Study On Modification Of Magnesium-Rich Primer On AZ91D Magnesium Alloy

Magnesium alloys have excellent properties including light weightand high strength, which makes them attractive in numerous applications,such as aerospace, vehicle and electronic fields. Unfortunately, theengineering applications of magnesium alloys are mainly limited by theirlow corrosion resistance. One of the most effective ways to preventcorrosion is to coat the magnesium substrate. An epoxy coating with puremagnesium particles called Mg-rich primer (MRP) was developed andapplied on AZ91D magnesium alloy in our previous work. Magnesiumparticles acted as sacrificial anodes in MRP so as to provide cathodicprotection.However, magnesium particles are highly active and the reaction timein MRP is short. In this thesis, various materials including silane couplingagents, ceria and carbon black were added in MRP to reduce the rate ofreaction between magnesium powder and electrolyte solution. Machu test,cross scratch test, salt-spray test, electrochemical impedance spectroscopyand potentiodynamic polarization measurement were used to study theprotection mechanism of MRP.1. The addition of2wt.%KH550,2wt.%KH560and3wt.%KH570silane coupling agents can improve the corrosion resistance of MRP.Compared with the MRP without silane coupling agents, Si-O-Si bonds areformed by hydrolytic condensation reaction in MRP with silane coupling agents during immersion, which results in the improvement of coatingdensity. Mg-rich primer adding2wt.%KH560silane coupling agentprovides the best protection to magnesium alloy.2. The addition of0.5wt.%ceria particles can apparently improve theprotection performance of the MRP. However, the addition of a smallamount of ceria (0.1wt.%and0.3wt.%) has no effect on the corrosionresistance of the MRP, while the addition of excessive particles (3wt.%and5wt.%) makes the corrosion resistance of the MRP reduce. The servicelife of the MRP is obviously prolonged by adding appropriate amount ofceria particles, a possible explanation is that ceria decreaseselectrochemical activity of magnesium particles and then corrosion rate ofmagnesium particles decreases.3. The addition of appropriate amount (0.3,0.5and0.7wt.%) carbonblack can improve the protection performance of the MRP. However, theaddition of a very small amount (0.1wt.%) or excessive amount (2wt.%)of carbon black particles can reduce the corrosion resistance of the MRP.Appropriate amount of carbon pigments in the primer can improveelectrical conduction between magnesium particles and the substrate. Thepossible reason is that more magnesium particles are involved in cathodicprotection and service life of the primer is prolonged. However, theaddition of excessive amount of carbon black may cause porosity increasesand barrier effect decreases.