Thermodynamic Evaluation of the Corrosion Reactions in a Novel Superlight Mg-Li-Al-Y-Mn Alloy

Due to ultra-low density, high specific stiffness, high specific strength, good damping properties and electromagnetic shielding properties, magnesium-lithium alloys are used in aerospace, military, 3C electronics and automotive manufacturing. Low strength and poor corrosion performance are the main problems puzzling the research of magnesiumlithium alloy. According to the literature research, the research on the corrosion and coating (protection) of magnesium-lithium alloy has become a hot topic in recent years, and people have developed various forms of coating, hoping to solve the corrosion resistance problem during the service of magnesium-lithium alloy [1-7]. However, although Esmaily et al. [8] mentioned the existence of a large number of corrosion chemical reactions in magnesium alloy corrosion, there is a lack of thermodynamic evaluation of the corrosion reactions in superlight magnesium-lithium alloys. Therefore, it is necessary to calculate the change of Gibbs free energy of chemical reaction from the perspective of thermodynamics, judge the role of reaction products, and deepen the understanding of corrosion mechanism. The severe plastic deformation methods, such as Equal Channel Corner Extrusion (ECAP), High Pressure Torsion (HPT), Friction Stir Machining (FSP), and Multi-Direction Forging (MDF) [9-12] are large strain deformation approaches which have been studied extensively in the past thirty years. In this paper, a method of MDF+AR (asymmetric rolling) combined forming is proposed. It is expected that MDF+AR can be used to refine the grain, improve the mechanical properties and corrosion resistance of the alloy. Therefore, Mg-Li-Al-Y-Mn alloy plate was prepared in this paper, and the corrosion resistance of the alloy was studied by potentiodynamic polarization curve and corrosion thermodynamics. This report is expected to arouse interest in the thermodynamics of chemical reactions of magnesium lithium alloy corrosion researchers.

Jackson flux-Argon gas protection method was used in the casting process of alloy ingot. The analytical composition of the present alloy was Mg-9.55Li-2.92Al-0.027Y-0.026Mn in wt.%. The ingot surface was milled and homogenized at 473K for 16 hours. The ingot was then cut into rectangular billets with a size of 40mm×32mm×22mm. Our previous MDF report was shown in Ref [13,14]. The cuboid billet was forged at 523K by changing the pressing direction continuously on a 300-ton press until it was subjected to 6 passes. The forged billets were then asynchronously rolled at 523K to 4mm thick with a reduction of 81.82% and further cold rolled to 2mm thick with a reduction of 50%. The speed ratio of asymmetric rolling was 1.2. In order to study the influence of processing deformation on the corrosion resistance of Mg-9.55Li- 2.92Al-0.027Y-0.026Mn alloy, the dynamic potential polarization curves of as-cast and rolled alloy were measured. Potentiodynamic polarization curve was measured on CS series electrochemical workstation. Before the test, the samples were first wrapped with denture base polymer and denture base resin, and only the test surface was exposed. The test surface size of each sample was 10mm×10mm. Before the experiment, the samples were polished on 800#, 1500#, and 2000# abrasive papers in order to remove the oxide skin and other impurities on the surface. Rinsed with alcohol after grinding and immediately dried with air dryer.

After the sample was prepared, the dynamic potential polarization curve was tested. During the measurement, a threeelectrode connection was adopted. The green current line (green jacket clip) was connected to the working electrode WE, the red current line (red jacket clip) was connected to the auxiliary electrode CE, and the yellow potential line (yellow jacket clip) was connected to the reference electrode RE. In this study, the alloy sample was used as the working electrode WE, the platinum electrode as the auxiliary electrode CE, the saturated calomel electrode as the reference electrode RE. The electrolyte was a solution of 3.5 wt.% analysis purity NaCl and distilled water. CorrTest software was used for measurement and data analysis during the experiment. The scanning range adopted in this study was the relative open circuit potential of -0.5V~1V, and the scanning rate was 5mV/s. Before the experiment, the sample was put into the 3.5wt. % NaCl solution for 5 min, and the test was carried out after the system was stabilized. In order to ensure the reliability of the experimental results, each group was tested three times.

