) alloys are shown in figures 2(a)-(c). The microstructure of the cast alloys are extremely resembled. Although the total area of Zn-rich particles did not differ substantially, it was mostly composed of the Zn phase, Zn/Al/MgZn 2 phases and the dark bulk-like Al 3 Ti phase distributed in the middle of the Zn grains. The EDS analysis of Zn-2.5Al-2Mg-0.1Ti alloy was shown in figure 3 and table 2. P1 with higher Al content is α-Al phase in the granular/lamellar region. The atomic ratio in   the dark area (P2) is close to Al: Ti=3: 1, which indicates that it is Al 3 Ti phase. P3 with lower Al content located at 4.27 at. % is Zn phase. The structure of Zn-2.5Al-2Mg-yZr (y=0.05, 0.1, 0.15) alloys can be identified in figures 4(a)-(c). Unlike the structure of Zn-Al-Mg alloy, the addition of Zr results in the appearance of Al 3 Zr phase. Figure 5 presents the SEM image and EDS-mapping of Zn-2.5Al-2Mg-0.1Zr alloy. It is noted that the lamellar region mainly includes Zn, Al and Mg. And the bright dendrite/globular region is mainly Zn with a small amount of Al dissolved. The gray block area is mainly the precipitated phase formed by Al and Zr. Additionally, through the detailed EDS analysis as shown in figure 6 and table 3, it can be confirm that the bulk-like phase is a type of Al 3 Zr phase (P1, P4) which exist as the enhanced phases in Zn-Al-Mg alloys. The comprehensive properties of the   It is considered that the Al 3 Zr phase will precipitate preferentially with the same amount Ti and Zr addition, when the temperature is above the peritectic reaction temperature of Al-Ti [34]. And when Al 3 Zr just precipitates and not aggregate, the Al 3 (Ti, Zr) phase was formed    due to Ti in the melt replaced part of Zr in Al 3 Zr phase, which can be used as effective nucleation particle to make the grains fine and uniform. Figure 9 shows the Rockwell hardness diagram of Zn-2.5Al-2Mg-xTi-yZr alloys. It can be evidently seen that the Rockwell hardness value of Zn-2.5Al-2Mg alloys were all increased by the addition of Ti and Zr elements.   With the addition of Ti, the hardness value of the Zn-2.5Al-2Mg-xTi (region I) increases first and decreases afterwards. When Zr is added, the hardness value of the Zn-2.5Al-2Mg-xZr (region II) significantly increased. When the content of the addition Zr reached to 0.15 wt%, the Rockwell hardness value of the alloy reached to 71.8 HRB. The hardness value of the Zn-2.5Al-2Mg-xTi-yZr alloys (region III) was higher than that of Ti added and slightly lower than that of Zr added. The reasons may be as follows: (1) The Al 3 Zr precipitated phase, which is hard and brittle, can improve the hardness of Zn-Al-Mg alloy. When the Al bath temperature is too high, aggregation is easy to occur in the solidification process due to Zr element has higher melting point, larger atomic radius and poor wettability. And the hardness of the alloy because of a large number of substitutional solid solution caused serious lattice distortion is greatly improved.
(2) The Ti has little effect on the hardness of the alloy because the Al 3 Ti phase has smaller atomic size, better coherence with the matrix and smaller lattice distortion.
(3) The hardness with the same amount Ti and Zr addition is not higher than that of the addition of Zr element, because adding Ti can reduce the lattice distortion caused by Zr added. Therefore, the same amount Ti and Zr addition can combine the advantages of Ti and Zr to achieve a perfect match between the microstructure and mechanical properties of Zn-2.5Al-2Mg alloy.   As seen in figure 10, all the curves show similar characteristics that the Nyquist plots consist of two complex capacitive loop and the corresponding Bode plots suggest two time constant in all case. The time constant at high frequency reveals the charge transfer impedance in the electric double layer, and the capacitive reactance loop in the low frequency region may characterize the resistance of surface conversion film of the alloy. The corresponding B-P diagrams in figures 10(b), (e), (h) shows the peaks at frequency around at the range of 10 2 -10 4 Hz and 10 −1 -10 0 Hz for tested specimens, respectively. In addition, comparing the Nyquist diagrams of Zn-2.5Al-2Mg-xTi-yZr alloys in figures 10(a), (d), (g), the loop length of these semicircles was increased by the addition of Ti and Zr, indicating that the modification enhanced the corrosion resistance of Zn-2.5Al-2Mg alloy. Based on the observation of Bode impedance |Z| diagrams in figures 10(c), (f), (i), the curve of Zn-2.5Al-2Mg alloy moved upward for a certain distance, which also indicated that the corrosion resistance of Zn-2.5Al-2Mg alloy was improved with the addition of Ti and Zr. The charge transfer resistance of the alloy and the width of the capacitance ring at high frequencies were enhanced with the addition of Ti and Zr. The reason may be that the defects in the structure are eliminated and the structure of alloy is more uniform with the addition of Ti and Zr.
