Study on the mechanism of melting oxidation of Zn-9Al-2.5Mg-Be alloy

The oxidation resistance of Zn-9Al-2.5Mg-xBe (x = 0,0.005,0.01,0.05,0.1) alloys was investigated in this study through isothermal oxidation experiments. The alloy microstructure, morphology, and composition of the oxide film were analyzed using scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), and x-ray diffraction (XRD). The oxidation kinetics and thermodynamics of Zn-9Al-2.5Mg were calculated, and the oxidation mechanism was discussed. The results show that the Zn-9Al-2.5Mg-0.05Be alloy exhibits a fine dendritic microstructure, with a large quantity of Zn-MgZn2 binary eutectic and Zn-Al-MgZn2/Mg2Zn11 ternary eutectic phases uniformly distributed. The Zn-9Al-2.5Mg-0.05Be alloy has the lowest oxidation rate among the tested alloys, with an oxide weight gain of 12 mg m−2, which is only 0.67 times that of the Zn-9Al-2.5Mg alloy. The oxide film on the surface of the alloy is relatively dense and maintains the appearance of the metal. The main constituents of the oxide film are Zn, Al, Mg17Al12, Mg2Zn11, and BeO, with no formation of Al2O3 or MgO. The oxidation mechanism of the Zn-9Al-2.5Mg-0.05Be alloy is attributed to the reaction of Be with oxygen, forming BeO, or the displacement reaction of Be with Al2O3 and MgO to form BeO, which inhibits the formation of Al2O3 and MgO.


Introduction
Hot-dip galvanized aluminum-magnesium steel sheets are widely used as primary materials in automotive, household appliance, and industrial machinery industries due to their excellent corrosion resistance in humid air and seawater environments [1].Therefore, there is significant importance in studying the hot-dip galvanized Zn-Al-Mg coating on steel sheets.The addition of appropriate amounts of Al and Mg to the hot-dip galvanizing bath leads to a noticeable improvement in the corrosion resistance of the coating [2][3][4][5][6][7].At present, a new type of open hot dip coating is being attempted in China.This process is exposed to the air.However, the inclusion of Mg in the Zn-Al-Mg alloy can result in the oxidation of the plating solution during the hot-dip process [8][9][10][11].Whenever the process lasts for a period of time, a thick oxide film will form on the surface of the hot-dip plating pool, leading to waste of raw materials.The resulting MgO oxide generates substantial heat and possesses a porous structure, which severely affects the performance of the hot-dip galvanized coating.
Efforts to enhance the oxidation resistance of Al-Mg alloys with Be elements have gained significant attention [12][13][14].Nevertheless, excessive Be content can lead to coarsening of the alloy microstructure, susceptibility to thermal cracking, and a decline in mechanical properties, thereby posing potential health risks [15].Qiyang Tan et al investigated the addition of 0.001%-0.01%Be to Mg-9Al-1Zn and observed that the inclusion of trace amounts of Be reduced alloy oxidation weight gain, forming a protective BeO layer on the alloy surface, which effectively inhibited oxidation [16].And it was found that the fine-grained (Mg, Be) O layer on AZ91 alloy with the addition of 60 wt ppm Be exhibited higher mechanical strength and crack resistance [17].Nicholas Smith et al demonstrated that adding trace amounts of Be to Al-5Mg alloys significantly decreased the alloy oxidation rate, with BeO and Al 2 O 3 existing in cluster form [18]. Bo Hu et al studied the oxidation mechanism of Al-5Mg-2Si-Mn-0.001Bealloy melt, which involves four stages: adsorption oxidation, accelerated oxidation, transitional oxidation, and stable oxidation.The resulting oxide layer consisted of a composite inner layer (MgO/Al 2 O 3 /MgAl 2 O 4 /BeO/SiO 2 ) and an outer MgO layer [19].Zeng xiaoqin added 0.3 wt% Be to significantly enhance the oxidation resistance of AZ91D alloy [11], and found the surface of Mg-9Al-0.5Zn-0.3Bealloy formed a protective oxide film composed of MgO and BeO.The inner oxide layer exhibited linear growth, while the outer layer exhibited parabolic growth.Finally the inner oxide layer acted as a barrier, effectively reducing the outward diffusion of Mg 2+ and achieving oxidation resistance [20].An appropriate amount of Be element was added to Zn9Al2.5Mgalloy to constrain the oxidation problem caused by the addition of Mg element.Finally, this study explored the mechanism of alloy oxidation.

