Study On Modified Mg-Rich Coating Having Excellent Performance For Protecting AZ91D Magnesium Alloy

The Mg-rich coating shows good corrosion resistance which may be attributed to the cathodic protection and the barrier effect by the pure magnesium particles. But the protective effect of Mg-rich coating for magnesium alloy still can not meet the requirement of practical application. Therefore, in order to improve the protective performance of Mg-rich coating, the mechanism of protective behavior of Mg-rich epoxy coating on AZ91D magnesium alloy and modification are investigated.In this thesis, a corrosion resistant coating for AZ91D magnesium alloy is prepared by adding pure magnesium particles, aluminium triphosphate (SAP) and ceria in epoxy coating. The performance and mechanism of protective behavior of the coating in3.5%NaCl solution are studied with methods of Machu test, electrochemical impedance measurement (EIS), scanning electron microscope (SEM). X-ray photoelectron spectroscopy (XPS), and open circuit potentials (OCP). In the Mg-rich coating, SAP and ceria can decrease the activity of magnesium particles and prolong the protection time of the coating. On the other hand, the compactness and the barrier property of the coating are improved.The Mg-rich primer provides protection to magnesium alloy substrate by a four-stage mechanism according to the reaction of magnesium particles:In the first stage, the electrolyte permeats into the coating and magnesium particles reacted with water and oxygen molecules gradually. Then in the next stage, the precipitation of corrosion product Mg(OH)2of magnesium in the coating provides the barrier protection to some degree. After that, in the third stage, magnesium particles are enshrouded by corrosion products, and this rate of cladding reaches a balance with the reaction rate of magnesium dissolution. In the last stage, magnesium particles lost the cathode protection to the magnesium alloy, and the coating deteriorats gradually.The influence of changing the size of magnesium powder in Mg-rich coating for protecting magnesium alloy is studied. The Mg-rich coating which contains smaller magnesium powder that has more activity shows better performance in strongly corrosion solution, while in the neutral solution, the protection of Mg-rich coating which contains bigger magnesium powder for magnesium alloy is better, because the porosity of this coating is lower, which improves the barrier property of the coating. On the other hand, the bigger magnesium powder has less activity, so it provids longer cathodic protection time and improves the lifetime of the coating.A silane film is prepared on AZ91D magnesium alloy and the effects of the silane treatment on the performance of the Mg-rich primer on AZ91D alloy are studied. After silane treatment, Si-O-Mg covalent bonds form between the silane film and magnesium substrate. Therefore, the adhesion of the aluminum tri-polyphosphate and ceria modified Mg-rich epoxy coating to AZ91D substrate increases obviously. The silane pre-treatment significantly improves the performance of the Mg-rich primer on AZ91D alloy, which is attributed to the strengthened barrier effect of the coating system.