Properties And Theory Research Of Novel Rare-earth Conversion Coating On Magnesium Alloys

In this dissertation, the new-type rare-earth conversion coating was obtained on thesurface of magnesium alloys. Optimized technologies and characters of the novel rare-earthconversion coating were analyzed in this paper. After that, the formation mechanism andcomputer simulation were carried out by incorporating the experimental phenomenon andrelevant theoretical knowledge. SEM, EDS, XRD, XPS, FTIR and the electrochemical stationwere employed to test the micro-morphology, crystal structure, composition andelectrochemical features of conversion coating.The new rare-earth elements were chose to be one of main elements in conversionsolution. The optimized process was determined by orthogonal experiment design.Open-circuit potential curves, formation kinetics curves and micromorphologies were appliedto monitor the coating formation during the process. And formation of cerium conversioncoatings on AZ31magnesium alloys and neodymium conversion coatings on AZ91magnesium alloys were discussed elaborately. Also, the micro-morphology, crystal structure,composition and electrochemical features of cerium conversion coatings, neodymiumconversion coatings, samarium conversion coatings and yttrium conversion coatings weredetected at the optimized process. Moreover, the post treatment was used to better thecorrosion resistance of rare-earth conversion coating and intensify the bond force with otherfollowing coatings. The influence of post treated by phytic acid to neodymium conversioncoatings was declared by comparing the two type coatings, which were post-treated or not, inmicromophology, chemical compounds, functional groups and corrosion resistance. It showedthat post-treated coatings were improved in corrosion resistance for matrix further and severalfunctional groups appeared in it. Furthermore, it analyzed the competitive mechanismbetween various rare-earth elements. Sm and Y were in a disadvantaged position comparedwith other elements. And it was resistant to form coatings on surface of second phase and nearthis area. Finally, formation mechanism during the formation was discussed preliminarilyincorporated the experimental phenomenon and relevant theories. There were two stages during this process: the matrix dissolved first and the released Mg2+reacted with OH-inconversion solution to form Mg(OH)2. Then the process of releasing Mg2+was impeded bydeposited Mg(OH)2. So OH-started to react with REn+and the rare-earth oxides formed onthe former coating. It presented computer simulation in the initial time of forming rare-earthconversion coating in this work. And the research integrated Monte Carlo method,crystallology, thermodynamics and experimental results together. The simulated results are inaccordance with experimental results basically.