Preparation and corrosion resistance of rare earth–silane composite conversion coatings on magnesium–lithium alloy surface

The poor corrosion resistance of magnesium–lithium alloy surface works against its application in aerospace, automobile, electronics, etc. In this research, some kinds of non-toxic and non-polluting rare earth and silane (RE–Si) composite conversion coatings were built up on Mg–Li alloy surface, and formation process of coatings was investigated. The parameters for coating preparation were determined, including immersion time and temperature, pH value of conversion solution and curing time. The optimized technological parameters for preparation of RE–Si composite coating were finally confirmed. The influence of doping nanoparticles on RE–Si composite coating was also discussed, and the microstructure of coatings shows that the addition of nanoparticles can effectively improve the compactness and uniformity of composite coating. The corrosion resistance of specimens with RE–Si composite conversion coating is improved to a great extent compared with those of substrate or specimens with single rare earth (RE) conversion coating or with silane coating. The result also indicates that corrosion resistance of RE–Si composite coating doping with nanoparticles is further improved. Composition of the composite coatings was analyzed, and the effect of RE elements and silane on coating formation process was investigated. Corrosion resistance mechanism of composite coatings on Mg–Li alloy surface was discussed.