Effect of T4 treatment on the corrosion resistance of Mg-4Al-6Er-0.3Mn alloy

The Mg-Al alloys exhibit poor corrosion resistance when they exposed to Cl- attack. To solve this problem, Erbium (Er) and Manganese (Mn) are added to the Mg-4Al alloy and the Mg-4Al-6Er-0.3Mn is T4 treated to enhance the corrosion resistance of the alloy. Then the corrosion behaviors of the as-extruded alloy and the T4 alloy are investigated in this paper. It is found that the effective cathode Mg17Al12 is significantly reduced in the alloy due to the precipitation of Mg17Al12 being suppressed by Al2Er and the dissolution of the Mg17Al12 in the alloy. It is observed that the corrosion products transform from needle-like to tetrahedral-shaped corrosion products during the transformation process, which leads to severe pit corrosion. The results show that the T4 treatment can delay the transformation of the morphologies of the corrosion products, thereby improved the alloy corrosion resistance during the early stages of corrosion.


Introduction
Magnesium alloys emerged as a promising candidate for lightweight structural materials [1][2][3][4] in engineering applications due to their low density [5], high specific strength [6], good damping [7,8] and electromagnetic shielding [9,10].The magnesium alloys have the potential for wide application fields such as aerospace, automotive, and telecommunications [11][12][13].With the increasing applications in automotive structural components [14] and medical fields [15], the magnesium alloys inherent traits have appeared.These include chemical activity [16][17][18], low electrode potential [19][20][21], and limited corrosion resistance [22].Among various magnesium alloys, Mg-Al-RE system alloys [23][24][25] stand out owing to better performance than those of the conventional magnesium alloys.However, it is important to note that Mg-Al-RE system alloys exhibit relatively poor corrosion resistance [16,26] compared to those of pure magnesium and Mg-Al alloys.The weak corrosion resistances of Mg-Al-RE system alloys requires careful consideration, especially in applications where corrosion resistance is of importance.Therefore, the development of Mg-Al-RE alloys with exceptional corrosion resistance is of utmost significance for practical utilization.
Efforts to enhance the corrosion resistance of magnesium alloys involve various techniques such as heat treatment [27][28][29], alloying with elements [9], and surface modification [30][31][32][33].Surface modifications can improve corrosion resistance and durability effectively, while it can be complex, damage the matrix and potentially impact dimensional tolerances.Changing in the compositions of the oxide films and surface microstructure densification of the magnesium alloys are achievable via addition of alloying elements.These elements not only can refine the grain sizes, but also play a role in the melt purification by eliminating oxide inclusions as well.During melting, manganese (Mn) forms high-melting-point and high-density intermetallic compounds with harmful iron (Fe) elements and thus remove the Fe from the melt and alloys [34].Furthermore, Mn encapsulates iron particles, reducing potential difference between iron and α-Mg [34].In corrosive aqueous environments, impurities or precipitates within the alloy can create corrosive cells, accelerating corrosion.However, the addition of rare earth element Erbium (Er) to the alloys purifies the alloy matrix and thus improving the corrosion resistance.Zhongjun Wang et al [35] and Haifei Zhou et al [36] confirmed that the suitable addition of Er elements enhance the corrosion resistance of magnesium alloys.Heat treatment plays a pivotal role in the corrosion resistance of magnesium alloys.It can change phase composition [37], grain size [38,39], and phase stability [27], leading to changes in corrosion behavior.It also influences the formation of corrosion products, which may densify the passivation films.The choice of the heat treatment process is supposed to accompany with specific alloy compositions and application requirements to optimize corrosion performance.However, the explanation of corrosion performance changes only focused on corrosion rate and electrochemistry in the literature [35,36].The underlying reasons for these changes and the subsequent effects of the formed corrosion products on the corrosion behavior were not revealed deeply.Magnesium alloy corrosion resistance is closely tied to the compactness of the oxide film, thus the investigation of the corrosion products and their effects on the corrosion behavior.
In this study, Mg-4Al-6Er-0.3Mnalloys are prepared and used to study the influence of Er element addition and heat treatment on the corrosion product film of magnesium alloys in chloride-ion-containing solutions.This investigation seeks to provide crucial insights into the effects of heat treatment and corrosion products on corrosion performance.