Experimental
Zn-9Al-2.5Mg-xBe(x = 0, 0.005, 0.01, 0.05, 0.1) alloys were prepared by melting Zn, Al, AZ91D, and Al-5Be alloy at 660 °C for 1 h and then casting them (table 1).The microstructure of the oxidized Zn-9Al-2.5Mg-xBealloys was observed using a metallographic microscope.The thickness and composition analysis of the oxide layer on the alloy were conducted using scanning electron microscopy (SEM) equipped with an energydispersive spectrometer (EDS).x-ray diffraction (XRD) was employed for phase analysis of the oxide film.The thermodynamic and kinetic mechanisms of oxide formation in the Zn-Al-Mg alloy melt were discussed.
This study adopts a commonly used alloy oxidation resistance test method.samples with dimensions of 15 mm × 15 mm × 12 mm were cut using wire cutting.After the samples were cleaned, polished, and dried, they were weighed together with the crucible.They were then placed in a GF12Q box furnace and exposed to ambient air at 450 °C for a constant temperature oxidation test.At regular intervals of oxidation time, the samples were removed, dried, and reweighed as Wi.The oxidation weight gain rate was calculated using formula (1), and the cumulative oxidation time was set to 200 h.Three parallel samples were used for the experiment.
Where F is the oxidation weight gain rate, g/m 2 •h; W0 is the initial total weight, g; Wi is the total weight after cooling for the i-th experiment, g; S is the surface area of the specimen, m 2 ; T is the experimental time, h.

Microstructural observation
As shown in figure 1, the Zn solid solution in the Zn-9Al-2.5Mgalloy exhibits slight dendritic tendencies using the M-31XM metallographic microscope, which is consistent with the findings in reference [21].The Zn-MgZn 2 binary eutectic is distributed throughout the entire alloy microstructure, while the Zn-Al-MgZn 2 /Mg 2 Zn 11 ternary eutectic is present in smaller quantities and has an uneven distribution.In the Zn-9Al-2.5Mg-0.005Bealloy, the Zn solid solution appears slightly coarser and some aggregates exhibit block-like distributions.The binary and ternary eutectic structures are distributed within the alloy microstructure.In comparison to the Zn-9Al-2.5Mgalloy, the Zn solid solution in the Zn-9Al-2.5Mg-0.01Bealloy shows reduced quantities and smaller sizes, while the binary and ternary eutectic structures increase in quantity and exhibit a more uniform distribution.The microstructure of the Zn-9Al-2.5Mg-0.05Bealloy is similar to that of the Zn-9Al-2.5Mg-0.01Bealloy.When the Zn-9Al-2.5Mg-0.1Bealloy contains 0.1% Be, the Zn solid solution tends to grow, and the microstructure still consists of the Zn solid solution, binary eutectic, and ternary eutectic.Therefore, the Be content has a certain influence on the quantity, size, and distribution of the Zn solid solution dendrites in the Zn-9Al-2.5Mg-xBealloys, but no new phases are formed, and the microstructure maintains the Zn solid solution, Zn-MgZn 2 binary eutectic, and Zn-Al-MgZn 2 /Mg 2 Zn 11 ternary eutectic structures.During the fitting process, a value of b = 0 was chosen, resulting in a multiple regression coefficient R 2 close to 1 and low SSE, as shown in table 2. Comparing with the Zn-9Al-2.5Mgalloy, except for the alloy with 0.005% Be added, the other alloy systems exhibit a lower tendency towards oxidation than the Zn-9Al-2.5Mgalloy.The alloy with the lowest tendency towards oxidation is Zn-9Al-2.5Mg-0.05Be.
From figure 4, it can be observed that the oxidation rate of the Zn-9Al-2.5Mg-xBealloy continues to decrease within the range of 0.005% to 0.05% Be, reaching a minimum value of 0.06 g m −2 •h −1 .As the Be content further increases, the oxidation rate of the alloy starts to rise, eventually reaching 0.072 g m −2 •h.The addition of an appropriate amount of Be element can effectively suppress the oxidation behavior of the Zn-9Al-2.5Mgalloy and improve its oxidation resistance.
It is worth noting that when a trace amount of 0.005% Be is added, an increase in the oxidation rate of the alloy is observed.This phenomenon can be attributed to the fact that the addition of very small or excessive amounts of Be to an alloy containing Mg can lead to coarsening of the alloy microstructure, a decrease in the mechanical properties and surface tension of the alloy, an increase in the susceptibility to thermal cracking, and the presence of pores and cracks in the oxide film, resulting in a less dense oxide layer [15,16,22].

