Positron Annihilation Spectroscopy Study On Corrosion And Protection Of AM60B Magnesium Alloy

Magnesium alloys have been widely used in commerce for over eighty years.With the continuous development of casting technology,more and more high performance magnesium alloys are being used in military,automobile,aerospace and electronic industries.Especially in automobile industry,magnesium alloy parts can effectively reduce the vehicle weight,the fuel consumption and emissions,owing to its low density,high specific strength,well damping property and so on.Magnesium alloys have a great application prospect,but the application range and level so far are still less than that of aluminum alloys.The poor corrosion resistance is a considerable factor hindering the development and application of magnesium alloys.Due to the high chemical activity,magnesium alloys are very susceptible to corrosion in industrial,marine atmosphere and rainwater environment.Improving the die-casting process can reduce the pores and impurities in magnesium alloys,which changes the mechanical properties of magnesium alloys but also has effect on its corrosion resistance.In recent years,a new super vacuum die-casting?SVDC?technique was developed by American General Motors Corporation?GM?.The SVDC process with powerful vacuum system and better die-casing process setting significantly decreases the porosity and impurity in magnesium alloys.The microstructure of alloy also can be optimized.The mechanical properties such as ductility,hardness and fatigue of magnesium alloys are improved.However,there are few studies on the correlation between micro-defects and corrosion behavior of magnesium alloy.In practical applications,the surface coating technologies are often used to provide corrosion protection for magnesium alloys.The rare-earth conversion treatment technology is environment-friendly and can improve the corrosion resistance of magnesium alloys significantly.However,little attention has been paid to the rare-earth conversion coating of magnesium alloys.Therefore,it is necessary to continue the research of rare-earth conversion treatment on magnesium alloys.In this paper,positron annihilation spectroscopy method combining with X-ray diffraction?XRD?,X-ray photoelectron spectroscopy?XPS?,electrochemical tests,scanning electronic microscopy?SEM?,thermogravimetric and differential scanning calorimetry?TG-DSC?analysis,and high-resolution transimission microscopy?HRTEM?were used to study the early corrosion behavior of super vacuum die-casting AM60B magnesium alloys,the thermal decomposition of Mg?OH?₂ and the cerium-based rare-earth conversion coating preparation on AM60B magnesium alloy.The main research content and conclusions are as follows:1.The early corrosion behavior of super vacuum die-casting AM60B magnesium alloys immersed in a 5 wt%NaCl solution was investigated.X-ray diffraction and X-ray photoelectron spectroscopy results showed that Mg?OH?₂ was main corrosion product in the salt solution.With prolonging the immersion time,a significant decrease of Doppler-broadened annihilation line-width parameter near the surface after corrosion was observed and interpreted that the pre-existing interfacial voids between oxide film and matrix might promote the formation of Mg?OH?₂ corrosion layer.Scanning electronic microscopy images showed that the main corrosion form in early corrosion stage of AM60B magnesium alloys was pitting,which gradually spread to the whole alloys surface with immersion time increasing.Polarization tests found that Mg?OH?₂ could provide a temporary protection for magnesium alloy,while with the immersion time up to 24 h,the alloy became more susceptible to corrosion.2.The thermal decomposition behavior of Mg?OH?₂ nanopowders was investigated by positron annihilation lifetime sepectroscopy?PALS?,XRD,TG-DSC,HRTEM and SEM.It was indicated that the microstructural changes started from 300? were earlier than the phase transformation from hexagonal Mg?OH?₂ nanopowders to face-centered cubic MgO occurred at 380 ? during decomposition process.The variation of positron annihilation parameters revealed the production of new vacancy defects and the aggregation of vacancy clusters in grain boundary areas and microvoids between particles due to the removal of H2O and rearrangement of interface atoms.A positron trapping model was proposed to help further understand the changes of microstructure and interfacial defects during Mg?OH?₂ thermal decomposition.3.The rare-earth cerium conversion coating was prepared on the surface of SVDC AM60B magnesium alloy using Ce?NO₃?₃ solution as the conversion solution.The effects of the concentration of Ce?NO₃?₃ solution and the conversion treatment time on the conversion coating were investigated.The main components of the coating were CeO2,Ce?OH?4 and Ce?OH?₃.The cerium-based conversion coating was rimous sticking to the surface of magnesium alloy.The positron lifetime spectroscopy analysis and SEM images indicated that the coating thickness and density increased with the concentration of Ce?NO₃?₃ solution and treatment time.The electrochemical tests results showed that the cerium conversion coating improved the corrosion resistance of magnesium alloy significantly.But too low or too high concentration was not conducive to forming a coating with good corrosion resistance.The conversion treatment time also had an effect on the coating corrosion resistance.