2023 Volume 64 Issue 12 Pages 2845-2848
An as-cast Mg–6Zn–0.9Zr–0.9Nd (mass%) alloy was isothermally heat treated to form semi-solid microstructure, and its corrosion behaviors before and after heat treatment were revealed. During the isothermal heat treatment, the microstructure transformed into homogenous spheroidized structure, and the liquid phase presented network-shaped morphology. The electrochemical results showed that there is no passivation behavior and obvious local corrosion in the semi-solid formed alloy, and homogeneous general corrosion is the primary corrosion form. The network-shaped liquid phase prevented the flow of corrosion media between solid phases, and therefore inhibited the formation of pitting corrosion.
It is well known that the poor corrosion resistance of magnesium alloy limits its applications.1) The semi-solid process has the advantages of low processing temperature and low deformation resistance, which result in homogenous spheroidized structure and improved mechanical properties.2,3) Studies have shown that semi-solid process can change the microstructure and composition distribution of magnesium alloy to form network-shaped liquid β phases, resulting in improved corrosion resistance of the alloy.4–7) Mathieu has found that the corrosion rate of semi-solid AZ91D magnesium alloy was 35% lower than that of as-cast magnesium alloy due to the different distribution of alloying elements and the smaller surface area of the cathode.8) Mingo attributed the good corrosion resistance of semi-solid magnesium alloys to the difference in microstructure.9) However, the mechanism affecting the corrosion behavior of semi-solid magnesium alloys has not been systematically studied.
Therefore, to reveal the mechanism how semi-solid formed microstructure affects the corrosion behavior of magnesium alloy, we prepared a semi-solid formed Mg–Zn–Zr–Nd alloy and investigated its corrosion behavior. Mg–Zn–Zr–Nd alloy is a typical magnesium alloy which has a wide semi-solid solidification range.10) Given that isothermal heat treatment process is an effective and efficient method for the preparation of semi-solid magnesium alloys, we employed it as the processing route to obtain a typical semi-solid microstructure.6,7) The microstructural evolution was studied at different heat treatment stages. The corrosion morphology as well as the electrochemical behavior of the alloys in different conditions were analyzed to draw the mechanism how semi-solid formed microstructure affects the corrosion behavior of this magnesium alloy.
The Mg–6Zn–0.9Zr–0.9Nd (mass%) alloy was prepared using pure magnesium ingots, pure zinc ingots, Mg–Zr and Mg–Nd master alloy. These raw materials were melted in a vacuum induction furnace, and the whole smelting process was under the argon gas atmosphere. In the next step, to create semi-solid formed microstructure, the as-cast ingots were heated at 620°C for 15, 30, 45, 60 minutes and quickly quenched in cold water.
Microstructural studies were accomplished using scanning electron microscopy (JSM-6510A, JEOL, Tokyo, Japan). An X-ray diffractometer (SmartLab 9kw, Rigaku, Tokyo, Japan) with Cu Kα radiation was used to analyze the phase composition of alloys. Simulated body fluid (SBF) are widely applied to determine corrosion properties in vitro including magnesium alloys. The electrochemical tests in SBF solution were carried out using an electrochemical workstation (LK2010, LANLIKE, Tianjin, China) for characterizing the electrochemical corrosion properties of semi-solid alloys. The cyclic voltammetry curve measurement was obtained by starting at the initial potential, returning to the initial potential at the same rate after reaching the termination potential. All the potentials mentioned are based on Saturated Calomel Electrode (SCE). Additionally, the immersion tests were used to evaluate the corrosion resistance of the alloys. The samples were soaked in SBF solution (pH = 7.4, inorganic ions concentration (mM): Na+ 142.0, K+ 5.0, Mg2+ 1.5, Ca2+ 2.5, Cl− 147.8, HCO3− 4.2, HPO42− 1.0, SO42− 0.5) for 1, 6, 24, 72 h at room temperature. The solution was renewed every 24 h in order to maintain a relatively stable immersion environment.
The typical microstructure of the as-cast alloy is constituted with continuous network-like eutectic phase (Fig. 1). The primary α-Mg grains shown in the upper right corner are surrounded completely by the precipitates, which continuously distribute along grain boundaries. According to the XRD results, the precipitates were mainly composed of α-Mg, MgZn2, Mg41Nd5, which were also identified in previous studies.11)
SEM image and XRD pattern of as-cast Mg–6Zn–0.9Zr–0.9Nd alloy.
Figure 2(a)–(d) displays the heat-treated semi-solid formed microstructure which means a homogeneous globular microstructure of α-Mg spheres surrounded by a fine and interconnected β-phase network at 620°C for different times.12) By isothermal heat treatment, grain boundaries of recrystallized grains separate due to the diffusion of liquid phase shown in the bottom left corner of Fig. 2(a), hence the microstructure consists of spheroidized α-Mg solid grains, continuous network-shaped liquid phase and the entrapped liquid inside the solid phase grains.7,12,13) It could be observed that with the extension of isothermal heat treatment time up to 45 min the shape of the solid phase grains become more spheroidized. A large number of second-phase grains were distributed on the liquid phase due to the diffusion of Zn/Nd atom (marked in yellow region of Fig. 2(c)).14) With increasing time from 15 to 60 min during the isothermal heat treatment, the average grain size and solid phase fraction rapidly decrease in the first 45 min, after the holding time lasts for 45 min the average grain size reaches the lowest point, then it increases with the time going on (Fig. 2(e)). With the isothermal time going on, the solid grains begin to grow under the effect of the Ostwald ripening mechanism (Fig. 2(d)).13)
The microstructure of the heated alloys at 620°C for: (a) 15 min, (b) 30 min, (c) 45 min, (d) 60 min. (e) The effects of treating time on the solid phase fraction and the average grain size.
