Effect Of Combinative Addition Of Strontium And Rare Earth Elements On Corrosion Resistance Of AZ91D Magnesium Alloy

Magnesium alloys are widely used in various fields such as aerospace,automobile, and electronic industry due to their unique advantages and abundant resources.However, magnesium alloys exhibit poor corrosion resistance which limits the most corrosion environment because of its remarkably higher electrochemical activity,lower electrode potential,and their less protective oxide film.Ce and La,as common alloying rare earth elements,can evidently improve the high temperature and casting properties,as well as the corrosion resistance.Alkaline earth elements,which can effectively improve the high temperature mechanical properties,especially for the creep resistance properties,upon which little systematic investigation has been made to clarify their effects on corrosion performance though,have become the research focus these years.Nevertheless,there are little reports on the combinative effect of rare earth elements and alkaline earth elements on the corrosion performance of magnesium alloys.For further comprehensive utilization it will be important to find application values of rare earth and alkaline earth elements,and ultimately explore novel corrosion-resistant magnesium alloys.In this work,the influence of strontium(Sr)and rare earth(RE)elements on the corrosion behavior of AZ91D magnesium alloy was investigated by conventional corrosion testing and electrochemical measurements in 3.5%NaCl solution.After comparing the weight loss and hydrogen evolution of the samples,microstructures of the alloys and morphologies of their corrosion product films were characterized by metalloscope,X-ray diffraction(XRD),electron probe microanalyzer-energy dispersive spectrometer (EPMA-EDS),Auger electron spectroscopy(AES)and field emission scanning electron microscope(FESEM).Comparing to individual addition of Sr and RE to AZ91D,combinative addition of 0.5%Sr and 1%RE to AZ91D successfully decreased the corrosion rate further.The synergic effects of RE and Sr on AZ91D alloy's microstructure and corrosion resistance are listed as follows:1)The combinative Sr and RE addition to AZ91D alloys has beneficial effect on fining and decomposing ofβ-Mg₁₇Al₁₂phase,which weakens the cathodic role ofβ-Mg₁₇Al₁₂phase.The weak cathodic property of the precipitated phases containing Ce and Sr can efficiently depresses the micro-galvanic couple.In a whole,the cathodic network made up ofβ-Mg₁₇A;₁₂phase and precipitated phases also lay some positive effect on corrosion reduction.2)The synergic effect of RE and Sr makes the corrosion product film more protective.The formed Al-enriched layer is stable and keeps intact in the media containing Cl^-,which contributes to the enhanced corrosion resistance of AZ91D alloy.Moreover,the formation of passive film can be demonstrated under anodic polarization.3)Compact and stable corrosion product film was formed,that profoundly reduces the corrosion rate and depresses the hydrogen evolution,which can be attributed to the uniformity,minuteness and weak cathodic character of the precipitated phases.4)The combinative addition of Sr and RE makes AZ91D alloy have lower stacking fault probability,which helps to produce integrated and uniform corrosion film.From this study,the combinative addition of strontium and rare earth elements to AZ91D magnesium alloy depresses the cathodic process and micro-galvanic couple corrosion,as well as improves the protection and integrality of the corrosion film,which appears to be a promising approach to increase its corrosion resistance.