Study On Composite Coatings Formed By Chemical Conversion And Sol-Gel Process On AZ91D Magnesium Alloys

Magnesium alloys,the lightest structural metallic materials,are intituled as the "environmental engineering materiasl in the 21^st century". Their potential applied fields include automobile,aerospace and consuming electronics,etc.However,the extensive use of magnesium alloys was limited du to poor corrosion resistance.In order to improve the corrosion resistance of magnesium alloys, sol-gel technology was combined with chemical conversion technology to prepare composite coatings on the AZ91D magnesium alloys.In this paper,ZrO₂-SiO₂ and TiO₂-SiO₂ sol-gel coatings by means of hydrolysis and condensation of a mixture of tetraethyl orthosilicate(TEOS), tetrabutyl orthotitanate(Ti(O-Bu)₄) and ZrOCl₂·8H₂O with variable molar ratios have been coated on AZ91D magnesium alloys surface pretreated by potassium permanganate-phosphate chemical conversion technology.Some samples of the composite coatings were heated at 300℃and 400℃to compare the change of morphology and properties with untreatment.The microstructure and properties were analysed and tested by SEM,XRD,EDS, DSC,FT-IR,polarization curves,electrochemical impedance and binding force test;in addition,the mechanism,process and influence factors of film-forming were investigated.The results show that the process time,temperature and pH of the phosphating solution have significantly influence on the quality of the chemical conversion films.The chemical conversion films,prepared at processing time for 1-2 minutes,temperature from 40℃to 60℃,pH for 1.3-1.7,have higher corrosion resistance and the best combination which can achieve level 1.The filmformation will loose and have a worse combination at too long time,high pH and high temperature.Composite coatings prepared by chemical conversion and sol-gel process can improve the corrosion resistance of the AZ91D magnesium alloys.The electrochemical tests of the composite coatings indicated that, compared with the AZglD magnesium alloys substruction,the corrosion potential(E_corr) increased 200-300mV,corrosion current density(j_corr) reduced about 2-3 orders of magnitude and the electrochemical impedance improved 4-5 orders of magnitude.It can be seen from binding force tests that the porous structure of chemical conversion films provides an excellent adhesion for the sol-gel coatings.The combination of the composite coatings could achieve level 0 after heat treatment at 300℃.Excessively high temperature would reduce the binding force of the composite coatings.The anatase and brookite phases formed in the TiO₂-SiO₂ composite coatings after heating at 300℃and 400℃.The sizes of grains calculated by Scherrer formula are 13-16nm.