Investigation On Corrosion Behavior Of As-cast Mg-Li-Al-Ca Alloys In Solution Of NaCl

Mg-Li alloys are the lightest alloys material people ever found. With increasing thecontent of Li in magnesium, the density of alloys decreases gradually. Because its dense isless than all the light metal people used, it is also named ultralight alloy. Mg-Li alloys havemany advantages that are similar to other magnesium alloys，such as higher specificstrength, higher specific stiffness, better aseismic performance, strong resistance to highenergy particles penetrate and low density，which makes it one of the most promisingstructure materials in the fields of aerospace, automotive, weapons industry,3C industry,nuclear industry, medical equipment and other aspect. Especially in the21century, theworldwide shortage of energy make many industries focus on light weight materials. Mg-Lialloys as the lightest alloys in the world are honoured as green materials which have greatpotential. But at the same time Mg-Li alloys also have some shortcomings，such as lowerstrength, bad thermal stability, lower corrosion resistent etc. These weaknesses severelylimit the application of Mg-Li alloys. This research focuses on corrosion behavior andmechanism and how to improve the corrosion resistance of Mg-Li alloys，which have beena hot spot in the worldwide. The theory value and realistic directive significance of thisresearch are very important.This paper studies the corrosion behavior of Mg-Li-Al-Ca alloys in the concentrationof3.5%NaCl solution.Weight-loss method, corrosion morphology observation, XRDcorrosion products calibration, polarization curve and electrochemical impedance energyspectrum are mainly used to analyse the effect of different content of Li and theaddition of rare earth elements Ce on the static corrosion rate in solution,corrosionmorphology,corrosion product and corrosion mechanism of Mg-Li-Al-Ca alloys.Throughthese study,we mainly get four conclusions which are listed as follows:1. Cast Mg-Li-Al-Ca alloys has a better corrosion resistance than ordinary AL seriesmagnesium alloys in NaCl solution, but worse than AZ and AM series magnesium alloy. The corrosion resistance of five different components alloys in the early corrosion stage issequenced as follow:Mg-7Li-Al-Ca>Mg-9Li-Al-Ca> Mg-11Li-Al-Ca> Mg-11Li-Al-Ca-Ce> Mg-12Li-Al-Ca. After a long term corrosion test, the corrosion resistancesequence is changed to be: Mg-11Li-Al-Ca-Ce> Mg-12Li-Al-Ca> Mg-11Li-Al-Ca>Mg-7Li-Al-Ca> Mg-9Li-Al-Ca. This is mainly because the increase of Li content makealloy into single phase area, which weakens the internal galvanic effect.2. During the corrosion process cast Mg-Li-Al-Ca alloys mainly suffers localcorrosion. The corrosion area spreads fast on the alloy surface through pitting corrosion.When alloy constituent is in Mg-Li binary phase area, corrosion preferentially occures inβ-Li phase and expands along the phase boundaries. When alloy constituent is insingle-phase area, corrosion preferentially occures in grain boundaries and around thesegregation compounds inside grains, expanding along the grain boundaries.3. The corrosion potential and electrochemical reactions resistance of cast Mg-Li-Al-Caalloy in NaCl solution both increased with the corrosion time prolonged. When alloyconstituent is in Mg-Li binary phase area, the corrosion potential and electrochemicalreactions resistance will have large fluctuation after corrosion for a period of time. Whenalloy constituent is in single-phase area, the corrosion potential and electrochemicalreactions resistance will keep basically constant with little fluctuation after corrosion for aperiod of time. This is mainly because the local corrosion of two phase alloys is more seriousthan single-phase alloys.4. The corrosion products attached on the surface of cast Mg-Li-Al-Ca alloys mainlycontain Mg（OH）₂and exists many holes, seams and other defects. The distribution ofcorrosion products is discontinuity and inhomogeneous, which can not effectively preventthe development of corrosion, resulting in the damage of the surface of alloys soon. Ascorrosion process going on, Al（OH）3and Ca（OH）₂appeared in the corrosion products.Adding rare earth elements Ce into alloys makes corrosion products more compact, whichprotects the alloy substrate to some extent.