Surface morphology of oxidation
Zn-9Al-2.5Mg-xBealloy samples after 200 h of molten salt oxidation at 450 °C were observed to have a macroscopic morphology as shown in figure 5.When Be was below 0.01%, the sample surfaces exhibited evident signs of burnout, appearing black and rough.The surface inside the crucible showed noticeable protrusions, indicating a higher surface tension of the liquid alloy.The presence of distinct grooves and wrinkles was attributed to poor flowability of the molten surface and a thicker oxide film.
When Be content exceeded 0.05%, no black burnout was observed on the sample surfaces.The alloys exhibited a metallic luster, with smooth and compact surfaces.Particularly, the Zn-9Al-2.5Mg-0.05Bealloy showed the best surface quality.But as the Be content reaches 0.1%, significant wrinkles appear on the alloy surface, which may be caused by a decrease in the alloy's flow performance.
The morphology of the oxide films on Zn-9Al-2.5Mg-xBealloys was observed using the SU3500 scanning electron microscopy (figure 6).When Be content was below 0.01%, the oxide films on the sample surfaces appeared porous with a large number of holes.Furthermore, the oxide films of Zn-9Al-2.5Mgand Zn-9Al- Table 2. Fit curve of oxidation kinetic curves.

Identification of oxidation products
The oxide film on Zn-9Al-2.5Mg-xBealloy was tested using the D8 ADVANCE x-ray diffractometer.withan incident angle of 0.2°.alloys gradually decreased and eventually disappeared.When the Be content exceeded 0.01%, BeO was detected in the alloy.

Morphology and composition analysis of oxidation cross-section
The observation and compositional analysis of the oxide films on Zn-9Al-2.5Mg-xBealloys are conducted (figure 8).Due to sample limitations, cracks are observed in all oxide layers of the Zn-9Al-2.5Mg-xBealloys, separating them from the substrate.The oxide film of the Zn-9Al-2.5Mgalloy is relatively thick, with small internal cracks.When the Be content is below 0.01%, the oxide layers of the alloys exhibit relatively high contents of O, Al, and Mg elements.Analysis indicates the presence of Al 2 O 3 and a small amount of MgO.The Zn-9Al-2.5Mg-0.005Bealloy shows more cracks in the layer and has the thickest oxide film.
When the Be content is greater than 0.05 wt%, in the Zn-9Al-2.5Mg-0.05Bealloy, although a small amount of cracks are observed in the oxide film, lower O, Al, and Mg element contents are detected, indicating less formation of Al 2 O 3 and MgO.The Zn-9Al-2.5Mg-0.01Bealloy exhibits transverse cracks in the oxide film, and the content of O, Al, and Mg elements in the oxide layer decreases again, indicating that Be effectively inhibits the oxidation of Al and Mg.

Thermodynamic analysis
Based on the experimental results mentioned above, a thermodynamic analysis of the oxidation resistance of the Zn-9Al-2.5Mg-0.05Bealloy suggests the occurrence of the following oxidation reaction during the molten salt oxidation process: According to the Van 't Hoff equation, To equations (3)-( 6), the change of Gibbs free energy is: To equations ( 7)-( 9), the change of Gibbs free energy is: Where ΔG is the corresponding change in free energy; ΔG θ is the standard Gibbs free energy; R is the gas constant with a value of R = 8.314 J/(mol•K); T is the thermodynamic temperature with T = 723 K; α is the activity; M is the metal involved in the oxidation reaction; A and B is the metals involved in the displacement reaction.In thermodynamic calculations, the activity of oxides such as MgO is often considered as 1 [23], hence αZnO = αAl 2 O 3 = αMgO = αBeO = 1.The activities of individual atoms can be approximated by their molar concentrations.Therefore, αZn = 0.7537, αAl = 0.1859, αMg = 0.0573, and αBe = 0.0031.The reference material suggests a value of pO 2 = 0.25.Through the use of data from references [24] and thermodynamic data [25], the values of ΔG θ and ΔG for each reaction can be obtained, as shown in table 3.
According to the experimental results, all changes in Gibbs free energy (ΔG) are negative, indicating that all reactions are possible during the initial stages of oxidation.Furthermore, ΔG 2 < ΔG 3 < ΔG 4 < ΔG 1 , indicating the following affinity order of alloy elements towards O: Al > Mg > Be > Zn.The decomposition pressures of the oxides are as follows: ZnO > BeO > MgO > Al 2 O 3 .According to the thermodynamic principles, the alloy will undergo selective oxidation, with the oxide with higher decomposition pressure located in the outer layer of the oxide film [26].
Based on the fact that ΔG 5 , ΔG 6 , and ΔG 7 are all negative, it can be concluded that Be is preferentially oxidized by Zn, Al, and Mg.Some ZnO, MgO, and Al 2 O 3 are replaced by Be, forming BeO.From a thermodynamic perspective, the addition of Be is beneficial for reducing the molten salt oxidation of Zn-9Al-2.5Mgalloy.