Alloy preparation
The Mg-4Al-6Er-0.3Mnalloy is created through a melting process in a crucible resistance furnace.This involves using essential materials including pure magnesium (>99.99%),pure aluminum (>99.99%),Mg-20 Er master alloy, and Mg-5 Mn master alloy.The process begins by preheating a carbon steel crucible inside the smelting furnace, inserting a thermocouple, and allowing the temperature to reach 500 °C.Next, pure magnesium is placed in the crucible and firmly secured using a tool clamp.At this stage, a protective gas mixture (consisting of 99% Ar and 1% SF6) is introduced into the crucible.Once the pure magnesium is completely melted, and the thermocouple registers a temperature of 750 °C, the Mg-Mn master alloy is added, followed by the Mg-Er master alloy, and eventually, pure aluminum.The molten alloy is then cast into a preheated steel mold maintained at 300 °C, with the pouring operation occurring at a temperature of 750 °C.Afterward, it is cooled within an ambient air environment.Subsequently, the ingot is placed into a small graphite crucible, which is positioned inside the ZXS1700 crucible resistance furnace.The samples are maintained at a temperature of 420 °C for a continuous 24-hour homogenization annealing process.Following this, the material is extruded using an IM-Y300 four-column hydraulic press, resulting in the desired extruded sample.The extruded material undergoes a solid-solution treatment at 420 °C for 24 h, achieving a solid-solution state.

Sample characterization
The metallographic microstructures of the samples are analyzed using an optical microscope (OM, ZEISS DMI3000M), and the average grain sizes are determined using Image-Pro 6.0 software.The phases of alloys and compositions of corrosion product were obtained by x-ray diffraction (XRD, Bruker XD 8 ADVANCE-A25X) with the Cu-K α radiation.To assess the corrosion characteristics of the samples, including surface morphology and depth, field emission scanning electron microscopy (SEM, ZEISS 6035) is employed, coupled with energydispersive x-ray spectroscopy (EDX, Oxford Instruments).
The material is ground to a thickness of 50 μm using sandpaper ranging from 800 to 7000 grit, following which it is prepared into a TEM sample through a double jet electrolysis process in an ethanol solution with 4 pct perchloric acid.The corrosion products on the samples are meticulously removed using plastic tweezers and then subjected to ultrasonic treatment in anhydrous ethanol for five minutes.These treated corrosion products are subsequently deposited onto a microgrid with a carbon film.The microstructures of the corrosion products are observed and analyzed using a transmission electron microscope (JEM-2100F, 200 kV).

Immersion test
The samples used for hydrogen evolution testing have dimensions of j 12 mm × 3 mm, with one surface meticulously polished by using 7000-grit silicon carbide sandpaper, to achieve a smooth and lustrous finish.The remaining surfaces are hermetically sealed with epoxy resin (HY604-Y) to prevent contact with the solution.Subsequently, the samples are immersed in a 3.5 wt% NaCl solution for durations of 24 h, 48 h, and 72 h, respectively.Following the hydrogen evolution testing, the samples are rinsed with distilled water and dried using a stream of cold air.To ensure the reproducibility of the data, three experiments are conducted on each sample in its respective state.

Electrochemical test
Electrochemical testing is conducted at room temperature in a 3.5 wt% NaCl solution using the Swiss Metrohm AutoLab electrochemical workstation.A conventional three-electrode configuration is employed, with a platinum sheet as the counter electrode and a saturated calomel electrode as the reference electrode.The sample serves as the working electrode surface.Prior to testing, the sample surfaces are polished using 2000# silicon carbide sandpaper.Each sample is immersed in a 3.5 wt% NaCl solution for 60 min to establish a stable open circuit potential (OCP).Potentiodynamic polarization (PDP) experiments were conducted from E OCP -0.4 V to E OCP + 0.4 V with a scan rate of 0.5 mV s −1 to study the corrosion behavior To ensure data reproducibility, each test is performed a minimum of three times.