Potentiodynamic polarization curves of as-cast and asrolled alloys

The samples for potentiodynamic polarization curve experiment are from as-cast and as-rolled alloys, and the experimental results are shown in (Figure 1). As shown in Figure 1, both polarization curves consist of two parts: A cathode branch related to hydrogen evolution reduction reaction and an anode branch related to anode dissolution of the electrode. The two sides of the anode and cathode branches of the as-cast sample are obviously asymmetrical, and the current density in the anode region is relatively high. This is because the as-cast alloy has a negative potential, and hydrogen evolution reactions also occur in the anode region, forming a negative difference effect. However, the potential of the rolled alloy is positive, so it does not show obvious asymmetry. The cathodic corrosion potential Ecorr(V) and corrosion current density Jcorr(mA·cm^-2) can be fitted by the tafel extrapolation method in CorrTest software, and the corresponding average corrosion rate Pi(mm·a^-1) can be obtained from the formula Pi=22.85Jcorr, and the analytical values are shown in Table 1. It can be seen from Figure 1 and Table 1 that the as-cast and as-rolled alloy samples have different corrosion potential and cathode branch slope, respectively. Compared with the as-cast alloy, the corrosion potential of rolled alloy is moving from -1.5936V to -1.3687V. The corrosion current density decreases from 1.1604×10^-1mA·cm^-2 to 1.7689×10^-4sup> mA·cm^-2, and the average corrosion rate decreases from 2.6515mm·a^-1 to 4.0419×10^-3mm·a^-1, a difference of three orders of magnitude, indicating that the corrosion resistance was greatly improved.

Figure 1: Potentiodynamic polarization curves of ascast and as-rolled samples in 3.5 wt.% NaCl solution.

Table 1:Corrosion potential, corrosion current density and average corrosion rates of as-cast and rolled alloy samples.

Thermodynamic evaluation of corrosion reactions

The cathode reaction and the anode reaction equations [15,16] are shown in (1) to (9).
Cathodic reaction

Where: v_B is the coefficient in the equation; ΔfHm^ϴ is the standard molar enthalpy of formation, in kJ·mol^-1; Sm^ϴ is the standard molar entropy, unit: J·K^-1.mol^-1; ΔrHm^ϴ is the standard molar enthalpy of reaction, in kJ·mol^-1; ΔrSm^ϴ is the standard molar entropy change, and the unit is J·K^-1·mol^-1; R is the molar gas constant, 8.314J·K^-1· mol^-1; T is the absolute temperature, 298K in this study; ΔrGm^ϴ is the standard Gibbs function variation, in kJ.mol^-1; Kϴ is the standard equilibrium constant.

The thermodynamic parameters of each chemical reaction can be calculated according to formula (10)~(13) [15,16], and the specific results are shown in Tables 2-4 [17,18]. Among them, the Gibbs function variation of each reaction is less than 0, which means that each reaction can proceed spontaneously. The value of the standard equilibrium constant represents the degree of reaction progress. Under a certain temperature, the greater the value of the standard equilibrium constant, the greater the degree of reaction is positive. According to the analysis of chemical valence change before and after the reaction, Mg becomes +2 from 0, Li from 0 to +1, H from +1 to -1, and O from 0 to -2. Therefore, all the chemical reactions here are REDOX reactions, in which Mg and Li lose electrons and become reducing agents, H2O and O2 gain electrons. It is an oxidant. According to the sequence of REDOX reaction, the same reducing agent is added into several oxidant solutions, and the oxidizing agent with strong oxidizing ability is reduced first. Since O₂ is more oxidizing than H₂O, when Mg-Li alloy corrodes, Mg and Li react with O₂ first to form MgO and Li₂O, and then react with H2O to form Mg(OH)₂ and LiOH. It was reported [19] that Mg-Li alloy corrodes in the atmosphere and forms a multi-component oxide film on its surface, which contains four layers. From the inside (near the matrix) to the outside (near the atmosphere) are Li₂O+MgO+Mg+Li, LiOH+MgO+Mg(OH)₂+Li₂O+Mg, Li>₂O+Mg(OH)₂+MgO and Li₂O+Mg(OH)₂, respectively. It can be seen that MgO and Li2O+Mg(OH)₂ are formed during the formation of the oxide film. Mg(OH)₂ and LiOH were regenerated. In terms of the equilibrium constant, the degree of Mg(OH)₂ formation was much greater than that of LiOH formation, which was consistent with the reaction order speculated in this paper. At the same time, due to the presence of atmospheric CO₂, Li₂O reacts with CO₂ to produce Li₂CO₃, and MgO reacts with CO₂ to produce MgCO₃. As per the equilibrium constant, the degree of Li₂CO₃ generation is far greater than the degree of MgCO₃ generation, indicating that the generated Li2CO₃ is more stable than MgCO₃.

Table 2:Thermodynamic data obtained from the literature [17,18].

Table 3:Standard molar reaction enthalpy change Δ_rH_m^ϴ and standard molar entropy change Δ_rS_m^ϴ of each chemical reaction.

Table 4:Standard Gibbs function change Δ_rG_m ϴ and standard equilibrium constant K^ϴ of each chemical reaction.