Zn-9Al-2.5Mg-0.05Be oxidation mechanism
The discussion was conducted on the oxidation kinetics curve of the Zn-9Al-2.5Mg-0.05Bealloy.The curve in figure 9(a) clearly delineated three oxidation intervals, which were named as initial film formation, film growth, and complete oxidation.The presence of stress, pores, and defects in the oxide film may have contributed to the observed phenomenon.Segments of the oxidation weight gain curve for the Zn-9Al-2.5Mg-0.05Bealloy were extracted, specifically for the time intervals of 0-52 h, 52-112 h, and 112-200 h.These segments were then subjected to parabolic fitting, as shown in figures 9(b)-(d).The fitting equation used was: The fitting results are presented in table 4. It can be concluded that the oxidation of the Zn-9Al-2.5Mg-0.05Bealloy in each respective oxidation interval follows the Wagner theory, exhibiting a parabolic trend.
To further investigate the mechanism of selective oxidation, XRD analysis was performed on the oxide films of the Zn-9Al-2.5Mgalloy and the Zn-9Al-2.5Mg-0.05Bealloy at 52 h, 112 h, and 200 h, as shown in figure 10.From figure 10(a), it can be observed that the primary components of the oxide film formed on the Zn-9Al-2.5Mgalloy are Mg 17 Al 12 , Mg 2 Zn 11 , and small amounts of MgO and Al 2 O 3 .In the case of the Zn-9Al-2.5Mg-0.05Bealloy, the primary film consists of Zn and Mg 17 Al 12 , with weaker diffraction peaks compared to the Zn-Table 3. Changes in free energy of each reaction and standard Gibbs free energy.After the formation of the initial oxide film, the oxide film remains porous, with defects and incompleteness, allowing oxygen to continue to penetrate through the first layer of oxide film and continue the oxidation of the Zn-9Al-2.5Mgalloy.The oxide film on the alloy surface still consists of a mixture of Mg 17 Al 12 , Mg 2 Zn 11 , MgO, Al 2 O 3 , and other compounds, leading to an increase in the thickness of the alloy's oxide film.

Oxidation Reaction
Figure 12 illustrates a schematic diagram of surface diffusion in the Zn-9Al-2.5Mg-0.05Bealloy melt (taking Mg as an example).In the figure, C represents concentration, X represents distance from the alloy interior, and J represents the oxide film.In the initial stage, the Mg 2+ concentration in the alloy matrix remains constant.During the initial oxidation of the alloy, BeO is first formed, blocking between the alloy and the initial oxide film.As the initial oxide film continues to oxidize, it thickens and becomes the growth oxide film.During the growth oxidation process, Be still preferentially forms BeO, and Be also undergoes displacement reaction to form BeO distributed on the surface of the growth film, preventing the formation of Al 2 O 3 and MgO.As the oxidation time of the alloy increases, with the influence of Be, a large amount of Mg and Al elements are replaced.With the increase in oxide film thickness, the content of Mg and Al elements significantly decreases, and the oxidation of the alloy is significantly restricted, achieving antioxidation.On the other hand, in the absence of Be in the Zn-9Al-2.5Mgalloy, the initial oxide film contains a large amount of Al 2 O 3 , MgO, etc, and the oxide film is porous and porous.Oxygen can continue to penetrate into the alloy matrix, not only causing oxidation corrosion of the alloy matrix but also further oxidizing the initial oxide film, forming an oxide film that cannot effectively shield oxygen from attacking the alloy matrix.Further analysis shows that BeO with a PB value of 1.71 and MgO with a PB value less than 1.BeO can effectively protect the underlying alloy from further oxidation [27].BeO helps to lower the diffusion rate of Mg and oxygen, slowing down the growth rate of the oxide film.During the alloy oxidation process, the produced BeO fills the loose gaps in MgO and is located above MgO, forming a dense BeO-MgO composite protective film, preventing further oxidation of magnesium and slowing down the growth rate of the outer MgO layer, thus enhancing the alloy's oxidation resistance.BeO, being lighter in density, floats on the surface of the melt, significantly reducing the diffusion of aluminum, magnesium, and oxygen through the oxide film, providing flame retardancy for Al and Mg.This demonstrates that the Zn-9Al-2.5Mg-0.05Bealloy exhibits higher oxidation characteristics.