To further interpret the interfacial reaction that occurred under the above situation, one equivalent circuit (EC) is employed for the EIS data fitting, as shown in figure 11. Rs is the solution resistance; Rct is the charge transfer resistance; Rc is the resistance of corrosion product layer; Q is the constant phase angle element; n is the dispersion coefficient, and the dimensionless constant range is 0-1.
The fitted EIS results for studied alloys were summarized in table 5. As compared in table 5, the Rs of all the tests are lower than 10 Ω cm 2 , which are much lower than Rct and Rc, indicating that all tests were in a stable environment. The equivalent circuit model did not change after adding the Ti and Zr, indicating that the corrosion behaviour of the Zn-2.5Al-2Mg alloy did not change at the initial stage of corrosion. The Rct of Zn-2.5Al-2Mg-xTi-yZr alloys were much better than Zn-2.5Al-2Mg alloy. Zn-2.5Al-2Mg-0.05Ti alloy has the max Rct which is 8070 Ω·cm 2 , about 100 times that of Zn-2.5Al-2Mg alloy. With the addition of Ti, the Rct of Zn-2.5Al-2Mg -xTi decreased. The Rct of Zn-2.5Al-2Mg -0.1Zr alloy is 2308 Ω cm 2 much higher than that of Zn-2.5Al-2Mg alloy, but only 1/4 of that of Zn-2.5Al-2Mg-0.05Ti alloy. The Rct of Zn-2.5Al-2Mg-0.05(TiZr) is 3049 Ω cm 2 , higher than Zn-2.5Al-2Mg-0.1Zr and lower than Zn-2.5Al-2Mg-0.1Ti. It is showed that the transfer of electric charge was significantly enhanced and the corrosion reaction of the alloy was delayed. Moreover, there is cooperation effect between Ti and Zr, which is lower than Ti but higher than Zr. Figure 12 shows the polarization curves of Zn-2.5Al-2Mg-xTi-yZr alloys in 5 wt% NaCl solution. All the curves show similar features that tend to present the addition of Ti and Zr contributed to the improvement of corrosion resistance, which was characterized by a significantly lower corrosion current density and more noble corrosion potential in figure 12. The polarization curves of Zn-2.5Al-2Mg-xTi alloys were showed in figure 12(a). It can be seen that the corrosion potential and corrosion current density of Zn-2.5Al-2Mg-xTi alloys decreases and increases gradually, respectively. The polarization curves of Zn-2.5Al-2Mg-xZr alloys are shown in figure 12(b). It can be seen that with the addition of Zr, the corrosion potential of Zn-2.5Al-2Mg alloy increases first and then decreases, the corrosion current decreases first and then increases. The polarization curves of Zn-2.5Al-2Mg-xTi-yZr are shown in figure 12(c). The trend of composite addition of Ti and Zr is similar to that of Zr added. The E corr (corrosion potential), I corr (corrosion current density) and ΔE (passivation region) of the polarization curve can be obtained by Tafel extrapolation, and the detailed electrochemical parameters are summarized in table 6. As can be seen from table 6, E corr and I corr of Zn-2.5Al-2Mg alloy are −1.46046827 V and 0.001584621 A·cm 2 , respectively. The E corr of Zn-2.5Al-2Mg alloy after adding Ti and Zr increased slightly and I corr decreased obviously. Among them, E corr and I corr of Zn-2.5Al-2Mg-0.05Ti, Zn-2.5Al-2Mg-0.1Zr and Zn-2.5Al-2Mg-0.05(TiZr) are −1.38660268 V, 0.000650273377 A·cm 2 , −1.40503879 V and 0.000725140257 A·cm 2 , −1.35529617 V and 0.000559848712 A·cm 2 , respectively. The E corr of Zn-2.5Al-2Mg-0.05Ti, Zn-2.5Al-2Mg-0.1Zr and Zn-2.5Al-2Mg -0.05(TiZr) alloys increased by 0.0739 V, 0.05543 V and 0.10517 V respectively, I corr decreased by 58.96%, 54.24% and 64.67% respectively. Moreover, the ΔE of Zn-2.5Al-2Mg alloy with Ti and Zr addition is slightly wider. And the ΔE of Zn-2.5Al-2Mg-0.05Ti, Zn-2.5Al-2Mg-0.1Zr and Zn-2.5Al-2Mg-0.05(TiZr) alloys increased by 0.01725 V, 0.02176 Vand 0.02996 V, respectively. It is showed that the corrosion resistance of the alloy can be improved significantly by addition of Ti and Zr elements.
The reasons why Ti and Zr elements can significantly improve the corrosion resistance of the alloy may be as follows: (1) Ti and Zr have higher corrosion potential and better corrosion resistance; (2) Ti and Zr will form a