| Magnesium alloys are good materials for many applications includingautomotive, aerospace and other industries due to their very high specificstrength and good formability. However, the engineering applications ofmagnesium alloys are limited by their poor corrosion resistance. In thisresearch work, a Mg-rich epoxy primer was prepared to protect AZ91Dmagnesium alloy and the protective performance of the Mg-rich primer onAZ91D magnesium alloy was improved. Meanwhile, various pretreatmenttechniques for magnesium alloy were investigated to improve the protectiveperformance of the Mg-rich primer.The Mg-rich epoxy primer was prepared by adding pure magnesiumparticles to an epoxy coating. The Mg-rich primer provided cathodicprotection effect for AZ91D alloy. And the Mg-rich primer provided betterprotection for the substrate than original epoxy coating without Mg addition.The precipitation of Mg(OH)2in the coating also provided some degree ofbarrier protection.Then, effect of magnesium content in the Mg-rich primer for AZ91D alloy on the mechanism of corrosion protection was investigated withelectrochemical research methods. The effects of Mg-rich primer on AZ91Dalloy are suggested as three stages: in the first stage, the MgRP showedbarrier effect. The electrolyte permeated into the coating and the magnesiumparticles were gradually activated. In the latter stage, cathodic protection wasactive. The time length of the second stage is decided by the total amount ofmagnesium available and the consumption rate of magnesium particles.Finally, precipitation of magnesium corrosion products in the primer leads toanother barrier effect. Mg-rich primer could retard the corrosion rate ofAZ91D alloy to some extent. Mg content should be kept in a proper level, onwhich MgRP could provide a long cathodic protection and lifetime.The protective performance of a Mg-rich primer on AZ91D magnesiumalloy was greatly improved by addition of aluminum tri-polyphosphatepigment into the primer. The effects of aluminum tri-polyphosphate pigmentin the Mg-rich primer were investigated. In the first stage, the pigment SAPprolongs the cathodic protection of the Mg-rich primer for AZ91D alloy by apH buffering effect. Then, the corrosion products of the magnesium particles,composed of phosphates and oxides of magnesium, form in the primer, whichenhances the barrier effect of the primer. Finally, a protective layer composedof magnesium phosphates and oxides forms on the substrate surface.A silane film is prepared on AZ91D magnesium alloy and the effects ofthe silane treatment on the performance of the Mg-rich primer on AZ91D alloy are studied. After silane treatment, Si-O-Mg covalent bonds formbetween the silane film and magnesium substrate and Si-O-Si structure formsin the silane film. Therefore, the adhesion of the Mg-rich primer to AZ91Dsubstrate increases obviously. The silane pre-treatment significantly improvesthe performance of the Mg-rich primer on AZ91D alloy, which is attributed tothe strengthened barrier effect of the coating system.The anodic oxidation and micro-arc oxidation pretreatment of magnesiumalloy could enhance the adhesion of the Mg-rich primer to AZ91D substrate.In this research, the effects of oxidation pre-treatment on the performance ofMg-rich primer on AZ91D alloy were investigated. The anodic oxidation andmicro-arc oxidation pre-treatment significantly improves the performance ofthe Mg-rich primer on AZ91D alloy, which is attributed to the retardedcorrosion rate of the substrate alloy. However, the micro-arc oxidationpre-treatment is superior to the anodic oxidation pre-treatment due to that thecorrosion resistance of the micro-arc oxidation film is better than that of theanodic oxidation film.Finally, the multi-layer coatings on pretreated AZ91D alloy was preparedby application of fluoro resin topcoat on Mg-rich primer. It is found that themicro-arc oxidation pre-treatment provides better protective performance thanthe silane pre-treatment in the coating system. The coating system withmicro-arc oxidation pre-treatment had good barrier effect after2400h of salt spray test and could provide good protective performance to AZ91D alloy. |
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