Conclusions
Through microstructural analysis, alloy melt oxidation experiments, and theoretical analysis of Zn-9Al-2.5Mg-xBe(x = 0, 0.005, 0.01, 0.05, 0.1) alloys, the following conclusions can be drawn: form BeO, effectively preventing the formation of Al 2 O 3 and MgO, thus imparting good oxidation resistance to the alloy surface.

Figure 3 .
Figure 3. Linear fit plot of oxidative weight gain curve.

Figure 7 .
Figure 7. XRD patterns of the oxide layer.

Figure 10 .
Figure 10.XRD patterns of the three-layer oxide films of Zn-9Al-2.5Mgalloy and Zn-9Al-2.5Mg-0.05Bealloy: (a) Initial oxide film after 52 h of oxidation, (b) Growing oxide film after 112 h of oxidation, (c) Oxide film after 200 h of oxidation.

Mg 17
Al 12 and BeO.The phase distribution of the oxide film at this stage is similar to that of the initial film formation stage for the Zn-9Al-2.5Mgalloy, indicating that Be hinders and delays the oxidation process.The composition of the oxide film after 200 h of oxidation is shown in figure10(c).For the Zn-9Al-2.5Mgalloy, the Mg 17 Al 12 diffraction peak continues to weaken, while the Mg 2 Zn 11 peak strengthens.Some diffraction peaks of Al 2 O 3 and MgO are present.In contrast, for the Zn-9Al-2.5Mg-0.05Bealloy, the Mg 17 Al 12 diffraction peak intensifies, the Mg 2 Zn 11 peak weakens, and very weak diffraction peaks of Al 2 O 3 are observed.No diffraction peaks of MgO are present, but there is still a diffraction peak of BeO.This suggests that Be not only participates in displacement reactions but also continues to exchange energy.Thermodynamically, the presence of BeO indicates the continuation of the displacement reaction, which slows down the formation of loose oxide films such as Al 2 O 3 and MgO in the molten alloy, thereby enhancing the oxidation resistance of the Zn-9Al-2.5Mg-0.05Bealloy.To further discuss the mechanism of selective oxidation in the Zn-9Al-2.5Mgalloy melt, the formation model of the initial film, growing film, and oxide film at three stages, as shown in figure11, is explained.Firstly, the eutectic structure components Mg 17 Al 12 and Mg 2 Zn 11 in the alloy preferentially combine with oxygen to form MgO, Al 2 O 3 , ZnO, etc These oxides are entrapped on the surface of the melt, increasing the surface tension of the liquid alloy and reducing the fluidity of the melt's liquid surface.This leads to the formation of a porous, loosely packed, semi-metallic oxide film containing MgO and Al 2 O 3 inclusions, known as the initial oxide film.In the presence of Be in the Zn-9Al-2.5Mg-0.05Bealloy, the formation of MgO and Al 2 O 3 is hindered, and oxidation is minimal, with no visible oxide products on the surface.As the oxidation time of the alloy melt progresses, the initial oxide film of the Zn-9Al-2.5Mgalloy, consisting of Zn, Al, Mg 17 Al 12 , and Mg 2 Zn 11 , continues to oxidize.The oxide film remains a mixture of Mg 17 Al 12 , Mg 2 Zn 11 , MgO, Al 2 O 3 , and other compounds, but its thickness significantly increases, marking this stage as the growing film.In the growing film of the Zn-9Al-2.5Mg-0.05Bealloy, weak diffraction peaks of Al 2 O 3 and MgO appear, along with diffraction peaks of Mg 17 Al 12 and BeO, similar to the initial oxide film stage of the Zn-9Al-2.5Mgalloy.Continuing the oxidation of the Zn-9Al-2.5Mgalloy melt, Zn, Al, Mg 17 Al 12 , and Mg 2 Zn 11 continue to be consumed, while the amounts of Al 2 O 3 and MgO increase, and even elemental loss occurs.This stage is referred to as the oxide film.However, the displacement reaction of Be in the Zn-9Al-2.5Mg-0.05Bealloy prevents the formation of loose oxide films such as Al 2 O 3 and MgO in the alloy melt, resulting in a significantly reduced oxidation in the Zn-9Al-2.5Mg-0.05Bealloy.

Figure 11 .
Figure 11.Schematic diagram of multiple film formation during alloy oxidation: (a) The original state of the alloy, (b) Initial oxide film, (c) Growing oxide film, (d) Oxide film.