Results and discussion
3.1.The phases and the microstructure analysis of the alloys Phase analysis is conducted on the Mg-4Al-6Er-0.3Mnalloy both before and after heat treatment.Figure 1.displays the x-ray diffraction results of the Mg-4Al-6Er-0.3Mnalloy before and after heat treatment.The addition of Er promotes the formation of the Al 2 Er phase in the alloy while reducing the formation of the Mg 17 Al 12 phase.It is evident that after heat treatment, there is a significant decrease of the peaks intensities of the Al 2 Er and Mg 17 Al 12 .This phenomenon results from the dissolving of Mg 17 Al 12 precipitates during the solid solution.The reduction of the Mg 17 Al 12 in alloy improves the corrosion resistance of the alloy.
To compare the microstructures of the alloys before and after T4 treatment, SEM characterizations were used to observe the distribution of the precipitates and grain sizes in the Mg-4Al-6Er-0.3Mnalloy as shown in figure 2. Grain size analysis of the alloy was performed using Image-Pro 6.0 software.the average grain sizes before and after T4 treatment are 6.92 μm and 11.70 μm, respectively.The T4 treatment reduces grain boundary corrosion due to the coarsened grain accompanied with the decreasing of the grain boundaries.Furthermore, after heat treatment, the Mg 17 Al 12 dissolved, resulting in a reduced cathode area in galvanic corrosion, which improves corrosion resistance of the alloy.
The microstructure and element distribution of the Mg-4Al-6Er-0.3Mnalloy are illustrated in figure 3. The blocky Al 2 Er is observed distributed throughout the alloy accompanied by an enrichment of Mn in the vicinity of the Al 2 Er phase based on the analysis of EDS and XRD.The Al 2 Er exhibited a blocky morphology, while the Al 8 ErMn 4 displayed a rod-like shape with submicron scales.After solution, a significant decrease in the size of Al 2 Er is observed.The corrosion resistance of the T4 treated alloy may be improved, attributed to the dissolution of Mg 17 Al 12 .
The higher-resolution TEM analysis of the matrix alloy is shown in figure 4. The nanoscale Al 8 ErMn 4 precipitates are identified by HR TEM.
Figure 5 is a mapping of Energy Dispersive Spectroscopy (EDS).After heat treatment, the mean size of the nanoscale Al 8 ErMn 4 precipitate increased significantly and their distribution became more dispersed than that of the as-extruded alloy.This is attributed to altering their atomic spacing and arrangement after the T4 treatment, resulting in a more homogeneous microstructure.Pitting corrosion is more likely to occur in the vicinity of the Al-Mn phase in magnesium alloys [40].As the Al-Mn phase aggregated, it can lead to severe pitting corrosion, resulting in the formation of large pitting pits that affect the corrosion resistance of the alloys.T4 treatment can prevent the aggregation of the Al-Mn phase, thereby enhancing the corrosion resistance of the alloys.

