The Effect Of Rare Earth Elements On Microstructure And Corrosion Behavior Of AZ91 Magnesium Alloys

Magnesium alloys are widely used in various fields such as aerospace, automobile, and electronic industry due to their unique advantages and abundant resources. Alloying rare earth elements and rare earth chemical conversion coating are effective approaches to improve the corrosion resistance of magnesium alloys. AZ91 is one of the most popular commercial magnesium alloys, but studies on the role of rare earth elements to the corrosion resistance of magnesium alloys and the mechanism are rare. The present study investigates the effects of rare earth elements on corrosion resistance of AZ91 alloy by series of experimental methods such as hydrogen evolution, electrochemical impedance spectroscopy, polarization, field emission scanning electron microscopy (SEM) and X-ray diffraction (XRD). The electrochemical measurements are mostly used to analyze the effect of rare earth elements on corrosion resistance of magnesium alloy.In order to develop an environmental friendly surface coating technology for magnesium, a chrome-free Ce-based process to form a conversion coating on AZ91 magnesium alloy is investigated. Electrochemical impedance spectroscopy (EIS) is chosen to evaluate corrosion resistance due to its low effect on electrochemical test system for getting high reproduction result. The effect of treatment time, temperature of electrolyte, concentration of Ce(NO₃)₃ and accelerant concentration on corrosion resistance are summarized, while a optimized technology is obtained, 0.02mol/l Ce(NO₃)₃, 4ml/l accelerant at 35℃for 35 min. The results indicates that the chemical conversion formed from optimized technology presented yellow and impact surface by naked eye, while taking on micro cracks on the outer layer. The content of Ce in the surface layer is high. Tafel plots indicated that conversion film restrained the anodic and cathodic reaction on the interface, while the corrosion potential shifted anodic 250mV and corrosion current density decreased about 100 times. EIS results of optimal film in NaCl solution for 96 hours also indicated that the conversion film protects the AZ91 magnesium alloy effectively in the first 60 hours. While a white lanthanum conversion film is obtained, the optimized condition is 0.005mol/l La(NO₃)₃, at 45℃for 35 min.The rare earth magnesium alloys with various cerium and lathanum are melted in order to study the effect of rare earth elements on corrosion resistance of magnesium alloy. The microstructure of AZ91 alloy has been improved with Ce and La addition. The Ce or La addition makes the grains of theβphase refine. A new phase MgAlRE (RE = Ce, La) appears in the alloy and its amount increases with the Ce or La increase. The hydrogen evolution results indicate that the corrosion resistance of magnesium alloy with cerium or lanthanum addition is over 14 times better than that without Ce or La addition as control. The optimal addition amount of Ce or La in AZ91 is about 1.32wt% (C3) and 1.82wt% (C6), respectively. Their corresponding hydrogen evolution rates are 68 and 47 better than that without Ce or La addition (AZ91), respectively. C3 and C6 also present the lowest corrosion rate comparing with the same rare earth elements magnesium alloy. Electrochemical test results indicate the corrosion resistance of the Ce or La addition magnesium alloys is improved. The result of polarization behavior in the 0.1mol/L NaCl solution indicates that Ce or La addition obviously decrease the corrosion current density by 100 times corresponding to AZ91 magnesium alloy. The EIS shows that the Zim and Zre of AZ91 with Ce or La addition both increase at the first stage of immersion process from the Nyquist plot. The corrosion rates decrease 2⁶ times analyzed from the simulation dates. In the analyses of rare earth magnesium alloys' corrosion microstructure, the corrosion area obviously decreases by the Ce or La addition. And the depth of the corrosion is lower than that without Ce or La addtion. The addtion of Ce or La can greatly enhance the corrosion resistance of AZ91. The analysis of alloys' surface corrosion product indicate that the Ce or La addition in AZ91 alloy greatly affects the formation of the corrosion product film of AZ91 alloy. There is more Al content and a little Ce and La in the corrosion product film of AZ91 alloy with Ce and La addition, so the film has better corrosion resistance than that of AZ91 without rare earth element.It can be seen that the addition of Ce or La both in conversion film or in AZ91 magnesium alloy can effectively enhance the corrosion resistance of AZ91, which indicates the rare earth element Ce and La is an useful reinforcement for improving corrosion resistance of magnesium alloy.