Study On The Corrosion Residual Strength Of Ce Modified AZ91D Magnesium Alloy

As a green material in the new century,AZ91D magnesium alloy, with high specific strength and rigidity, good thermal conductivity and other advantages, has a great space for development in aerial, electrical appliances and automotive industry field. However, the poor corrosion resistance of AZ91D magnesium alloy has been becoming the"bottle-neck"in their further application. Nowadays, there are many works concentrated on the corrosion behavour of AZ91D magnesium alloy. Rare earth modified magnesium alloys are widely used to improve the corrosion resistance of AZ91D magnesium alloy. Nevertheless, the dynamic-mechanics of AZ91D magnesium alloy in corrosive conditions are little discussed. A series of studies on the dynamic-mechanics of the rare earth Ce modified AZ91D magnesium alloy in corrosive conditions have been performed and some noble results have been obtained in this paper.In this paper, AZ91D, AZ91D+0.2% Ce, AZ91D+0.5% Ce, AZ91D+0.8% Ce, AZ91D+1.0% Ce and AZ91D+1.5% Ce magnesium alloy were adopted as the target alloy. The effect of rare earth Ce on the corrosion residual strength of AZ91D magnesium alloy was investigated chiefly by analyzing microstructure, comparing corrosion morphologies, determining extreme depth of corrosion pit statistically , testing electrochemical polarization curves, examing tensile protperty, contrasting corrosion residual strength and analyzing fractography, the AZ91D+1.0% Ce magnesium alloy exhibits the best comprehensive performance, i.e., the appropriate amount of rare earth Ce is 1.0%. Results of microstructure analysis indicate that the microstructure of AZ91D magnesium alloys is refined with the addition of rare earth Ce. Simultaneously, rare earth compound Al4Ce phase emerges, the amount and size of which increase with increase of rare earth. Rare earth Ce significantly suppresses the nucleation of corrosion pits, AZ91D+1.0% Ce magnesium alloy performs better than other rare earth modified AZ91D magnesium alloys by comparing their corrosion morphologies. Adding 0.8%-1.0% rare earth Ce, the extreme depth of corrosion pit of AZ91D magnesium alloy is smaller in contrast to other rare earth Ce modified AZ91D magnesium alloys. Electrochemical polarization curves indicates that rare earth Ce has great influence on the cathodic reaction of AZ91D magnesium alloy during immersing process by refining microstructure and reducing the micro-galvanic effect. However, when the content of rare earth is up to 1.0%, AZ91D magnesium alloy obtains the best corrosion resistance. Furthermore, the cathodic hydrogen evolution reaction is accelerated by excessive rare earth compounds. The mechanical properties of AZ91D magnesium alloy are improved with increase of rare earth Ce through solid solution strengthening, grain refinement strengthening and precipitation strengthening. Comparing the corrosion residual strength of the rare earth Ce modified AZ91D magnesium alloys after 108h immersion, the corrosion resistence of AZ91D magnesium alloy with more than 1.0% addition of rare earth Ce gets worse, which leads to the excessive development of corrosion pit and the rapid drop of corrosion residual strength. The decay rate of corrosion residual strength of AZ91D+1.0% Ce magnesium alloy is relatively slower and the dynamic-mechanics property is the best. Rare earth Ce exhibits little effect on fracture mode of AZ91D magnesium alloy according to the fractography analysis, but the rare earth compounds could inhibite the propagation of cracks, thus the mechanical properties of AZ91D magnesium alloy is markedly promoted.Further investigation on the regularity and the mechanism of the corrosion residual strength of AZ91D+1.0% Ce magnesium alloy indicates that the variation of the corrosion residual strength of AZ91D +1.0% Ce magnesium alloy is similar to that of AZ91D magnesium alloy. The curve fitting shows that the residual strength of the two alloys follows the negative exponential decay function with increase of immersion time. Comparing the two corrosion residual strength curves for each alloy, the lanthanon Ce significantly improves the dynamic-mechanics of AZ91D magnesium alloy in corrosive conditions, and the corrosion residual strength of rare earth Ce modified AZ91D magnesium alloy is higher than that of AZ91D magnesium alloy at the same corrosion time. The microstructure analysis of AZ91D+1.0% Ce magnesium alloy shows that Ce apparently refines the microstructure of AZ91D magnesium alloy. The bright white Al4Ce phase precipitates in the matrix of AZ91D+1.0% Ce magnesium alloy. The result from weight loss test and the corrosion morphology of corrosion show that the corrosion rate of 1.0% Ce modified AZ91D magnesium alloy decreases gradually with the increase of corrosion time, and tends to be uniform, and comparing to AZ91D magnesium alloy, the corrosion rate of 1.0% Ce modified AZ91D magnesium alloy decreases and advances into equilibrium. Electrochemical potentiodynamic polarization curves show that the cathodic hydrogen evolution reaction is the key reaction which determines the rate of electrochemical reaction of AZ91D magnesium alloy, i.e., the rate-determining reaction step. The self-corrosion potential shifts negatively after Ce is added into the AZ91D magnesium alloy. This result proves that Ce improves the corrosion resistance of AZ91D magnesium alloy once again. The reason is that the addition of Ce effectively suppresses the cathode hydrogen evolution reaction of AZ91D magnesium alloy. The maximum corrosion pit depth variation of Ce modified AZ91D magnesium alloy is obtained using the statistical method, and the functional relationship between the value of maximum corrosion pit depth and the corrosion residual strength is established. The maximum pit depth can be used to evaluate the corrosion residual strength of the Ce modified AZ91D magnesium alloy. The curve fitting result shows that the corrosion residual strength of the Ce-modified AZ91D magnesium alloy is negative proportional to extreme depth of corrosion pit (EDCP), i.e.,σCRS = 188.38 ? 0.12EDCP. The variation mechanism analysis of corrosion residual strength indicates that the nucleation and growth of corrosion pit is the main reason for the decrease of the corrosion residual strength of the Ce modified AZ91D magnesium alloy. The nucleation and growth of corrosion pits changes the geometry of alloy surface and leads to the decrease of the effective bearing area and stress concentration. In particular, the concentration of stress causes a rapid decay of the corrosion residual strength for the Ce modified AZ91D magnesium alloy.The variation of the stress field in the matrix of AZ91D magnesium alloy with the applied load is simulated and calculated by ANSYS 11.0 finite element simulation software though changing the depth and the curvature of corrosion pits. The simulation results verify that stress concentration emerges at the bottom of corrosion pit. The maximum von stress at the bottom of corrosion pit increases exponentially with the depth of corrosion pit increasing, and as the curvature increases, the stress concentration at the bottom of corrosion pit becomes severer. The results of finite simulation and calculation further reveal that the root cause of the drop of corrosion residual strength is stress concentration in the corrosion pit caused by the nucleation and growth of corrosion pit.In a word, after adding proper rare earth Ce, not only the corrosion rate of AZ91D magnesium alloy decreases, the mechanical property inhances, simultaneously the corrosion residual strength is improved significantly and the decay rate of corrosion residual strength is inhibited. So the dynamic-mechanics of AZ91D magnesium alloy in corrosive conditions is improved obviously.