| Owing to their low density and be friendly to the environments, magnesium alloys have been increasingly used in the automobile industry, aerospace components communications. Unfortunately, their poor corrosion resistance not only prevents their further application in many fields, but also is a difficult problem on research and development of magnesium alloys. Therefore, it is very important region to improve the corrosion resistance of magnesium alloys. It also has important economic and social significance to investigate the corresponding basic research.This paper carried out a study on the magnesium alloy by molten salt surface treatment and corrosion resistance of the Mg-Al intermetallic coating. A new process for surface modification of magnesium alloy substrate called for dipping the alloy into molten bath of equal molar mixture of AlCl3 and NaCl salt, and obtaining the diffusion aluminized coating. Using the characteristics of its high activity of magnesium alloy, active aluminum atoms and new phase were obtained at low molten salt temperatures. These temperatures are lower than the traditional solid diffusion temperatures. With the diffusion process progresses, the phase layered distribution of the Mg-Al intermetallic coating is observed. This coating improved the corrosion resistance of magnesium alloy. The main conclusions are shown as follows:(1) When the diffusion temperature is lower or the diffusion time is shorter, only the formation of a single-phase (Al12Mg17) layer, and accompanied byδ(Mg) solid solution precipitation on the magnesium alloy surface; when the temperature is higher or diffusion time is longer, from magnesium substrate to the surface the diffusion alloying layer was mainly composed of Mg-Al intermetallic compounds: a transitionδ(Mg) solid solution layer, gray phase (Al12Mg17) layer, linear structure and light gray phase (Al0.58Mg0.42) layer, white phase (Al3Mg2) layer and the scattered light distribution ofα(Al) solid solution at the outermost layer of intermetallic coating. Diffusion temperature, diffusion time and the initial atoms concentration in substrate were the main factors, which effected the formation of Mg-Al intermetallic coating. (2) Thermodynamic analysis on the formation of Mg-Al intermetallic coating of molten salt bath treatment is discussed, which includes thermodynamic analysis on the process of replacement reaction,diffusion and alloy phase formation. The replacement reaction between magnesium alloy and molten salt can produce active atoms, which was divided into two stages. First phase: the initial contact of magnesium alloy and molten salt; Second phase: the formation of a certain thickness Mg-Al intermetallic coating on magnesium alloy surface. The two-stage thermodynamic analysis shows that the displacement reactions spontaneously occur and Al element continually diffuse in the molten salt bath on the magnesium alloy. Thermodynamic analysis on diffusion process shows that Al element has the driving force of diffusion if Al element exist the concentration gradient. Thermodynamic analysis on phase formation shows that Al12Mg17 phase can be obtained at 115.86 < T <450°C, as long as there continue to provide the aluminum elements; At T <415.13°C condition, Al0.58Mg0.42 phase can be obtained; At 228 .22< T <450°C conditions, Al3Mg2 phase can be obtained.(3) The physical model of replacement and diffusion at magnesium alloy/molten salt interface has been established. The core of model is using the replacement reaction product active aluminum atoms and the new phase. With the diffusion process gradually advancing, from magnesium substrate to the surface the diffusion alloying layer was mainly composed of Mg-Al intermetallic coating:δ(Mg) layer→γ(Al12Mg17) layer→ε(Al0.58Mg0.42) layer→β(Al3Mg2) layer→α(Al) layer. Among them, the diffusion process is dominant. The displacement reactions which provide active atomics always accompanied the diffusion process to occur at the same time.(4) Dynamic analysis on the formation of Mg-Al intermetallic coating of molten salt bath treatment is discussed. At different temperatures and diffusion time, the apparent diffusion coefficients of Mg diffusing in Al and Al diffusing in Mg were obtained. The apparent diffusion coefficient of Mg diffusing in Al with molten salt treatment significantly increased than that of pure magnesium solid diffusion Al powder, while the apparent diffusion coefficient of Al diffusing in Mg with molten salt treatment is lager 3 4 orders of magnitude than that of pure magnesium solid diffusion Al powder. The activation energy for the growth of layer has been fitted that the value was 51666 J·mol-1; Deducing the growth model of each phase. The growth model of each phase had been verified that one phase generation is consumed another phase, leading to the phase boundary movement. The mathematical model of replacement and diffusion at magnesium alloy / molten salt interface has been established.(5) When the sample of molten salt treatment without a major structural defects, the continuousγphase can be completely isolated from magnesium alloy substrate and the corrosive media. The coating was an effective barrier to hinder the transmission of electronic, so that the charge transfer polarization resistance increased and improved the corrosion resistance. The sample of molten salt treatment appeared brittleβphase, which would lead to cracks and other structural defects on the surface of magnesium alloy. The electrode potential ofγphase is lower than that ofβphase, the surface of sample would occur galvanic corrosion in the corrosion solution. Theγphase as an effective micro-galvanic cathode would accelerate the corrosion rate and decrease the corrosion resistance of the sample.The main innovations are shown as follows:(1) The distribution of gradient compositions and layered phases in Mg-Al intermetallic coating on magnesium alloy with molten salt surface bath treatment can be obtained at low temperatures. These temperatures are lower than the traditional solid diffusion temperature. The layered phases distribution of Mg-Al intermetallic coating can remarkably improve the corrosion resistance of magnesium alloy. Molten salt treatment is a new surface modification on magnesium alloy for the surface protection.(2) Thermodynamic and dynamic analysis and on the formation of Mg-Al intermetallic coating with molten salt bath treatment is discussed. The physical model of replacement reaction and diffusion at magnesium alloy/molten salt interface has been established. The research on the structure of Mg-Al intermetallic coating and the corresponding basic of materials science had been carried out, which can lay the foundation for structure design and process optimizing.
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