Preparation And Corrosion Resistance Of Modified Silane Film On Magnesium Alloy Surface

Magnesium alloys are the lightest commonly used engineering material with their high strength,elastic modulus,and good processing performance,and no pollution to the environment,and are considered as green metal engineering materials.However,its poor corrosion resistance limits their application areas.In this work,in order to improve the corrosion resistance of magnesium alloys,silane treatments were developed toreplace the traditional toxic and polluting chromium treatment and phosphating treatment.It is very significative to their practical applications.In this thesis,the main works are to study the preparation processes of the silane films on AZ91D magnesium alloys for improving their corrosion resistance.In prepared series of silane films,the film by KH560 silane treatment possessed the best corrosion resistance.The optimized process was obtained by investigating the effect of volume ratio of water to alcohol,pH,hydrolysis time,and silane volume content in silane hydrolysate.The corrosion resistance of the silane films were studied by copper sulfate drop test,potentiodynamic polarization curve and AC impedance test.The surface morphology and composition of silane film were studied by SEM-EDS,Fourier infrared spectrum and XPS.An optimum process by KH560 silane treatment was:volume ratio of alcohol:water:silane=45:45:10,the pH value of silane hydrolysate=5,hydrolysis time=24 hours,immersion time in silane hydrolysate=5 min,curing time=1 hour,and curing temperature=130?.The corrosion current density of the silanized samples in 3.5%NaCl solution decreased significantly.EIS test shows that the capacitive arc radius of the treated sample is much larger than that of the untreated sample.It maybe is mainly due to the fact that KH560 silane molecules contain epoxy groups,which can open the ring to form a more compact and stable structure during curing,thus improving the physical barrier properties of the silane film.The influence of the pretreatment with phytic acid and cerium nitrate on Mg alloy sample surface before silane treatment was studied.The results from SEM,polarization test,AC impedance test indicate that the pretreatment with phytic acid and cerium nitrate can obviously improve the integrity and corrosion resistance of the silane film samples,because phytic acid pretreatment increased the number of hydroxyl groups on the substrate surface and enhanced the bond of the silane film to the substrate surface.Insides,the presence of Ce4+in the film also can improve their corrosion resistance.In addition,the modified silane films were prepared by adding cerium nitrate or nanoparticles in silane hydrolysate,and electrochemical assisted deposition method.The superhydrophobic/silane composite film also was studied.The test showed that the above modifications or treatment methods can improved the corrosion resistance of the silane films.