Preparation And Properties Of SiO₂ Based Sol-gel Anticorrosion Coatings On Magnesium Alloy

Magnesium alloy is widely used in many fields for its excellent properties such as low density,machinability,high specific strength and high specific rigidity.However,the magnesium alloy has low free corrosion potential and poor corrosion resistance,which largely limit its widely application.The passivation coatings formed during the corrosion process was loose and porous,which couldn't effectively shield the invasion of corrosive medium.In order to improve the corrosion resistance of magnesium alloys,the SiO₂ based sol-gel films were prepared on AZ91 D magnesium alloy by sol-gel method.The SEM,EDS were used to analyse the morphology,structure and composition of these coatings.The influence of different preparation conditions on the corrosion resistance of SiO₂ based sol-gel film were investigated.The polarization curves,electrochemical impedance spectroscopy and electrochemical noise analysis were used to evalutate the corrosion behaviour during immersion in 3.5 wt.% NaCl solution.The results indicate that:?1?The silane coupling agent can effectively regulating the hydrolysis and condensation rate of sol-gels.Different contents of silane coupling agent affect the corrosion resistance significantly.The concentration increase of KH-570,which leads to higher organic functional group concentration,is helpful to obtain better hydrophobicity and compactness.The coating consisting KH-570 of 5.96 g shows less micro defects,lower corrosion current density at polarization curves and larger capacitance arcs at Nyquist plots.It has been concluded that the coating consisting KH-570 of 5.96 g has better property of corrosion resistance than magnesium alloy.?2?SiO₂-ZrO₂ sol-gel coating catalysted by cerium nitrate shows less film defects and better hydrophobicity than the sol-gel coating catalysted by hydrochloride.Compared with the coating catalysted by hydrochloride,the coating catalysted by cerium nitrate has lower corrosion current density at polarization curves and larger capacitance arcs at Nyquist plots.The coating catalysted by cerium nitrate possesses a better corrosion protection than the coating catalysted by hydrochloride.?3?With different heat treatment methods and temperature,the coatings preform different corrosion resistance.The coating with heat treatment at 80? for 2 h and then at 180? for 30 minutes is smooth enough for its less obvious white particles.The contact angel of the coating is 84.2 °.Compared with the coatings at different heat treatment temperature,the coating shows lower corrosion current density at polarization curves and larger capacitance arcs at Nyquist plots.The Electrochemical noise analysis shows the noise resistance increases and the fluctuates of noise current decreases of the coating compared with the coating treated at room temperature.Therefore,the coating can effectively inhibit metastable pitting transient to the stable pitting.It has been concluded that heat treatment at 80? for 2h and then at 180? for 2h is proper and enough for the coatings.?4?Adding hydrophobic organic component of methyl methacrylate,the coating becomes porous and the surface is attached with white particles and becomes more roughness.As the interaction of functional groups and surface roughness,the contract angel increases from hydrophilic to hydrophobic.With lower corrosion current density at polarization curves and larger capacitance arcs at Nyquist plots,the PMMA-SiO₂-ZrO₂ coating has better corrosion protection.The Electrochemical noise analysis shows that the 1/Rn of PMMA-SiO₂-ZrO₂ coating always maintains on a low value and the fluctuates of noise current is far less than the SiO₂-ZrO₂ coating.The PMMA-SiO₂-ZrO₂ coating shows higher pitting corrosion resistance than the coating of SiO₂-ZrO₂.