Corrosion behaviors of Mg-4Al-6Er-0.3Mn alloy
Hydrogen evolution tests were conducted in times of 24 h, 48 h, and 72 h. Figure 6 displays the macroscopic corrosion morphologies of the alloys after undergoing hydrogen evolution for varying time intervals at room temperature.It is evident that both alloys generate uniform corrosion products that resemble a film during the initial stages of corrosion.However, as the corrosion time extends, the corrosion pits appear on the surface of the samples and the corrosion rates increase.The extruded alloy starts to display localized pitting corrosion after 24 h of exposure and becomes more pronounced after 48 h of exposure.The pitting corrosion expands across approximately half of the surface after 72 h of exposure.However, no obvious pitting corrosion on the surface of    the T4 treated alloy is observed both after 24 h and 48 h of exposure, while the localized pitting corrosion is observed on the surface of the heat-treated alloy after 72 h of exposure, which is very similar to those observed on the surface of the extruded alloy after 24 h of exposure.The results indicate that the corrosion resistance of the T4 treated alloy is markedly superior to that of the extruded alloy due to a coarser grain size, fewer grain boundaries and less Mg 17 Al 12 in the T4 treated alloy.To determine the corrosion rate of the two alloys in terms of hydrogen evolution rate, a conversion calculation is conducted based on the hydrogen evolution rate obtained during the aforementioned hydrogen evolution test.The calculation formula is as follows [41]: ´- In this equation, V 1 and V 2 represent the volumes of hydrogen evolution (ml), S denotes the total surface area (cm 2 ) of the material exposed to the corrosive liquid, and t signifies the duration (d) for which the material is immersed in the hydrogen evolution test.The resultant corrosion rate is expressed in units of millimeters per year (mm/y).The macroscopic corrosion morphologies of the two alloys after varied immersing times (24 h, 48 h and 72 h) at room temperature are shown in figure 6.It shows that the corrosion pits are observed obviously in figure 6(a), while the corrosion product film is almost complete and no corrosion pits exist on the surface of the samples as shown in figure 6(d).Comparing figure 6(b) to figure 6(a), it can be observed that the corrosion pits become deeper and larger.However, comparing figures 6(d)-(e), the corrosion morphology in figure 6(e) remains almost unchanged.When the sample of as-extruded alloy immersed in 3.5 wt% NaCl solution for 72 h, the corrosion pits cover nearly half of the surface of sample, while the T4 treated sample shows the initial appearance of corrosion pits after 72 h of immersion similar to figure 6(a).Based on the above mentioned analysis, it is concluded that The corrosion resistance of T4 heat treated alloy is much better than that of the asextruded alloy owing to dissolving of the Mg 17 Al 12 .Figure 7 is hydrogen evolution rates diagrams of two alloys immersed in 3.5 wt% NaCl solution for 24 h, 48 h, and 72 h, respectively.It can be seen from the figure 7 that hydrogen evolution rates of the T4 heat treated alloy are much lower than those of the as-extruded alloy immersed for same duration.The hydrogen evolution rates for the as-extruded alloy increase rapidly from 11.08 mm y −1 after immersed 24 h to 15.05 mm y −1 after immersed 48 h, while the hydrogen evolution rates increase slower from 48 h to 72 h, due to the probably changing of the corrosion products.The T4 treated alloy exhibited a slow increase in hydrogen evolution rate with immersed time from 24 h to 48 h, while the hydrogen evolution rates surge from 1.56 mm y −1 to 4.25 mm y −1 with immersed time from 48 h to 72 h owing to the delayed changing of the corrosion products.To investigate the phases of corrosion products, the XRD analysis of the corrosion products is applied.After immersed in a 3.5 wt% NaCl solution, the alloy underwent varying degrees of corrosion, leading to the formation of corrosion products.The corrosion products are separated from the samples and ground into powder for subsequent XRD analysis, and the XRD patterns of the corrosion products are shown in figure 8.The corrosion products consist of Mg(OH) 2 , MgO, and Mg.The macroscopic corrosion morphology reveals the formation of passivation films, which protect the matrix from the Cl − attack.
To understand the corrosion morphology deeply, both samples were immersed in a 3.5 wt% NaCl solution for durations of 24 h, 48 h and 72 h, respectively.Figure 9. shows the corrosion morphologies of the two samples after immersion in a 3.5 wt% NaCl solution.From the high-magnification SEM images as shown in figures 9(d)-(f) and (j)-(l), it is obviously observed that the corrosion products of the as-extruded alloy sample exhibit a needle-like morphology and are relatively dense after 24 h of immersion.It is reported that these needle-like corrosion products are likely to be Mg 2 Cl(OH) 3 •4H 2 O passivation film, which provides a more effective protective passivation film compared to that of Mg(OH) 2 [42].When this film is damaged, the corrosion resistance of the alloy decreases.As the corrosion time goes to 48 h, tetrahedral-shaped corrosion products appear.It is interesting that, the needle-like corrosion products still distribute under the tetrahedral-shaped corrosion products.The corrosion products entirely transform into block-shaped corrosion products when the immersion time reaches 72 h.Moreover, the tetrahedral-shaped corrosion products are grown with the     Additionally, the T4-treated alloy demonstrates a less pronounced pseudo-passivation region than that of the asextruded alloy.When the potential surpasses the critical value, there is a rapid increase in corrosion current density due to the degradation of the corrosion product film.Figure 11(c) is the Nyquist curves of the two alloys.The Nyquist curve of the T4-treated alloy exhibits the larger capacitance loop radius than that of the as-extruded alloy, which signifies the higher charge transfer resistance of the active corrosion electrode in T4 treated alloy than that of the as-extruded alloy, indicating superior corrosion resistance for the T4 treated alloy.The second capacitor loop in the intermediate frequency region is a result of the passivation film existing on the surfaces of the samples.It is can be seen from the figure 11(d) that the galvanic corrosion resistance of the T4 treated alloy with the larger impedance modulus is better than that of as-extruded.For the Mg-4Al-6Er-0.3Mnalloy, there is  only one time constant, and the maximum phase angle is less than 90°, suggesting the presence of an ideal capacitive behavior.

