Investigation On Microstructures And Corrosion Behaviors Of Squeeze-cast Mg-Al-RE-Ca Magnesium Alloys

Mg-Al-RE-Ca magnesium alloys have good heat-resistant ability.The current researches focus on the room temperature mechanical properties,high temperature mechanical properties and high temperature creep properties of the alloys.The investigation of corrosion behavior of these alloys is few,especially for corrosion behavior of alloy in different corrosive environments.Therefore,investigation of the corrosion mechanism for low rare-earth Mg-Al-RE-Ca magnesium alloy is very important for advancing the application of the alloys.The effects of different Ca and mixed rare earth content on the microstructure,corrosion rate,electrochemical properties,corrosion products and corrosion morphologies of low rare-earth Mg-Al-RE-Ca alloys were investigated by using X-ray diffraction?XRD?,optical microscopy?OM?,scanning electron microscopy?SEM?,electron probe micro-analyzer?EPMA?,gravimetric measurements and electrochemical testing.At the same time,two kinds of alloys with the best corrosion-resistance properties were selected and the effects of different corrosion environments on the corrosion rate,electrochemical behavior and corrosion morphology of the alloys were also studied.?1?SEM and XRD analysis show that compared with AE42 alloy,dendritic and acicular Al-RE phases are suppressed and Al-RE phases are dispersed in the?-Mg matrix after adding Ca element to Mg-4Al-0.5RE-xCa alloys.With increasing Ca additions,Ca-containing phases change from?-Mg₁₇Al₁₂ phase containing Ca to thin acicular Al2Ca or bone-shaped?Mg,Al?2Ca phases.Mg-4Al-2RE alloy presents the highest mass loss.With the increasing of Ca content,the corrosion rate of alloy first decreases and then accelerates.Mg-4Al-0.5RE-0.8Ca alloy has the most excellent corrosion resistance.Compared with AE42 alloy,the corrosion rate of Mg-4Al-0.5RE-0.8Ca alloy decreases by 71.4%.The anodic polarization curve of the alloy appeared obvious passivation phenomenon.That utilizing Ca to replace RE element can improve the corrosion resistance can be contributed to the following points.?a?The reduction of RE element decreases the volume fraction of the cathode precipitation phase of the alloys,resulting in a decrease in the micro-cell corrosion rate.?b?After addition of Ca,the generated Al₂Ca and Mg₂Ca phases have a lower potential,which reduces the potential difference between the precipitated phase and the?-Mg matrix,and resulting in a weaker galvanic-corrosion of the alloys.?c?The mixed oxide film of Ca and RE is more stable and dense,which prevents the erosion of magnesium matrix caused by Cl^-.?2?When mixed rare earths?Y and Ce?are added to Mg-4Al-3Ca alloy,the coarser Chinese-character-like?Mg,Al?₂Ca phase is significantly refined and sparsely particulate Al-RE phase is dispersed in?-Mg matrix.With the increase of RE element,continuous slivers with eutectic structure of Al2Ca and Mg2Ca and rod-like and granular Al-RE phases dispersed in?-Mg matrix are formed.The gravimetric measurements and electrochemical testing show that with the addition of RE element,the corrosion rate of the alloys first decreased significantly and then slightly increased.The Mg-4Al-3Ca-0.5RE alloy has the best corrosion resistance.Compared with the AX43 alloy,the corrosion rate of Mg-4Al-3Ca-0.5RE alloy is reduced by 89.5%.The anodic polarization curve of the alloy shows obvious passivation phenomenon.The mixed rare earth elements can change the corrosion resistance of the alloys due to the following points:?a?The addition of trace amounts of RE elements purifies the microstructure of alloys and further achieves the effects of dehydrogenating and removing impurities.?b?Trace amounts of RE elements significantly change the corrosion film of the alloys,making the corrosion film stable and dense and impeding the attack of Cl^-on the surface of the alloy.?c?Excessive RE increases the volume fraction of the cathodic precipitation phase,causing an increase in micro-cell corrosion rate.?3?In the initial stage of corrosion,the Mg-4Al-3Ca-0.5RE alloy is dominated by filiform corrosion.As the corrosion reaction progresses,filiform corrosion gradually extends.The corroded area serves as a corrosion cathode,and the magnesium matrix at the front of filiform corrosion serves as a corrosion anode.Therefore,an electrical coupling reaction occurs,resulting in dissolution of the magnesium matrix.The corrosion of Mg-4Al-0.5RE-0.8Ca alloy on the same corrosion environment is mainly pitting corrosion.With the increasion of the immersion time,the pitting pits gradually extend longitudinally,and corrosion occurs inside the magnesium matrix,eventually leading to severely localized corrosion.