According to Fig. 3(a), by increasing the heat treatment time, the corrosion potential moves towards more positive potential. The most positive potential, corresponds to the sample heat-treated for 45 min, is equal to −1662 mV. The corrosion current increased a little from approximately 0.439 mA/cm2 to approximately 0.527 mA/cm2 with increasing the time of heat treatment but not much. Liquid phase modified by isothermal heat treatment provides the site for the accelerated cathodic reaction. This phenomenon shows that the sample heat-treated for 45 min has more uniform and spheroidized structure, which results in suppressing the galvanic coupling reaction.15) According to the reasons mentioned above, the sample heat-treated for 45 min has better corrosion resistance tendency due to the nobler corrosion potential and the nearly unchanged corrosion current. The samples heat-treated for different times both have hysteresis behavior indicating that all the samples underwent pitting corrosion to some extent in SBF solution.16) It is possible to see that the sample heat-treated for 45 min acquire less pitting corrosion susceptibility. The electrochemical results shown in Fig. 3(b) hint that the pitting corrosion resistance is brought by the uniform and spheroidized structure. The pitting potential of heat-treated samples are more anodic than as-cast alloy in SBF solution, indicating that more pitting corrosion happened in the as-cast alloy.16)
(a) Tafel polarization curves and (b) cyclic voltammetry curves of the samples obtained by isothermal heat treatment of 620°C for different times in SBF solution.
The precipitates in semi-solid alloys have more positive corrosion potential than that of α-Mg matrix, and therefore act as cathodic sites, resulting in initiation of corrosion in the matrix next to the second precipitates.17) The local and pitting corrosion (marked in white arrows of Fig. 4(a)) was initiated next to the interface of the precipitates, suggesting that the interface consisted of the network-shaped liquid phase was more cathodic and susceptible to severe micro-galvanic corrosion, which corresponds to the hysteresis behavior shown in Fig. 3(b). When the immersion time reached 6 h, the entrapped liquid inside the solid phase grains showed exfoliation (Fig. 4(b)). As the immersion time increased to 24 h, the upper left corner of Fig. 4(c) exhibited a groove type propagation of corrosion indicating the occurrence of intergranular corrosion. Such a micro-galvanic corrosion occurs predominantly at the phase interface, and as a result, α-Mg as anode is corroded preferentially, leading to deep grooves in the original position of the interface where corrosion products accumulate to form a protective film to inhibit the cathodic reaction. The obvious intergranular corrosion was caused by the continuous network-shaped liquid phase.18) Similarly, the image of the sample immersed after 72 h showed severe intergranular corrosion that the groove was spreading along the grain boundary (marked in yellow region of Fig. 4(d)).
Corrosion morphology of the samples heat-treated for 45 min immersed in SBF solution after the removal of corrosion product for: (a) 1 h, (b) 6 h, (c) 24 h, (d) 72 h.
In the heat-treated semi-solid microstructure, the liquid phases act as cathode whilst the solid phases act as anode.19) The average corrosion rate is determined by the cathode to anode area ratio.8) The solid phases are fully encapsulated by liquid phases in the semi-solid microstructure, which assures the galvanic only happen inside each separated solid phase. Viewed from a larger scale, it is speculated that the continuous network-shaped cathodes prevent the flow of corrosion media between solid phases, and therefore inhibit the formation of severe local corrosion represented by pitting corrosion. Based on the above results, we can conclude that the spheroidized microstructure with appropriate solid phase ratio is the primary reason for the improved corrosion resistance of the semi-solid formed Mg–6Zn–0.9Zr–0.9Nd alloy.
The isothermal heat treatment process successfully completed the microstructural transformation of Mg–6Zn–0.9Zr–0.9Nd alloy to non-dendritic structure. The increasing heat treatment time in the process reduced the average solid phase area as well as the α-Mg grain size. Optimized spheroidized microstructure could be obtained by isothermal heat treatment of 620°C for 45 min. The heat-treated alloy showed improved corrosion resistance in both electrochemical and immersion tests. The network-shaped liquid phase prevented the flow of corrosion media between solid phases, and therefore inhibited most local corrosion. Our results show that tailoring the microstructure of magnesium alloy via semi-solid forming process is an effective route to improve its anti-corrosive properties especially the resistance to local corrosion.
The authors appreciate the financial support given by National Key R&D Program of China (2022YFE0126400), National Natural Science Foundation of China (52171235, 51974083, 52111540263), and Liaoning Provincial Department of Education (LJKMZ20220856).
