Study On The Self-sealing Ceramic Coating Preparation Of Micro-arc Oxidation On AZ91D Mg Alloy In Zirconium-salt Solution

The traditional micro-arc oxidation(MAO) electrolytic can perform MAO coatings on Magnesium alloy.The surface of micro-arc oxidation film evenly distributes a large number of volcanic-like microporous.These micropores are the weak zones of the coatings, aggressive mediums can pass through them and corrode alloy substrates.In order to improve the coating corrosion resistance, this paper utilise self-sealing type zirconium salt electrolyte system to perform self-sealing type ceramic coatings on AZ91 D Magnesium alloy surface, the sealing element and the film have the same component. In this way, film growth and micropores closure can be done in one operation, and film also can improve the corrosion resistance.With the help of Scanning Electron Microscope(SEM) and X-ray Diffraction Analysis(XRD), we can explore the influence of the concentration of Ammonium fluoride, Zirconyl carbonate and Ammonium zirconium carbonate on MAO coatings. The results show that the main phase composition of self-sealing type ceramic coatings are MgF2, ZrO2 and Mg2Zr5O12. Furthermore, Ammonium fluoride is helpful to reduce the arc voltage and motivate development of the film. When the additive quality of Zirconyl carbonate is 10g/L, the porosity reaches the minimum value of 2.1%. Meanwhile, ammonium zirconium carbonate can increase the density of unit area arc, decrease process time and reduce the film porosity significantly. Besides, with the rise of its concentration, the film porosity goes down to the minimum value of around 2% and then tends to be stable. It is found that, the best concentration of Ammonium fluoride, Zirconyl carbonate and Ammonium zirconium carbonate are 15g/L, 10g/L and 30mL/L respectively.This paper adopts the control voltage and processing time, establish relationship curves of processing voltage with porosity and thickness, processing time with porosity and thickness. Establishing micropores sealing model with SEM photos and EDS element maps of micropores sealing morphology in every period is easy to conclude that micropores sealing has three different stages which are the formation of micropores and colloidal phase & ion adsorption, the filling of granular sinter, the cementation and seaming of cracks.Study on the corrosion of MAO coatings performed by different processes in 3.5% Sodium Chloride Solution finds that corrosion resistance of the film is closely related on its porosity and thickness. Using natural aging weather resistance test, drop corrosion test, CuCl2 solution immersion corrosion test and electrochemical test to analyse the corrosion resistance of AZ91 D Magnesium alloy samples and MAO samples processed respectively by self-sealing electrolyte and unself-sealing electrolyte. The results show that self-sealing film has better corrosion resistance than unself-sealing film, and micropores sealing has dramatic effects on increasing film’s corrosion resistance.