The products of Mg(OH)2(s), LiOH(s), MgCO3 and Li2CO3 are stable films, which prevent further corrosion. Li et al. [20] studied the corrosion and electrochemical behavior of Mg-4Li(α), Mg-7.5Li (α+β) and Mg-14Li (β) alloys and investigated the effect of crystal structure on the corrosion resistance of Mg-Li binary system. The corrosion resistance was tested in 0.1M NaCl solution, and the corrosion rates of the three alloys were found to be Mg-7.5Li >Mg- 4Li>Mg-14Li from high to low. The results show that the corrosion resistance and electrochemical behavior of Mg-Li alloy vary with the composition and crystal structure. The corrosion resistance of α+β biphase Mg-Li alloy is worse than that of single-phase Mg- Li alloy. Moreover, the β-Li phase of BCC structure contributes to the formation of protective surface films. The single phase β-Li solid solution Mg-Li alloy with BCC structure has higher corrosion resistance than the single phase α-Mg solid solution Mg-Li alloy with HCP structure. Zhao et al. [21] studied the corrosion behavior of α-Mg based Mg-4Li- (3AlSi) alloy and β-Li base Mg-12Li-(3AlSi) alloy and found that the corrosion resistance of α-Mg is better than that of β-Li, and the corrosion is related to the phase composition. Despite the differences in the above-mentioned research results, the existence of Li2CO3 and MgCO3 corrosion films produced by thermodynamic evaluation prevents further corrosion, which is the reason for the increase in corrosion resistance of β-Li phase alloys. Dobkawska et al. [22] recently found that the corrosion resistance of α+β biphasic Mg-Li alloys is reduced due to the action of microgalvanic couples.

Sun et al. [23] studied the microstructure and corrosion behavior of three homogenized LAZ532-0.20Zr, LAZ832-0.20Zr and LAZ1132-0.20Zr alloys with α, α+β and β as the matrix, respectively, and found that the homogenized LAZ832-0.20Zr with α+β as the matrix had the best corrosion resistance. This indicates that due to the different processing history, the corrosion experiment results of different authors are different. At present, there are two views on the effect of grain refinement on corrosion resistance: opposing and supporting. The opposing view is that even though the grain size of the biphase alloy is finer, the corrosion resistance of the alloy is reduced due to the action of the micro-galvanic couple. Our research results support the view that grain refinement improves corrosion resistance. This is because the severe plastic deformation of MDF+AR results in grain refinement and second phase breakage, which increase the homogeneity of the structure, weaken the micro-galvanic couple effect and improve the corrosion resistance.

He et al. [24] studied the effects of Li, Al and Y elements on the microstructure and corrosion behavior of Mg-Li-Al-(Y) alloy, and the results showed that the alloy elements affecting the corrosion behavior of Mg-Li-Al-(Y) alloy were Al>Li>Y from strong to weak. In addition, it is found that Mg-10Li-3Al-0.6Y alloy has better corrosion resistance. The main reason is that the α-Mg phase of the alloy has uniform microstructure, high area fraction and uniform distribution, which acts as the corrosion barrier to inhibit the severe local dissolution of β-Li matrix. According to the XRD patterns of the as-cast and as-rolled state [25], compared with the as-cast state, the intensity and number of the characteristic peaks of α-Mg phase and Li2MgAl phase in the rolled state are slightly decreased, while the intensity of the characteristic peaks of β-Li phase is significantly increased, which indicates that the content of α-Mg and Li₂MgAl phase is decreased, while the content of β-Li phase is significantly increased. The process may promote the transformation of α-Mg into β-Li phase. On the one hand, the increase of β-Li phase content contributes to the formation of surface protective film, on the other hand, the micro-galvanic coupling between phases is greatly reduced. Therefore, since the rolled grain size of Mg-9.55Li-2.92Al 0.027Y-0.026Mn alloy is finer to 1.9μm because of the dominance of the β-Li phase, the corrosion resistance of rolled state is greatly improved compared with that of as-cast state.

Mg-9.55Li-2.92Al-0.027Y-0.026Mn alloy plate was prepared by multidirectional forging and rolling. The corrosion resistance of the alloy was studied by potentiodynamic polarization curve and corrosion thermodynamics. i. The potentiodynamic polarization curve showed that compared with the cast state, the corrosion potential of rolled state moved from -1.5936V to -1.3687V, and the corrosion current density decreased from 1.1604×10-1mA.cm-2 to 1.7689×10^-4mA.cm^-2. The average corrosion rate decreases from 2.6515mm.a-1 to 4.0419×10^-3mm.a-1, a difference of three orders of magnitude, showing that the corrosion resistance is greatly improved. ii. Thermodynamic evaluation of corrosion reaction shows that the reaction film formed by Mg(OH)₂(s), LiOH(s), Li₂CO₃(s) and MgCO₃(s) is beneficial to prevent further corrosion on the surface.

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