Discussion
Figure 12 illustrates the mechanism of corrosion morphologies changes in this article: the corrosion products of the as-extruded alloys transformed from needle-like to tetrahedral-shaped, while those of the T4treated alloys transformed from needle-like to mixture of needle-like and tetrahedral-shaped products.It can be inferred that the initial deposition products is mostly like needle-like Mg 2 Cl(OH) 3 •4H 2 O, as the reaction progresses, subsequent deposition of other corrosion products, like tetrahedral-shaped Al 2 O 3 /Al(OH) 3 and MgO/Mg(OH) 2 is gradually formed.The transformation rate of this corrosion product affects the corrosion rate.

Conclusion
Based on the above mentioned analysis, it can be concluded that: 1.The effective cathode Mg 17 Al 12 is greatly reduced in the alloy due to the precipitation of Mg 17 Al 12 being suppressed by Al 2 Er and the dissolving of the Mg 17 Al 12 in the alloy.
2. The corrosion resistance of T4 treated Mg-4Al-6Er-0.3Mnalloy is much better than that of the as-extruded due to dissolving of the Mg 17 Al 12 and decreasing of the grain boundary.
3. The corrosion products transformed from needle-like to tetrahedral-shaped corrosion products, leading to severe pit corrosion during the transformation process.4. T4 treatment can delay the morphologies transformation of corrosion product, thereby providing the alloy better corrosion resistance in the early stages of corrosion.

Figure 2 .
Figure 2. The SEM image of the microstructure.(a) the as-extruded alloy, (b) the T4 treated alloy at 420 °C for 24 h.

Figure 3 .
Figure 3. SEM and Mapping image (a) the as-extruded alloy, (f) the T4 treated alloy after solution treatment at 420 °C in 24 h, the distributions of related elements are: Mg (b) and (g), Al (c) and (h), Mn (d) and (i), Er (e) and (j).

Figure 5 .
Figure 5. TEM and Mapping diagrams; (a) as-extruded alloy; (f) the T4 treated alloy at 420 °C in 24 h; The distribution of related elements is: Mg (b) and (g), Al (c) and (h), Mn (d) and (i), Er (e) and (j).

Figure 7 .
Figure 7. Hydrogen evolution rates diagrams of two alloys immersed in 3.5 wt% NaCl solution for 24 h, 48 h, and 72 h.

Figure 8 .
Figure 8. XRD patterns of corrosion products produced by hydrogen evolution corrosion of alloy in 3.5 wt% NaCl Solution.

Figure 10
shows the mappings of the EDS affiliated with HR TEM for the corrosion products of the asextruded alloy.It is detected that there are the O, Mg, Al, Cl and Er elements in the corrosion products.It is inferred that there are likely Al(OH) 3 /Al 2 O 3 and Mg 2 Cl(OH) 3 •4H 2 O in the corrosion products, which were not detected in the XRD patterns in figure 8 due to very small amount in the corrosion products.The exact phases will be discussed in our next work.

Figure 11
Figure 11 shows OCP curve, polarization curve, Nyquist diagram and Bode diagram.It is found that the potential of the material decreases after T4 treatment, as shown in figure 11(a), owing to the dissolving of the Mg 17 Al 12 .The polarization potential of the alloy after T4 treatment is considerably lower than that of the asextruded alloy, as shown in figure11(b).Furthermore, the cathodic current density of the T4 treated alloy is also lower than that of the as-extruded alloy, indicating the superior corrosion resistance of the T4 treated alloy.

Figure 9 .
Figure 9.The microscopic corrosion morphologies of the samples after immersion in a 3.5 wt% NaCl solution.The as-extruded alloy samples (a) and (d) for 24 h, (b) and (e) for 48 h, (c) and (f) for 72 h; T4 treated alloy samples (g) and (j) for 24 h, (h) and (k) for 48 h, (i) and (l) for 72 h.

Figure 10 .
Figure 10.TEM image and element mapping of corrosion products produced by alloy immersed in 3.5 wt% NaCl solution.

Figure 12 .
Figure 12.The mechanism diagram of the transformance of the corrosion morphologies.